United States Patent (19) 11) 4,021,322 Dolan 45 May 3, 1977

(54) ELECTROLYTIC PREPARATION OF 3,437,439 4/1969 Carroll ...... 23/165 PHOSPHORUS ACID FROM ELEMENTAL 3,437,440 4/1969 Carroll ...... 231165 3,528,772 9/1970 Whyte et al...... 231165 PHOSPHORUs 3,532,461 10/1970 Whyte et al...... 23/165 (75) Inventor: Michael J. Dolan, Manchester, Mo. OTHER PUBLICATIONS 73) Assignee: Monsanto Company, St. Louis, Mo. Inorganic Syntheses, vol. IV, 1953, pp.55-58, “Phos (22 Filed: Dec. 15, 1975 phorus Acid." (21) Appl. No.: 640,506 Primary Examiner-R. L. Andrews Attorney, Agent, or Firm-Wendell W. Brooks; John D. 52 U.S. Cl...... 204/103 Upham (51 Int. Cl'...... C25B 1122 (58) Field of Search ...... 204/103 57 ABSTRACT (56) References Cited Elemental phosphorus is oxidized via indirect electro UNITED STATES PATENTS lytic oxidation to yield phosphorous acid, (HO),HPO. 3.437438 4/1969 Carroll ...... 231165 12 Claims, 2 Drawing Figures

OVERFLOW HX / H2O

EVAPORATOR

CRUDE HO)a HPO

14 CRYSTALLIZER (HO)2 HPO FILTER CRYSTALS

( Ho), HPo Li QUOR U.S. Patent May 3, 1977 4,021,322

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SNSS arr an NNar NNN a C2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2:2

FIG... l.

OVERFLOW HX / H2O

EVA PORATOR

CRUDE (Ho). HPo

CRYSTALLIZER (HO)2 HPO FILTER CRYSTALS

(HO)2 HPO LIQUOR 4,021,322 1 2 pared by the indirect electrolytic oxidation of elemen ELECTROLYTIC PRE PARATION OF tal phosphorus. PHOSPHORUS ACID FROM ELEMENTAL PHOSPHORUS DETALED DESCRIPTION OF THE INVENTION 5 The indirect electrolytic oxidation of elemental phos BACKGROUND OF THE INVENTION phorous to phosphorous acid conveniently represented This invention relates to the indirect electrolytic by reactions (2) through (5). oxidation of elemental phosphorus to phosphorous acid, (HO)HPO. ANODE REACTION 3 X -> 1% X + 3e (2) Phosphorous acid is available commercially as 30 10 percent and 70 percent aqueous solutions. The conven SOLUTION REACTIONS 1% X, + AP - PX (3) tional method of preparation of phosphorous acid com PX-3HO - (HO), HPO +3H +3X (4) prises hydrolyzing phosphorus trichloride according to the diagrammatically simplified reaction: CATHODE REACTION 3H + 3e - 1% H, (5) 15 PC-3HO - (HO)HPO +3HC (1) The net effect of reactions (2) through (5) of the present process are summarized as shown in reaction and evaporating the excess water and the hydrogen (6). chloride which are formed. However, the methods AP +3HO - (HO),HPO - 1% H. (6) involving these raw materials suffer from a number of 20 While not desiring to be bound by the theory of the disadvantages, most of which are inherent in the prior present invention, it is believed that the process com art methods. prises: One of the more obvious and vexing difficulties asso a generation of molecular halogen from the corre ciated with the known methods of preparation of phos sponding halide ion by electrolytic oxidation at the phorous acid is the absence of simple, effective, and 25 anode of an aqueous solution of hydrogen halide; efficient means of disposal for the large volume of b. oxidative reaction of the molecular halogen pro hydrogen halide produced during the conversion (ei duced during the electrolysis with elemental phos ther hydrolysis or otherwise) of phosphorus trihalide to phorus to form phosphorus trihalide, and the desired acid. Means which have been proposed for c. hydrolysis of the thus-formed phosphorus trihalide this purpose are generally expensive and less than satis 30 to produce phosphorous acid and halide. factory. Electrolytic reduction of hydrogen ions (protons) at As a result of the difficulties and disadvantages asso the cathode completes the electrochemical reaction. ciated with the known methods of preparation, phos Elemental phosphorus is a non-metallic element that phorus acid remains a relatively expensive chemical 35 exists in several allotropic forms (white or yellow, red, compound. and black or violet). All of these forms can be used in It has now been discovered that the difficulties and the present invention but the white or yellow (the terms disadvantages of the prior art methods are overcome by are used interchangeably) and red forms are preferred. the process of the present invention which represents a Of these, the white or yellow form is particularly pre substantial improvement in the sense that: 40 ferred. The term "elemental phosphorus,' as used a. the hydrogen halide generated is disposed of in situ herein, designates these allotropic forms. in a manner which facilitates it being recycled for White phosphorus exists as P, having a tetrahedral continued use with the result that only catalytic molecular structure. It is a brittle, waxy solid which has amount of hydrogen halide is required to be added a melting point of 44.1 C and a boiling point of 280.5 initially; and 45 C. Its vapor density corresponds to a formula of P. It is b. phosphorous acid is obtained relatively inexpen virtually insoluble in water and alcohol, moderately sively with a possible resulting decrease in the com soluble in chloroform, hexane, and benzene, and very mercial price of this important chemical com soluble in carbon disulfide. pound. Because elemental phosphorous is insoluble in water, A further advantage of the present invention is the 50 coupled with the fact that in the practice of the present ready availability of the essential reactants. The essen invention the electrolysis medium comprises elemental tial reactants are elemental phosphorus, hydrogen ha phosphorus and an aqueous solution of hydrogen ha lide (which is reusable), water, and electric current. lide, the elemental phosphorus is conveniently sus Various other advantages of this invention will be pended in the aqueous solution of hydrogen halide. come apparent from the accompanying description and 55 And further, since white phosphorus, the preferred claims. allotropic form, melts at 44.1 C, the electrolysis is conveniently carried out with the elemental phospho BRIEF DESCRIPTION OF THE DRAWINGS rus existing as a molten suspension. FIG. 1 is a schematic diagram of a cross section of an Thus, it is preferred to agitate the electrolysis me undivided electrolysis cell suitable for batch operation 60 dium vigorously so as to insure a relatively uniform of the present invention. dispersion of the elemental phosphorus throughout the FIG. 2 is a schematic illustration of a typical flow aqueous solution of hydrogen halide. Such agitation diagram of a process suitable for continuous operation also facilitates the dispersal of impurities and thereby of the present invention. minimizes the tendency of such impurities gradually to 65 coat the surface of the elemental phosphorus and pre SUMMARY OF THE INVENTION vent further reaction, particularly at process tempera According to the present invention, it has been dis tures in excess of its melting point. Still further, such covered that phosphorous acid is conveniently pre agitation insures thorough mixing the elemental phos 4,021,322 3 4 phorus and the molecular halogen produced during the The de Nora-type dimensionally stable anodes em electrolysis. Moreover, the preferred vigorous agitation ploy precious metal oxides plated on a titanium sub of the reaction mixture facilitates relatively rapid hy strate. Other materials include, for example, ruthenium drolysis of the phosphorus trihalide to phosphorous oxide, mixed with oxides of titanium and tantalum, also acid. The agitation can be carried out in any conven plated on a titanium substrate. Dimensionally stable tional manner such as by flow mixers, jet mixers, injec anodes suitable for use in the present process are cur tors, turbulence mixers, circulatory mixer systems, rently commercially available from the Diamond centrifugal pumps, and the like; by paddle and propel Shamrock Company, Cleveland, Ohio. ler mixers of various designs as well as by turbine or The anode materials of choice, by analogy with the centrifugal impeller mixers, colloid mills, and homoge 10 electrolysis of hydrochloric acid in chlorine cells which nizers. involves oxidation of chloride ion at the anode and The concentration of the aqueous solution of hydro reduction of hydrogen ions at the cathode, are graphite gen halide can vary widely, for example, from about 0.5 and de Nora-type dimensionally stable anodes. Graph percent to about 50 percent or more by weight of the ite functions in the present invention satisfactorily ex aqueous phase, but preferred concentrations will often 15 cept when an aqueous solution of hydrogen chloride is be in the range of about 1.0 percent to about 10 per employed as the source of molecular halogen. In such cent by weight, or on a molar basis, often in the range instances the electrolysis causes significant anode cor of about 0.1 molar to about 3.0 molar. It is to be noted, rosion to occur. It, therefore, becomes advantageous to however, that the concentration of the aqueous solu employ the de Nora-type anodes which are sufficiently tion of hydrogen halide has little effect on the current 20 stable under the reaction conditions utilized so as to efficiencies and product distribution (although there eliminate any corrosion problems. A further advantage might be a lower limit which would depend on the resulting from the use of dimensionally stable anodes is current density employed). the lowering of the halogen overvoltage with a concur Various current densities can be employed in the rent lowering of energy requirements. present process. It will be desirable to employ high 25 Any suitable electrode material may be employed as current densities in order to achieve high use of elec the cathode so long as it is relatively inert under the trolysis cell capacity which will result in increased pay reaction conditions and does not promote the produc load. Therefore, for production purposes it will gener tion of undesirable side products, such as, for example, ally be desirable to use as high a density as feasible, phosphine in any significant amount. Graphite serves taking into consideration sources and cost of electrical 30 admirably as a satisfactory cathode material, even current, resistance of the electrolysis medium, heat when aqueous hydrogen chloride is employed as the dissipation, effect upon yields, etc. Over broad ranges source of molecular halogen. If, therefore, is the mate of current density, the density will not greatly affect the rial of choice. Low hydrogen overvoltage metals, such yield. And while low densities are operable, suitable as, for example, platinum, palladium, and the like are ranges for efficient operation will generally be in the 35 also suitable as cathode material, although they suffer range of a few hundred amperes per square meter of from the disadvantage of being relatively expensive. anode surface, up to 10,000 to 20,000 or more amperes High hydrogen overvoltage metallic cathodes, such as, per square meter. for example, mercury, zinc, lead, and the like may be In effecting the present process, the cell voltage must used, but is advantageous and desirable to avoid their be sufficient to pass the desired current (amperes) and 40 use in that they promote the direct reduction of phos to effect electrolytic oxidation for hydrogen halide. phorus to phosphine. Generally this value should be as close to the theoreti While a divided cell may be employed in the practice cal cell voltage as possible, although it is recognized of the present process, an undivided cell is generally that the cell voltage will vary with electrode materials preferred. Such a cell offers marked advantages over and their surface conditions, the distance between the 45 divided cells for commercial production purposes in electrodes, various materials in the electrolysis me that electrical resistance across a cell-divider is elimi dium, resistance of the electrolysis medium, and resis nated. It is to be noted, however, that when high hydro tance of cell dividers, when employed. For example, gen overvoltage metallic cathodes are employed di under the conditions employed in the procedural and vided cells may be preferred so as to avoid reduction of illustrative Examples described hereinbelow the cell SO phosphorus to phosphine. voltage is between about +6.0 volts and +8.0 volts. The electrolysis cell employed in the procedural Ex The present process can be conducted in the various amples herein is primarily for laboratory demonstration types of electrolysis cells known in the art. In general, purposes. Production cells are usually designed with a such cells comprise a container made of material capa view to the economics of the process, and characteristi ble of resisting action of electrolytes, that is, material 55 cally have large electrode surfaces and short distances which is inert under the reaction conditions, for exam between the electrodes. ple, glass or plastic and a cathode and an anode, which The electrolysis cell utilized in the procedural and are electrically connected to sources of electric cur illustrative examples described hereinbelow is shown in rent. The anode may be of any electrode materialso FIG. 1 except for four other necks, one of which is used long as it is relatively inert under the reaction condi 60 for addition of reactants and periodic sampling. It is tions. Suitable anode materials include, for example, stoppered during the electrolysis. The remaining three graphite, de Nora-type dimensionally stable anodes, of the not-shown necks are used for gas-tight attach the precious metals such as platinum, palladium, ruthe ment of a water-cooled condenser topped with a mer nium, rhodium, and the like, and the precious metals cury-sealed gas outlet and vent, a thermometer, and a plated onto other metals, such as, for example, titanium 65 gas-inlet tube. and tantalum, although the precious metal type anodes Referring to FIG. 1, electrolysis cell 1 comprises a suffer from the disadvantage of being relatively expen glass reaction vessel consisting of two sections--bottom SWe. section 2 and top section 3-joined together at flange 4,021,322 S 6 joint 4 and secured by fastening means, such as, for are satisfactory. If desired, cooling can be effected by example, metal fastening clamps. permitting a component, for example, water reflux Cell 1 is equipped with graphite electrodes (or de through a cooling condenser, or by immersing the elec Nora-type dimensionally stable anode and graphite trolysis cell in an ice or ice-salt bath. The amount of cathode) 5 which are spaced apart a suitable distance 5 cooling capacity needed for the desired degree of con by Teflon rods 6. The Teflon rods 6 are extended to the trol will obviously depend upon the cell resistance and sides of section 2 of cell to maintain the electrode the electrical current drawn. assembly rigid. Cell 1 is also equipped with a mechani Pressure can be employed to permit electrolysis at cal stirrer which is fitted with a large Teflon paddle 7 higher temperatures, but unnecessary employment of capable of effecting vigorous agitation of the reaction O pressure is usually undesirable from an economic mixture. standpoint. Moreover, it is to be noted that phospho For a general description of various laboratory scale rous acid, on being subjected to excessive heat, under cells see Lund et al., “Practical Problems in Electroly goes disproportionation to phosphoric acid and phos sis, "in Organic Electrochemistry (Baizer, ed.), Marcel phine and/or hydrogen as illustrated in reactions (7) Dekker, New York, 1973, pp. 165-249, and for some 15 and (8). consideration of industrial cells designs see Danly, "In dustrial Electroorganic Chemistry," in Ibid., pp. 4(HO),HPO Real- 3(HO)PO + PH (7) 907-946. The present process is suited to either batch or con (HO),HPO + HOlea (HO)PO + H, (8) tinuous operations. Continuous operations may in 20 volve, after product removal, recirculation of the aque Therefore, the preferred temperature is any tempera ous electrolyte (aqueous solution of hydrogen halide) ture not sufficient to cause substantial disproportion in a manner similar to that illustrated in FIG. 2. ation. More particularly, the preferred temperature is in order to facilitate the explanation of one such less than 180° C and greater than the melting point of continuous operation as contemplated herein, refer 25 the elemental phosphorous utilized because attemper ence is made to FIG. 2. Electrolysis cell 1 is as shown in atures greater than 180°C reactions (7) and (8) occur FIG. 1 except that it contains additional inlets and at a fairly rapid rate and at temperatures greater than outlets sufficient to accommodate any desired addi the melting point of the elemental phosphorous uti tions, withdrawals, and recycling of materials. For ex lized, the elemental phosphorous will exist in molten ample, elemental phosphorus is added from reservoir 8 30 form which facilitates its dispersal throughout the and water is added from reservoir 9. aqueous solution of hydrogen halide. As the reaction of the present process proceeds, the The process of the present invention involves an aqueous solution containing dissolved phosphorous indirect electrolytic oxidation and therefore requires a acid flows by overflow line 10 into evaporator 11 where source of oxidizing agent. Aqueous hydrogen halide the water and hydrogen halide (aqueous solution of 35 which is employed in catalytic amounts admirably hydrogen halide) are removed by evaporation. The serves this purpose. The preferred molar ratio range of aqueous solution of hydrogen halide is recycled to cell elemental phosphorous to hydrogen halide present in 1 by line 12 for repeated use as a source of molecular the aqueous solution is between about 1:1 and about halogen. The crude phosphorous acid is discharged by 20:1, although the molar ratio can be considerably line 13 to crystallizer-filter 14 where it is crystallized 40 higher or lower as desired. and filtered by suction filtration. The crystals are re While it is recognized that any means known to the moved and collected through line 15 while the filtrate art, such as, for example, employing (a) a greater or is discharged through line 16. lesser volume of the aqueous solution of hydrogen A number of options, including but not limited to halide having the same concentration; (b) a greater or those described hereinbelow, are available for utilizing 45 lesser molar quantity of elemental phosphorus; or (c) the filtrate discharged through line 16. It can be (a) some combination of (a) and (b) can be employed to recycled to cell 1 to facilitate isolation of additional effect a change in the molar ratio of elemental phos phosphorous acid on a repeated run through the reac phorus to hydrogen halide, a convenient means of ef. tion system; (b) transferred to a cell similar to cell 1, fecting such a change is simply to increase or decrease with an aqueous solution of hydrogen halide and ex 50 the concentration of the aqueous solution of hydrogen haustively oxidized to phosphoric acid, which is useful halide. And as noted hereinbefore, the concentration as an article of commerce; (c) exhaustively oxidized to of hydrogen halide has little effect on the current effi phosphoric acid by any other means known to the art, ciencies and product distribution. for example, catalytic oxidation; or (d) separated into The hydrogen halides preferred for use in the present its component acids by any suitable means known to 55 process are hydrogen chloride, hydrogen bromide, and the art, for example, counter current extraction as hydrogen iodide. Of these, hydrogen bromide and hy described in Kovacs et al., U.S. Pat. No. 3,769,384. drogen iodide are particularly preferred because of (a) It is obvious that the counter current extraction pro the stability of graphite anodes in their aqueous solu cess as described in Kovacs et al, U.S. Pat. No. tions under process conditions; and (b) the high selec 3,769,384 can also be applied to the aqueous solution 60 tivity towards phosphorous acid which is observed flowing through overflow line 10 as well as the crude when they are employed. It is recognized, however, phosphorous acid discharged by line 15. that in view of its lower cost, hydrogen chloride might The electrolysis of the present process can be con be the hydrogen halide of choice. ducted at a broad range of temperatures-ambient, or The term “selectivity” is employed herein to mean higher or lower temperatures-without any significant 65 the percentage of reacting molecules of elemental effect upon the course of the reaction and the yield of phosphorus converted to phosphorus acid. the desired phosphorous acid. For example, tempera Without limiting the present invention in any way, it ture ranges from about 20°C or lower to about 180°C is believed that in accordance therewith, the hydrogen 4,021,322 7 8 halide is electrolytically oxidized to molecular halogen, denser topped with a mercury sealed gas outlet and which in turn oxidatively reacts with elemental phos vent, and a gas-inlet tube. The remaining neck was used phorus to form phosphorus trihalide. The thus-formed for addition of reactants and was stopped during the phosphorus trihalide is hydrolyzed to the desired phos electrolysis. The bottom section had a usable volume phorous acid and hydrogen halide. In this case, how up to the flange joint of about 800 milliliters. The cell ever, the need for external means for disposing of the was equipped with graphite-plate electrodes measuring hydrogen halide generated thereby is eliminated; it is 10 x 6 x 1.2 centimeters and spaced 3 centimeters disposed of in situ by recycling by means of electrolytic apart by Teflon rods which were extended to the sides oxidation to molecular halogen for reuse as a reactant. of the glass reaction vessel to maintain the electrode That is, the halide ions are electrolytically oxidized to 10 assembly rigid. Vigorous agitation of the reaction mix regenerate molecular halogen which further reacts with ture was accomplished by a mechanical stirrer fitted elemental phosphorous to produce additional phospho with a large Teflon paddle. rous trihalide. At the same time the hydrogen ions are A mixture of white phosphorous (100.0 grams, 3.23 electrolytically reduced at the cathode to generate moles) and aqueous hydrogen bromide (400 milliliters, hydrogen gas which, being non-polluting, is safely 15 2.4 percent, 0.176 mole; prepared from 20 milliliters of vented into the atmosphere, or, alternatively, flared to 48 percent aqueous hydrogen bromide and 380 milli produce gaseous water as the only product. liters of water) was placed in the electrolysis cell and This means of disposing of the hydrogen halide gen heated to about 50° C under a nitrogen atmosphere. erated during the hydrolysis of phosphorus trihalide to The mixture was vigorously agitated to disperse the phosphorous acid provides obvious advantages over now molten phosphorus and electrolyzed with a con procedures described in the prior art. stant current of 10 amperes for 26 hours (which is The phosphorous acid produced in the present inven equivalent to 260 ampere-hours which equal 9.7 Fara tion is conveniently recovered in the form of the free days which equal 3.0 Faradays per mole of white phos acid. However, it is to be understood that the isolation phorus). Throughout the electrolysis, vigorous agita procedures employed in the procedural Examples and 25 tion and nitrogen purging were maintained. The initial discussed herein are primarily for illustrative purposes. cell voltage of about --8.0 volts gradually decreased Other procedures can be employed, and may be pre over the electrolysis period to about +6.0 volts. The ferred, for commercial purposes. passage of current maintained the reaction mixture at a Upon completion of the reaction, the aqueous solu temperature of between about 70° C and about 80°C tion is filtered, if necessary, to remove unreacted ele 30 throughout the electrolysis. Upon completion of the mental phosphorous and, if desired, analyzed to deter reaction, the cell and its contents were allowed to cool mine the total yield of phosphorous acid. Utilizing this to ambient temperatures, whereupon the reaction mix value and the total amount of elemental phosphorous ture was filtered in a nitrogen atmosphere to remove consumed during the reaction, the percentage yield of unreacted white phosphorus. The filtrate was analyzed phosphorous acid can also be determined. And while 35 by hydrogen-1 and phosphorus-31 nuclear magnetic any method of phosphorous acid analysis known to the resonance spectroscopy and found to contain phospho art can be employed, a method suitable for use herein rous acid (1.02 moles), hypophosphoric acid (0.08 is hydrogen-1 and phosphorous-31 nuclear magnetic mole) and phosphoric acid (0.79 mole). Evaporation resonance spectroscopy which provides a convenient of the filtrate in vacuo at moderate temperatures of and efficient method of analysis. 40 between about 70° C and about 80° C gave a viscous The aqueous filtrate is evaporated in vacuo at moder liquid which was seeded with a crystal of phosphorous ate temperatures to yield a viscous liquid which upon acid. The crystallized mass was filtered with prolonged cooling to ambient temperatures partially crystallizes. suction under a nitrogen atmosphere to give phospho More complete and more rapid crystallization can be rous acid (39.0 grams) containing about 5 mole per induced by using subambient temperatures and by the 45 cent phosphoric acid and having an overall purity of addition of a seed crystal of phosphorous acid to tie about 92 percent which corresponds to a yield of 0.44 viscous liquid. Filtration of the crystallized mass with mole. prolonged suction under a nitrogen atmosphere yields The current efficiency for the combined production phosphorous acid as white crystals. of phosphorus acids (phosphorous acid, hypophos The filtrate from the isolation of phosphorous acid 50 phoric acid, and phosphoric acid) was 61 percent, the can be recycled to electrolysis cell 1 to facilitate isola apparent conversion of white phosphorus to phospho tion of additional phosphorous acid upon repeating the rus acids as determined by hydrogen-1 and phos present process. It can also be utilized by employing phorus-31 nuclear magnetic resonance spectroscopic any of the remaining available options as described analysis was also 61 percent; the percentage yield of hereinabove. 55 phosphorous acid isolated was 22 percent based on the The following examples illustrate the present inven apparent conversion of white phosphorus; and the se tion and the manner by which it can be practiced. lectivity to phosphorous acid was 52 percent. EXAMPLE 1. EXAMPLE 2 The reaction was carried out in an undivided cell 60 Following the procedure of EXAMPLE 1 above, a (FIG. 1) comprising a 1-liter glass reaction vessel con mixture of white phosphorus (40.0 grams, 1.3 moles) sisting of two sections-a bottom section and a top and aqueous hydrogen iodide (400 milliliters, 5.7 per section-joined together at the flange joint and secured cent, 0.3 mole; prepared from 40 milliliters of 57 per by fastening means, such as, for example, metal fasten cent aqueous hydrogen iodide and 360 milliliters of ing clamps. The top section had seven necks, with 65 water) was electrolyzed with a constant current of 10 standard-taper inner joints, used for gas-tight attach amperes for 10 hours (which is equivalent to 100 am ment of a mechanical stirrer, two platinum wire elec pere-hours which equal 3.7 Faradays which equal 2.8 trode connections, a thermometer, a water-cooled con Faradays per mole of white phosphorus). The initial 4,021,322 9 10 cell voltage of about +7.0 volts gradually decreased tions, and variations as fall within the spirit and broad over the electrolysis period to about +6.0 volts. Hydro scope of the invention. gen-1 and phosphorus-31 nuclear magnetic resonance What is claimed is: spectroscopic analysis of the aqueous filtrate following 1. A process for the indirect electrolytic oxidation of removal of unreacted white phosphorus showed the elemental phosphorus to phosphorous acid which com presence of phosphorous acid (0.36 mole), hypophos prises subjecting an electrolysis medium comprising phoric acid (0.03 mole), and phosphoric acid (0.15 elemental phosphorus and an aqueous solution of hy mole). drogen halide to electrolysis, and thereafter recovering The current efficiency for the combined production phosphorous acid. O 2. The process of claim 1 wherein of phosphorus acids was 55 percent; the apparent con a molecular halogen is generated from the corre version of white phosphorus to phosphorus acids was sponding halide ion by electrolytic oxidation at the 44 percent; and the selectivity to phosphorous acid was anode of an aqueous solution of hydrogen halide; 63 percent. b. said molecular halogen produced during the elec Phosphorous acid has a number of useful purposes. It 15 trolysis oxidatively reacts with the elemental phos is useful as a reducing agent where a strong but rela phorus to form phosphorus trihalide; tively slow-acting reducing agent is desirable. It is also c. said phosphorus trihalide is hydrolyzed to produce useful as a starting material for the production of phos phosphorous acid and hydrogen halide; phitic esters such as diethyl phosphite, which is useful d. said hydrogen halide is recycled; and as a lubricant additive, antioxidant, and solvent. 20 e. said phosphorous acid is recovered. Phosphorous acid is also employed as a starting mate 3. The process of claim 2 wherein the phosphorus rial in the preparation of valuable phosphonate com trihalide is phosphorus trichloride. pounds such as ethane-1-hydroxy-1,1-diphosphonic 4. The process of claim 2 wherein the phosphorus acid which, including water soluble derivatives thereof, trihalide is phosphorus tribromide. are valuable builders for detergent compositions as 25 5. The process of claim 2 wherein the phosphorus described in Diehl, U.S. Pat. No. 3, 159,581. In addi trihalide is phosphorus triiodide. tion, phosphorous acid is useful in the preparation of 6. The process of claim 2 wherein the hydrogen ha various phosphonomethylamines. Such compounds are lide generated during the hydrolysis of phosphorus known agents for various water treating and similar trihalide to phosphorus acid is disposed in situ by recy purposes, particularly as scale inhibiting agents as de 30 cling by means of electrolytic oxidation to molecular halogen for reuse as a reactant. scribed in Railson, U.S. Pat. No. 3,336,221, and as 7. The process of claim 1 wherein the cell voltage is metalion sequestering agents as described in Irani, U.S. sufficient to pass the desired current and to effect elec Pat. No. 3,234,124. In addition to scale inhibition in trolytic oxidation of hydrogen halide. boiler waters, and the like, such agents are effective in 35 8. The process of claim 7 wherein the cell voltage is inhibiting corrosion of iron, steel, and other metals between about 6.0 volts and about --8.0 volts. coming into contact with such water under highly oxy 9. The process of claim 1 wherein the elemental genated or otherwise possibly corrosive conditions. phosphorus is white phosphorus. And, because of their inhibiting, anti-precipitant, che 10. The process of claim 1 wherein the molar ratio of lating, and sequestering properties, such agents are 40 elemental phosphorous to hydrogen halide present in usefully employed in various soaps, detergents, and the aqueous solution is between about 1:1 and about cleaning compounds. 20:1 and the process temperature is between about 45° While the invention has been described with respect and about 150 C. to various specific examples and embodiments thereof, 11. The process of claim 1 wherein a graphite anode it is to be understood that the invention is not limited 45 and a graphite cathode are used. thereto and that many alternatives, modifications, and 12. The process of claim 1 wherein a de Nora-type variations will be apparent to those skilled in the art in dimensionally stable anode and a graphite cathode are light of the foregoing description. Accordingly, it is used. intended to embrace all such alternatives, modifica k k k k 50

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