Precipitation Hydrogen Sulphide

Sulphides. Acquaint yourself with the solubility products of zinc and cadmium sulphides (see Appendix 1, Table 12). Prepare the sulphides of these metals from solutions of zinc and cadmium nitrates. What should be used as the precipitant—hydrogen sulphide or ammonium sulphide Note the colour of the precipitates and test their reaction with 1 N solutions of sulphuric and hydrochloric acids. 

In presence of hydrochloric acid, tin(II) in aqueous solution (1) is precipitated by hydrogen sulphide as brown SnS, and (2) will reduce mercury(II) chloride first to mercury(I) chloride (white precipitate) and then to metallic mercury. 

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

When hydrogen sulphide is bubbled into an acidic solution of an antimony or a bismuth salt an orange precipitate, SbjSs, or a brown precipitate, BijS, is obtained. Bismuth(III) sulphide, unlike antimony(IIl) sulphide, is insoluble in lithium hydroxide. 

Hence the orange colour of a dichromate is converted to the green colour of the hydrated chromium(III) ion, Cr ", and sulphur is precipitated when hydrogen sulphide is passed through an acid solution.)... 

Since most metallic sulphides are insoluble, many are precipitated when hydrogen sulphide is passed through solutions containing ions of the metals. Some are precipitated in acid, and others in alkaline... 

These are practically insoluble in water, are not hydrolysed and so may be prepared by addition of a sufficient concentration of sulphide ion to exceed the solubility product of the particular sulphide. Some sulphides, for example those of lead(II), copper(II) and silver(I), have low solubility products and are precipitated by the small concentration of sulphide ions produced by passing hydrogen sulphide through an acid solution of the metal salts others for example those of zincfll), iron(II), nickel(II) and cobalt(II) are only precipitated when sulphide ions are available in reasonable concentrations, as they are when hydrogen sulphide is passed into an alkaline solution. 

For a cobalt(ll) salt, the precipitation of the blue->pitik cobalt(II) hydroxide by alkali, or precipitation of black cobalt(II) sulphide by hydrogen sulphide provide useful tests the hydroxide is soluble in excess alkali and is oxidised by air to the brown CoO(OH) . 

Addition of an alkali metal hydroxide solution to an aqueous solution of a nickel(II) salt precipitates a finely-divided green powder. nickel(II) hydroxide NilOHfj on heating this gives the black oxide. NiO. which is also obtained by heating nickel(II) carbonate or the hydrated nitrate. Black nickel(II) sulphide, NiS, is obtained by passing hydrogen sulphide into a solution of a nickel(II) salt. 

CopperilT) sulphide, CuS, is obtained as a black precipitate when hydrogen sulphide is passed into a solution of a copper(II) salt. 

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 ... 

Cadmiumdl) sulphide, CdS, is a canary-yellow solid, precipitated by addition of hydrogen sulphide (or sulphide ion) to an acid solution... 

Complexes of cadmium include, besides those already mentioned, a tetracyanocadmiate [Cd(CN)4] which in neutral solution is sufficiently unstable to allow precipitation of cadmium(II) sulphide by hydrogen sulphide. Octahedral [CdCl ] ions are known in the solid state, as, for example, K4CdCl5. 

Mercuryill) sulphide, HgS, again appears in two forms, red (found naturally as cinnabar) and black, as precipitated by hydrogen sulphide from a solution containing Hg(Il) ions. 

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. 

This concentration has been chosen since it is that at which the sulphides of certain heavy metals are precipitated. The total concentration of hydrogen sulphide may be assumed to be approximately the same as in aqueous solution, i.e. 0.1 M the [H + ] will be equal to that of the completely dissociated HC1, i.e. 0.25M, but the [S2 ] will be reduced below 1 x 10 14 (see Example 6). 

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 ... 

A. Precipitation of sulphides. In order to understand fully the separations dependent upon the sulphide ion, we shall consider first the quantitative relationships involved in a saturated solution of hydrogen sulphide. The following equilibria are present ... 

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). 

Mendeleev predicted that scandium would be precipitated by hydrogen sulphide. This is not the case. 

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