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As,Og add a drop of HCl, and pass H,S through the solution, and observe the yellow precipitate of As, S.
(c) Attach another tube containing an arsenic mirror to an apparatus evolving dry H2S, and warm the mirror gently : it will be converted into yellow As,S3. Now pass dry HCl through the tube (without warming), and observe that the sulphide remains unaltered. (Compare corresponding Sb reactions, 74, 6, c).
(d) Allow a current of AsHz to pass through a solution of silver nitrate : a black precipitate of Ag will be produced, and As,Og will be found in solution along with HNO3 liberated from the AgNO3. Filter from the Ag, and very cautiously neutralize the free acid with highly diluted (NH)HO, when a yellow precipitate of AgzAsOwill be formed. (Compare corresponding Sb reaction, 74, 7.)
6. Compounds of arsenic, when treated with Zn and strong solution of KHO, are converted into AsH, by the action of the nascent hydrogen. If this reaction be performed in a test tube, and the gas escaping be held near a piece of paper moistened with AgNO3, a bluish black coloration is produced by the formation of AsAgz. (Sb compounds give no similar reaction.)
7 (Reinsch’s Test).—Add to the solution of arsenic, HCl and a few strips of bright copper wire or foil : As is deposited on the copper, which may be removed from the solution, dried by filter paper, and heated in a dry test-tube to obtain the octahedral crystals of As2O3.
8. Dry reactions.
Place the dry arsenic compound at a (Fig. 57), in a drawn-out hard glass tube. Then place above it at b a small rod of well-ignited charcoal, and heat the portion containing the charcoal until it is red hot. This wiil
cause the glass to soften, and the tube will bend so as to bring the portion a into the flame. The arsenic compound will volatilize and be decomposed by the red-hot charcoal, and a metallic mirror will form at c.*
9. Place the dry arsenic compound in a bulb tube as at a (Fig. 18), along with a mixture of equal parts of dry Na,CO2 and KCN, and heat the bulb. A mirror
of As will form at b, which may be further tested by the reactions mentioned for the mirror obtained in Marsh's test (75, 5, 6). If any moisture be deposited on first heating the tube, remove it by inserting a small coil of purpose, the arsenic compound is pounded in a mortar with a perfectly dry mixture of three parts Na, CO, with one part of KCN, and placed at a in the tube ab (Fig. 19), through which a slow current of dry CO, is
10. The above reaction is more delicate when the mixture is heated in a current of dry CO2. For this
* Non-volatile compounds of As must be mixed with dry charcoal powder, and heated in a similar tube, having in addition a small bulb at the lower end to contain the mixture.
led, and the whole tube gently heated until every trace of moisture is expelled. When this is the case the tube is more strongly heated at a, and the mirror is obtained at b; traces of arsenic escape condensation, and therefore a slight garlic odour is observed at the extremity of the tube. Antimony compounds treated in this way yield no metallic mirror.
11. Arsenic compounds, when mixed with Na,C03 and heated on charcoal by the blow-pipe flame, are reduced to metallic arsenic, which at once volatilizes, and may be recognized by the characteristic odour resembling garlic.
76. Separation of Arsenic, Antimony, and Tin
Solution in (NH4)2S. cc-tains sulphides of As, Sb, and Sn. Add HCI until acid : the metals 74 re-precipitated as sulphides. Filter, wash precipitate with lot irne: till 'ree from HCI; digest precipitate with one or two pieces of olid (NH4)2CO3 and H2O. Filter.
SnS2, S6,S3. Wash and dissolve in strong boiling HCl, dilute with water, and add to the solution a piece of zinc and platinum : Sb forms a black stain on the platinum. Dissolve in HNO3, boil down to a small bulk, and pass HQS through the solution ; orange precipitate indicates presence of Antimony. The Sn deposits on the zinc; dissolve in HCT, boil down, and test with HgCl2. White precipitate indicates Tin.
As. Add HCl until acid; wash precipitated sulphide, and dissolve in HCl and a little KCIO3, boil down to a small bulk, and apply Marsh's test (75,5). Metallic mirror, yielding octahedral crystals on heating, indicates Arsenic. Dissolve in H2O, and confirm by adding AgNO3 and dilute (NH) HO, to obtain yellow precipitate of AgzAsO3 (75, 56).
Reactions of the Metals of the Iron Group. 77. Metals whose sulphides and hydrated oxides are insoluble in water, and are precipitated on addition of the group reagent (NH4)2S in presence of (NH)HO and NH Cl.
Iron, Nickel, Cobalt, Zinc, Aluminium, Manganese,
IRON. Fe, c.w. 56. Ferrous Salts.
1. (NH)2S produces a black precipitate of Fes, insoluble in alkalies, but soluble in HCl. In dilute solutions of ferrous salts (NH4)2S produces at first a green colour; on standing, however, FeS separates as a black precipitate.
2. KHO or (NH)HO produces a white precipitate of ferrous hydrate Fe(HO), which rapidly acquires a dirty green, and ultimately a reddish brown colour, owing to absorption of oxygen and conversion into ferric hydrate Fe (HO)
3. Carbonates of the alkalies precipitate FeCO3
(white), which rapidly darkens in colour owing to absorption of oxygen.
4. K, Fe(CN)6 produces a white precipitate of K,Fe(CN)6, which rapidly becomes blue by oxidation to Fe (CN)12 (Prussian blue). It is insoluble in acids, but is decomposed by alkalies.
5. K,Fe(CN)6 produces a blue precipitate of Fez Fe, (CN)12 (Turnbull's blue), also insoluble in acids, but decomposed by alkalies.
6. KCNS produces no coloration.
7. Ba CO3 produces no precipitate in cold solutions of ferrous salts,
8. Fused with borax in the oxidizing flame, yellowish red beads are produced ; in the reducing flame the beads become green. (See also 59.)
Ferric Salts.-I. H,S in acid solutions produces a precipitate of sulphur, and the salt is reduced to protosalt, thus : Fe,Cl6 + H2S = 2 FeCl2 + 2 HCl + S.
2. (NH4)2S produces a black precipitate of FeS mixed with sulphur, insoluble in excess of the reagent and in alkalies, but soluble in HCl and in HNO3. In dilute solutions of iron only a greenish coloration is produced.
3. KHO or (NH4)HO produces a brownish red precipitate of Fe,(HO)o, insoluble in excess of either reagent.
4. K4Fe(CN)6 produces a precipitate of Fe (CN)2 (Prussian blue), insoluble in HCl, soluble in C,H,O4, and decomposed by KHO or NaHO with separation of Fe(HO).
5. KgFe(CN)6 changes the colour of the solution to reddish brown, but does not produce a precipitate.