Sulphide precipitation

It must be pointed out that the above calculation is approximate only, and may be regarded merely as an illustration of the principles involved in considering the precipitation of sulphides under various experimental conditions the solubility products of most metallic sulphides are not known with any great accuracy. It is by no means certain that the sulphide ion S 2 is the most important reactant in acidified solutions it may well be that in many cases the active precipitant is the hydrogensulphide ion HS , the concentration of which is considerable, and that intermediate products are formed. Also much co-precipitation and post-precipitation occur in sulphide precipitations unless the experimental conditions are rigorously controlled. 

B. Precipitation and separation of hydroxides at controlled hydrogen ion concentration or pH. The underlying theory is very similar to that just given for sulphides. Precipitation will depend largely upon the solubility product of the metallic hydroxide and the hydroxide ion concentration, or since pH + pOH = pKw (Section 2.16), upon the hydrogen ion concentration of the solution. 

Thiosulphate as Ag2S. Add a slight excess of 0.1 M silver nitrate solution to the cold, almost neutral, thiosulphate solution. Heat at 60 °C in a covered vessel and, after cooling, filter and wash the silver sulphide precipitate with ammonium nitrate solution, water and finally with ethanol. Dry at 110°C and weigh as Ag2S (Section 11.76). 

Flower shaped crystalline deposit on the surface of the solid non-crystalline mass of platinum sulphide was probably due to the precipitation of elemental sulphur, which deposited as a floral growth on the non-crystalline platinum sulphide precipitate. Ultrasonic irradiation seemed to have broken tender sulphur flakes and cleaned the surface. The free sulphur, however, did not deposit further. This was probably due to the formation of other compounds of sulphur such as H2S, S02, etc. which could have been removed from the solution due to the phenomenon of degassing. 

Treat the copper glycerate and cadmium chloride solutions with a sodium sulphide solution up to the complete precipitation of the copper and cadmium sulphides. Filter the sulphide precipitates using a glass filter No. 4 and dry them at 100 C. Weigh the dry substances and calculate their yield. 

Prepare a lead sulphide precipitate by reacting solutions of a lead salt and ammonium sulphide. Treat a small amount of the precipitate, washed with water by decantation, with a hydrogen peroxide solution. Why does the colour of the precipitate change Write the equations of the reactions. What properties does hydrogen peroxide exhibit in this and the preceding reactions ... 

Preparation of Crystalline Cadmium Sulphide. Dissolve 3 g of cadmium sulphate in 80 ml of water, add 20 ml of a 1 N sulphuric acid solution to the reaction vessel, and pass a stream of hydrogen sulphide through it up to complete precipitation of cadmium sulphide. Filter off the cadmium sulphide precipitate, wash it several times with water saturated with hydrogen sulphide, remove it from the filter, and dry it in a drying... 

Tin(n) and Tin(IV) Sulphides. Pass a stream of hydrogen sulphide through hydrogen chloride solutions of tin (I I) and tin(IV) chlorides in separate test tubes. What is the colour of the sulphide precipitates Write the equations of the reactions. 

The presence of alkali and alkaline earth chlorides may also hinder precipitation of the sulphide thus, from a 0001 molar solution of lead chloride in water at 20° C., precipitation is completely inhibited by hydrogen chloride alone if in a concentration of 1-riV, and by decreasing concentrations of the acid in the presence of increasing quantities of calcium, ammonium or potassium chloride. Cadmium sulphide, precipitated from hydrochloric acid solution, contains adsorbed chlorine,1 the amount depending on the conditions of the precipitation the precipitation is incomplete at 80° C. 

Sulphur, as sulphide ion, may be detected by predpitation as black lead sulphide with lead acetate solution and acetic add or by the purple colour produced on addition of disodium pentacyanonitrosoferrate(m). Halogens are detected as the characteristic silver halides by the addition of dilute nitric acid and silver nitrate solution. Cyanide and sulphide ions both interfere with this test for halide by forming silver cyanide and silver sulphide precipitates. If nitrogen or sulphur has been detected, therefore, the interfering ions must be removed by boiling the acidified fusion solution as detailed later, before the silver nitrate solution is added to detect the halogen. 

The solubility-product of copper sulphide, and of the other sulphides which are not soluble in dilute acids, is still less hence hydrogen sulphide precipitates them from... 

Ammonium sulphide precipitates the disulphide from solution it is soluble in excess of the yellow ammonium sulphide, yielding the thio salt, (NfI4)2PtS3. 

If similar calculations are carried out for a number of other metal sulphide precipitates it is easy to classify these metals into two distinct groups. Metal ions like Ag+, Pb2+, Hg3+, Bi3+, Cu2+, Cd2+, Sn2+, As3+ and Sb3+ form sulphides under virtually any circumstances e.g. they can be precipitated from strongly acid (pH = 0) solutions. Other metal ions, like Fe2+, Fe3+, Ni2+, Co2+, Mn2+, and Zn2+ cannot be precipitated from acid solutions, but they will form sulphides in neutral or even slightly acid (buffered) solutions. The difference is used in the analytical classification of these ions the first set of ions mentioned form the so-called first and second groups of cations, while the second set are members of the third group. The separation of these ions is based on the same phenomenon. 

From the colourless solution of tetracyanocuprate(I) [Cu(CN)4]3- ions however, hydrogen sulphide does not precipitate copper sulphide, indicating that the tetracyano complex is more stable than the tetrammine complex. On the other hand, cadmium(II) ions form both tetrammine [Cd(NH3)4]2+ and tetracyano [Cd(CN)4]2- complexes, but hydrogen sulphide gas can precipitate the yellow cadmium sulphide from both solutions, even though cadmium sulphide is more soluble than the copper(I) sulphide precipitate (cf. Table 1.12). This fact indicates that the tetracyanocadmiate(II) [Cd(CN)4]2 complex is less stable than the tetracyanocuprate(I) [Cu(CN)4]3 ... 

Example 27 Cadmium sulphide precipitate can be dissolved in hot nitric acid. Let us try to express the equation of this reaction. 

The sulphide precipitates formed will therefore also contain some free sulphur. The reaction equations are as follows ... 

This however decomposes on dilution (a) or on further addition of hydrogen sulphide (b) and black lead sulphide precipitate is formed ... 

Lead sulphide precipitate decomposes when concentrated nitric acid is added, and white, finely divided elementary sulphur is precipitated ... 

Cations of the second group are traditionally divided into two sub-groups the copper sub-group and the arsenic sub-group. The basis of this division is the solubility of the sulphide precipitates in ammonium polysulphide. While... 

Note that while tin is bivalent in the tin(II) sulphide precipitate, it is tetravalent in the thiostannate ion. 

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

If the test is made with the sulphide precipitated in acid solution, only mercury will interfere by converting the arsenic into magnesium ammonium arsenate and heating to redness, the pyroarsenate Mg2As207 remains and any mercury salts present are volatilized. This forms the basis of a delicate test for arsenic. 

In hydrochloric acid the black iron(II) sulphide precipitate dissolves and the white colour of sulphur becomes visible ... 

The hiunid iron(II) sulphide precipitate, when exposed to air, is slowly oxidized to brown iron(III) hydroxide ... 

The reaction is exothermal. Under certain conditions so much heat may be produced that the precipitate dries out, and the filter paper, with the finely distributed sulphur on it, catches fire. Sulphide precipitates therefore should never be disposed of into a waste bin, but should rather be washed away under running water only the filter paper should be thrown away. 

VI.14 SEPARATION AND IDENTIFICATION OF GROUP IIIB CATIONS ON THE SEMIMICRO SCALE The separation scheme outlined in Table VI. 16 commences with the sulphide precipitates obtained according to the prescriptions of the general separation table (Table VI. 11 in Section VI.9). It is a semimicro adaptation of the hydrochloric acid-hydrogen peroxide method, described in Table V.24 (Section V.15). 

Silver subsulphide, Ag4S.—Hydrogen sulphide precipitates from silver subfluoride a black, amorphous substance, possibly Ag4S,.but probably only a mixture of silver and its sulphide.6... 

Gold monosulphide, AuS.—Hydrogen sulphide or an alkali-metal sulphide precipitates the monosulphide from solutions of auric chloride,8 and it is also produced by the action of hydrogen sulphide on sodium aurothiosulphate. It is a black substance, insoluble in acids except aqua regia, but soluble in alkali-metal sulphides.8... 

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