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Notes on Qualitative Analysis Procedures
Eolmn'n NOTE: The following papers have been collected over s. period of months. I t is the Editor's hope that collection into one place will prove more attractive to resdeders than the more frequently used editorial procedure of scattering items such as these through the pages of several issues. We apologize to authors who thus have bad to wait to see their work in print. We commend to readers' attention the fact that severalof these represent thesolution of probl e m given to freshman students in qualitative analysis courser. We hope that publication in these pages can stimulate just such activity. If readers merely incorporate these ideas into their own laborstoryinstructions, our battle is only half won. If, beyond that, theseidess suggest to the readers that they use their own ingenuity in similar fashion, we are gratified.
TRE COLORIMETRIC DETECTION OF ARSENIC AS COPPER ARSENIDE JOHN T. STOCK and LUCINDA M. GARCIA University of Connedicut, Storm
SEVX~AL methods for the detection of arsenic either suffer from interference, especially by antimony, or are not convenient in introductory courses in qualitative analysis. It has been known for over a century that arsine precipitates copper arsenide from a copper salt solution (I, S), while in 1871 Hager noted that the gas darkened a test paper soaked in such a solution (8). Until the recent work of Vial de Sachy (6, 7), however, little analytical use has been made of these facts. He devised a simple test for arsenic in the presence of antimony. We have now adapted this method to the 0.5ml. scale. Filter-paper strip A is 2 to 3 mm. wide and is supported in attachment B, as shown in the figure. The attachment is made by collapsing one end of a short length of 6-mm. outside diameter glass tubing until the bore is abont 1 mm.; its principal use in qualitative analysis is for treatment of solutions with hydrogen sulfide by the "pressure method," which is simple, rapid, and economical on the 0.5-ml. scale (4, 5). Rubber tubing sleeve C fits snugly on the attachment and into the mouth of the 75- X 10-mm. test tube D. Procedure is as follows: Apply one drop of 10% comer sulfate solution to the Daoer strip andinsert the latter wet end first into the attachment. Place 2 t o 4 drops of the sample solution in the test tube a i d follow with about 10 drops of 4 N sulfuric acid. Add a "knifepoint" of zinc granules (abont 20 mesh), quickly insert a small cotton plug and at once push home the attachment. A dark stain appearing on the paper within 2 minutes indicates the presence of arsenic; quantities greater than about 1 mg. cause
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VOLUME 35, NO. 8, AUGUST, 1958
staining to occur within 40 seconds. Run an occasional blank test to prove absence of reagent contamination. Although the test works equally well in the presence of antimony and tin, these common Groups IIB elements cause no staining when arsenic is absent. The entire Group IIB precipitate is therefore dissolved in a few drops of aqua regia and the solution is partially evaporated to expel nitric acid. After dilution with water t o 0.5 t o 1 ml., portions are used not only for the detection and confirmation of arsenic by the test described and by formation of arsenomolybdate, hut also for the detection and confirmation of antimony and tin (5). The concentration of the copper sulfate solution is not critical. Although slower in appearing, satisfactory stains were obtained on test papers impregnated with a solution containing only about 1 g. of copper sulfate peotahydrate per liter. The dark stain due to copper arsenide is formed as follows: .480,-'
+
+
1
+ +
11H+ 4Zn = AsHa 4Hz0 X u + + 2AsH8 = CusAsl 6Ht
+
+ 4Znt+
If antimony is present, stibine is also formed. This gas diiers from arsine in not staining the test paper. LITERATURE CITED (1) DUMAS, J. B. A., Ann. Chirn. Phys., 33,355 (1826). (2) HAGER,H., Jahrb. prakt. P h . , 35, 92 (1871). (3) S ~ ~ B E I R E., A NAnn. , Chim. Phys., 43, 417 (1830). J. T.,AND P. HEATH,Metallurgia, 42,44 (1950). (4) STOCK, (5) STOCK,J. T., AND P. HEATH,"Small-scale Inorganic Qualitative Analysis," Chemical Publishing Co., New York, 1954.
(6) VIALDE SACHY, H., Chim. anal., 38, 435 (1956). (7) VIALDE SACHY, H., Ann. Falaif., 50,53 (1957).
