Rapid Potentiometric Determination of Zinc D. G. STURGES, Fisher Scientific Co., Pittsburgh, Penna.
T
HE increasing availability of apparatus for precise and convenient potentiometric titration suggests the possibility of its use in the determination of zinc. This is a common industrial analytical procedure, yet one of the most difficult. An investigation of the potentiometric titration of zinc was undertaken in the hope of finding a method at least as accurate as present ones, but more convenient and rapid. Kolthoff and Furman (6)and Kolthoff and Verzijl (3) have discussed the potentiometric titration of zinc and suggest the use of potassium ferrocyanide and potassium permanganate according to the following equations: 3Zn++ + 2Feoc---- + 2K+ KnZns(Feoc)2 Feoc----(excess) MnOJ-
+ 8H+ + 5Feoc----
Excellent reproducibility was obtained in standardization of the ferrocyanide solution, and the results obtained with three samples of pure zinc are shown in Table 11. The zinc used was c. P., granular, 40-mesh, WITH PUREZINC TABLE 11. RESULTS
Sample Gram
0.1094 0.1065 0.1038
+ C= Mn++ + 4Hz0 + 5Feic---
ZD Equivalent t o 1 M1. of K4Fe(CN)s
KdFe(CN)s M1. 38.92 37.90 38.61
Grana
Erratic results were obtained with a series of commercial zinc samples and a search for the causes was undertaken. Here as in the permanganate back-titration, hydrochloric acid seemed to decrease the inflection potential, but did not affect the precision.
Tanaev (4) also investigated this method with apparent success. The initial work was a check on his procedure.
Equipment and Procedure The equipment used was a Fisher titrimeter, a standard vacuum tube-voltmeter setup for potentiometric titration. The platinum-tungsten electrode pair was used throughout, except where otherwise noted. TABLE I. PERMANGANATE-FERROCYANIDE METHOD (About 5- t o 10-ml. excess of ferrocyanide was added and the solutions ere titrated immediately) Zn Equivalent t o 1 M1. of KdFe(CN)e Zinc Solution KaFe(C N)I M1. M1. Uram 13.67 0.002634 12 13.65 0.002637 12 13.67 0.002634 12 27.35 0.002634 24 27.38 0.002631 24 Zinc solution, 3 grams per liter. 0.025 M . K ~ F ~ ( C N )approximate1 B, KMn04, 1 ml. c 1.436 m? of KaFe(CN)a.
10
0
20
30
MI. OF CalS04)z .2(NHdzS04
FIGURE1. TITRATION OF APPROXIMATELY 0.025 M POTABSIUM FERROCYANIDE WITH APPROXIMATELY 0.02 N CERIC AMMONIUM SULFATE
A thorough check of the permanganate-ferrocyanide method, wherein an excess of potassium ferrocyanide is added to the hot zinc solution and the excess is then titrated potentiometrically with potassium permanganate, showed certain undesirable features. In low concentrations excellent results were obtained, as is shown in Table I. As the zinc concentration was increased, however, it became more difficult to obtain consistent results. I n many instances no distinct end point was observable. An investigation was then conducted to determine the source of the difficulties. It was found, as is pointed out by Kolthoff ( I ) , that the presence of hydrochloric acid tended to decrease the inflection potential, sufficiently in many instances to obscure the, end point. To obviate this, samples dissolved in hydrochloric and perchloric acids were neutralized with ammonium hydroxide and acidified to approximately 1.5 N with sulfuric acid. In those determinations run above 70" C. a perceptible odor of hydrocyanic acid indicated the necessity of maintaining the temperature below this point. Acid concentration as high as 2 N produced no appreciable differences. The effect of higher acidity was not studied. In an effort to avoid any possible effects due to breakdown of the permanganate, ceric sulfate was substituted as the oxidizing agent. A study of the ceric sulfate-ferrocyanide titration showed a large inflection potential (Figure 1) which was somewhat sharper than that obtained with permanganate.
If the ferrocyanide was added to a hot solution of zinc and the excess titrated immediately, there was no effect on the precision as long as operation was normal. However, a t times the electrodes behaved erratically and the observable formation of hydrocyanic acid made it obvious that such a procedure is not to be encouraged. TABLE 111. EFFECT OF PHYSICAL HANDLING OF SOLUTIONS Sample
KdFe(CN)a
Zn Equivalent t o 1 M1. of KaFe(CN)a
Gram
M1.
Gram
No. 1.
0.1013 0,1056 0.1048 0.1031
No. 0.1052 0.1013 0,1029 0.1031 0.1035
0.1051 0.1022 0.1036
0.1002 0.1056
267
Cold, back-titrated immediately 43.70 0.002316 0,002370 44.55 44.50 0.002355 44.33 0.002326 Av. 0.002342 2. Cold, stood 15 minutes after addition of excess Feoc----; then back-titrated 44.71 0.002353 42.94 0.002359 43.74 0.002363 43.88 0.002350 43.97 0.002354 Av. 0.002363 No. 3. Hot, back-titrated immediately 0.002372 0.002331 0.002340 Av. 0.002348 No. 4. Same as No. 2 but run hot 43.28 0.0023 15 44.45 0.002376 Av. 0.002346
INDUSTRIAL AND ENGINEERING CHEMISTRY
268
OF COMMERCIAL ZINC SAMPLES TABLEIV. ANALYSIS
Sample
KdFe(CN)s
Zinc
Gram
M1.
%
0.1024 0.1046 0.1028 0.1030 0.1041
40.10 40.90 40.13 40.27 40.67
97.15
Zinc dust, NO. 1
0.1011 0.1026
39.25 39.82
Zino dust, No. 2
0.1040
40.55
Zinc mossy
Zina die casting
0.1033
0.1044 0.1010 0.1040
40.30 40.15 38.95 40.10
97.01 96.95
97.00 96.93 Av. 97.01 96.32 96.32 Av. 96.32 96.74
96.79
Av. 96.77 95.43 95.67 95.66 Av. 95.59
Ammonium chloride had no effect. Sulfuric acid up to 2 N caused no appreciable differences. It seemed possible that the dense potassium zinc ferrocyanide precipitate was affecting the tungsten electrode. However, no satisfactory results could be obtained with a polarized platinum-platinum electrode pair. Finally it was decided to vary the physical handling of the solutions, in an effort to determine whether these factors were responsible for the difficulties. Four procedures were investigated. I n each instance the samples were dissolved in hydrochloric acid and diluted to 150 ml. with distilled water. The excess hydrochloric acid was neutralized with ammonium hydroxide and the solution made acid with 1 to 1 sulfuric acid. At this point two of the procedures involved adding an excess of potassium ferrocyanide to the cold solution, in the first, titrating the excess ferrocyanide immediately with approximately 0.02 N ceric ammonium sulfate; in the second, titrating the excess after the solution had been allowed to stand 15 minutes. A similar procedure was followed with two series of solutions which were heated to 70” C. The results obtained are shown inTable 111. A further check confirmed the results shown in the second part of Table IIIthat is, that by permitting the cold solutions to stand for 15 minutes after the excess ferrocyanide has been added, a high order of precision is obtainable. This would seem to be in line with the known low velocity of attainment of electrochemical equilibrium of ferrocyanide-zinc solutions (2). The method as outlined was applied to the analysis of commercial zinc samples. The solutions were allowed to stand 15 minutes before titration, as indicated in T a b l e 111. T h e r e s u l t s a r e given in Table IV. Potassium permanganate was then substituted for ceric sulfate in the procedure. The results obtained were much less satisfactory.
VOL. 11, NO. 5
The actual titration can be done with much greater speed than can be attained using indicators. The 15-minute period required to allow the solutions to come to equilibrium, after the addition of excess ferrocyanide, is of no disadvantage where a large number of samples are to be titrated, since the first ones treated with ferrocyanide will be ready for titration when the excess reagent has been added to all the samples. Potassium permanganate is a satisfactory oxidizing agent for ferrocyanide where the zinc concentration is not in excess of 20 to 30 mg. per 100 ml. Above this concentration, the system behaves erratically. Hydrochloric acid decreases the inflection potential sufficiently, with both ceric sulfate and potassium permanganate, to make the end point difficult to obtain. The titration should be carried out a t room temperature and the solutions allowed to stand 15 minutes before titrating the excess ferrocyanide. Addition of the potassium ferrocyanide to a hot zinc solution should be avoided because of decomposition of the ferrocyanide a t even moderately high temperatures,
Literature Cited (1) Kolthoff, I. M., and Furman, N. H., “Potentiometric Titrations”, p. 264, New York, John Wiley & Sons, 1931. (2) Ibid., p. 324. (3) Kolthoff, I. M., and Verzijl, E. J. A. H., Rec. trav. chim., 43, 380 (1924). (4) Tanaev, I., J. Applied Chem. (U.S. S. R.), 5, 86 (1932).
Summary The volumetric determination of zinc, by adding an excess of potassium ferrocyanide and back-titrating the excess potentiometrically with ceric sulfate, using a platinumtungsten electrode pair, can be carried out with a precision equivalent in magnitude to methods employing external or internal indicators.
APPARATUS FOR ELECTROMETRIC DETERMINATION OF ZIXC
