Qualitative analysis / by William Briggs and R.W. Stewart.

  • William Briggs
Date:
1913
Catalogue details

Licence: Public Domain Mark

Credit: Qualitative analysis / by William Briggs and R.W. Stewart. Source: Wellcome Collection.

    48/180 page 32
    ARSENIC,* As. Solution—Arsenious Oxide, As.Q6, dissolved in dilute HC1 (which contains Arsenious Acid, H3AsOa). 1. Sulphuretted hydrogen, H0S, gives (in acid solutions) a lemon yellow precipitate of arsenious sulphide, As2S3. 3 H2S + 2 H3As03 = As2S3 + 6 H20. The precipitate is soluble in ammonium sulphide, forming ammonium arsenite, (NH4)3AsS3, and in caustic alkalies forming a mixture of arsenite and thioarsenite. As2S3 + 3 (NH4)2S = 2 (NH4)3AsS3. As2S3 + 6 KOH = K3As03 4- K3AsS3 + 3 H20. Potassium Potassium arsenite thioarsenite It is also readily soluble in a concentrated solution of ammonium carbonate again forming a mixture of arsenite and thioarsenite. As2Ss + 3 (NH4)2C03 = (NH4)3As03 + (NH4)8AsS3 Ammonium Ammonium arsenite thioarsenite + 3 C02. 2. Add cupric sulphate, CuS04, and then sodium hydrox¬ ide, NaOH, drop by drop, till in excess (to render the solution alkaline): a fine green precipitate of arsenite of copper (CuHAs03) is obtained. HsAs03 + CuS04 + 2 NaOH = CuHAs03 + NTa2S04 + 2 H,0. 3. Marsh’s Test.—Hydrogen is generated from zinc + and dilute- sulphuric acid, both free from artenic, in the apparatus described on p. 13, and the jet lit as soon as all the air in the apparatus is dis- * Arsenic, when present as an arsenate, that is, as Asv, is only precipitated as As2S3 by passing the gas through the solution for some time. In this form arsenic will be detected as an acid (see page 72). f As already stated on p. 13 ordinary commercial zinc contains arsenic and is therefore useless for this test; special arsenic free from zinc must be employed.
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