) US005431,792A United States Patent (19) 11 Patent Number: 5431,792 Morgan et al. 45 Date of Patent: Jul. 11, 1995

54 METHOD OF MAKING 5,135,626 8/1992 Mani et al...... 204/1824 HYPOPHOSPHOROUSACD 5,139,632 8/1992 Chlanda et al...... 204/1824 5,162,076 11/1992 Chiao et al...... 204/1824 75 Inventors: Russell J. Morgan, Grand Island; 5,225,052 7/1993 Takikawa et al...... 204/90 Robert L. Zeller, III, Youngstown, both of N.Y.; Joseph Dealmeida, FOREIGN PATENT DOCUMENTS Carrollton, Tex. 0459751A1 12/1991 European Pat. Off. . 73 Assignee: Occidental Chemical Corporation, 92/11080 7/1992 WIPO. Niagara Falls, N.Y. OTHER PUBLICATIONS 21 Appl. No.: 169,021 K. N. Mani, “Electrodialysis Water Splitting Technol 22 Filed: Dec. 20, 1993 Yournal of Membrane Science, 58, (1991) pp. 51) Int. Cl...... so a sease oeuvoo BOD 61/44 Abstract, G. E. Revzin et al., Ser. Khim. Nauk (2), 52 U.S. C...... 204/1824; 204/90; 125-9 (1987). 204/98; 204/103 VanWazer, “ And Its Compounds', vol. I, 58 Field of Search ...... 204/103, 1824, 90, p. 359 (1958). 204/98 Primary Examiner-John Niebling (56) References Cited Assistant Examiner-Arun S. Phasge U.S. PATENT DOCUMENTS Attorney, Agent, or Firm-Wayne A. Jones; Richard D. Fuerle 2,976,117 3/1961 Pahud ...... 23/107 3,052,519 9/1962 Bianchi et al...... 23/107 57 ABSTRACT 4,082,8353,787,304 4/19781/1974 Chlanda et al...... 204/182.4 E. Pisclosed is a method of making hypophosphorous aid> 4,107,015of 3/16788/1978 Ninia".Chlanda et al...... 42.3/si fromlytic watersodium splitting hypophosphite upon an aqueous by performing solution electrodia of sodium 4,219,396 8/1980 Gancy et al...... 204/1824 hypophosphite. The process can be tied into an existing 4,265,866 5/1981 Arzoumanidis et al...... 423/304 process for producing wherein 4,391,680 7/1983 Mani et al...... 204/98 the product of the sodium hypophosphite process is 4,504,373 3/1985 Mani et al...... 204/1824 used as a starting material in the hypophosphorous 4,521,391 6/1985 Estes it..." 423/307 process and the depleted sodium hypophosphite solu : A. inci et al...... 2: tion from the hypophosphorous acid process, which 496,838E. 12/1990E. ManiE.M., et al... ..", thecontains pH in some the sodiumhypophosphorous hypophosphite acid, process. is used to adjust 4,999,095 3/1991 Chlanda et al...... 204/1824 5,006,211 4/1991 Paleologou et al...... 204/182.4 23 Claims, 3 Drawing Sheets

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|-!2.‘9 5431,792 1. 2 8.0x10-2 (pKa=1.1). On the other hand, K. Mani in METHOD OF MAKNG HYPOPHOSPHOROUS WO 92/11080 defines a weak acid for the purpose of ACD electrodialytic water splitting as one with a pKa of 3 or greater, but generally less than 11. The electrodialytic BACKGROUND OF THE INVENTION 5 water splitting of sodium hypophosphite would there This invention relates to a method of making hypo fore be expected to produce hypophosphorous acid (HPA) using electrodialytic water having a concentration of about 1 normal at a current splitting. It also relates to an improved method of mak efficiency of 80%. Another reason, relating to produc ing sodium hypophosphite wherein the sodium hypo tion in conventional electrolytic membrane cells, may phosphite is used in an electrodialytic water splitting be due to product quality concerns associated with process to make hypophosphorous acid and the de oxidation of the hypophosphorous acid at the anode to pleted sodium hypophosphite from the electrodialytic produce phosphite anion contamination of the product. water splitting process is used to adjust the pH in the process for making sodium hypophosphite. SUMMARY OF THE INVENTION Electrodialytic water splitting is a process in which as We have discovered that when hypophosphorous solution of a salt is subjected to a direct current, decom acid is produced by the electrodialytic water splitting of posing water and causing the anions and cations to pass sodium hypophosphite at 80% current efficiency, the through anion exchange and cation exchange mem concentration of the hypophosphorous acid produced is branes, respectively, forming an acid and a base in sepa rate compartments. For example, if a solution of sodium 20 not the 1 normal concentration that would be expected chloride is placed between a cation exchange membrane for a strong acid, but instead is about 2.2 normal. While and an anion exchange membrane and is subjected to a we are unable to account for the unusually high concen direct current, hydrochloric acid will form on the other tration of hypophosphorous acid that can be obtained, side of the anion exchange membrane and sodium hy the result is very beneficial because it reduces the droxide will form the other side of the cation exchange 25 amount of evaporation that is needed to produce high membrane. The concentration of the acid formed is a concentrations of the acid. As a result, the production function of the current density, but the higher the con of hypophosphorous acid by electrodialytic water split centration of the acid, the lower will be the current ting may be competitive with other methods of produc efficiency. ing that acid. There is also a relationship between the strength of 30 We have also discovered that the production of hy the acid (i.e., how strongly it dissociates to liberate H) pophosphorous acid by electrodialytic water splitting and the concentration of the acid that can be obtained at can be tied into an existing process for producing so a particular current efficiency. At the same current dium hypophosphite. That is, the sodium hypophos efficiency, it is possible to obtain higher concentrations phite product from an existing process can be used as of weak than of strong acids. The relationship 35 the starting material in the electrodialytic water split between the strength of the acid and the concentration ting process and the depleted sodium hypophosphite of the acid that can be obtained at a given current effi salt from the electrodialytic water splitting process, ciency can be found in an article by K. N. Mani titled, which contains some hypophosphorous acid, can be "Electrodialysis Water Splitting Technology,” Journal used to adjust the pH in the existing sodium hypophos of Membrane Science, 58 (1991) pps. 117-138 at page 40 phite process. When this is done, the existing sodium 122. (This article is hereinafter referred to as "Mani, hypophosphite process is improved in several ways. 1991'.) In that article it states that at a current effi The product value of the hypophosphorous acid in the ciency of 80% or higher and a current density of 100 depleted salt stream is recovered instead of being neu mA/cm2 the concentration of a strong acid that can be tralized with the dilute caustic stream prior to resatura obtained is about 1 normal and the concentration of a 45 tion with sodium hypophosphite crystal. This not only weak acid that can be obtained is about 3 to about 6 allows product acid, which would otherwise be lost, to normal. Because hypophosphorous acid is a strong be effectively utilized in the process, but also allows monobasic acid, the normality and molarity are equal more of the weak caustic product solution to be utilized values (i.e., 1N H3PO2=1M H3PO2). as feed in the sodium hypophosphite process. The use of Hypophosphorous acid is now produced by the acidi- 50 acids other than hypophosphorous acid in the existing fication of sodium hypophosphite. For example, one sodium hypophosphite process to adjust the pH is elimi can load a cation exchange resin with ions nated, and therefore the sodium hypophosphite product and pass a solution of sodium hypophosphite over the will contain hypophosphite instead of foreign anions resin so that the sodium ion is exchanged for the hydro and be purer. This will also reduce the amount of cal gen ion and hypophosphorous acid is produced. Until 55 cium salt waste material that is generated, which has a now, the production of hypophosphorous acid by elec significant disposal cost. Also, the salt waste trodialytic water splitting has not been suggested or material is thixotropic, which means that it releases attempted. One possible reason for this is that the water when it is shaken, and since it must be disposed of readily available references for HPA describe it as a as a solid waste, this is unacceptable for companies that strong acid. As a strong acid, the concentration of HPA 60 accept solid waste for disposal. The substitution of hy that could be produced at a given current efficiency pophosphorous acid for other acids in the existing so would be low, requiring a choice between high power dium hypophosphite process unexpectedly eliminates consumption to make a high concentration of acid and this problem. high energy consumption to evaporate water from a In addition, the dilute caustic generated in the elec low concentration of acid. Van Wazer, Phosphorus and 65 trodialytic water splitting process can be incorporated Its Compounds, Vol I., page 359 (1958), the definitive into the sodium hypophosphite process without penalty reference on phosphorous chemistry, describes HPA as as a source of NaOH in the phosphorous hydrolysis a strong monobasic acid with a dissociation constant of portion of the process. The dilute caustic can also be 5,431,792 3 4 used to generate calcium from lime needed in would detrimentally impact product quality. For this the sodium hypophosphite process. reason, the use of electrodialytic water splitting tech While there are a number of ways of tying together nology represents a distinct improvement over conven the electrodialytic water splitting process for producing tional electrolysis and is probably the reason why elec hypophosphorous acid with existing processes for pro trolytic production of HPA has not been reported in the ducing sodium hypophosphite, in one method the open literature. (Cationic exchange membranes and mother liquor from an existing sodium hypophosphite anionic exchange membranes are commercially avail process, which contains both sodium hypophosphite able. Bipolar membranes are manufactured by Aqua and sodium phosphite, could be used in the electrodia tech Systems, a Division of Allied-Signal, and are de lytic water splitting process. This eliminates the need to 10 scribed in U.S. Pat. Nos. 2,829,095; 4,116,889; 4,024,043; purify the liquor and evaporate water from it. Although and 4,082,835; herein incorporated by reference.) this liquor contains both phosphite ion and hypophos The compartments are sealed by means of gaskets phite ion, it is believed that the two ions can be at least (not shown) and the fluids in the compartments are partially separated in the electrodialytic water splitting recirculated to prevent the build-up of ions near the process because the hypophosphite ion is smaller than 15 membranes. The fluid in acid compartments 1A, 2A, the phosphite ion and can therefore pass more easily 3A, and 4A is recirculated through lines 7 and 8 by through the anion exchange membrane. European Pa means of pump 9. Hypophosphorous acid product is tent Application 0459751A1 also teaches that adjust removed through line 10 and additional water is added ment of the depleted salt feed to alkaline conditions will through line 11. The capacity of the recirculating sys improve the separation by forcing the phosphorous 20 tem is increased by means of storage tank 12. Similarly, anion to become divalent. fluid in salt compartments 1S,2S,3S, and 4S is recircu The electrodialytic water splitting process for pro lated through lines 13 and 14 and storage tank 15 by ducing hypophosphorous acid can also be tied into the means of pump 16. Depleted salt solution is removed existing ion exchange process for producing hypophos through line 17 and additional fresh salt solution is phorous acid. That is, instead of using the ion exchange 25 added through line 18. Fluid in base compartments 1B, process to produce all the hypophosphorous acid from 2B, 3B, 4B, and 5B is recirculated through lines 19 and sodium hypophosphite, it could instead be used to ex 20 and storage tank 21 by means of pump 22 and sodium change hydrogen ions for the small concentration of hydroxide is removed through line 23 while freshwater sodium ions in the hypophosphorous acid produced by is added through line 24. Finally, the electrode rinse the electrodialytic water splitting process of this inven 30 solution in compartments ER and ER is recirculated tion. This would lower the overall cost of producing through lines 27, 28, and 30 and storage tanks 25 and 29 hypophosphorous acid of high purity. by means of pump 26. A purge, if required, can be re moved through line 31 while freshmake-up solution can BRIEF DESCRIPTION OF THE DRAWINGS be added through line 32. Oxygen and hydrogen formed FIG. 1 is a diagrammatic side view which illustrates 35 in the electrode rinse compartments ER and ER disen an electrodialytic water splitting process according to gage in vessels 29 and 25 and exit through lines 33 and this invention for producing hypophosphorous acid. 34, respectively. The removal of product solution from FIG. 2 is a block diagram which illustrates an existing the respective recirculation loops can be accomplished process for producing sodium hypophosphite and at any point in the loop as long as the pressure differen shows how the electrodialytic water splitting process tial between compartments is not influenced by the for producing hypophosphorous acid according to this removal process. invention ties into that existing process. The production of gaseous hydrogen 34, and oxygen FIG. 3 is a graph giving the results of Examples 3, 4, 33, occurs only at the anode and cathode. Since the and 5. In FIG. 3 the ordinate is current efficiency and production of these gases increases energy consumption the abscissa is normality (Eq/L). 45 per unit of product, it is advantageous to juxtapose a large number of units using common bipolar membranes DESCRIPTION OF THE INVENTION in between them to minimize the production of these FIG. 1 shows an electrolytic cell according to this gases relative to the product streams. However, if the invention. The cell is formed from four juxtaposed number of units in the cell is too large, power consump units, 1, 2, 3, and 4. Each unit has an acid compartment, 50 tion again begins to increase and the total voltage across 1A, 2A, 3A, and 4A, a salt compartment, 1S,2S,3S, and the stack becomes excessive. While the drawing illus 4S and a base compartment, 1B, 2B, 3B, and 4.B. The trates only four units, any number of units can be simi anode 5 is in the anode side electrode rinse (ER) acid larly juxtaposed and a typical cell may contain about compartment and the cathode 6 is in the base side elec one to about two hundred units. trode rinse (ER") compartment. Separating the acid and 55 To operate the cell, it is assembled and HPA and salt compartments of each unit are anion exchange weak NaOH solutions are circulated through the re membranes 1, 2, 3, and 4 and separating the salt spective acid and base compartments while an aqueous and base compartments are cation exchange membranes solution of sodium hypophosphite and HPA is circu 1, 2, 3 and 4t. Beginning each unit is a bipolar lated through the salt compartment. A weak solution of membrane 1, 2, 3- and 4. The two elec caustic is also circulated through the electrode rinse trode rinse membranes, ER, separate the beginning compartments. The salt compartment contains both and ending of the unit cells from the electrode compart sodium hypophosphite and HPA because of the inequal ments and generally consist of commercially available ity between the acid and base current efficiencies. When Nafion (R) type membranes. The rinse compartments the base current efficiency is greater than the acid cur which are fed dilute caustic are segregated from the unit 65 rent efficiency, the salt compartment contains sodium cells in hypophosphorous acid production because the hypophosphite and HPA. When the acid current effi oxidizing environment of the anode compartment could ciency is greater than the base current efficiency, the oxidize the reducing acid, generating phosphites which salt compartment contains sodium hypophosphite and 5,431,792 5 6 NaOH. Under most conditions, the base conductivity about 5 wt.%. In order to maximize product quality, will be greater than the acid conductivity. The concen the hydrodynamic head of the HPA recirculating loop tration of sodium hypophosphite in the salt loop that is should be 1 to 2 psi higher than the salt or base recircu used can vary from about 1 wt.% (based on solution lating loops. The higher pressure discourages mass weight) up to saturation (about 55 wt.%), but it is pref erably about 5 to about 30 wt.% because at less than transport of NaOH and sodium hypophosphite into the about 5 wt.% the solution conductivity drops to a point acid compartment. If a pinhole develops in a membrane, where cell voltage and heat generation associated with operating at higher differential pressures will minimize the increased ohmic drop make the operation of the cell HPA contamination. difficult, and at over about 55 wt.% there is a risk that Keeping the sodium hydroxide concentration low sodium hypophosphite may precipitate in the men 10 increases the acidity of the spent salt solution, which is branes. The HPA concentration in the salt loop can advantageous if the spent salt solution is to be used for vary from 0 wt.% to the same concentration as the acid pH adjustment in the existing process for producing compartment (a maximum of about 55 wt.%). The cell sodium hypophosphite. For that purpose, a pH of about can be operated at temperatures between 10 C. and 1.5 to about 4 for the depleted salt solution is desirable. about 50 C., but it is preferable to operate at about 25' 15 Also, if the acid content of the spent salt solution is to be to about 40' C. as the cell generates heat and cooling used for pH control in the existing process for produc requirements are reduced by operating above room ing sodium hypophosphite, the concentration of acid temperature. No pH adjustments are normally required and base in the acid compartment and base compart to operate the cell. The current density used is prefera ment, respectively, can be controlled so that the acid bly about 50 to about 155 mA/cm2 and the cell voltage 20 content of the spent salt solution is appropriate for that can vary from about 1.6 to about 2.5 volts/unit, depend purpose (i.e., so that at least no additional acid is needed ing on cell geometry, its operation and current density, in the sodium hypophosphite process to adjust the pH). but is typically about 1.8 to about 2.0 V/unit, direct The concentration of the hypophosphorous acid in current (DC). While recirculation of the fluids is re the acid compartment can be increased up to about 55 quired to prevent damage to the membranes, the rate 25 wt.%, but the current efficiency falls off as the concen can vary widely although about 5 to about 15 cm/sec is tration increases and the concentration of sodium in the recommended. The overal reaction in the cell is: product acid increases. A current efficiency of 80% at a concentration of 2.23 normal hypophosphorous acid 30 (14.3 wt.%) can be achieved. This is unusually high, a There are a number of ways of operating the cell. In little more than double what the teachings of the prior the batch mode, most or part of the fluid in the recircu art would lead one to expect. lation system of the acid compartment is removed as FIG. 2 illustrates an existing process for producing product and is partially replaced by fresh water when sodium hypophosphite and shows how the electrodia ever the acid concentration reaches a certain predeter 35 lytic water splitting process of FIG. 1 ties into that mined value. For example, when the hypophosphorous process. Calcium hydroxide, produced in mixing vessel acid concentration reaches a predetermined concentra 35 is made by mixing water from line 36 with lime tion between 5 and 55 wt.% sufficient acid can be (CaO) from line 37. The calcium hydroxide passes removed and replaced with water to reduce the acid through line 38 to reactor 39 where it is mixed with concentration to the lower limit. Preferably, this is done phosphorus-containing water from line 40 and elemen when the acid concentration reaches a predetermined tal phosphorus from line 41 along with sodium hydrox concentration between about 10 and about 30 wt.%. ide from line 42. The reaction: The base and salt loops can be operated in a similar fashion and combinations of batch and continuous pro cesses for the product and feed streams can also be 45 employed. In the continuous mode of operating the system, acid is believed to occur in reactor 39. and hydro product is continuously removed and replaced with gen gases are removed from reactor 39 through line 43. fresh water to maintain the hypophosphorous acid con The products, a slurry of precipitated calcium phos centration at a certain predetermined concentration. 50 phite and dissolved calcium hypophosphite in sodium The concentration of the acid is maximized when there hypophosphite solution, pass through line 44. They are is no water addition to the cell at a given set of operat combined with mother liquor from solid/liquid separa ing conditions. While the batch mode of operation is tor 45 from line 46 and are fed to solid/liquid separator more energy efficient, it requires more careful monitor 47, where the precipitated calcium phosphite is re ing of the system and more controls in the system than 55 moved through line 48. The solution of sodium and does the continuous mode. calcium hypophosphite passes through line 49 to cal In both modes it is advantageous to keep the sodium cium precipitator 50 where sodium carbonate in line 51 hypophosphite salt concentration feed to the recircula is added to precipitate calcium carbonate. The slurry tion loop at a uniform level and preferably between 5 passes through line 53 to calcium carbonate solid/liquid and saturation (about 55 wt.%). It is also preferable to separator 54 where the calcium carbonate is removed keep the sodium hydroxide concentration low to pre through line 55. The liquor of sodium hypophosphite vent sodium ions from migrating from the base com passes through line 56 where, in the existing process, an partment into the acid compartment through the bipolar acid is added through line 57 to lower the pH and pre membrane, which would reduce the concentration of cipitate sodium hypophosphite in evaporator/crystal acid and contaminate the remaining acid with salt. A 65 lizer 58. The slurry passes through line 59 to solid/liq concentration of sodium hydroxide between about 1. uid separator 45 which separates the mother liquor in and about 20 wt.% is recommended and it is preferable line 46 from crystalline sodium hypophosphite. The to keep the sodium hydroxide concentration below crystalline hypophosphite in line 60 is dried in dryer 61 5431,792 7 8 and forms the product in line 62. The mother liquor in line 46 is recycled and combined with line 44 which feeds the solid/liquid separator 47. Further details on #BV ppm Na wt % H3PO2 1.1 2.2 2.8 existing processes for producing sodium hypophosphite 3.2 5.8 14.0 which can be combined with the process of this inven 5.4 32 15.4 tion for producing hypophosphorous acid can be found 7.5 219 15.4 9.6 57 15.3 in U.S. Pat. Nos. 3,052,519; 2,976,117; 4,521,391; 11.7 118S 15.2 4,380,531; and EP 459751A1, herein incorporated by 13.9 1194 15.1 reference. 16. 1509 15. If crystalline sodium hypophosphite is used in the O 18.2 1537 15.2 hypophosphorous acid process, it is dissolved in tank 20.3 1565 15.0 63, then passed through line 64 to cation exchange ma 22.5 1560 15.1 terial 65, which removes any divalent (e.g., calcium and The experiment was stopped because the IX resin was magnesium) ions that may be present as these may dam 5 saturated with Na. Approximately 420 ml of deion age the ion-exchange membranes. (Depending on the ized water was used to backwash the IX bed and to purity of the product formed in the sodium hypophos remove residual H3PO2. Then, 190 grams of 10 wt.% phite process, ion exchange material 65 may be unneces HCl was used to regenerate the resin, which was subse sary.) The purified sodium hypophosphite passes quently backwashed with approximately 190 ml of de through line 66 into the salt compartment (and through 20 ionized water. The column was placed in service again. its recirculation system) of electrolytic cell 67. Water is Based upon the amount of sodium removed by the IX added to the cell from line 68 and sodium hydroxide resin, the capacity was estimated to be 0.61 meq/ml of produced in the process is removed through line 69 resin. The reported capacity is 2 meq/ml of resin. where it is sent to vessel 35 or to reactor 39 (dotted line Therefore, the resin Hi/Nat ratio is 70) or to the calcium precipitator 50 (dotted line 71). 25 (2-0.61)/0.61=2.27, indicating a much better Nat When the sodium hydroxide or a portion thereof is sent selectivity than expected from the literature (e.g., 3.65). to vessel 50 through dotted line 71, CO2 is added through dotted line 52 in order to generate carbonate in EXAMPLE 2 situ. The sodium carbonate entering through line 51 can Hypophosphorous acid (14.9 wt.% with 1540 ppm then be reduced nearly stoichiometrically. 30 Na) was pumped from a reservoir to the top of the Depleted salt from the hypophosphorous acid pro regenerated bed used in Example 1. The solution was cess can be recycled through line 72 to crystallizer fed at a rate of 0.149 BV/min. Samples were collected /evaporator 58. If this is done, it is not necessary to add and analysis performed in an identical manner as given an acid from line 57 to the sodium hypophosphite li in Example 1. quor. A portion of the depleted salt can be recycled to 35 dissolving tank 63 through line 73. Hypophosphorous #BV ppm Na wt % H3PO2 acid, which is typically about 10 to about 50 wt.%, is 1.3 57 5.7 removed through line 74. It can be taken as a product 3.5 63 4.1 from line 75 or it can be sent to evaporator 76 and con 5.7 88 5.0 centrated to about 25 to about 70 wt.% and taken as a 7.9 288 15. 10. 810 5. concentrated product from line 77. 12.4 121 14.9 The sodium hypophosphite used in the hypophospho 14.6 461 15.0 rous acid process can be obtained from various loca 6.8 1309 17.9 45 9.1 1542 14.9 tions in the sodium hypophosphite process. If sodium 21.3 1535 14.9 hypophosphite crystals are used they can be obtained 23.6 1537 14.8 from solid/liquid separator 45 through line 78 or from the product drier through line 79. Alternatively, sodium hypophosphite liquor in line 56 or mother liquor in line This example shows that the column can be regenerated 50 and be used to remove sodium from hypophosphorous 46 can be sent through lines 80 or 81, respectively, to acid. ion exchange bed 65. Based upon the amount of sodium removed by the IX The following examples further illustrate this inven resin, the capacity was estimated to be 0.66 meq/ml of tion. resin. The reported capacity is 2 meq/ml of resin. EXAMPLE 1. 55 Therefore, the resin H--/Nat ratio is (2-0.66)/0.66=2.03, indicating a much better Nat A bed volume (BV) of 22 cm3 of Dowex G-24 ion selectivity than expected from the literature (e.g., 3.65). exchange (IX) resin was placed in a column. The IX resin bed had a length to diameter ratio of 6.8. Hypo CELLEXAMPLES phosphorous acid (15.0 wt.%) with 1522 ppm Na was In the following examples, a laboratory cell setup was pumped from a reservoir to the top of the bed. The utilized which was similar to those described in WO solution was fed at a rate of 0.144 BV/min. Samples 92/11080. A laboratory cell which is configured as were collected as a function of time and related to the shown in FIG. 1, was used to accomplish the splitting of total number of BV which had passed through the col sodium hypophosphite into hypophosphorous acid and umn. The ppm Na was measured using a sodium ion sodium hydroxide utilizing a combination of anion, selective electrode and the wt.% H3PO2 was measured cation, and bipolar ion exchange membranes. The cell by NaOH neutralization to pH 7. The data were col consisted of two end blocks which were fabricated to lected and are: form the respective anode and cathode electrode rinse 5,431,792 10 compartments for the cell. The ion exchange mem Ion exchange was accomplished by passing the salt branes, separated one from the other with gaskets, solution through a column packed with the Duolite formed the individual acid, base, and salt compartments. resin, which is manufactured by Rohm & Haas. The The thickness of the gaskets was 50 mils, and the gaskets initial Ca concentration of the 25 wt.% solution was were cut so as to create an open area of 3.6 square measured by Inductively Coupled Argon Plasma (ICP) inches central to the gasket to allow solution flow to be 8 ppm and that of the final solution <1 ppm. through the exposed faces of the membranes. As an aid to solution distribution across the face of the mem EXAMPLE 3 branes, expanded plastic mesh was set into the compart This example shows that higher hypophosphorous ment formed by the open area of the gasket and 10 acid concentrations are obtained at a given current bounded by adjacent membranes. The gaskets were efficiency relative to those reported for other strong manifolded at the bottom and top of each compartment acids produced during the electrodialytic splitting of to create channels of flow for the acid, base, and de sodium hypophosphite salt into its acid and base compo pleted salt solution entering and exiting the cell stack nents using a three-compartment cell equipped with and to prevent cross contamination of the solutions 15 anion, cation, and bipolar membranes. between the flow channels. In this test, the salt and base streams were operated in The order of the membrane types, starting from the a continuous bleed and feed mode. The base loop caus anode end block, consisted of a cation membrane fol tic concentration was maintained at about 5 wt. 26 lowed by four sets of three membranes which repeated NaOH with water addition. The salt loop was operated the pattern bipolar, anion, and cation. The bipolar mem to maintain a conductivity of 40-50 millisiemens/centi branes were oriented so that the anion side always faced meter (ms/cm) though the continuous addition of the 25 the anode. An additional cation exchange membrane wt.% sodium hypophosphite feed solution. followed just ahead of the cathode compartment. Thus, The hypophosphorous acid loop was operated in a there were a total of 15 compartments consisting of 2 batch configuration without water addition. Hypophos electrode rinse compartments, 5 base compartments, 4 25 phorous acid (0.5N) was used to initially inventory the acid compartments, and 4 salt compartments. The cell. Product was periodically removed from the acid anode and cathode compartments were fed an electrode recirculation loop to reduce the product liquor inven rinse solution which carried gas generated at the elec tory. trodes out of the cell, where it separated from the solu This experiment operated for a total of about 450 tion in the recirculation reservoir and was vented. 30 minutes. During that time, a total of approximately 1100 Nafion (R) 324 was used as the cation exchange mem ml of 2.2N hypophosphorous acid product was ob brane forming the electrode rinse compartments. The tained including about 600 ml of 2N acid removed dur bipolar membranes were prepared according to U.S. ing the operation of the cell and about 510 ml of 2.5N Pat. No. 4,766,161. The cation exchange membranes acid remaining in the acid recirculation loop. The con were prepared according to U.S. Pat. No. 4,738,764. 35 The anion exchange membranes were from Asahi Glass centration of sodium in the product acid was measured Co. (sold under the tradename Selemion (R) AMV anion to be 900 and 1080 ppm, respectively. The concentra permselective membranes). The cell was equipped with tion of the depleted salt was about 0.8 molar (50 a anode and a stainless steel cathode. ms/cm). The potential across the cell was found to be Four pumps were used to circulate the acid, base, about 1.8 volts/unit cell. The information in the at depleted salt, and the combined electrode rinse solu tached tables details this experimental run. tions in and out of the cell and through calibrated recir TABLE 1. culation vessels. These pumps were operated in a such a EXAMPLE 3 - ACD LOOP way so that the linear velocity of the solution in the cell Avg. was maintained at about 15 cm/sec. This allowed each 45 Cumulative Current H3PO2 Acid Efficiency for cell stream to be operated independently in one of two H3PO2 Vol- Current Per Sample different modes-batch/semi-batch and continuous. Time Normality ume Efficiency Sampling Period The cell was powered by a DC constant current (Min.) (Eq/L) (ml) (%) Period (%) (N) rectifier capable of providing 20 amps of current at up O 0.5 800 to 50 V. A current density of 100 amps per square foot 50 78 0.95 852 84 84.42 0.725 187 1485 935 85 85.44 1.2175 was maintained in all examples. The temperature in the 234 1.66 965 83 73.03 15725 cell was maintained at 35 C. 234 1.66 865s A 10 wt.% NaOH solution was used for the elec 297 1.93 909 82 81.30 1.795 trode rinse solution. The anode and cathode electrode 297 1.93 809 55 368 2.2 861 81 75.39 2.065 rinse solutions exiting the cell were continuously re 368 2.2 661 combined into a common recirculation tank. The acid 420 2.4 696 79 66.87 2.3 and base compartments were initially inventoried with 420 2.4 496 hypophosphorous acid and 5% NaOH. 450 2.525 513 78 56.25 2.4625 A 25 wt.% sodium hypophosphite solution with a Solution removed from the Acid Recirculation Loop calcium concentration of less than 1 ppm was used to 60 replenish the depleted salt stream and maintain a con stant solution conductivity. The 25 wt.% salt solution TABLE 2 was prepared by dissolving 25 g of technical grade EXAMPLE 3 - BASE LOOP Base sodium hypophosphite monohydrate crystal in 75 g of NaOH Flow Current Water DI water per 100 g of feed solution. The pH of this 65 Interval Normality Rate Efficiency Make-up solution was 7.2. The salt solution was adjusted with (Min.) (Eq/L) (ml/Min) (%) (ml/Min) caustic in order to raise its pH to 10 and thus facilitate 178 1.53 3.6 89 3.302 calcium removal via a Duolite C467 ion exchange resin. 10 1.457 3.65 87 3.306 5431,792 11. 12 TABLE 2-continued EXAMPLE 4 EXAMPLE 3 - BASE LOOP Base This example demonstrates the continuous mode of NaOH Flow Current Water operation for a bipolar membrane cell and supports the Interval Normality Rate Efficiency Make-up conclusion expressed in Example 3 that concentrations (Min.) (Eq/L) (mi/Min) (%) (ml/Min) of hypophosphorous acid significantly higher than 1N are possible at a given current efficiency during the 142 1.425 3.55 81 3.321 electrodialytic splitting of sodium hypophosphite salt into its acid and base components using a three-com TABLE 3 O partment cell equipped with anion, cation, and bipolar EXAMPLE 3. SALT LOOP membranes. Depleted H3PO2 In this test, the salt and base streams were operated in Feed Salt Normality Conductivity a continuous bleed and feed mode. The base loop caus (ml/Min.) (ml/Min) (Eq/L) (ms/cm) tic concentration was maintained at about 5 wt. 9% 15 2.2 0.94 0.23 41.5 NaOH with water addition. The salt loop was operated 2.259 1.091 0.21 48.5 to maintain a conductivity of 70 ms/cm through the continuous addition of the 25 wt.% sodium hypophos phite feed solution. The cumulative current efficiencies (CE) shown in The hypophosphorous acid loop was operated in a Table 1 show that 2.2N hypophosphorous acid can be 20 continuous configuration without water addition. In generated on a batch basis. This is significantly higher this mode of operation, product was allowed to continu than the 1N concentration and 80% CE reported in the ously overflow the hypophosphorous acid recirculation open literature for strong acids (Mani, 1991). Table 1 loop into a product collection vessel. Hypophospho also shows that if current efficiencies are calculated for rous acid (5.0N) was used to initially inventory the cell. each discrete sampling period, the concentration of 25 This experiment operated for a total of about 430 hypophosphorous acid can be expected to be about 1.87 minutes. During that time, a total of approximately 210 at 80% CE (see FIG. 3, line A). As the FIG.3 indicates, ml of 5.7N hypophosphorous acid product was ob this CE is lower than the cumulative efficiency since the tained. The concentration of sodium in the product acid early part of the batch cycle operates at high current was measured to be 3156 ppm. The concentration of efficiency when the hypophosphorous acid concentra 30 hypophosphorous acid approached 6N on a steady state tion is relatively low. This data provides some insight as basis. The depleted salt concentration was about 0.9 to how a cell operating in a continuous bleed and feed molar NaH2PO2.H2O. The potential across the cell was mode would operate at a given current efficiency. For found to be about 1.8 volts/unit cell. The information in example, a 56% CE would be predicted for a cell oper the attached tables details this experimental run. ating at 2.5N H3PO2. 35 The sodium concentration in the 600 ml of 2.0 normal TABLE 5 product hypophosphorous acid collected during the EXAMPLE 4 - ACID LOOP Time Cumulative Acid Flow Current experimental run was found to be 900ppm, while that of Interval Time (H3PO2) Rate Efficiency the residual product liquor in the acid recirculation loop 4 (Min) (Min.) (M) (ml/Min.) (%) was 1080 ppm. These concentrations are slightly lower 97 97 5.45 0.535 46.89 than those reported as being typical by Mani, 1991, at 60 157 5.7 0.517 47.40 equivalent acid and base concentrations. The base nor 45 202 5.75 0.52 48.09 46 248 5.8 0.509 47.48 mality was 1.32. Table 4 summarizes the experimental 54 302 5.85 0.518 48,74 results for this example and Examples 4 and 5 and com 45 54 356 6 0.51 49.31 pares the results to those predicted in Mani, 1991. 40 396 6 0.525 50.66 TABLE 4 Acid Loop TABLE 5 Sodium Contamination EXAMPLE 4 - ACEO LOOP Mole 2% 50 Na Time Cumulative Acid Flow Current 100% Enterval Time (H3PO2) Rate Efficiency Acid HPA Wt 2 Dens. % Na (Min.) (Min.) (M) (ml/Min.) (%) Nor. Basis HPA g/cc As-is Source 97 97 5.45 0.535 46.89 60 57 5.7 0.517 47.40 200 2.05 12.30 1.100 0.0897 Actual - 55 45 202 5.75 0.52 48.09 Example 3 46 248 5.8 0.509 47.48 2.50 2.01 5.10 1.16 0.1077 Actual - 54 302 . 5.85 0.518 48.74 Example 3 54 356 6 0.511 49.31 5.70 2.65 33.33 1.180 0.3156 Actual - Example 4 40 396 6 0.525 50.66 5.70 4.42 33.33 1.180 0.5370 Actual - Example 5 1.00 2.5 6.75 1,081 0.0379 Predicted, TABLE 6 Mani 1991 EXAMPLE 4 - BASE LOOP Time Cumula Base Caustic Inter tive Flow Water Current The table shows that at 2.0 normal hypophosphorous 65 val Time (NaOH Rate Make-up Efficiency acid, the actual product contamination was about 2.0 (Min) (Min) (M) (ml/Min) (ml/Min.) (%) mole % sodium vs. 2.5 mole % predicted by Mani, 101 10 1.375 3.47 3.29 76.74 1991. 114 215 1.4 3.036 2.89 68.36 5,431,792 13 14 TABLE 6-continued ously overflow the hypophosphorous acid recirculation EXAMPLE 4 - BASE LOOP loop into a product collection vessel. Hypophospho Time Cumula Base Caustic rous acid (5.0N) was used to initially inventory the cell. Inter tive Flow Water Current This experiment operated for a total of about 450 val Time NaOH) Rate Make-up Efficiency 5 minutes. During that time, a total of approximately 240 (Min) (Min) (M) (ml/Min) (ml/Min.) (%) ml of 5.7 N hypophosphorous acid product was ob 102 317 1495 2,894 2.71 69.59 tained. The concentration of sodium in the product acid 97 414 1.52 2.771 2.7 67.74 was measured to be 5370 ppm. The concentration of hypophosphorous acid approached 5.8N on a steady 10 TABLE 7 state basis. The depleted salt concentration was about EXAMPLE 4 - SALT LOOP 0.85 molar NaH2PO2.H2O. The potential across the cell Depleted was found to be about 1.8 volts/unit cell. The informa Time Cumulative Salt tion in the attached tables details this experimental run. Enterval Tirne Feed Rate Flow (H3PO2 15 (Min.) (Min.) (ml/Min.) Rate (M) TABLE 8 97 97 2.17 1.38 0.47 12 218 2.34 1.44 0.64 EXAMPLE 5 - ACID LOOP 208 426 2.35 1.44 0.68 Time Cumulative Acid Flow Current interval Time (H3PO2 Rate Efficiency The acid product data in Table 5 shows that a current 20 (Min.) (Min.) (M) (mi/Min.) (%) 81 81 5.1 0.47 38.55 efficiency of 51% is possible at a hypophosphorous acid 71 152 5.35 0.55 47.33 concentration of 6 molar. Contrasting this information 45 197 5.45 0.56 49.09 with the batch operation described in Example 3, Table 90 287 5.8 0.54 50.37 1, it can be seen that at 2.5 molar, the current efficiency 92 379 5.8 0.54 50.37 per sampling period is about 56%. These two drastically 25 different hypophosphorous acid concentrations at about equivalent current efficiencies provide additional sup TABLE 9 port to the conclusion that acid concentrations signifi EXAMPLE5 - BASE LOOP cantly higher than those reported in the open literature Base Water Current for strong acids are possible during the bipolar mem- 30 Time Cumulative Flow Make- Effi brane electrodialyses of sodium hypophosphite into Interval Time (NaOH) Rate up ciency hypophosphorous acid and sodium hydroxide. The data (Min.) (Min.) (M) ml/Min m/Min (%) also indicates that the concentrations attainable in Ex 95 95 1.32 2.99 2.75 63.48 37 132 1.4 2.84 63.95 ample 3 at various current efficiencies for sampling 20 152 1.26 3.36 2.78 68.09 periods may in fact be conservative in contrast to what 3 39 191 1.26 3.26 66.06 is attainable under continuous operation. Evidenced by 140 331 1.26 the shape of the curve for the batch/semi-batch data 36 367 1S6 3.1 2.78 77.78 (see FIG. 3, line B), the nearly 6 molar acid concentra tion would not be expected based on the batch/semi batch data. 40 TABLE 10 The concentration of sodium in the hypophospho EXAMPLE5 - SALT LOOP rous acid was measured in a 5.7 molar composite sample Depleted collected during the run. The analyses showed the sam Time Cumulative Salt ple contained 3156 ppm Na. Table 4 compares this re Enterval Time Feed Rate Flow H3PO) sult to those of Example 3. The result on a mole percent 45 (Min.) (Min.) (ml/Min.) Rate (M) 75 75 2,033 1.17 0.79 basis is about that predicted by Mani, 1991 for 1 molar 41 16 2.033 1.17 0.69 acid vs the 5.7 molar acid generated in this example. 85 20 2.033 1.14 0.63 EXAMPLE 5 35 236 2,033 1.14 0.64 This example demonstrates the continuous mode of 50 operation for a bipolar membrane cell and supports the The acid product data in Table 5 shows that a current conclusion expressed in Example 3 that concentrations efficiency of 50% is possible at a hypophosphorous acid of hypophosphorous acid significantly higher than 1N concentration of 5.8 molar. Again, contrasting this in are possible at a given current efficiency during the formation with the batch operation described in Exam electrodialytic splitting of sodium hypophosphite salt 55 ple 3, Table 1, it can be seen that at 2.5 molar, the cur into its acid and base components using a three-com rent efficiency per sampling period is about 56%. The partment cell equipped with anion, cation, and bipolar difference in hypophosphorous acid concentrations at membranes. about equivalent current efficiencies, as in Example 4, In this test, the salt and base streams were operated in supports the conclusion that acid concentrations signifi a continuous bleed and feed mode. The base loop caus- 60 cantly higher than those reported in the open literature tic concentration was maintained at about 5 wt. 26 for strong acids are possible during the bipolar mem NaOH with water addition. The salt loop was operated brane electrodialyses of sodium hypophosphite into to maintain a conductivity of 70 ms/cm through the hypophosphorous acid and sodium hydroxide. A com continuous addition of the 25 wt.% sodium hypophos parison of Example 4 with Example 5 shows that the phite feed solution. 65 concentration and current efficiency results are repro The hypophosphorous acid loop was operated in a ducible. The data also indicates that the concentrations continuous configuration without water addition. In attainable in Example 3 at various current efficiencies this mode of operation, product was allowed to continu for sampling periods may in fact be conservative in 5,431,792 15 16 contrast to what is attainable under continuous Opera rinse compartment and is separated therefrom by tion. Evidenced by the shape of the curve for the batch a cation exchange membrane, and the salt com /semi-batch data (see FIG. 3, line C), the nearly 5.8 partment at the opposite end of said series is molar acid concentration would not be expected based adjacent to an additional base compartment on the batch/semi-batch data. which is adjacent to said cathode electrode rinse The concentration of sodium in the hypophospho compartment, and said additional base compart rous acid was measured in a 5.7 molar composite sample ment is separated from said cathode electrode collected during the run. The analyses showed the sam rinse compartment by a cation exchange mem ple contained 5170 ppm Na. Table 4 compares this re brane; sult to those of Examples 3 and 4. The result on a mole 10 (4) means for circulating said aqueous solution of percent basis is higher than that predicted by Mani, sodium hydroxide product through said base com 1991. partments; We claim: (5) means for circulating said aqueous solution of 1. A method of making hypophosphorous acid and sodium hypophosphite and hypophosphorous acid sodium hydroxide from sodium hypophosphite com 5 through said salt compartments; prising performing electrodialytic water splitting upon (6) means for circulating said aqueous solution of an aqueous solution of said sodium hypophosphite. hypophosphorous acid through said acid compart 2. A method according to claim 1 wherein said elec ments; trodialytic water splitting is performed in an electro (7) means for circulating aqueous electrode rinse lytic cell having the structure Anode-A-AM-S-CM-B- 20 solutions through said electrode rinse compart BM-A-AM-S-CM-B-Cathode where CM is a cationic ments; exchange membrane, B is a base compartment contain (8) means for adding water to said acid compartment; ing an aqueous solution of sodium hydroxide product, (9) means of adding water to said base compartment; BM is a bipolar ion exchange membrane, A is an acid and compartment containing an aqueous solution of hypo 25 phosphorous acid product, AM is an anion exchange (10) means of adding an aqueous solution of sodium membrane, S is a salt compartment containing an aque hypophosphite to said salt compartment. ous solution of sodium hypophosphite, and n is 1 to 200. 5. A method according to claim 4 wherein the con 3. A method according to claim 1 wherein said elec centration of said aqueous solution of sodium hypo trodialytic water splitting is performed in an electro 30 phosphite is about 5 wt.% to about saturation. lytic cell having the structure Anode-ER-CM-B-BM 6. A method according to claim 4 wherein the con A-AM-S-CM-B-BM-A-AM-S-CM-B-CM-ER-Cath centration of said aqueous solution of sodium hypo ode where CM is a cationic exchange membrane, B is a phosphite and hypophosphorous acid in the salt com base compartment containing an aqueous solution of partment is about 1 to 50 wt.% and 0 to 55 wt.%, sodium hydroxide product, BM is a bipolar ion ex 35 respectively. change membrane, A is an acid compartment contain 7. A method according to claim 4 wherein the tem ing an aqueous solution of hypophosphorous acid prod perature of said cell is about 10 to about 45 C. uct, AM is an anion exchange membrane, S is a salt 8. A method according to claim 4 wherein the current compartment containing an aqueous solution of sodium density in said cell is about 50 to about 155 mA/cm2. hypophosphite, ER is an electrode rinse compartment, 9. A method according to claim 4 wherein the linear and n is 1 to 200. solution velocities of said aqueous solutions through 4. A method according to claim 1 wherein said elec said cell are about 2 to about 15 cm/sec. trodialytic water splitting is performed in an electro 10. A method according to claim 4 wherein a portion lytic cell comprising of said hypophosphorous acid and sodium hydroxide (1) an anode electrode rinse compartment housing an 45 solutions is continuously drawn off and replaced with anode; water and a portion of said sodium hypophosphite and (2) a cathode electrode rinse compartment housing a hypophosphorous acid solution is continuously drawn cathode; off and replaced with fresh sodium hypophosphite solu (3) a series of at least two juxtaposed units in between tion. said anode electrode rinse compartment and said 50 11. A method according to claim 4 wherein, when cathode electrode rinse compartment, where each ever a predetermined concentration of hypophospho unit comprises rous acid is reached in said hypophosphorous acid solu (a) a base compartment containing an aqueous solu tion, a portion of said hypophosphorous acid solution is tion of sodium hydroxide product; drawn off. (b) an acid compartment containing an aqueous 55 12. A method according to claim 4 wherein the con solution of hypophosphorous acid product; centration of sodium hydroxide in said sodium hydrox (c) a salt compartment containing an aqueous solu ide solution is maintained at less than 10 wt.%. tion of sodium hypophosphite and hypophos 13. A method according to claim 4 wherein the pres phorous acid; sure in the acid compartments is about 1 to about 3 psi (d) a bipolarion exchange membrane between each greater than the pressure in the base and salt compart base compartment and adjacent acid compart ments. ment; and 14. A method according to claim 4 including the (e) an anion exchange membrane between each additional last step of using at least a portion of said acid compartment and adjacent salt compart sodium hypophosphite and hypophosphorous acid solu ment, where each salt compartment is separated 65 tion to adjust the pH of a second aqueous sodium hypo from each base compartment cation exchange phosphite solution prior to the evaporation and crystal membrane and the base compartment at one end lization of said second aqueous sodium hypophosphite of said series is adjacent to said anode electrode solution. 5431,792 17 18 15. A method according to claim 14 wherein the mA/cm2, whereby said sodium hypophosphite concentration of said hypophosphorous acid in said hypophosphorous acid solution and the concentration and water react in said cell to produce an aque of said sodium hydroxide in said sodium hydroxide ous solution of hypophosphorous acid in said solution are controlled so that the acid content of said acid compartments and an aqueous solution of sodium hypophosphite and hypophosphorous acid solu sodium hydroxide in said base compartments; tion is the content needed to reduce the pH of said (E) maintaining the concentration of sodium hy second aqueous sodium hypophosphite solution to the droxide in said base compartments at less than desired pH. about 10 wt.% by replacing at least a portion of 16. A method according to claim 4 including the said sodium hydroxide solution with water; additional last step of passing at least a portion of said 10 (F) maintaining the concentration of sodium hypo hypophosphorous acid solution through cation ex phosphite in said salt compartments at about 5 to change material loaded with hydrogen ions to replace about saturation by drawing off depleted sodium sodium ions that may be present in said solution with hypophosphite solution from said salt compart hydrogen ions. ments and replacing said drawn off solution with 17. A method of making hypophosphorous acid in an 15 fresh aqueous sodium hypophosphite solution electrodialytic water splitter cell having about 2 to having a concentration of about 5 to about satu about 200 juxtaposed units, where said cell comprises ration; (1) an anode electrode rinse compartment housing an (G) removing hypophosphorous acid solution from anode; said acid compartments and replacing said re (2) a cathode electrode rinse compartment housing a 20 moved hypophosphorous acid solution with wa cathode; ter; and (3) a series of juxtaposed units in between said anode (H) maintaining the pressure in the acid compart electrode rinse compartment and said cathode elec ment at about 1 to about 3 psi greater than the trode rinse compartment, where each unit com pressure in the base and salt compartments. prises 18. A method according to claim 17 wherein a por (a) a base compartment containing an aqueous solu 25 tion of said hypophosphorous acid solution is removed tion of sodium hydroxide product; from said acid compartments and is replaced with water (b) an acid compartment containing an aqueous whenever the concentration of hypophosphorous acid solution of hypophosphorous acid; in said compartments reaches about 10 to about 55 wt. (c) a salt compartment containing an aqueous solu %. tion of sodium hypophosphite and hypophos 30 19. A method according to claim 18 wherein a por phorous acid; tion of said hypophosphorous acid solution is continu (d) a bipolarion exchange membrane between each ously removed from said acid compartments and is base compartment and adjacent acid compart continuously replaced with water in order to maintain ment; and the concentration of hypophosphorous acid in said (e) an anion exchange membrane between each 35 compartments below a concentration between about 10 acid compartment and adjacent salt compart and about 30 wt.%. ment, where each salt compartment is separated 20. A method according to claim 17 wherein a por from each base compartment by a cation ex tion of said hypophosphorous acid solution is continu change membrane and the base compartment at ously removed from said acid compartments without one end of said series is adjacent to said anode replacement with water in order to obtain the maximum electrode rinse compartment and is separated concentration of hypophosphorous acid possible in said therefrom by a cation exchange membrane, and compartments. the salt compartment at the opposite end of said 21. A method according to claim 17 wherein the series is adjacent to an additional base compart concentration of hypophosphorous acid in said salt ment which is adjacent to said cathode electrode 45 compartments is maintained at about 0 to about 55 wt. rinse compartment, and said additional base % by drawing off depleted salt solution from said salt compartment is separated from said cathode compartments and replacing it with about 0 to about 55 electrode rinse compartment by a cation ex wt.% aqueous sodium hypophosphite solution. change membrane; 22. A method according to claim 17 including the (4) means for circulating said aqueous solution of additional last step of passing at least a portion of said sodium hydroxide product through said base com SO hypophosphorous acid solution through cation ex partments; and change material loaded with hydrogen ions to replace (5) means for circulating said aqueous solution of sodium ions that may be present in said solution with sodium hypophosphite and hypophosphorous acid hydrogen ions. through said salt compartments; comprising and 23. In a process for making sodium hypophosphite (6) means for circulating said aqueous solution of wherein the pH of a sodium hypophosphite aqueous hypophosphorous acid through said acid compart liquor is lowered with an acid other than hypophospho ments; rous acid prior to evaporative crystallization of sodium (7) means of circulating aqueous electrode rinse solu hypophosphite, a method of producing hypophospho tions through said electrode rinse compartments; rous acid from at least a portion of the sodium hypo (A) adding water to said acid and base compart 60 phosphite thereby produced, comprising ments; (a) performing electrodialytic water splitting upon an (B) adding an aqueous solution of about 5 to about aqueous solution of at least a portion of said sodium saturation of sodium hypophosphite to said salt hypophosphite to produce said hypophosphorous compartments; acid and a depleted sodium hypophosphite solution (C) maintaining the temperature in said series of 65 containing hypophosphorous acid; and units at about 10 to about 45 C.; (b) replacing said other acid with at least a portion of (D) passing direct current through said series of said depleted sodium hypophosphite solution. units at a density of about 50 to about 155 k k . k k k