INDUSTRIAL A N D ENGINEERING CHEMISTRY
370
Vol. 19. KO. 3
A Simple Reference Electrode for Potentiometric Titrations' By Theodore R. Ball
~vASHINGTONUNIVERSITY,
T
H E calomel electrode is the most conimonly used reference electrode in potentiometric titrations. It has several disadvantages in routine analysis, among which are the necessity of using a salt bridge in cases where chlorides interfere with the reaction in the titration vessel and of frequent flushing to prevent diffusion and remoye air bubbles. The electrode here described is simple, practically unbreakable, requires no flushing, and may be used for 2 or 3 weeks without refilling. Although it is not a substitute for t,he calomel electrode in all cases, it is satisfactory for oxidation-reduction and precipitation react'ions and for acidimetric titrations in which accurate values of pH are not required. Construction and Properties of the Electrode A piece of 5-mm. glass tubing about 150 nini. long is heated in a gas flame until the end is almost closed. A fine thread of acid-washed, long-fiber asbestos is inserted into the opening, projecting a few millimeters on either side of the constriction, and the tube strongly heated until the fiber is securely sealed in. The tube is filled with a suitable electrolyte, and air pressure applied to the open end until a drop of the liquid is forced t'hrough the asbestos. Connection to the potentiometer is made by a platinum wire inserted into the electrolyte. Three normal sulfuric acid has been used as t'he elect'rolyte for oxidation-reduction reactions in acid solution and normal potassium chloride for acidimetric titrations. The resistances of several electrodes were measured when filled with normal potassium chloride. A No. 26, B. & S. gage, platinum wire was inserted into the tube, extending clear to the bottom, and served as one terminal. The other terminal was a bright platinum foil, 2 X 2.5 em., welded onto a heavy wire of the same material. The foil was bent a t right angles to the wire and was in contact with the sealed end of the tube. The whole was immersed in normal potassium chloride. The results are shown in Table I, together with the volume of liquid which leaked out of each electrode tube in 48 hours, when filled with water to an initial depth of 150 mm. and left suspended in an upright position. The average volume of each tube was 2.51 cc. when so filled. Experimental All experiments were run with a Leeds and Northrup student's potentiometer and a lamp and scale galvanometer having a rated deflection of 40 to 50 mm. per microampere. Satisfactory results were also obtained with the Hildebrand set-up, using a millivoltmeter and a portable d'Arsenva1 galvanometer, and with the Roberts electrometric titration unit. Unless otherwise stated, the indicator electrode was a No. 26, B. & S. gage,'platinum wire 1 cm. long and the reference electrode was N o . 3 (Table I) filled with 3 N sulfuric acid. All electrometric end points were determined by reading the points of inflection of the curves, which were plotted on paper having twenty divisions per inch, each division corresponding to 0.05 cc. Potentiometer readings were taken after each drop (about 0.03 cc.) of reagent near the end point and all solutions were over-titrated by a t least 1 CC. TITRATION OF DICHROMATE WITH FERROUS SULFATETen cubic. centimeters of 0.1 N potassium dichromate, diluted with 65 cc. of water and 10 cc. of 3 N sulfuric acid re1
Received October 2, 1926.
S T . LOUIS,
MO.
quired 9.90 cc. of ferrous sulfate to give an electrometric end point with the asbestos reference electrode. Two titrations, using a calomel electrode, required 9.90 and 9.92 cc., respectively. A sample of chrome-nickel steel from the Bureau of Standards, containing 0.89 per cent of chromium, was prepared for analysis by the rapid method of Kelley and Wright? and the chromium titrated with ferrous sulfate which had been standardized against pure potassium dichromate. Two titrations showed, respectively, 0.87 and 0.92 per cent chromium. Satisfactory end points were obtained in dichromate titrations, using electrode No. 5 (Table I) which had a resistance of 175,000 ohms. PRECIPITATION > ~ ~ ~ ~ o ~ s - - B i c h ohas ~ s kshown y~ that zinc may be determined electrometrically with pota,;' ~~lUlI1 ferrocyanide. With the asbestos electrode and a platinum wire indicator electrode the breaks were sharp and reproducible in hot hydrochloric acid solution. Two titrations against pure zinc chloride showed 1 cc. of the ferrocyanide to be equivalent t o 5.922 and 5.926 mg. of zinc, respectively. The precipitation of silver from the neutral nitrate gave satisfactory end points with both sodium sulfide and ammonium sulfocyanate, using a silver ribbon as a reference electrode. Ten cubic centimeters of a 0.10 N silver nitrate solution required 16.56 cc. of ammonium sulfocyanate by the Yolhard method and 16.52 cc. electrometrically. Table I-Resistance
1 2 3
4 5
and Rate of Leakage of Electrodes
Long fiber acid-washed Long fiber' acid-washed Long fiber' acid-washed Untreated'fiber from large crystal Untreated fiber from large crystal
Ohms 6,100 8,200 40,000 120,000 175,000
CC
1 66 1 40 0 2h 0 15 0 10
TITRATIOK OF ACETICACID WITH SODIUM HYDROXIDEThis titration was carried out with the usual hydrogen electrode and a normal calomel and asbestos electrode in the same solution. The asbestos electrode tube was filled with 1 N potassium chloride. A double-throw switch made it possible to take readings with either reference electrode after each addition of alkali. The asbestos-hydrogen electrode system gave readings about 70 millivolts higher than the calomel-hydrogen electrode system during the entire titration. This difference is only approximate, as potentiometer settings were not sensitive to within about 4 millivolts with the asbestos electrode. The end points were identical. Because of the relative inaccuracy of the readings, the asbestos electrode is not recommended as a substitute for the calomel, except for rough determinations of pH. It may, however, be used successfully for end-point determinations. Work is contemplated on the effect on the potential of a calomel electrode of an asbestos fiber in the capillary outlet, and the application of the fiber to non-siphoning salt bridges. Acknowledgment The writer's thanks are due to H. Lee Ward for making the resistance measurements. 2 Iron Age, 103, 1507 (1909). 8 THISJ O U R N A L , 9, 668 (1917).
