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 HS, 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.

I. 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. H2SO4 decomposes chlorates with evolution of Cl2O4, a greenish yellow gas, having a powerful odour

49

I

If heated, violent explosions occur: the mixture ought therefore to be kept cold, and only very small quantities 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. Chlorates are reduced by SO2 with liberation of chlorine or its oxides, hence if the solution of a chlorate be coloured blue with indigo solution, it is decolorized on adding H2SO, and solution of Na2SO3. (Distinction from perchlorates).

*4. HCl decomposes chlorates with evolution of Cl and ClO4, a mixture called euchlorine.

5. Heated with charcoal, chlorates deflagrate violently.

PERCHLORIC ACID. HCIO4, C.W. 100'5.

1. H2SO4 does not act upon perchlorates in the cold, and on heating white fumes of HCIO4 are given off, but no explosions occur. (Compare 119, I.)

*2. KCl produces in strong solutions a white precipi. tate of KC104.

3. Indigo solution is not decolorized when added to perchlorates warmed with HCl, as euchlorine is not evolved.

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.)

6. Perchlorates are not reduced by SO2. (Compare 119, 3.)

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, H2S2O3, HCO3, HS, Si(HO), H2CrO4, H3AsO3, and H3AsÔ4.

(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 H2SO4 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 remain, H2SO4 was present.

2. To another portion of the neutralized solution add AgNO3: n precipitate indicates one or more of the following acids :

(a) HCl, H Br, HI, HCN, H4Fe(CN)6, H3Fe (CN)6, H2S.

(b) H3PO4, H3 As O4. H3 A9 O3, H2CrO4, Si(HO)4, B(HO)3, Ox, T, and Ci. To the precipitate add cold dilute HNO3. Acids under (a) are insoluble, those under (6) soluble.

Ox, Ci, and T are contractions for oxalic, citric, and tartaric acids (For the further separation of these organic acids, see Table K.)

TEST FOR THE REMAINING ACIDS BY THE FOLLOWING REACTIONS GIVEN UNDER EACH ACID.

For H3 Fe(CN)6, by tests 2 and For B(HO)3 by tests 4 and 5, 99. 3, 127.

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 HClO3, by tests 1 and 2, For CO2 by test 2, 102.

(6) 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 Fe SOA + 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 BaCl

NOTE. In mixtures of chlorides, bromides, and iodides, or any two of them, proceed as follows. Place a small quantity of the mixture in a test tube, add water and a few pieces of MnO2 * (free from chlorides), then one drop only of dilute sulphuric acid and boil: violet vapour indicates Iodides. Add another drop of the dilute acid and boil again. Proceed in this way till no more violet vapour is given off. Then add about 2 c.c. dilute sulphuric acid and boil: brown vapour indicates Bromides. Boil till all bromine is expelled, allow to cool completely, add to the residue an equal bulk of strong sulphuric acid and warm a green gas indicates Chlorides. Confirm by observing if a piece of moistened red blotting-paper held in the mouth of the tube is bleached.

* Powdered MnO2 produces too much "bumping" to be used for this purpose.