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2. HQS in presence of HCl reduces the solution to Cr,Cl(green), with separation of S. In neutral solutions, Cr,(HO), is precipitated along with S.

3. SO, reduces solutions of chromates to the chromic salt, the colour of which is green. Chromates are likewise reduced by zinc and a dilute acid, by oxalic acid and dilute sulphuric acid, by strong H,SO,, by strong HCl, and by boiling the solution acidified with HCl or H2SO4 along with alcohol.

4. AgNO, produces a dark red precipitate of Ag2CrO4, soluble in HNO3 and in (NH)HO.

5. Lead acetate produces a bright yellow precipitate of PbCrO4, soluble in Naho, but soluble with difficulty in dilute HNO3.

(See also reactions for Chromium, 83.)

Reactions of the Acids belonging to Group III.

111. Acids precipitated by AgNO3, and not by BaCl2.

Hydrochloric, Hydrobromic,Hydriodic, Hydrocyanic, Hypochlorous, Nitrous, and Hydrosulphuric Acids.

HYDROCHLORIC ACID. HCl, c.w. 36.5.

1. AgNOą produces a white curdy precipitate of AgCl, which becomes violet on exposure to light. The precipitate is insoluble in HNO3, but soluble in (NH4)HO, in KCN, in Na2S2O3, and also to some extent in NaCl.

2. Heated with H2SO4 and MnO2, chlorides yield chlorine gas, recognized by its smell, bleaching action, and green colour.

3. Dry chlorides, when heated in a retort with H2SO4 and K,Cr2O7, yield Cro,Cl, (chromium oxychloride), which distils over into the receiver as dark red liquid, decomposed by addition of water or (NH)HO, yielding a yellow solution, which, on addition of a lead salt, gives a yellow precipitate of PbCrO4.

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112. HYDROBROMIC ACID. HBr, c.w. 81.

1. AgNO, produces a pale yellow precipitate of AgBr, insoluble in dilute HNO3, soluble in strong (NH)HO, and readily in KCN and Na2S2O3.

2. Chlorine passed through a solution of a bromide decomposes it with liberation of Br, which dissolves in the liquid and colours it yellow. If this solution be shaken up with ether, the bromine is dissolved by it, and the yellow ethereal solution floats above the liquid which becomes colourless. If the ethereal solution be then separated from the liquid, and NaHO be added, the yellow colour disappears, and NaBr and NaBrO2 are produced. On evaporation and ignition, oxygen is evolved and NaBr alone remains, which may be tested as in 3.

3. Heated with H2SO4 and MnO2, bromides yield red vapours of Br, recognized by its powerful odour.

4. Heated in a retort with K,Cr,O, and H2SO4, dry bromides yield dark red vapours, which condense in the receiver to a liquid of the same colour, which consists of pure bromine, and is decolorized on adding excess of (NH)HO. (Compare Hydrochloric Acid test, 111, 3.)

113. HYDRIODIC ACID. HI, C.W. 128.

1. AgNO, produces a pale yellow precipitate of Agl, insoluble in dilute HNO3, and very difficultly soluble in (NH)HO, but readily in KCN and Na2S2O3.

2. Cuprous sulphate * produces a dirty-white precipitate of Cu,l2, which separates most completely if the solution be made slightly alkaline with Na,C0g. The reagent produces no precipitate in solutions of chlorides or bromides.

3. KNO, produces no reaction in solutions of iodides until a few drops of HCl or H2SO4 are added, when iodine is at once liberated and colours the solution yellow. If a little starch solution be now added, a deep blue coloration results from the formation of starch iodide. On warming the blue liquid the colour disappears, but reappears on cooling. The production of blue starch iodide is the most characteristic test for iodine.

4. Chlorine water (or the gas) liberates iodine from iodides, but excess of Cl causes the formation of ICl, which is colourless, and gives no blue coloration with starch solution. If therefore chlorine water be added drop by drop to a solution of an iodide mixed with starch solution, a blue coloration is produced,

* Prepared by dissolving a mixture of two parts CuSO4 and five parts FeSO4 in water, or by the action of So, on ĈuSO4.

which disappears on further addition of the reagent.

5. Free iodine (liberated by either of the above methods) is dissolved by CS2, forming a violet-coloured solution. If then, a solution of iodine be shaken up with CS2, the latter acquires a violet colour. Chloroform may be substituted for CS2.

6. Heated with MnO, and dilute H2SO4, violet vapours of iodine are obtained, which colour paper moistened with starch, blue.

114. HYDROCYANIC ACID. HCN, C.W. 27.

1. AgNO3 produces a white precipitate of AgCN, insoluble in HNO3, with difficulty in (NH4)HO, but readily in KCN and Na2S2O3. AgCN is decomposed on ignition, and metallic Ag remains ; this serves to distinguish it from AgCl, which is not decomposed on ignition.

2. If a solution of FeSO4, which has become oxidized by exposure to the air, be added to the solution of a cyanide made alkaline with Naho, a bluish green precipitate is formed, which is a mixture of Prussian blue with the hydrated oxides of iron. On adding HCl, these last are dissolved, and the blue precipitate remains.

3. HCl decomposes nearly all cyanides with evolution of HCN, recognized by its odour, resembling bitter almonds. If a cyanide be thus decomposed in a small porcelain basin, covered by a similar basin on which a drop of (NH),S, (yellow) adheres, the latter is converted into (NH)CNS, which gives a blood-red coloration on addition of Fe,Clo and HCI.

NOTE.—Hg(CN), cannot be detected by the above methods. The dry substance is detected by igniting in a small tube, when cyanogen gas is evolved, or the solution is decomposed by H, S and filtered from the HgS; the filtrate contains HCN.

115. HYPOCHLOROUS ACID. HCIO, c.w. 52-5. 1. AgNO, produces a white precipitate of AgCl.

2. Pb(NO3)2 produces a white precipitate, which changes in colour to red, and ultimately to brown from formation of PbO2

3. MnCl, produces a dark brown precipitate of MnO(HO)

4. Indigo and litmus solution are decolorized, especially on addition of an acid.

5. Dilute acids decompose hypochlorites with evolution of Cl. HNO3 evolves HClO from hypochlorites.

116. NITROUS ACID. HNO2, C.W. 47.

1. AgNO3 produces a white precipitate of AgNO, soluble in a large excess of water.

2. H,S, in presence of acid, produces a precipitate of S, and (NH4)NO, remains in solution.

3. FeSO4, in presence of an acid, produces a black coloration from solution of NO in the FeSO4.

(See also 113, test 3.)

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