Patented Sept. 5, 1950 2,521,082

UNITED STATES PATENT OFFICE

ELECTROLYTIC PRODUCTION OF SILVER CHLORIDE . John B. Mullen, Chicago, Ill., assignor, by mesne assignments, to Burgess Battery Company, Freeport, Ill., a corporation of Delaware No Drawing. Application July 9, 1947, Serial No. 59,91f. Claims. (C. 204-56) 1. 2 This invention relates to an improved method It is an additional object of the invention to for forming silver chloride electrolytically, and provide a method of the character described ini particularly to a method of electrolytically form which the formed layer of silver chloride is of ing an adherent coating of silver chloride upon uniform thickness. a body of silver. It is a further object of the invention to pro In a known type of primary cell, one of the vide an improved method of anodically oxidizing electrodes is composed of silver and has a layer Silver to form silver chloride in which the eff. of silver chloride applied thereto, which layer ciency of silver chloride formation is increased. serves as the depolarizer of the cell. In a cell Briefly, the objects are accomplished by car structure which has been used satisfactorily, the O rying out the anodic oxidation in a cell in which silver electrode is in the form of a thin sheet the electrolyte is a dilute aqueous solution of a having the thickness of foil, and the layer of Soluble chloride having an alcohol such as silver chloride is formed in situ upon the silver methanol dissolved therein. sheet by electrolytic action. Such method of As an example of a satisfactory method in ac forming the layer of silver chloride has advan 15 Cordance With the invention, an electrolytic cell tages in that it adheres well to the silver, is com is made up having an electrolyte of the following pact and exhibits low electrical resistance dur Composition: ing cell action. It is to an improvement in the Pounds method of forming such layer of silver chloride Water ------95 that the present invention is directed. Sodium chloride ------6 The layer is formed by providing an electrolytic Hydrochloric acid (38% solution in water) - 1 cell in which the sheet of silver is arranged as Methanol ------5 the positive electrode, or anode, in a bath of elec trolyte comprising a dilute aqueous Solution of The above solution has a pH of approximately a soluble chloride, and passing electric current 1.0. The composition and pH are maintained through the cell. At the surface exposed to the approximately as given by suitable replenish electrolyte, the Silver electrode undergoes anodic ments of the various ingredients as they are oxidation to Silver chloride, which forms as a Consumed during operation of the cell. The cell dense, compact layer in adherent relation to the is completed by arranging the electrodes there surface of the silver sheet. 30 in, one of which is composed of silver and the The method described has advantages, as pre other of which may be composed of any con viously mentioned, but also has the disadvantage ductive composition which is not attacked by the that the silver sheet is not uniformly attacked, electrolyte, such as carbon, silver, gold, platinum, with the result that the layer of silver chloride etc., and the electrodes are connected to a source is of non-uniform thickness and the thin silver of direct current energy in such manner that sheet is frequently penetrated, that is, completely the silver electrode is the positive electrode, or converted to silver chloride, at local areas before anode. Any suitable voltage and current density the silver chloride layer as a whole has been may be used, for example, 2 volts and 12 amperes built up to the desired thickness. When the per Square foot of anode surface. The voltage sheet is subsequently used in a primary cell, such 40 usually must be increased as operation progresses local areas represent a complete Waste of silver in order to maintain the current at a constant chloride because that portion of the electrode Value. As stated heretofore, if the silver chlo is inactive with respect to the function and oper ride coated sheet is to be used subsequently in ation of the cell. It is not unusual to employ a a primary cell, the silver body may be in the foil which initially has a thickness of only .003 form of a thin sheet or foil. It may be desired inch and convert two-thirds of the Silver to Silver that the formation of silver chloride take place chloride, so that the importance of preventing on both sides of the sheet or upon one side only. penetration is apparent. If the latter is the case, the cell may be arranged It is the object of the invention to provide an So that the action is confined to one side of the improved method of forming silver chloride by sheet, as by applying a suitable resistant com anodic oxidation of a body of silver, in which position to the surface at which anodic oxida the Oxidation reaction proceeds at a uniform rate tion of the Silver is to be prevented, such as Over the area of the body of silver, and when the varnish, wax, the copolymer of vinyl chloride latter is in the form of a thin sheet, there is and vinyl acetate in a suitable solvent, or the no objectionable local penetration, 55 like. During the electrolytic action, the elec 2,521,082 3 4. trolyte bath is maintained at a temperature be of 25° C. and an electrolyte in which the liquid tween approximately 40° C. and 60° C. constituent is composed of 20% methanol and It has been found that when the method is 80% water, a uniformity is obtained which is sub carried out in accordance with the above ex stantially equal to that obtained with a tem ample, a layer of silver chloride is formed upon 5 perature of 50° C. and a liquid constituent of 5% the surface of the silver anode and that the ac nethanol and 95% water. tion progresses with surprising uniformity over The Water Soluble alcohols other than the area of the anode, such that the Silver methanol may be employed to improve the uni chloride continues to build up in the form of a formity of anodic oxidation. Ethanol is an ex layer of substantially uniform thickness duling 10 ample of another alcohol and an electrolyte the operation of the cell, and the sheet of Silve' composition similar to the example given here becomes progressively and uniformly thinner tofore, in which 5 pounds of ethanol is substi throughout its area and is not penetrated or tuted for the 5 pounds of methanol, has been completely converted to silver chloride at any found to produce a satisfactory reduction in the local areas thereof. The electrolytic action may 5 local penetration of the silver sheet. be discontinued when the desired proportion of Salts other than sodium chloride may be used. the silver has been converted to silver chloride. Any other soluble chloride, such as potassium In addition to the advantage of unifornity of chloride may be used, and hydrochloric acid may anode oxidation, the use of methanol. in the be used in the absence of a chloride salt. For best electrolyte results in an increase in the anode 20 results, the pH of the electrolyte should be main efficiency of the electrolytic cell. For example, tained between approximately 1 and approxi when the method is carried out in accordance mately 3. The soluble chloride and hydrochloric with the example given heretofore, that is, emir acid provide the desired conductivity and pH ploying a temperature of from 40° C. to 60° C. and also the Supply of chloride ions for the anodic and an electrolyte in which the liquid constit 2 5 Oxidation function. uent is composed of 5% methanol and 95% What is claimed is: water, the same current input produces, a de l. In anodically coating silyar chloride upon posit of silver chloride which is greater by ap a Silver body in an acidic aqueous electrolyte con proximately 10% than is produced under similar taining a water-soluble chloride, the improvement circumstances but without the use of methanol : which consists in including in the electrolyte a in the electrolyte, that is, with a liquid consti Substantial amount of an alcohol from the group tuent which is 100% water. In terms of anode consisting of methanol and ethanol. efficiency, with an electrolyte in which the sol 2. The method as claimed in claim in which vent is 100% water, the anode efficiency is ap the proportion of alcohol is approximately 5% proximately 85% to 95%, while when 5% or more 35 to 95% by weight of the liquid constituents of the of methanol is used, the anode efficiency is in electrolyte. creased to substantially 100%. 3. The method as claimed in claim 1 in which While the above is given as an example of a the pH of the bath is maintained at approxi satisfactory method in accordance with the in mately 1 to 3. vention, many variations may be made therein 40 4. The method as: claimed in claim 1, in which While preserving the desired results. For ex the temperature of the bath is maintained be ample, the proportion of methanol may be varied, tween approximately 40° C. and approximately In general, the uniformity of anodic oxidation 60° C. is improved as the concentration of methanol 5. The method as claimed in claim 1 in which is increased, and the proportion of methanol the body of silver is in the form of a sheet. may be increased until the solution, contains 6. In anodically coating silver chloride upon Only Sufficient water and conductive salt to in a Silver body in an acidic aqueous electrolyte part the desired conductivity. In such case, the containing a water-soluble chloride, the in liquid constituent of the solution may contain provement which consists in including in the as much as 95% by weight of methanol and as 50 electrolyte a substantial amount of methanol. little as 5% of water. To incorporate, the de 7. The method as claimed in claim 6 in which Sired amount of sodium chloride, such a solu the amount of methanol is maintained at not tion becomes a substantially saturated solution more than approximately 10% of the weight of of Sodium chloride. The use of such a solution provides the desired. uniformity of anodic oxi the liquid constituents of the electrolyte. dation, but the conductivity of the cell is less 55 JOHN B. MULLEN. and the operation is more expensive than the REFERENCES, CEO operation of the cell-employing a lower propor The following references are of record in the tion of methanol. Usually, a concentration of file of this patent: 5% of methanol, as is given in the foregoing ex ample, produces satisfactory results, and the use 30 UNITED STATES PATENTS of more than 10% of methanol is undesirable Number Nanne Date for economical reasons, 902,755 Meyer ------Nov. 3, 1908 The temperature. of the electrolyte may also 1,226,165 Axtell ------May 15, 1917 be varied. In general, the uniformity of anodic 2,428,850 Lawson ------Oct. 14, 1947 oxidation is improved as the temperature is in creased, but it is preferred not to. employ a tem FOREIGN PATENTS perature above 60° C., because objectionable loss Numbe: Country C3te of methanol by vaporization occurs above that 123,936 Great Britain ----- Mar. 13, 1919 temperature. 70 OTHER REFERENCE There is, therefore, a correlation between the Jahn: Z. Phy. Chem., vol. 33 (1900), p. 556. factors of temperature and methanol concentra Moore et al.: Chemical Abstracts, vol. 41. tion. With higher concentrations of methanol, (1947), p. 5031, a lower temperature may be used with equally Jour. Amer. Chem, -' -- Soc., voi. i. (i9 i3), pp. good results. For example, with a temperature 3 1898-1902.