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121. GROUPING OF THE ORGANIC ACIDS.

GROUP I. (TARTARIC ACID GROUP).

Group reagent, CaCl2.

Tartaric Acid and Citric Acid (Oxalic Acid, see 100).

Acids which are precipitated by CaCl, in the cold or on boiling.

GROUP II. (BENZOIC ACID GROUP).

Group reagent, Fe, Cla.

Benzoic and Succinic Acids.

Acids which are not precipitated by CaCl2, but which give precipitates with Fe,Cl. in neutral solutions.

GROUP III.

Group reagent, AgNO3.

Ferro-cyanic, Ferri-cyanic, Acetic and Formic Acids.

Acids precipitated by AgNO, in neutral solutions, and not by CaCl2, or Fe,Cle. Acetates and Formates are only precipitated in concentrated solutions.

Reactions of the Organic Acids belonging to Group I.

(Tartaric Acid Group.)

Acids precipitated by CaCl, in the cold or on boiling.

Tartaric and Citric Acids.

122. TARTARIC ACID. C4H 08.

1. CaCl, in neutral solutions produces a white precipitate of CH4CaOg, soluble in acids, and in ammoniacal salts. The precipitate is soluble in KHO, but is re-precipitated when the solution is boiled, and on cooling is re-dissolved.

2. KCl produces in solutions containing T in excess a white crystalline precipitate of C&H KO., soluble in mineral acids and alkalies, insoluble in acetic acid. The precipitation is promoted by stirring, or by addition of alcohol.

3. Lime-water produces in neutral solutions a white precipitate CH.CaO(flocculent at first, afterwards crystalline), soluble in tartaric acid and NH_Cl, but re-precipitated in crystals from these solutions after standing some time.

4. AgNO3 in neutral solutions produces a white precipitate of C&H Ag2O6, soluble in HNO3 and in (NH)HO. If the solution in (NH)HO be warmed in a test tube kept at a temperature of about 66° C. for some time, the glass becomes coated with a mirror of silver.

5. Heated with H2SO4, the mixture darkens rapidly from separation of carbon, and SO, CO, and Co, are evolved.

6. Heated to redness, the substance darkens in colour and gives off the characteristic odour of burnt sugar.

123. CITRIC ACID. C6H8O.

1. CaCl, produces no precipitate in neutral solutions in the cold, but on boiling, Caz(C6H307)2 is precipitated, and is not soluble in KHO, but soluble in (NH)HO.

2. Lime-water produces no precipitate in cold neutral solutions, but on boiling, Caz(C6H307)2 is precipitated.

3. AgNO3 produces in neutral solutions a white flocculent precipitate of C H Ag2O7, soluble in (NH)HO: this solution does not blacken on boiling.

4. Heated with H2SO4, CO, and CO are evolved without any darkening in colour ; on continued heating, however, the mixture darkens, and SO, is evolved.

5. Heated to redness, irritating fumes are given off, readily distinguished from those given off by heating the preceding acid.

Reactions of the Acids belonging to Group II.

(Benzoic Acid Group.)

124. Acids precipitated by Fe,Cla, and not by CaCl.

Benzoic and Succinic Acids.

BENZOIC ACID. C,H,Oz.

1. Fe,Cle produces, in neutral solutions, a buffcoloured precipitate of ferric benzoate, decomposed by (NH)HO with formation of a more basic benzoate and ammonium benzoate. Ferric benzoate is soluble in HCl with liberation of benzoic acid.

2. Heated with H2SO4, benzoic acid does not blacken.

3. Heated in an open tube, the acid sublimes in needle-shaped crystals, and an irritating vapour is given off. When kindled, the crystals burn with a smoky flame.

125. SUCCINIC ACID. C4H204.

1. Fe,Cle produces, in neutral solutions, a reddish brown bulky precipitate of ferric succinate, soluble in mineral acids, and decomposed by (NH4)HO in a similar manner to ferric benzoate.

2. Lead acetate produces a white precipitate of lead succinate, soluble in excess of the reagent and in HNO3.

3. BaCly, in presence of (NH)HO and alcohol, produces a white precipitate of barium succinate ; this reaction serves to distinguish this acid from benzoic, which does not give a similar precipitate.

4. Heated in an open tube, the acid sublimes in fine needles ; and when kindled, the crystals burn with a bluish but not smoky flame.

Reactions of the Acids belonging to Group III.

126. Acids precipitated by AgNO3, in strong neutral solutions.

Ferro-cyanic, Ferri-cyanic, Acetic, and Formic

Acids.

FERRO-CYANIC ACID. HFe(CN).

1. AgNO3 produces a white precipitate of Ag4Fe(CN), insoluble in dilute HNO3 and in (NH)HO, but soluble in KCN.

2. CuSO4 produces a reddish brown precipitate of Cu, Fe(CN)6

3. Fe,Cle produces a deep blue precipitate of Prussian blue, insoluble in dilute mineral acids, soluble in oxalic acid, and decomposed by NaHO with separation of Fe (HO)

4. FeSO4 gives a light blue precipitate, which rapidly darkens in colour by oxidation.

127. FERRI-CYANIC ACID. H Fe(CN).

1. AgNO3 produces an orange-coloured precipitate of AgzFe(CN)6, insoluble in dilute HNO3, soluble in (NH4)HO and KCN.

2. FeSO4produces a blue precipitate of Fe Fe(CN)12 (Turnbull's blue), insoluble in acids, but decomposed by alkalies.

3. Fe,Clo produces no precipitate, but alters the colour to a greenish brown.

128. ACETIC ACID. C2H4O2

1. AgNO3 produces, in neutral solutions, a white crystalline precipitate of C,H,AgO2, soluble in (NH)HO and in hot water.

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