Mercury precipitation, sulphide

Appreciable errors may also be introduced by post-precipitation. This is the precipitation which occurs on the surface of the first precipitate after its formation. It occurs with sparingly soluble substances which form supersaturated solutions they usually have an ion in common with the primary precipitate. Thus in the precipitation of calcium as oxalate in the presence of magnesium, magnesium oxalate separates out gradually upon the calcium oxalate the longer the precipitate is allowed to stand in contact with the solution, the greater is the error due to this cause. A similar effect is observed in the precipitation of copper or mercury(II) sulphide in 0.3M hydrochloric acid in the presence of zinc ions zinc sulphide is slowly post-precipitated. 

Some precipitates are deposited slowly and the solution is in the state of supersaturation for a considerable time. Thus, when calcium oxalate is precipitated in the presence of larger amounts of magnesium ions, the precipitate is practically pure at first, but if it is allowed to remain in contact with the solution, magnesium oxalate forms slowly (and the presence of calcium oxalate precipitate tends to accelerate this process). Thus, the calcium oxalate precipitate becomes contaminated owing to post-precipitation of magnesium oxalate. Post-precipitation often occurs with sparingly soluble substances which tend to form supersaturated solutions, they usually have an ion in common with the primary precipitate. Another typical example is the precipitation of copper or mercury(II) sulphide in dilute acid solution, which become contaminated, if zinc ions are present, by post-precipitation of zinc sulphide. Zinc ions alone may not be precipitated with sulphide ions under identical circumstances. 

Hydrogen sulphide in neutral or dilute acid medium black precipitate, which is a mixture of mercury(II) sulphide and mercury metal... 

After removing the mercury metal by filtration, black mercury(II) sulphide can again be precipitated by acidification with dilute mineral acids ... 

Mercury(II) sulphide, which was originally present in the precipitate, reacts with disulphide ions yielding disulphomercurate(II) and trisulphide ions ... 

Group reaction precipitates of different colours mercury(II) sulphide HgS (black), lead(II) sulphide PbS (black), copper(II) sulphide CuS (black), cadmium sulphide CdS (yellow), bismuth(III) sulphide Bi2S3 (brown), arsenic(III) sulphide As2S3 (yellow), arsenic(V) sulphide (yellow), antimony(III) sulphide Sb2S3 (orange), antimony(V) sulphide (orange), tin(II) sulphide SnS (brown), and tin(IV) sulphide SnS2 (yellow). 

Hydrogen sulphide (gas or saturated aqueous solution) in the presence of dilute hydrochloric acid, initially a white precipitate of mercury(II) chlorosulphide (a), which decomposes when further amounts of hydrogen sulphide are added and finally a black precipitate of mercury(II) sulphide is formed (b). 

Mercury(II) sulphide is one of the least soluble precipitates known (Ks = 4x 10"54). 

Adding ammonium chloride to the solution, mercury(II) sulphide precipitates again. 

Hydrogen sulphide orange-red precipitate of antimony trisulphide, Sb2S3, from solutions which are not too acid. The precipitate is soluble in warm concentrated hydrochloric acid (distinction and method of separation from arsenic(III) sulphide and mercury(II) sulphide), in ammonium polysulphide (forming a thioantimonate), and in alkali hydroxide solutions (forming antimonite and thioantimonite). 

Hydrogen sulphide yellow precipitate of tin(IV) sulphide SnS2 from dilute acid solutions (0 3m). The precipitate is soluble in concentrated hydrochloric acid (distinction from arsenic(III) and mercury(II) sulphides), in solutions of alkali hydroxides, and also in ammonium sulphide and ammonium polysulphide. Yellow tin(IV) sulphide is precipitated upon acidification. 

The characteristic colours and solubilities of many metallic sulphides have already been discussed in connection with the reactions of the cations in Chapter III. The sulphides of iron, manganese, zinc, and the alkali metals are decomposed by dilute hydrochloric acid with the evolution of hydrogen sulphide those of lead, cadmium, nickel, cobalt, antimony, and tin(IV) require concentrated hydrochloric acid for decomposition others, such as mercury(II) sulphide, are insoluble in concentrated hydrochloric acid, but dissolve in aqua regia with the separation of sulphur. The presence of sulphide in insoluble sulphides may be detected by reduction with nascent hydrogen (derived from zinc or tin and hydrochloric acid) to the metal and hydrogen sulphide, the latter being identified with lead acetate paper (see reaction 1 below). An alternative method is to fuse the sulphide with anhydrous sodium carbonate, extract the mass with water, and to treat the filtered solution with freshly prepared sodium nitroprusside solution, when a purple colour will be obtained the sodium carbonate solution may also be treated with lead nitrate solution when black lead sulphide is precipitated. 

Mercury(II) cyanide is decomposed by hydrogen sulphide, when mercury(II) sulphide is precipitated (Ks = 4 x 10 53). If the precipitate is filtered off, cyanide ions can be tested for in the solution ... 

Tri-)8-naphthylarsine sulphide. —The corresponding bromide in alcoholic solution is treated with hydrogen sulphide. The sulphide crystallises in plates, M.pt. 162° C., soluble in benzene and carbon disulphide, less soluble in ctlier or alcohol. When its benzene. solution is heated under reflux with mercury, mercuric sulphide is precipitated and pure tri-)8-naphthylarsinc remains. 

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. 

The U.S.P. assay of this ointment is by shaking about 3 g, dissolved in 50 ml of ether, in a separator with a mixture of equal volumes of concentrated hydrochloric acid and water until the ammoniated mercury has dissolved, filtering the aqueous layer into a beaker, repeating the extraction and filtration with successive portions of water and precipitating the mercury as sulphide. 

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. 

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

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). 

Post-precipitation involves the deposition of a sparingly soluble impurity of similar properties to the precipitate on the surface of that precipitate after it has been formed. It is particularly a problem where similar materials are being separated on the basis of their different rates of precipitation, e.g. calcium and magnesium oxalates or zinc and mercury sulphides. Copreci-... 

Neuberg and Popowsky, as also Abderhalden and Kempe, have introduced a few alterations in the procedure, such as evaporation in vacuo, and Levene and Rouiller suggested in 1906 that the tryptophane, on account of its proneness to decompose on evaporation of its solution with consequent loss, be estimated colorimetrically the mercury sulphate precipitate is decomposed, and the solution, freed from hydrogen sulphide, is titrated with bromine water in presence of amyl alcohol. Both cystine and tyrosine react with bromine water the latter can, however, be removed, but for the former a correction has to be made. Up to the present no values concerning the amount of tryptophane in various proteins have appeared, and it will be of interest to see if the values so obtained are very much higher than those obtained by crystallisation of the tryptophane. 

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