117. HYDROSULPHURIC ACID (Sulphuretted Hydrogen). H2S, c.w. 34. 1. AgNO3 produces a black precipitate of Ag2S, insoluble in dilute acids. 2. Lead acetate, even when highly dilute, produces a black precipitate of PbS. 3. Sodium nitro-prusside, in presence of NaHO, produces a reddish violet coloration, even in very dilute solutions. The colour disappears in a short time. 4. HCl or H2SO decomposes most sulphides with evolution of H2S, recognized by its disagreeable odour and by its blackening paper moistened with solution of lead. Reactions of the Acids belonging to Group IV. 118. Acids not precipitated by any reagent. Nitric, Chloric, and Perchloric Acids. NITRIC ACID. HNO3, c.w. 63. 1. Nitrates when heated evolve oxygen, and in some cases nitrous vapours also. On fusing a nitrate and adding a fragment of charcoal, vivid deflagration occurs. (For the reactions of Nitric Acid, see 22.) 119. CHLORIC ACID. HCIO3, c.w. 84°5. 1. H2SO, decomposes chlorates with evolution of ClO4, a greenish yellow gas, having a powerful odour. I If heated, violent explosions occur: the mixture ought therefore to be kept cold, and only very small quanti. ties should be used. 2. When chlorates are heated, oxygen is evolved and a metallic chloride remains, which may be dissolved in water and precipitated, as AgCl, by AgNO3. (See 5.) 3. Indigo solution, if added to the solution of a chlorate, is decolorized on addition of H2SO4, and solution of Na2SO3. 4. HCl decomposes chlorates with evolution of Cl and ClO4, a mixture called euchlorine. 5. Heated with charcoal, chlorates deflagrate violently. PERCHLORIC ACID. HClO4, c.w. 100 ̊5. 1. H2SO4 does not act upon perchlorates in the cold, and on heating white fumes of HCIO, are given off but no explosions occur. (Compare 119, 1.) 2. KCl produces in strong solutions a white precipitate of KCIO4. 3. Indigo solution is not decolorized when added to perchlorates mixed with HCl. 4. Dry perchlorates evolve oxygen on heating. 5. Anhydrous perchloric acid is a colourless fuming liquid, which explodes with great violence when dropped on wood charcoal. (Roscoe.) TABLE J. 120. Detection of Inorganic Acids in Mixtures. [The following acids are found in the examination for bases, which ought always to precede the examination for acids:-H2SO3, H2CO3, H2S, Si(HO)4, H2CrO4, H3 As O3, and H3 As O4.] (a) Acids in Soluble Bodies. 1. Neutralize a portion of the solution with (NH4) HO and add BaCl, (or Ba(NO3)2 if Ag, Hg2, or Pb be present): precipitate indicates H2SO H3PO4, H3 As O3, H3 As O4, Si (HO)4, H2CrO4, Ox, Ci, T,* and large quantities of B(HO)3 and HF. To precipitate, add water and then HCl: if a precipitate remair, H2SO4 was present. 2. To another portion of the neutralized solution add AgNO3: & precipitate indicates one or more of the following acids : (a) HCl, H Br, HI, HCN, H4Fe(CN)6, H3Fe(CN)6, H2S. (6) H3PO4, H3 As O4. H3 As O3, H2CrO4, Si(HO)4, B(HO)3, Õx, T, and Ci. To the precipitate add cold dilute HNO3. Acids under (a) are insoluble, those under (b) soluble. Ox, Ci, and T are contractions for oxalic, citric, and tartaric acids. (For the further separation of these organic acids, see Table K.) DETECTION OF ACIDS UNDER (a). To a portion of the solution add starch paste and one drop of a solution of N2O3 in H2SO4. Blue coloration indicates HI. Add now chlorine water till the blue colour disappears, and shake with chloroform. If this becomes reddish brown in colour, the presence of HBr is indicated. HCl is detected in the presence of the others by boiling down the solution to dryness and distilling the residue with K2Cr2O7 and H2SO4. (See 111, 3.) DETECTION OF ACIDS UNDER (6). Test separately for each acid by the methods already given. SEPARATION OF H3ASO3, H3AsO4, AND H3PO4. Acidify solution with HCl, ad l Na2SO3, and heat until no smell of SO2 is given off. Pass H2S through the hot solution, filter, and test for H3PO4 with ammonium molybdate: yellow precipitate indicates H3PO4. Precipitate another portion with magnesia mixture, and test both precipitate and filtrate for arsenic. TEST FOR THE REMAINING ACIDS BY THE FOLLOWING REACTIONS GIVEN UNDER EACH ACID. For HCN by test 3, 114. For H4 Fe(CN)6 by tests 3 and 4, 126. For H3 Fe(CN)6, by tests 2 and 3, 127. For H2 S, by test 4, 117. *For H NO3, by tests b and c, 22. For HClO3, by tests 1 and 2, 119. For Ox and HF, by Ca Cl2 + acetic acid. Confirm Ox by test 4, 100, and HF by test 3, 101. For B (HO)3, by tests 4 and 5, 99. For Si(HO)4, by tests 2 and 4, 103. For H2CrO4, by tests 4 and 5, 110. For H2SO3 by test 3, 105, and smell of SO2 on adding HCl, For CO2 by test 2, 102. (B) Acids in Insoluble Bodies. Some idea of the nature of the compound is generally obtained by the preliminary examination (Tables A and B). If not dissolved by the * If HI be present, it must first be removed by addition of FeSO4 + CuSO4 (See 113, 2.) ordinary reagents, the substance must be fused with about four times its weight of a mixture of Na2CO3 and K2CO3. When cold, extract the fused mass with water, and filter if necessary. The filtrate contains the acid, and is neutralized with HCl or HNO3, and examined by the methods given under (a). The sulphates of barium, strontium, and calcium are decomposed by boiling with a concentrated solution of Na2CO3. Filter, and examine the filtrate for the acid. Nitric acid and aqua regia oxidize sulphides to sulphates: hence the solution of a sulphide in these acids always contains H2SO4. In such cases a separate portion of the substance must be examined for H2SO4 by boiling with HCl, diluting with water, and then testing with BaCl2. |