The Silver Chloride Electrode 0. REDLICH and L. F. MARANVILLE Department of Chemistry, The State College of Washington, Pullman, Washington
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CONSIDERABLE number of the methods and apparatus developed in Ostwald's laboratory half a century ago are still in use. Some devices are so efficient that hardly any better can be found. The calomel electrode is not. Indeed, the silver-silver chloride electrode, early developed by Jahn' and successfully used by numerous earlier2 and recent authors, is superior not only in precision work but also in the students' laboratory, in potentiometric titrations, and routine work of any kind. It is simpler to prepare, cheaper, and can be used in any position and arrangement. Complete and fairly accurate cells consisting of a silver chloride and a hydrogen electrode have been set up with not more than one-half to one gram of liquid for an early determination of the ionization constant of heavy water.8 This could hardly be done with a calomel electrode. Above all, the use of the silver chloride electrode saves mercury, and eliminates one of the opportunities to spill mercury, an important advantage especially in those laboratories where the danger of mercury vapor4is still underestimated. Silver chloride electrodes are very quickly prepared. Clean the surface of a piece of silver wire by means of soap and water, and turn a spiral using a thin glass rod as a mandrel. Sufficient length should be left to be used as a lead. Cover the spiral with silver chloride by electrolyzing i t for 5 to 15 minutes in hydrochloric acid (1 part concentrated acid 3 parts water) a t about 4
volts, using a piece of platinum as a cathode. The coat should be dark gray or brownish rather than white. Wash and keep in water. We prepared three electrodes according to this recipe and three more with porous ~ i l v e r . ~ All six electrodes checked within one millivolt. If higher accuracy is wanted, a more elaborate method should be used.8 Both the calomel and the silver chloride electrodes are known to be slightly sensitive to oxygen dissolved in the electrolyte. The silver chloride electrode is particularly convenient in cells without liquid junction. Direct contact between the electrode and hydrogen or quinhydrone, of course, must be avoided as an irreversible reaction would take place. The electrode can be used in alkaline solutions provided the ratio of the activities of chloride ion and hydroxyl ion exceeds a certain minimum value. A concentration ratio of 1 : 1 is amply sufficient. The potential of the cell7 Ag, AgCl(s), chloride, HgzC12(s),Hg is 0.0455 volts (silver being the positive electrode). For thermodynamical reasons i t is independent of the nature and concentration of the chloride solution. Silver wire can be fused to copper wire without difficulty, just as copper-copper and copper-platinum connections are made. Hold the end of the copper wire in a small, moderately hot flame of a Bunsen burner until i t melts, forming a tiny globule. Put the end of the silver wire in the liquid globule and remove the connection quickly from the flame without pulling ' JAHN,Z. physik. Chem., 33,545 (1900). a t the connection. ' HALLA, Zeit. Ekktroch., 17, 179 (1911). MCINNES AND PAR- KER, 3.Am. Chem. Soc., 37, 1445 (1915). LEWIS,BRIGHTON, AND LEWIS,1.Am. C h m . Soc., 28, 139 (1906); Z. physik. Ckem.,
+
SEBASTIAN. ibid.. 39.2245 (1917).
55, 449 (1906). Cf. RANDALL ~ I N DYOUNG. J. Am. Chem. Soc., 50, 989 (1928). 11 0'7;) PATTERSON AND FELSING, ibid., 64, 1478 (1942). DOLE,"The ,A""",. ' STOCK AND CUCWL.Ber., 67,122 (1934). SHEPHERD, ET AL., glass electrode," John Wiley and Sons. Inc., New York. 1941, 3.Research Nat. Bur. Standards, 26, 357 (1941), and many other p. 98. papers. 7 GERKE, 3.Am. Chcrn. Soc., 44, 1684 (1922). Cf. ref. 5.
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R ~ D L IZC. ~physik. , Chem., A173, 353
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