The Removal of Interfering Anions in Semimicro Qualitative Analysis L. J. CURTMAN and S . KRAKAUER The City College, College of the City of New York, New York, New York URTMAN and Wigler' pointed out that in the analys~sof an inorganic mixture, i t is imperative to test for and eliminate the anions prior to analyzing for cations. They then developed a procedure for the removal of interfering anions. In view of the growing popularity and proved reliability of semimicro techniques in qualitative chemical analysis, i t was considered desirable to reinvestigate the method of Curtman and Wigler with a view to ascertaining its value when scaled down and adapted to semimiao work. Following the method outlined by Curtman and Wigler, some preliminary experiments were made in which the quantities of all reagents were reduced to a one-twentieth scale. The following results were obtained: (1) The maximum quantity of tartrate was not destroyed; some remained as a charred residue. (2) The time designated for fuming was excessive for the quantity of acid used. (3) Sulfurous acid, introduced to reduce chromate and arsenate, was found to be inconvenient because the volume required was relatively large. Its use, moreover, was considered unnecessaryfirst, because adequate provision is now made for the detection and removal of pentavalent arsenic in systematic cation analysis and, second, because experiments had shown that, on prolonged fuming with H2SOd, the greater part of the chromate is reduced to insoluble anhydrous Crz(SO& according to the equation :
C
.
(4) Finally, there was need for more information as to the influence of the procedure on losses of volatile metals such as mercury. With a view to overcoming these and similar objections, Curtman and Fuhrmanz reinvestigated the procedure and proposed certain modifications. However, the new procedure did not result in the complete destruction of maximum quantities of tartrate nor was i t tried out on a suilicient number of compounds to prove its e5iciency. Its chief novel and useful proposal related to the treatment of the residue remaining after the action of HNOs and H2S04. It was proposed that, after transposing i t with Na2C03, the residue should be treated 6rst with dil. HNOs and the remainChcm. N m ,139,353 (1929).
'Dissertation of A. F u h a n , City College of New York, 1942.
ing residue with dil. HCl. The two solutions were then combined. Following this procedure it was found that when Pb++ was present, a precipitate of PbClz formed when the two solutions were combined. This fact is taken into account in our procedure which follows. PROCEDURE
In a 5-ml. beaker, treat 50 mg. of the substance with 3 ml. of 3 M HzSOa. Stir and place the beaker, supported on a wire triangle, in the hot air bath. The latter consists of a 50-ml. pyrex beaker resting on a wire gauze and heated by a Tirill burner. The gauze is supported by a tripod or ring stand. Heat the mixture to boiling and boil vigorously with constant stirring to prevent bumping until SO3 fumes are given off. Then fume strongly, with stirring, for five minutes. Allow the mixture to cool thoroughly and then add 10 drops of conc. HNOI, washing down, in so doing, any precipitate that may have adhered to the sides of the beaker. Boil again and when fumes come off, fume strongly for one minute. Cool the mixture. Add 10 drops of conc. HNOe boil once more, and fume strongly for one minute. Allow the mixture to cool. Add 2 ml. of water, stir, and transfer the mixture to a 4-ml. test tube. Wash the beaker with 0.5 ml. of water, adding the washings to the test tube. Heat in the water bath for five minutes. Cool. Centrifuge and separate the solution. Label the latter Solution 1, and set it aside to be analyzed for all cations except Pb, Ba, and Sr. Wash the residue twice with water, using five drops for each washing. Centrifuge and discard washings. Add 1 ml. of 1.5 M NazC03, stir, and beat in a water bath for five minutes. Cool. Centrifuge and discard solution. Wash the residue free of SOn- by several washings, using two.to five drops of water each time. Treat the residue with 10 drops of dil. HN03. Heat in water bath, centrifuge, and separate the solution. Label it Solution 2 and set i t aside. To the residue add five drops of dil. HCl and heat in a water bath.3 Combine the solution with Solution 2. If a precipitate forms on mixing the solutions, centrifuge and test the residue for Ag+ and Pb++. Test Solution 2 for cations, combining group precipitates obtained in the analysis of this solution with like ones formed in the examination of Solution 1.
Any residue at this point vill consist of untransposed sulfates or substances not decomposed by the HSOI treatment. If the residue is large, it should be fused in a nickelcruciblewithNanC08 to which a little NaNOs is added.
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EXPERIMENTS
Except where otherwise stated, the quantity of sample used in each of the following experiments was 50 mg. (1) Na2CJLO6.2H20. After treatment with HzS04 and HN03 a clear straw-colored solution was obtained containing no tartrate. This was shown by the failure of the solution to char when boiled with conc. H1S04, and again when dried and heated strongly. (2) PbBrz left a white residue after the HzS04, HNOa treatment. The precipitate after heating with Na2C03solution dissolved in the dil. HN03. (3) SnCl2.2IfZ0formed a white precipitate after the HZS04, HN03 treatment. The precipitate did not dissolve in dil. HNOs after being heated with NazCOa solution. It dissolved, however, in dil. HC1. (4) A mixture of 25mg. PbBrzand 25mg. SnCL.ZHz0, when subjected to the HzS04,HN03 treatment, gave a precipitate which partly dissolved in dil. HNOa after transposition with NazC03. The remaining residue dissdved in dil. HC1. The two sdutions were combined, yielding a precipitate soluble in hot water. (5) CrCZO4. The action of H2S04and HNOJ resulted in the formation of a precipitate which on further treatment with Na2C03did not dissolve either in HNOs or HC1. However, the aqueous extract, i. e., Solution 1 in the procedure, was green, showing the presence of Cr+++. After oxidizing the Cr+++ to CrO4- with NazOnthe resulting yellow solution after acidification gave a positive test with diphenyl carbazide reagent. Similar results were obtained when 10 mg. of CrCzO' were used. (6) KzCrOngave the same results as above except that the water extract contained a a u c h larger quantity of Cr+++.
(7) SnC14.51f20,same as (3) above. (8) KaFe(CN)e.3H20yielded, after the HzSOr, HNOa treatment, a brown solution and a precipitate. The solution gave a test for Fe+++ but none for Fe(CN)P or CN-. The precipitate was transposed with NazCOa and dissolved in dil. HN03. (9) Znz[Fe(CN)6]a,same as (8) above. (10) BiOCl after treatment with HnSOa, HNOa left a precipitate which after transposition with NazC03 dissolved in dil. HN03. Solution 1 (see procedure) when separated from the residue gave a positive test for Bi+++ as bismuth sulfide. From the size of the precipitate, it was estimated that more than one-half of the BiOCl went into Solution 1. (11) Losses of Mercury. (a) 25 mg. of Hg' gave a clear solution after treatment with HSOa, HNOs. Treated with H2S, a test was obtained equivalent to 3 to 4 mg. of Hg. (b) 10 mg. of Hg++ gave a similar result, i. e., the final test with H a swas equivalent to 3 mg. Hg++. (c) 1 mg. of Hg++ gave a similar result yielding a test equivalent to 1 mg. of Hg". (12) SbOCl when subjected to the HaO4, HNOa treatment left a precipitate. After separation of the residue from the solution, the latter gave a positive test for Sb. The precipitate after transposition did not completely dissolve in either the dil. HNOa or dil. HCl. However, the solution obtained after treatment with dil. HCl gave a positive test for Sb. The same results were obtained when samples of 10 mg. of SbOCl were used. A solution of HgClt in NaCl solution of strength 50 mg. of Hg per ml. was used. The solution contained 0.025 mg. NaCl per ml.
