POTASSIUM FERROCYANIDE AS A QUALITATIVE TEST FOR ZINC GEORGEW. B E N NAND ~ F. G. MCKEB, GROVECITY COLLEGE, GROVECITY,PENNSYLVANIA
In the literature there are occasional references to the use of potassium ferrocyanide as a qualitative test for zinc, but the use of this test has not become as general as its desirability warrants. Ganassinil mentions this test. Werner2 precipitated zinc with the ferrocyanide, and then treated the precipitate with bromine water, whereupon a deep yellow color forms which is characteristic of zinc under the conditions of the experiment. Feigl and Stema utilized this test for a spot reaction on black paper as a micro test for zinc. HutTered4 tests zinc by both the sulfide and ferrocyanide tests. More recently a brief article by Mehlig5 has shown that the potassium ferrocyanide test in the hands of student analysts leads to a greater percentage of correct reports for zinc than do the older tests. At the time our experiments were being made we were unaware of this latter paper, but it seems desirable to present our results in view of the excellence of the test and because we have examined its limitations and somewhat modified its application. Method of Applying the Test The filtrate from the sodium hydroxide-sodium peroxide separation is acidified and aluminum precipitated as usual. The filtrate, containing the zinc and chromium as chromate, a t this point has a volume of 60 to 80 cc. in the procedure used in this laboratory. The solution is neutralized, and then 2 to 4 cc. of 6 N acetic acid are added, and the resulting solution is allowed to stand for 10 to 15 minutes to permit the precipitation of silica if present. Any silica is filtered off and the solution is divided into two equal parts. To one part, 1 cc. of normal potassium ferrocyanide is added. The usual amounts of zinc give a flocculent precipitate immediately, but smaller amounts give a turbidity which may take 5 to 10 minutes to form. The portion tested is compared with the untested portion to detect a turbidity, by holding the beakers between the eye and a strong light source. A turbidity in both beakers indicates incomplete removal of silica before the test was applied. The reference portion of the original solution is then tested for chromate ion in the usual manner. Ganassini, Soc. med. chi,. Pavia sedut., Jan. 29, 1909; C. A , , 4,2246 a
F. F. Werner, Z. anal. Chent., 51,481 (1912). Feigl and Stern, Z. anal. C h . ,60, 1 (1921). Huffered, Proc. Indiana Aced. Sci., 36, 183 (1926). J. P. Mehlig, THISJOURNAL, 4,722 (June, 1927).
(1910).
Sensitivity of the Test Using solutions containing zinc chloride prepared from zinc carbonate it is possible to detect 1mg. of zinc very readily while the limit of the sensitivity seems to be about 0.2 mg. of zinc in 80 cc. of solution, or one part in 400,000. TABLE I MB. 01 zinc
5.0 3.0 1.0 0.8 0.6 0.4 0.2
Nature of ppt.
Time of appearance
Turbid Turbid Turbid Turbid Turbid Slight turbidity Faint turbidity
1min. 2 mi".
3 min. 4 min. 5 min. 20 min. 20 min.
The presence of 120 milliequivalents of acetic acid does not interfere with, although it modifies, the results of the test. In the presence of much acid, small amounts of zinc produce a turbidity with the ferrocyanide very much more quickly than without the acid, but the intensity of the turbidity so produced is very much less than that which appears when the acid is absent. This effect6is, no doubt, due to the two salts Zn2Pe(CN)6 and KzZna[Fe(CN)sI2. The presence of NH40H, however, rapidly decreases the sensitivity of the test due to the formation of the complex zinc ion. Amounts of aluminum up to 500 mg. give no precipitate with the ferrocyanide in two hours, but on long standing a jelly may form which is colored blue due to decomposition of the ferrocyanide ion. A solution of ferrocyanide which is increasingly add, of course, will, by atmospheric oxidation, give enough ferric ion to result in the appearance of a blue color by reaction with the ferrocyanide. Less than 10 mg. of chromic ion gives no precipitate with the reagent in half an hour. The presence of these amounts of aluminum or chromic ion, moreover, does not prevent the immediate detection of one milligram of zinc. In any case such amounts of aluminum and chromic ions cannot be present a t the stage of the analysis a t which the test for zinc is made. One milligram of zinc is readily detected in the presence of 750 mg. of chromium as chromate or dichromate ion, and the dark orange color offers no hindrance to the detection of a turbidity. With larger amounts of zinc in the presence of this much chromium as chromate, but in the absence of acid zinc chromate may form, thus obscuring the test. The amount of acid specified, when added, however, does not permit zinc chromate to precipitate. It is possible, therefore, to detect 500 mg. of 6
Mahin, "Quantitative Analysis," McGraw-Hill Book Co., 1924, p. 251.
zinc in the presence of 500 mg. of chromium present as chromate or dichromate ion. While the students in this laboratory have not shown as high a percentage of correctness as Mehlig' reports, our results on the old and new tests are of the same order as he mentions. The number of students reporting zinc present when it was not present in an unknown has decreased materially with the new test. Students have had no difficulty in detecting one milligram of zinc in the presence of 50 milligrams of three or four other metals. Summary 1. The use of potassium ferrocyanide as a qualitative test for zinc has been described. The test is shown to be sensitive to the extent of 21/2 milligrams in one liter. 2. Aluminum and chromate ions do not interfere, and small and large amounts of zinc are readily detected in the presence of large amounts of chromium as chromate or dichromate. 3. The test is applied at the proper point in the usual qualitative scheme of analysis and, as previously shown by Mehlig, is readily adapted to student use.
' Mehlig, loc. cit.
