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 hypophospho- rous 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, irid- ium 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).