MODIFICATION in the CONFIRMATORY TEST for ZINC ION MARGARET W. KELLY AND ELISABETH L. JOHNSON /
Connecticut College, New London. Connecticut
A new qualitative test for zinc ion, based on the fusion of zinc nitrate with solid ammonium-nitrate, is d~scribed. For 300 mg. Zn++, a characteristic rose-colored residue i s obtained which on high heating yields the ordinary yellow zinc oxide. For amounts of zinc ion ranging from 5 to 25 mg., the same technic yields the ordinary zinc oxide with its characteristic colors.
A
CCORDING to the work of Artur Kutzelnigg,' a brick-red oxide of zinc can be .obtained by dissolving the ordinary zinc oxide in fused ammonium nitrate and heating the melt over a Bufisen flame. A deflagration occurs and a brickish red residue is thrown out on the sides of the flask. This reddish compound is not affected by boiling water, but when heated its color fades, leaving a residue of the ordinary zinc oxide, bright yellow when hot and cream-colored when cool. The purpose of the work to be described was to determine whether or not the formation of this reddish oxide and subsequently of the ordinary cream-colored oxide could be used as a confirmatory test for zinc ion under the conditions obtaining in the NoyesZand Hammett3 schemes of qualitative analysis. The work 1 KUTZELNIGG, ARTUR." ~ b e reine ziegelrote Form des Zinkonyder," 2. anorg. allgem. Chcm., 208, 2318 (1932). NOYES.A. A., "Qualitative chemical analysis of inorganic substances," 9th ed., The Mamillan Company, New York City, 1932, pp. 98-110. HAWEIT, L. P., "Solutions of electrolytes," McGraw-Hill Book Company, Inc.. New York City. 1929, pp. 179-80. 186.
showed that the test is wholly satisfactory for 300 mg. of zinc ion, but that certain modifications of the Kutzelnigg technic are necessary for 30 mg. or less of zinc ion. This modified test is considerably easier to manipulate and is more definite than the Rinwnn's green test employed by Noyes. For the ordinary purposes of a qualitative analysis a coniirmatory test may be considered satisfactory which will identify as little as 5 mg. of a constituent in the presence of as much as 300 mg. of other ions of its group. In the case of zinc it is particularly important to have a reliable confirmatory test, sirice zinc ion is subject to occlusion by the hydroxides of the iron group and may therefore appear in either the aluminum or the iron group. Also, a small amount of zinc sulfide may be indistinguishable from the precipitate of sulfur often obtained when hydrogen sulfide is passed into an alkaline solution. When 5 mg. of zincion must be identified in the presence of large quantities of the other ions of the aluminum and iron groups, the Rinmann's green test frequently proves unsatisfactory as a confirmatory test. It is difficult to adjust the amount of cobalt nitrate to the amount of zinc ion, and, with only slight overheating, enough cobalt oxide may be produced to obscure the green color completely. Thus time-consuming repetitions of the operation are necessitated and the final result is apt to be very doubtful. The first step in attempting to determine the usefulness of the oxide described by Kutzelnigg was to repeat his experiment. Accordingly a mixture of 9 g. of zinc oxide and 25 g. of d i d ammonium nitrate was heated
in a 50-cc. pyrex Erlenmeyer flask over a Bunsen flame. After fusion, the reaction proceeded quietly a t first, then suddenly increased in speed and a deflagration occurred with sudden evolution of gases. A brickish rose residue was obtained, encrusted on the walls of the flask. On further strong heating, oxides of nitrogen were evolved and the residue lost its rose color, becoming bright chrome yellow when hot and fading to cream color when cooled. As is characteristic of the ordinary zinc oxide, the bright yellow color could be renewed with reheating. The reaction corresponded to the description of Kutzelnigg's results except that the residue was rose, rather than red in color. The procedure was then repeated, using varying amounts of a solution of zinc nitrate of known concentration (10 mg. zinc ion per cc.) in place of zinc oxide. These samples of zinc nitrate were evaporated to about 2 cc., solid ammonium nitrate was then added, and the mixture was heated. The results showed that a definite deflagration and a rose-colored residue were obtained only for amounts of zinc ion in excess of 50 mg. However, in all cases, the familiar cream-colored oxide, bright yellow when hot, was obtained as a linal residue. Since in the usual analytical procedure zinc is separated as zinc sulfide, solutions of zinc nitrate containing different known amounts of zinc ion were next treated, in the presence of excess ammonium hydroxide, with hydrogen sulfide, the precipitate was filtered off,washed, and redissolved by pour in^ 5-10 cc. 6 N nitric acid through the f i l t e ~ ~ ~ The ~ ~ eresulting r. solution was collected in a 50-cc. pyrex Erlenmeyer flask and concentrated to about 2 cc. Two grams of solid ammonium nitrate was then added and the flask was heated until a deflagration occurred. The amount of deflagration and the color of residue were observed, and the heating of the flask continued a t a high temperature until all oxides of nitrogen had been expelled. Finally the amount and color of the final residue were noted. The results of such a series of experiments are summarized in Table 1. TABLE 1 Nature of solu- Amount tioo used of pretipitate Other Mg. eonstituwith Zn+' entr H I 3W none heava 100 none heavy none moderate 50 25 none moderate 5 none slight slight 1 none
" " "*. "."
none
sulfur
none
Amount of deflanation lare large small small small slight slight
Color of residue after deflagatlon
E
Residue after high heating Color Amount Hot Cold rose laree . .ello ow cream pale rose large yellow cream pale rose moderate yellow m n m small yellow u e a m yellowinh ycllowi~h small yellow cream yellowirh slight yellow u e a m
....
none
.... ..
each group, zinc was tested for by precipitating in alkaline solution with hydrogen sulfide. The resulting precipitates were dissolved in nitric acid, the solution was neutralized with ammonium hydroxide, evaporated to small volume, and given the ammonium nitrate treatment described earlier. Results are summarized in Table 2. The most rigorous condition which the confirmatory test would usually have to meet would be the detection of 5 mg. of zinc ion in a solution containing originally 300 mg. each of the other ions of these groups. Therefore, parallel determinations were carried through the analytical procedure; one with 5 mg. of zinc ion in the presence of 300 mg. each of aluminum, chromium, manganese, ferric iron, cobalt, and nickel ions, the other containing no zinc ion, but 300 mg. of each of the other ions. The results are summarized in Table 2. TABLE 2' Mg. of each of other ion8 of Amount precipitate with Results after NHtNOa treatment aluminum H B i o test for Zoi+ in Amouof Mr. and iron Alumiovm Iron final Color of residue group residue Hot Cold Zn soups soup 30 30 small small small bright cream (pale yellow) (pale yellow) yellow none 30 small small tree white white (pale yellow) (pale yellow1 5 300 rmall small smdl bright pale green(pale yellow) (pale yellow) yellow ish yellow none 300 small small trace whitish whitish (pale yellow) (pale yellow) + +
In view of these results, the following procedure* is recommended as a confirmatory test for zinc ion. After separation from the other ions of these groups by the usual methods, and precipitation of zinc as zinc sulfide, filter and wash .any' precipitates obtained. Dissolve the zinc sulfide so obtained by pouring 5-10 cc. of 6 N nitric acid through the filters, collecting the combined extracts in a 50-cc. pyrex Erlenmeyer flask. Add 1g. of solid ammonium nitrate and heat on a wire gauze under the hood until a deflagration occurs. Then continue with strong heating until alPthe oxides of nitrogen are completely expelled. If zinc is present, a residue remains which is bright yellow when hot and cream colored when cold. Attempts were made to substitute a small porcelain casserole for the Erlenmeyer flask, but the residue was so scattered that its color was uncertain. Furthermore, there is considerable danger of loss of residue out of the open dish. Substitution of a hard-glass test-tube was also unsatisfactory, as the gases evolved during the later stages of the boiling operation cause much frothing and the solution is kept in the test-tube with great difficulty. It might be suggested that ammonium nitrate, although obviously necessary for the formation of the reddish oxide, would not be needed in the formation of the yellow oxide, since the zinc is already in the form of zinc nitrate a t this point in the procedure. But when
Next, two determinations were carried through, one on a sample containing 30 mg. of zinc ion in the presence of 30 mg. each of aluminum, chromium, manganese, ferric iron, cobalt, and nickel, the other on a sample containing 30 mg. of each of these ions, but excludmg zinc. The samples were carried through the analytical procedure according to NoyesZexcept that the Ham- mett procedure%as used for removal of aluminum and *Successfully used by students in our course in qualitative chromium. After separation of interfering ions, of analysis.
tests were made without ammonium nitrate, the oxide was so spread out on the walls of the flask that small quantities were very hard to see. Ammonium nitrate is useful because, by careful manipulation of the melt, practically all of the zinc oxide can be brought together in one place, so that when excess ammonium nitrate is driven off, the oxide of zinc is left in a compact mass. Its color, therefore, is readily distinguished. When the presence or absence of a small amount of zinc is in question, it is important to be sure that all
oxides of nitrogen are driven off by heating at red heat for a few minutes, since occlusion of small amounts of nitrogen oxides may give a temporary yellowish discoloration. The precipitate of sulfur resulting from oxidation of hydrogen sulfide, and closely resembling zinc sulfide, is removed in greater part during the filtering operation, but traces carried through during the washing with nitric acid are eliminated later by oxidation with ammonium nitrate.
