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Reactions of the Metals of the Silver Group.
63. Metals whose chlorides are insoluble in water, and which are precipitated on addition of the group reagent, HCI:
Silver, Mercury, Lead. SILVER. Ag, combining weight 108.
1. HCl produces a white curdy precipitate of AgCl, insoluble in hot water and in HNO3, but readily soluble in (NH, HO.
2. H,S or (NH4)2S produces a black precipitate of Ag S, soluble in boiling HNO3, with separation of sulphur.
3. NaHo produces a light brown precipitate of Ag20, insoluble in excess of NaHO, but soluble in (NH)HO.
4. K,CrO produces a dark red precipitate of Ag,CrO4, soluble in hot HNO3 ; this solution deposits on cooling an acid chromate in needle-shaped crystals.
5. Heated on charcoal with Na,CO2, in the reducing flame of the blow-pipe, yields bright, malleable metallic beads, soluble in HNO3 (56, a).
64. MERCURY. Hg, C.W. 200.
1. HCl produces a white precipitate of Hg2Cl2 (calomel), insoluble in cold HNO3, and blackened by (NH2)HO, from formation of Hg2Cl(NH2).
2. HQS or (NH)2S produces a black precipitate of Hg,S, not dissolved by boiling HNOą.
3. NaHO produces a black precipitate of Hg20, insoluble in excess of NaHO or (NH)HO.
4. SnCl2 produces a grey precipitate of Hg. If the fluid be poured off and the residue boiled with HCI distinct globules are obtained.
5. If a drop of a neutral or only slightly acid solution of a mercurous salt be placed on a bright piece of copper, metallic mercury is deposited and the stain becomes bright on rubbing : it disappears on heating, owing to the volatility of the mercury.
6. Heated in a small tube with NaHCO3, yields grey deposit of Hg, which on rubbing appears in distinct globules (59).
65. LEAD. Pb, c.w. 207.
1. HCl produces a white precipitate of PbCl2, which is converted into a basic salt on adding ammonia, but without change of appearance. PbCl, is soluble in a small quantity of hot water, or in a large quantity of cold water.
2. H.S04 produces a heavy white precipitate of PbSO4, soluble in NaHO. In dilute solutions this precipitate appears only on standing; if therefore there is no immediate precipitation, the solution should be concentrated by evaporation. PbSO4 is soluble in boiling HCl, and the solution on cooling deposits needle-shaped crystals of PbCl.z.
3. K, Cr, produces a bright yellow precipitate of PbCrO4, readily soluble in NaHO, but with difficulty in HNO3.
4. KI produces a bright yellow precipitate of Pbl, soluble in boiling water; the solution on cooling depo sits the salt in brii iant golden hexagonal scales.
5. Heated on charcoal with NaHCO3, yields malleable beads, and at the same time a yellow incrustation of PbO on the charcoal (56, a).
SILVER GROUP (I.).
66. Separation of Silver, Mercury, and Lead.
Add HCl and filter from the precipitated chlorides.
AgCi, Hg2012, PbC1z. Groups II., III., IV. & V. Wash precipitate twice with cold water, and add washings to filtrate ; then twice with hot water, and test part of this for lead with H2'04. White precipitate indicates Lead. Boil the remaining part down to obtain the needle-shaped crystals of PbClz. If lead be found, wash the precipitate free from it with hot water, and treat the residue with warm (NH4) Ho; filter.
Reactions of the Metals of the Copper Group.
67. Metals whose sulphides are insoluble in HCI and are precipitated in presence of that acid by the group reagent H S.
Mercury, Lead, Bismuth, Copper, Cadmium, Arsenic,
Antimony, and Tin.
SUB-GROUP A.-Sulphides of the above metals insoluble in (NH4)2S2, viz., Mercury, Lead, Copper, Bismuth, and Cadmium.
MERCURY. Hg, C.W. 200. Mercuric Salts.
1. H S produces, when added by degrees, first a white precipitate, which changes to orange, then to brownish red, and finally to a black precipitate of HgS. These successive changes of colour on the addition of H2S are exceedingly characteristic. This precipitate is insoluble in HCl and in HNO3, even on boiling ; it is soluble, however, in KHS and in aqua regia.
2. KHO produces a yellow precipitate of HgO, which is insoluble in excess of the precipitant, except when added to very acid solutions.
3. (NH4)HO produces in solutions of HgCl, a white precipitate of HgCI(NH), (“white precipitate”).
4. SnCl, produces, when added in small quantities, a white precipitate of Hg2Cl2, but on adding an excess of the reagent, metallic mercury precipitates as a grey powder, and may be united into a coherent globule by boiling with HCl.
5. Reactions 5 and 6 for mercurous salts (64) are also produced with mercuric salts.
68. LEAD. Pb, c.w. 207.
1. H2S produces a black precipitate of Pbs, even in solutions of PbCl2, so that a weak solution of a lead salt which has not been precipitated with HCl will be precipitated with H,S. Hence lead occurs both in the silver and copper groups.
2. Reactions 2, 3, 4, and 5, for lead, in Group I. (65), are also applicable in this group.
69. BISMUTH. Bi, c.w. 210.
1. H S produces a black precipitate of Bi2S3, insoluble in KHS and KHO, but soluble in HNO3
2. KHO or (NH4)HO produces a white precipitate, which on boiling becomes yellow (Bi,O3); the precipitate is insoluble in excess of either reagent.
3. H,O, when added in considerable quantity to normal salts of bismuth, produces an immediate white precipitate of a basic salt of bismuth.
Bismuth trichloride is most easily precipitated by H,O. If another salt of this metal is being examined, it is best to precipitate the oxide first by ammonia ;