Hydrogen sulphide precipitation with

N. A. E. Millon found that a soln. of alkali chlorite gives a yellow precipitate of lead chlorite, Pb(C102)2, or of silver chlorite, AgC102, when treated respectively with lead or silver nitrate. On recrystallization from hot water, lead and silver chlorites are obtained in yellow plates. J. Schiel also made lead chlorite by treating a soln. of barium or calcium chlorite with lead nitrate an excess of the lead nitrate is to be avoided because of its solvent action. Fine sulphur yellow crystals can be obtained from a warm soln.—50°-60°—of the salt in calcium chlorite. The dry salt explodes at 126° according to N. A. E. Millon, at 100° according to J. Schiel. It explodes when triturated with flowers of sulphur or antimony sulphide. It behaves like silver chlorite with hydrogen sulphide and with sulphuric acid. 

Explain why precipitates form only in some of the solutions. What substances form when hydrogen sulphide reacts with solutions of iron(II) salts Classify the studied metal sulphides according to their olubility in water and acids. 

When dilute solutions of nickel sulphate and an alkali sulphide are slowly mixed at the ordinary temperature in the absence of air, 85 per cent, of the precipitate consists of oc-NiS. This sulphide is stable in the absence of air if kept in contact with pure water. In contact with solutions that dissolve it to a slight extent it yields the p and y varieties. -nickel sulphide results, mixed with a little y, when a solution of nickel acetate acidified with acetic acid is treated with hydrogen sulphide. Boiling with acetic acid converts it into the y-form. It appears to be crystalline, y-nickel sulphide may be obtained in a pure crystalline condition by boiling the mixed sulphides with 2-normal hydrochloric acid. 

This compound with potassium cyanide or concentrated acids (not acetic acid) fields ethylene. From an aqueous solution of the chloride, hydrogen sulphide precipitates all the mercury as sulphide, but from an alkaline solution potassium hydrosulphide precipitates ethanol mercuric sulphide (CHgOHCH 2Hg) gS. 

Precipitation with hydrogen sulphide operations with hydrogen sulphide are of such importance in qualitative inorganic analysis that they merit detailed discussion. Hydrogen sulphide gas is highly poisonous, its lethal dose... 

Dilute 5 ml to 100 ml with water add 15 ml of concentrated hydrochloric acid. Boil under a reflux condenser for three hours, add 200 ml of hot water and pass in hydrogen sulphide for fifteen minutes. Filter while hot through a Gooch crucible, wash the precipitate first with solution of hydrogen sulphide, then with ethanol and finally with carbon disulphide. Dry at 110° and weigh. 1 g = 0 8622 g Hg. 

Tin(IV) in aqueous acid gives a yellow precipitate with hydrogen sulphide, and no reaction with mercury(II) chloride. 

Copper(II) ions in aqueous solution are readily obtained from any copper-containing material. The reactions with (a) alkali (p. 430), (b) concentrated ammonia (p 413) and (c) hydrogen sulphide (p. 413) provide satisfactory tests for aqueous copper(II) ions. A further test is to add a hexacyanoferrate(II) (usually as the potassium salt) when a chocolate-brown precipitate of copper(II) hexacyanoferrate(II) is obtained ... 

Mercury 11) compounds in solution give a black precipitate with hydrogen sulphide or a yellow precipitate with alkali hydroxide (pp. 437. 438). 

Arsenic. The presence of arsenie in an organie eompound is generally revealed by the formation of a dull grey mirror of arsenic on the walls of the test-tube when the eompound is fused with sodium in the Lassaigne teat. Usually sufficient arsenic is found in the fusion solution to give a yellow precipitate of arsenic trisulphide when the solution is acidified with hydrochloric acid and treated with hydrogen sulphide. 

It is recommended that the eompound be fused with a mixture of sodium carbonate (2 parts) and sodium peroxide (1 part) as in the test for Plvoaphoms. Extract the fused mass with water, filter, and acidify with dilute hydrochloric acid. Pass hydrogen sulphide through the hot solution arsenic is precipitated as yellow arsenic sulphide. If antimony is present, it will be precipitated as orange antimony trisulphide. 

Neuberg and Tiemann propose the following method, depending on the fact that most aldehydes form a compound with thiosemi-car-bazide. The oil containing aldehyde is heated in alcoholic solution on a water-bath, with thiosemi-carbazide. Various salts of the heavy metals will form insoluble precipitates with the thiosemi-carbazone formed, and such precipitate is dissolved in alcohol, and a current of hydrogen sulphide passed through until the metal is precipitated, leaving the thiosemi-carbazone dissolved in the alcohol. 

For reasonably satisfactory results, the sulphide solution must be dilute (concentration not greater than 0.04 per cent or 0.01M), and the sulphide solution added to excess of acidified 0.005M or 0.05M iodine and not conversely. Loss of hydrogen sulphide is thus avoided, and side reactions are almost entirely eliminated. (With solutions more concentrated than about 0.01M, the precipitated sulphur encloses a portion of the iodine, and this escapes the subsequent titration... 

Excellent results are obtained by the following method, which is of wider applicability. When excess of standard sodium arsenite solution is treated with hydrogen sulphide solution and then acidified with hydrochloric acid, arsenic(III) sulphide is precipitated ... 

As shown in Section 2.15, in a solution of 0.25M hydrochloric acid saturated with hydrogen sulphide (this is the solution employed for the precipitation of the sulphides of the Group II metals in qualitative analysis),... 

Mercury (II) ( 3> 100mg in 100 mL solution). Add a few millilitres of dilute hydrochloric acid and saturate the cold solution with washed hydrogen sulphide. Allow the black precipitate to settle, filter and wash with cold water (if the presence of sulphur is suspected, wash the precipitate with hot water, ethanol, or carbon disulphide). Dry at 105-110 °C and weigh as HgS (Section 11.33). 

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