Silver Chloride Electrode in Acid-Base Titrations
used in this laboratory for some years and may he recommended as a distinct improvement over the more usual calomel and salt bridge arrangement.
Literature Cited
JOHN E. VANCE Department of Chemistry, Yale University,
(1) Cavamgh, B.. 3. C h a . Soe., 1927,2207. (2) Hrtmer. W.J.. Trans. Electrocha. Soc., 72,45 (1937). (3) Hamer. W. J.. and Aerec, S. F.,3. Research Natl. BUT.Standards. 23, 647 (1939). (4) Harned. H. S., and Brumbaugh, N. J., J . Am. C h a . Soe.. 44, 2729
New Haven, Conn.
T
HE use chloride electrode in potentio. of. the. silver-silver '
(1,322).
metric titrations has been suggested by Cavanagh ( 1 , 8) and by Hiltner (6). Cavanagh used the electrode in an ingenious scheme which obviated the necessity for a potentiometer in certain titrations, while Hiltner used the electrode, as well as the other silver-silver halide electrodes, in the titration of soluble halides. The general use of such an electrode in acid-base titrations, where it may replace the calomel electrode, does not seem to have been given the prominence it deserves. The advantages over the calomel electrode are that there is no possibility of plating mercury out on the hydrogen electrode; the liquid junction is avoided; and the experimental setup is simplified since the silver-silver chloride electrode is immersed directly in the acid being titrated. The charxcteristics of the silver-silver chloride electrode which make it unusually satisfactory are that it is a truly reversible electrode in the thermodynamic sense; it attains equilibrium almost instantaneously; and the developed potential is extremely stable. The preparation of the silver chloride electrode has been described in detail numerous times by Harned and others. Briefly, it may be prepared hy heating silver oxide on a platinum spiral at 450" C. and then electrolyzing in a hydrochloric acid solution (4, Q), or by heating a mixture of 90 per cent silver oxide and 10 per cent silver chlorate on a platinum spiral a t ahout 500" C. (7). While either type of electrode is satisfactory, the latter type is preferable, since it may be used for titrations over a period of months. Following a titration the electrode is washed carefully and kept in distilled water. I n titrating acids other than hydrochloric, such as sulfuric, the weak organic acids, etc., it is necessary to provide chloride ions or silver ions, this being done most conveniently by adding a small quantity of sodium chloride. The addition of a salt to the solution of a weak acid also has the virtue of causing a slight increase in the dissociation of the acid (5). For titrations of hydrobromic or hydriodic acid the corresponding halide electrode may be prepared according to the methods outlined above. The chief limit to the use of the electrode is that ions whose silver salts are very much less soluble than silver chloride should not be present. During the titration the original concentration of chloride ions will be decreased, owing to the increase in volume, as the base is added. This change in chloride-ion concentration will alter the electrode potential of the silver chloride electrode somewhat, but in no case will the change of the hydrogen electrode potential at the end point be masked. The shape of the titration curve will thus he slightly different from that obtained with the calomel cell with liquid junction, but the position of the end point will not be shifted. Iftho quinhydrone electrode is used in place of the hydrogen electrode, as may he done, the pH of the solution should not exceed 8.5 during the titration, because of the effect of more alkaline solutions on quinhydrone. This limitation does not apply to the silver chloridehydrogen electrode combination. The experimental procedure suggested here should not be used directly for the determination of pH without modification. The same pair of electrodes has been applied successfully to the exact determination of the activity and concen-, tration of hydrogen ions by Hamer (9)and by Hamer and Acree (3). For titrations, however, both in instruction and in practical determinations, the electrode as described has been
( 5 ) Harned. A. S., and Owen, B. B.,Ibid., 52,5079 (1930). (6) Hiltner. W..2. anal. C h a . . 95,37 (1933).
(7) Keston, A. S., 3. Am. Chem. Soc.. 57. 1671 (1935).
(8) . . Kolthoff. I. M.. and Furman. N. H.. "Potent.iomet,rieTitr= tions": pp. 1 2 6 3 1 , New York, John'Wiley & Sons. 1931. (9) Noyes, A. A., and Ellis, J. H.. 3. Am. C h a . Sac., 39, 2632 (1917). ~
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A Time-Saving Adaptation for Photomicrography JOHN A. QUENSE AND WILLIAM M. DEHN University of Washington, Seattle, Wash.
1
NUSINGacbemical microscope t o select a representative field, crossed nicols are often necessary. If a photomicrographic camera carrying a view tube is connected with the microscope, the camera must be removed for manipulation of a cap nicol analyzer and replaced to photograph the selected field; but if the photomicrograph is to be taken without the analyzer, with either polarized or unpolarized illumination, theadaptationdepicted rendersunnecessary the removal of the camera. The analyzer support is clamped onto a slotted metal rine that fits L n r tlii. v i w t h r ucwlar. The analyzer YIII compensntinx dev i m can then be used without disturbing t1.e camera. Most view tubes are constructed with a reflecting prism in the light path above the ocular of the microscope. Because reflection reduces light vibrations parallel to the plane of incidence, the vibration direction of the analyzer should he adjusted perpendicular to the plane of incidence of the reflecting prism, so i t can function in the same way as when crossed nicols are used with a microscope alone. This is easily effected by ascertaining the vibration directions of the analyzer and the substage polarizer and turning them to the proper positions relative to the prism. The same result can be attained by using a microscope with a tube analyzer, but i t is frequently impossible to obtain such an instrument, although the ordinary chemical microscope equipped with a cap nicol is fairly common. 68
