Electrolytic Production of Hypophosphorous Acid

Patentamt Europaisches || || 1 1| || || || || || || 1 1|| || || (19) J European Patent Office

Office europeen des brevets (11) EP 0 701 860 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) |nt. CI.6: B01 D 61/44, C01 B 25/1 65, 20.03.1996 Bulletin 1996/12 C25B 1/22 (21) Application number: 95111697.9

(22) Date of filing: 25.07.1995

(84) Designated Contracting States: • Thomson, Donald CH DE FR GB IT LI NL Northport, New York 1 1 768 (US) • Garay, Luis Henry (30) Priority: 16.09.1994 US 307923 Rockville Center, New York 1 1 570 (US)

(71 ) Applicant: LeaRonal, Inc. (74) Representative: Hansen, Bernd, Dr. Dipl.-Chem. et Freeport, N.Y. 11 520 (US) al Hoffmann, Eitle & Partner, (72) Inventors: Patentanwalte, • Nobel, Fred I. Arabellastrasse 4 Sands Point, New York 1 1 050 (US) D.81 925 Munchen (DE) • Brasch, William Nesconset, New York 1 1 767 (US)

(54) Electrolytic production of hypophosphorous acid

(57) Methods for preparing hypophosphorous acid the insoluble anode to a cathode (24, 124) in electrical are disclosed comprising contacting an insoluble anode contact with the aqueous solution to generate H+ ions in (12,112) with an aqueous solution of hypophosphite ani- the aqueous solution thereby forming a hypophospho- ons (27, 127) and applying a current (30, 130) through rous acid solution.

FIG. 1 o CO CO o r»- o Q_ LU Printed by Rank Xerox (UK) Business Services 2.9.13/3.4 1 EP 0 701 86050 A1 2

Description the anions are transported across an anion permeable membrane into the anolyte, where they are converted to FIELD OF THE INVENTION acids or halogens. It is therefore of considerable interest to develop a The present invention relates to the manufacture of s novel and more economical method for producing hypo- hypophosphorous acid. More particularly, the present phosphorous acid for the various uses discussed above. invention relates to the manufacture of hypophosphorous acid for use in electroless nickel plating systems. SUMMARY OF THE INVENTION Still more particularly, the present invention relates to the manufacture of hypophosphorous acid from an anolyte 10 In accordance with the present invention, a method system including hypophosphite anions. has been discovered for preparing hypophosphorous The present invention provides a significant acid which comprises electrodialysis using anionic and improvement in the means by which hypophosphorous cationic exchange membranes to convert hypophosphite acid may be produced efficiently and inexpensively. salts, such as alkali metal hypophosphite salts, into 15 hypophosphorous acid and alkali hydroxide. These BACKGROUND OF THE INVENTION results are achieved by conducting electrodialysis through an insoluble anode and an anolyte comprising Hypophosphorous acid (H3PO2) also known as hypophosphite ions and a cathode and a catholyte com- phosphinic acid, is currently a staple article of commerce prising alkali (sodium) ions. Electrolysis of the water sup- which is sold by various companies for purposes such 20 plies hydrogen ions to the anolyte and hydroxyl ions to as the manufacture of hypophosphite salts, as well as to the catholyte. prevent the discoloration of phosphate esters, in esteri- In accordance with this invention, methods for pre- fication catalysts, and for the manufacture of cooling paring hypophosphorous acid are disclosed comprising water treatment chemicals. It is also sold for use in metal providing an insoluble anode in an electrically conductive finishing procedures, as a reducing agent for electroless 25 anolyte and a cathode in an electrically conductive plating, and as a sealer for phosphated steel. catholyte, providing a hypophosphite salt solution sepa- The manufacture of hypophosphorous acid, how- rated from the anolyte anode by an anionic exchange ever, has generally been carried out by somewhat com- membrane resistant to cation diffusion and from the plex and expensive methods utilizing ion exchange catholyte by a cationic exchange membrane resistant to procedures. In these procedures, for example, the 30 anionic diffusion, and applying a direct current through sodium ion of sodium hypophosphite is exchanged for a the insoluble anode to the cathode to transfer hypophos- hydrogen ion using an ion exchange resin therefor. phite anions through the anionic exchange membrane These procedures result in hypophosphorous acid being into the anolyte and to generate hydrogen ions in the a rather expensive commodity, generally at over $7.00 anolyte, thereby forming hypophosphorous acid in the per pound. 35 anolyte. Preferably, the hypophosphite salt solution is an By way of background, Liaukonis et al., Issled. Obi. alkali metal hypophosphite salt solution. In a preferred Osazhdeniya Met. (1 985), pp. 1 34-9 sets forth a detailed embodiment, the insoluble anode comprises a precious study of the anodic polarization of the Ni-P electrode in metal surface, such as platinum, iridium or ruthenium. an acetate solution of hypophosphite as a function of the Most preferably, the insoluble anode includes an inert pH. Furthermore, in Makarov et al., Zasch. Met. 18(6) 40 inner support for the precious metal surface, such as tita- pp. 918-919 (1982) the rate dependence of hypophos- nium, zirconium or tantalum. phite anion oxidation and the evolution of hydrogen on In accordance with one embodiment of the method titanium is investigated for sodium hypophosphite solu- of the present invention, the solution of hypophosphite tions. These authors thus describe the application of a anions has a pH of below about 2. current to titanium electrodes in contact with nickel hypo- 45 In accordance with another embodiment of the phosphite solutions therein. In accordance with the dis- method of the present invention, the cathode is com- closure of this article, a conventional electroless nickel prised of stainless steel, steel, graphite, or platinum- bath is contained in a titanium tank and the article con- coated titanium. In a preferred embodiment, the direct cerns the tendency of the bath to plate onto that tank. current is applied to the anode at a current density of Furthermore, Sadikov et al., Zasch. Met, 19(2). pp. 314- so between about 1 0 and 400 asf . 317 (1983), sets forth yet another investigation of the In a preferred embodiment of the method of the behavior of titanium hypophosphite electrolyte solutions. present invention, the hypophosphite salt solution com- Electrodialysis is also a known process which has prises at least a 1 molar solution of hypophosphite ani- been utilized for various purposes, such as that of U.S. ons. Patent No. 5,264,097. In that patent an alkali salt-con- 55 In accordance with a preferred embodiment of the taining aqueous solution including salts and complexes method of the present invention, the method includes an of metal anions and cations is fed to the catholyte, and electrically conductive catholyte which comprises a the metal cations are removed therefrom as insoluble dilute alkali metal hydroxide solution. Preferably, the hydroxides by controlling the pH therein. In this process, alkali metal hydroxide solution comprises about a 0.1

2 3 EP 0 701 860 A1 4 molar solution of sodium hydroxide. In another embodi- at the anode, does not result in oxidation of the hypo- ment, the electrically conductive anolyte is a dilute solu- phosphite ions, to either orthophosphite or phosphate tion of hypophosphorous acid. Preferably, the ions. Although this is clearly what one of ordinary skill in hypophosphorous acid solution comprises a 0.05 molar this art would have expected, this oxidation does not take solution of hypophosphorous acid. 5 place in the method of the present invention. To the con- In accordance with another embodiment of the trary, the reaction taking place at the anode does not method of the present invention, the method includes result in destruction of the hypophosphite ions, which recovering the hypophosphorous acid, preferably com- remain in tact, and which in the presence of hydrogen prising concentrating the hypophosphorous acid solu- ions, unexpectedly produces hypophosphorous acid in tion. 10 the anolyte compartment. In accordance with a preferred embodiment of the The solution of hypophosphite anions in contact with method of the present invention, the hypophosphite salt the insoluble anode can have a pH of less than about solution is separated from the anode by a pair of anionic 0.5. Preferably, the pH of the solution is below about 2. exchange membranes resistant to cation diffusion and Essentially any cathode material is suitable for use from the cathode by a pair of cationic exchange mem- is as the counter-electrode. Examples of suitable cathode branes resistant to anion diffusion, thereby providing an materials include stainless steel, steel, graphite, plati- anolyte buffer solution between the pair of anionic num-coated titanium, and the like. The preferred cathode exchange membranes resistant to cation diffusion and a material is stainless steel. catholyte buffer solution between the pair of cationic The anode materials suitable for use herein are exchange membranes resistant to anion diffusion. 20 insoluble in the aqueous hypophosphite anion solutions hereof. Examples of suitable insoluble anode materials BRIEF DESCRIPTION OF THE DRAWINGS include precious metal surfaces, such as platinum, iridium or ruthenium, and preferably precious metal sur- A more complete appreciation of the invention and faces on an inert inner support, the latter being a metal many other intended advantages can be readily obtained 25 such as titanium, zirconium or tantalum. It is understood, by reference to the following detailed description when however, that the precious metal surfaces can be in the considered in connection with the following drawings, form of an oxide of the precious metal, again such as the wherein: oxides of platinum, iridium or ruthenium. The preferred insoluble anode material is platinum-coated titanium. FIG. 1 shows a side, cross-sectional view of a three- 30 Sufficient voltage should be supplied to the anode compartment electrodialysis cell embodying the to create an anode current density between about 1 0 and method of the present invention; and about 400 amp/ft2 (asf). A current density of between FIG. 2 shows a side, cross-sectional view of a five- about 20 and about 200 asf is preferred, and a current compartment electrodialysis cell embodying the density of between about 50 and 1 00 asf is most pre- method of the present invention. 35 f erred. With the application of current, oxygen is generated It should be noted that the drawings are not neces- at the anode by oxidation of the hydroxyl ions in the water sarily to scale, but that certain elements have been in which the hypophosphite anions are dissolved, thus expanded to show more clearly the various aspects of leaving behind H+ ions, which build up within the anolyte the present invention and their advantages. 40 compartments. At the same time, acid protons are dis- placed at the cathode, where they are liberated as hydro- DETAILED DESCRIPTION gen gas, with the generation of hydroxyl anions. In order to carry out this method, a source of hypo- In its simplest form, the method of the present inven- phosphite anions must be provided. A rather inexpensive tion involves converting alkali hypophosphite salts into 45 commercial source of such anions are the alkali metal hyXophosphorous acid and alkali (sodium) hydroxide by hypophosphites, such as sodium hypophosphite, potas- electrodialysis utilizing anionic and cationic exchange sium hypophosphite, and the like. Alkali metal hypophos- membranes. In particular, these results can be achieved phites may thus be used as a source of the by conducting electrodialysis through an insoluble anode hypophosphite anions in a three-compartment electrodi- and an anolyte comprising hypophosphite ions and a 50 alysis cell. cathode and a catholyte comprising alkali (sodium) ions. In such a three-compartment cell, such as that Electrolysis of the water supplies hydrogen ions to the shown in FIG. 1 , the middle compartment contains the anolyte and hydroxyl ions to the catholyte. alkali metal hypophosphite solution. The middle com- The simplicity and effectiveness of the present partment is separated from the compartment containing invention is that much more surprising in view of the 55 the insoluble anode by an anionic exchange membrane known fact that hypophosphite ions are a strong reducing resistant to cation diffusion, and from the compartment agent which can be readily oxidized. Thus, it is quite sur- containing the cathode counter-electrode by a cationic prising that the strong oxidizing power of the anode, as exchange membrane resistant to anion diffusion. The well as the liberation of a considerable amount of oxygen solution in the center cell is an alkali hypophosphite solu-

3 5 EP 0 701 860 A1 6 tion which contains alkali ions and hypophosphite ions. widely, it is preferred to maintain the concentration at Under the influence of a direct current which is applied between 100 and 200 g/l. through the system, these ions are separated. That is, The hypophosphite anions of the center compart- the hypophosphite ions transfer through the anionic ment diffuse across the anionic exchange membrane exchange membrane resistant to cation diffusion into the 5 resistant to cation diffusion to the anolyte, but are unable anolyte while the alkali ions transfer through the cationic to diffuse across the cationic exchange membrane exchange membrane resistant to anionic diffusion into resistant to anion diffusion to the catholyte. Similarly, the the catholyte. The hypophosphite ions entering the alkali metal cation of the alkali metal hypophosphite is anolyte combine with the hydrogen ions which are gen- unable to diffuse across the anionic exchange mem- erated at the anode in the anolyte to produce hypophos- 10 brane resistant to cation diffusion to the anolyte, but dif- phorous acid. On the other hand, the alkali ions entering fuses across the cationic exchange membrane resistant the catholyte combine with hydroxyl ions generated at to anion diffusion to the catholyte. the cathode in the catholyte to produce alkali hydroxide. When direct current is applied to the anode, the H+ The overall reaction can next be simplified to produce ions are anodically generated into the anolyte, forming hypophosphoric acid and sodium hydroxide from aque- 15 hypophosphorous acid solution with the hypophosphite ous alkali hypophosphite. anions from the center compartment. The anionic Anionic ion exchange membranes resistant to cation exchange membrane resistant to cation diffusion retains diffusion include membranes such as R5030 brand avail- the H+ ions in the anolyte. Similarly, the hydroxyl anions able from the Pall Corporation. Cation ion exchange generated at the cathode with the liberation of hydrogen membranes resistant to anion diffusion include mem- 20 gas are retained in the catholyte by the cationic branes available from DuPont under the brand name exchange membrane resistant to anion diffusion. NAFION. Those of ordinary skill in this art will understand that The anode compartment contains a solution known electrodialysis cells having even greater numbers of in the art as an anolyte, and the cathode compartment compartments can be devised, sandwiching additional contains a solution known as a catholyte. Before the 25 compartments, which may contain alkali metal hypo- application of a DC voltage supplied by a rectifier, gen- phosphite or a buffer solution, between the insoluble erator, or battery, both the anolyte and the catholyte must anode compartment containing the above-described be made conductive so that current will begin to flow as anolyte and the cathode counter-electrode compartment soon as a voltage is applied. The anolyte should thus containing the above-described catholyte, which com- contain a dilute solution of hypophosphorous acid, and 30 partments are separated by the above-described ion the amount needed in the anolyte is merely enough to exchange membranes. Such an arrangement allows for render the solution conductive to an electric current. An the large-scale highly efficient commercial production of amount of about 4 g/l or greater of hypophosphorous hypophosphorous acid. acid or the like is therefore sufficient. The anolyte should One example of a highly preferred embodiment of preferably start with a dilute solution of hypophospho- 35 such a multi-compartment cell is shown in FIG. 2. It is rous acid, and not some other conducting salts or ions, known that ion exchange membranes used in electrodi- because the anolyte should be kept free of extraneous alysis are not 100% perfect, and that a small amount of ions that might interfere with the purity of the final prod- undesired leakage can take place thereacross. For this uct; namely, the hypophosphorous acid itself. reason, the five-compartment cell shown in FIG. 2 can The catholyte can be made electrically conductive 40 be utilized. That is, when the hydroxyl ion concentration with any suitable conducting salt, provided only innocu- in the catholyte in the cell of FIG. 1 builds up to a suffi- ous ions are introduced. Since the catholyte will eventu- ciently high value, a small amount of that hydroxyl ion ally build up in alkali hydroxide due to the liberation of can tend to leak across the cation exchange membrane hydrogen gas at the cathode during electrolysis, an alkali into the center compartment. This, in turn, can adversely metal hydroxide is the preferred starting material, gen- 45 effect the pH in that compartment. Similarly, when the erally up to about a 4 g/l solution of, for example, sodium H+ concentration in the anolyte compartment builds up hydroxide. Sodium hydroxide is thus preferred, and the to a sufficiently high value, a small amount of these H+ amount required is merely enough to make the catholyte ions can leak across the anion exchange membrane into conductive to an electric current. About 5 g/l of sodium the center compartment. This would represent an unde- hydroxide is sufficient. 50 sirable loss of a source of acid from the anolyte. The concentration of hypophosphite salts such as Referring to FIG. 2, buffer cells can be utilized to pro- alkali hypophosphite contained in the center compart- tect the center compartment. This is, by adding a second ment is not critical. It is, in fact, limited only by saturation cationic exchange membrane to the first cationic mem- at the upper end and by the need for electrical conduc- brane, a catholyte buffer cell is created between the tivity of the solution at the lower end. In addition, this solu- 55 catholyte compartment and the center compartment. tion can be replenished from time to time with additional Therefore, even in the case where the hydroxyl ion con- alkali hypophosphite or the like during the electrodialysis. centration in the catholyte compartment builds up to the While the concentration of hypophosphite salt can very extent that some leakage occurs into the catholyte buffer cell, since the hydroxyl ion concentration in the catholyte

4 7 EP 0 701 860 A1 8 buffer cell would not build up to any significant extent, the liberation of hydrogen gas from the catholyte solution leakage of hydroxyl ion into the center compartment 22. would be effectively prevented. In order to produce an A five-compartment electrodialysis cell is depicted operating cell, the catholyte buffer cell should include a in FIG. 2. Anionic exchange membranes 140 and 140' starting solution which contained a dilute solution of elec- 5 and cationic exchange membranes 1 50 and 1 50' sepa- trically conductive innocuous ions just as in the case with rate electrodialysis cell 110 into five compartments. the catholyte compartment as discussed above. Anolyte compartment 1 1 6 contains anolyte solution 1 1 8 Similarly, by adding a second anionic exchange in contact with insoluble anode 112. Anolyte buffer com- membrane to the first anionic exchange membrane, an partment 116' contains anolyte buffer solution 118'. anolyte buffer cell can be created between the anolyte 10 Catholyte compartment 120 contains catholyte solution compartment and the center compartment. In this case, 122 in contact with stainless steel cathode 124. even if the H+ ion concentration in the anolyte compart- Catholyte buffer compartment 120' contains catholyte ment built up to the extent that some leakage occurred buffer solution 122'. Center compartment 125 contains into the anolyte buffer cell, since the H+ ion concentration a 200 g/l solution 127 of sodium hypophosphite. The in the anolyte buffer cell would not build up to a significant 15 anolyte solution 1 18 is a solution containing about 4 g/l extent, leakage of H+ ions into the center compartment H3PO2. Anolyte buffer solution 1 1 8' is a solution contain- would be effectively prevented. In this case, as with the ing a dilute solution of hypophosphorous acid. Catholyte catholyte buffer cell, in order to produce an operating cell, solution 122 is about a 5 g/l solution of NaOH. Catholyte the anolyte buffer cell should initially include a starting buffer solution 122' is a dilute solution of alkali metal solution containing a dilute solution of electrically con- 20 hydroxide. ducting ions of the type used in the anolyte compartment The present invention satisfies a heretofore unmet as discussed above. need for a method by which hypophosphorous acid may For each embodiment of the method of the present be prepared from inexpensive raw materials. This invention, the hypophosphorous acid is recovered by reduces the cost of preparation of hypophosphorous drawing off the solution in contact with the anode, i.e., 25 acid. the anolyte solutions of these electrodialysis cells. The The following examples illustrate particular condi- hypophosphorous acid thus produced can be sold in tions, steps and materials within the scope of this inven- solution form or concentrated therefrom. tion, it being understood that these examples are given In the electrodialysis cells, the hydroxyl ion concen- only by way of illustration and not limitation. tration of the catholyte solution increases as the hypo- 30 phosphorous acid solution is produced in the anolyte. EXAMPLES Upon completion of the reaction, the catholyte solution may be discarded, or diluted for reuse. In electrodialysis EXAMPLE 1 - Three-Compartment Electrodialysis Cells cells containing three or more compartments, the concentration of the alkali metal hypophosphite compart- 35 A 6 x 1 8 x 1 1 inch container was separated into three ment(s) decreases as the reaction progresses. This compartments essentially as depicted in FIG. 1 using a solution may be replenished and reused. R 5030 anionic exchange membrane from Pall Corpora- A three-compartment electrodialysis cell embodying tion and a Nation cationic exchange membrane from the method of the present invention is depicted in FIG. DuPont. The container thus had a center compartment 1. Anionic exchange membrane 14 and cationic 40 between ionic exchange membranes, with an anionic exchange membrane 1 5 separate electrodialysis cell 1 0 exchange membrane between the anolyte solution com- into three compartments. Anolyte compartment 16 con- partment and the center compartment and a cationic tains anolyte solution 18 in contact with insoluble anode exchange membrane between the catholyte solution 12. Catholyte compartment 20 contains catholyte solu- compartment and the center compartment. tion 22 in contact with stainless steel cathode 24. Center 45 To the anolyte compartment 50% H3PO2 was added compartment 25 contains a 200 g/l solution 27 of sodium to provide an initial hypophosphite anion concentration hypophosphite. The anolyte solution 18 is a solution ini- of 1 0 g/l and a pH of 1.16. To the center compartment tially containing about 4 g/l H3PO2. Catholyte solution 22 was added 4.0 L of a sodium hypophosphite solution is about a 5 g/l solution of NaOH. having a concentration of 203.0 g/l and a pH of 5.3. To The application of direct current from power source so the catholyte solution compartment NaOH was added to 30 anodically oxidizes OH" from the water in the aque- form an NaOH solution having a concentration of 10.0 ous anolyte solution 18, thus liberating oxygen at the g/l. A platinized titanium mesh, insoluble anode having insoluble anode 12 and generates H+ ions in the anolyte a dimension of 8" x 5.75" was immersed in the solution solution 18. Hypophosphite anions from the solution 27 in the anolyte compartment, and a stainless steel cath- in compartment 25 diffuse across membrane 1 4 to form ss ode having a dimension of 8" x 5.75" was immersed in hypophosphorous acid solution in anolyte solution 18 the solution in the catholyte compartment. All three solu- with the H+ ion generated anodically at the anode 12. tions were at room temperature. Hydroxyl anions are generated at the cathode 24, with A direct current of 4.0 amps at 1 5 volts was supplied, providing an approximate anode current density of 18

5 9 EP 0 701 860 A1 10 asf. Oxygen was produced at the anode, and hydrogen Claims gas was produced at the cathode. After 1 5 hours, the hypophosphite anion concentration in the anolyte com- 1 . A method for preparing hypophosphorous acid compartment was 80 g/l, and the concentration of sodium prising the steps of: hypophosphite in the center compartment was 129.0 g/l. 5 providing an insoluble anode (12, 1 12) in an The center compartment was then replenished with electrically conductive anolyte (18, 118) and a cath- 70 g/l of additional sodium hypophosphite. ode (24, 1 24) in an electrically conductive catholyte After a total of 28 hours at a continued current of 4 (22, 122), amps, the reaction was stopped. Over the time of the providing a hypophosphite salt solution (27, reaction, a voltage drop from the initial 1 5 volts to 1 0 volts w 127) separated from said anolyte by an anionic was experienced. The concentration of hypophosphite exchange membrane (1 4, 1 40') resistant to cationic anions in the anolyte compartment was now 1 35 g/l. The diffusion and from said catholyte by a cationic solution was free, or substantially free, of orthophosphite exchange membrane (1 5, 1 50') resistant to anionic anions. The pH had decreased to 0.5. diffusion, and In the center compartment, the sodium hypophos- is applying a direct current (30, 130) through phite concentration had decreased to 153.0 g/l and the said insoluble anode to said cathode to transfer pH had decreased from 5.3 to 1 .9. hypophosphite anions through said anionic exchange membrane into said anolyte and to gen- EXAMPLE 2 erate hydrogen ions in said anolyte, thereby forming 20 hypophosphorous acid in said anolyte. In a second experiment, the same container was separated into three compartments in essentially the 2. The method of claim 1 characterized in that said same manner as discussed above. In this case, to the hypophosphite salt solution (27, 1 27) comprises an anolyte compartment hypophosphorous acid was ini- alkali metal hypophosphite salt solution. tially added to provide a hypophosphite anion concen- 25 tration of 13.8 g/l at a pH of 1.0. To the center 3. The method of claim 1 characterized in that said compartment was added 4.0 L of a sodium hypophos- insoluble anode (12, 112) comprises a precious phite solution having a concentration of 1 90 g/l and a pH metal surface. of 4.6. To the catholyte solution compartment, NaOH was added to form an NaOH solution having a concen- 30 4. The method of claim 3 characterized in that said pre- tration of 10 g/l. Cathodes having the dimensions in the cious metal surface is selected from the group con- experiment discussed above were again utilized and all sisting of platinum, iridium and ruthenium, and the three solutions were at room temperature. The platinized oxides thereof. titanium mesh anode was reduced in size to provide a high anode current density. 35 5. The method of claim 4 characterized in that said A direct current of 4.5 amps was supplied, providing insoluble anode (12, 112) includes an inert inner an anode current density of approximately 1 00 asf. support for said precious metal surface. After seven hours, the reaction was stopped. The volume of the solution in the anolyte compartment had 6. The method of claim 5 characterized in that said increased from 1 .2 to 1 .4 L. The pH had decreased from 40 inert inner support comprises a metal selected from 1 .0 to 0.75. The hypophosphite anion concentration had the group consisting of titanium, zirconium and tan- increased from 13.8 g/l to 65 g/l. The solution was free, talum. or substantially free, of orthophosphite anions. In the center compartment, the volume of the solu- 7. The method of claim 1 characterized in that said tion was maintained the same, namely, 4.0 L. The 45 cathode (24, 1 24) is selected from the group con- sodium hypophosphite concentration had decreased sisting of stainless steel, steel, graphite, and plati- from 1 90 g/l to 1 67.0 g/l. The pH had decreased from 4.6 num-coated titanium. to 2.86. The foregoing examples demonstrate the high effi- 8. The method of claim 1 characterized in that said ciency at which hypophosphorous acid may be produced so direct current (30, 1 30) is applied to said anode at a from inexpensive raw materials by the method of the current density of between about 1 0 and about 400 present invention. As will be readily appreciated, numer- asf. ous various and combinations of the features set forth above can be utilized without departing from the present 9. The method of claim 8, characterized in that said invention as set forth in the claims. Such variations are 55 direct current (30, 1 30) is applied to said anode at a not to be regarded as a departure from the spirit and current density of between about 20 and about 200 scope of the invention, and all such modifications are asf. intended to be included within the scope of the following claims.

6 11 EP 0 701 860 A1

10. The method of claim 1, characterized in that said electrically conductive anolyte (18, 118) comprises a dilute solution of hypophosphorous acid.

11. The method of claim 1 characterized in that said 5 hypophosphite salt solution (27, 1 27) has a concentration of between about 1 00 and 200 g/l grams of said hypophosphite salt per liter.

12. The method of claim 1 characterized in that said 10 electrically conductive catholyte (22, 122) comprises an alkali metal hydroxide solution.

13. The method of claim 1 characterized by the step of recovering said hypophosphorous acid. 15

14. The method of claim 1 characterized in that said step of recovering said hypophosphorous acid comprises the step of concentrating said hypophosphorous acid from said anolyte (1 8, 1 1 8). 20

15. The method of claim 1 characterized in that said hypophosphite salt solution (27, 1 27) is separated from said anode (12, 1 12) by a pair of said anionic exchange membranes (140, 140') resistant to cation 25 diffusion, thereby providing an anolyte buffer solution (1 1 8') between said pair of anionic exchange membranes resistant to cation diffusion.

1 6. The method of claim 1 5 characterized in that said 30 hypophosphite salt solution (27, 127) comprises an alkali metal hypophosphite.

17. The method of claim 15, characterized in that said anolyte buffer solution (118') comprises a dilute 35 solution of hypophosphorous acid.

18. The method of claim 15, characterized by the step of recovering said hypophosphorous acid. 40 19. The method of claim 1 characterized in that said hypophosphite salt solution (27, 1 27) is separated from said cathode (24, 1 24) by a pair of said cationic exchange membranes (150, 150') resistant to anion diffusion, thereby providing a catholyte buffer solu- 45 tion (122') between said pair of cation ionic exchange membranes resistant to anion diffusion.

20. The method of claim 1 9 characterized in that said catholyte buffer solution (122') comprises a dilute so solution of alkali metal hydroxide.

55 EP 0 701 860 A1

FIG. 2 EP 0 701 860 A1

Patent Application Number J) European EUROPEAN SEARCH REPORT Office EP 95 11 1697

DOCUMENTS CONSIDERED TO BE RELEVANT Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE Category of relevant passages to claim APPLICATION (Int.CI.6) P,X JS-A-5 431 792 (MORGAN) 1,2, B01D61/44 7-14,19, C01B25/165 20 C25B1/22 * abstract; claims 1-3; figure 1 column 1, 1 1 ne 1 - line 29 column 1, line 50 column 2, 1 ine 12 * column 3, line 48 column *, 1 ine 6 * column 4, 1 i ne 55 column 5, 1 ine 30 * example 1 column 8, line 58 - column 9, line 39

CHIMIE & INDUSTRIE, vol . 87, no. 6, June 1962 pages 759-763, R.R. PAHUD 1 L1 acide hypophosphoreux et les hypophosphites1 * page 759, column 2, line 6 - line 9 *

GB-A-751 856 (THE PERMUTIT CO LTD) * claim 3; figure 2 * * 2, line 82 - 3, line 2 * TECHNICAL f lfcLUa page page SEARCHED (Int.CI.6) EP-A-0 459 751 (RINKAGAKU K0GY0 CO LTD) B01D * abstract; claims 1,4,5; figure 1 * C01B * page 3, line 31 - line 41 * C25B PATENT ABSTRACTS OF JAPAN vol. 18 no. 468 (C-1244) ,31 August 1994 & JP-A-06 145995 (CANON INC) 17 May 1994, * abstract * & DATABASE WPI Section Ch, Week 9426 Derwent Publications Ltd., London, GB; Class D15, AN 94-211307 * abstract *

The present search report has been drawn up for all claims Place of icarch Date of conpletioa of the search ExmAma THE HAGUE 8 December 1995 Hoornaert, P CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document