Metal sulphides production

Sulphur dioxide, SO2, m.p. — 72-7°C, b.p. — I0"C. Colourless gas with characteristic smell. Formed by burning S, metal sulphides, H2S in air or acid on a sulphite or hydrogen sulphite. Powerful reducing agent, particularly in water. Dissolves in water to give a gas hydrate the solution behaves as an acid - see sulphurous acid. Used in the production of SO3 for sulphuric acid. 

With the aid of a table of solubility products of metallic sulphides, we can calculate whether certain sulphides will precipitate under any given conditions of acidity and also the concentration of the metallic ions remaining in solution. Precipitation of a metallic sulphide MS will occur when [M2 + ] x [S2 ] exceeds the solubility product, and the concentration of metallic ions remaining in the solution may be calculated from the equation ... 

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. 

Hydrogen sulphide occurs naturally, e.g. in natural gas and petroleum, volcanic gases, and from decaying organic matter. It may be present near oil wells and where petroleum is processed. Commercially it is obtained as a by-product from many chemical reactions including off-gas in the production of some synthetic polymers (e.g. rayon, nylon) from petroleum products, and by the action of dilute mineral acids on metal sulphides. Physical properties are summarized in Table 9.14 and effects of temperature on vapour pressure are shown in Figure 9.5. 

Similarly, doping Ag2S with a divalent metal sulphide enables the electrode to respond to the corresponding metal (e g. Pb, Cu or Cd). In all cases the electrode responds by virtue of the solubility product equilibria... 

A list of such solubility products for metal sulphides, as well as updated conventional ones, has been given by Licht [1], In this book, we will continue to use the more conventional solubihty products, partly because they are more common and partly because the relevant equihbria are less unwieldly to describe. 

This can certainly be extended to other metal sulphides, using other complexes of sulphur (and also selenium). However, the complex and anion of the metal salt need to be chosen so that all the by-products of the pyrolysis reaction are volatile, otherwise the film will be contaminated with the nonvolatile by-products. For example, using cadmium nitrate and thiourea, all the by-products are volatile ... 

It should be kept in mind that many of these decomposition reactions are equilibria. The decomposition of thiourea in the absence of a metal ion will normally be much slower than in the presence of such an ion. The metal ion removes sulphide as metal sulphide—the less soluble the sulphide, the more effective the removal at very low sulphide concentrations. This continuous removal of sulphide shifts the equilibrium to the direction of more sulphide production. The same principle holds for many other anion precursors. 

Considering that homogeneous precipitation of metal chalcogenides (mainly sulphides) by reaction between metal ions and dissolved chalcogen is well established, the main difference between this deposition and similar reactions seems to be that the products adhere to a substrate to give a visible fdm (in this case) rather than only precipitate. Whether this is connected with the redissolu-tion/redeposition process that occurs with the Sn-S system or has some other explanation is important. If the former, it may be limited to only those systems that behave similarly. Otherwise it is not unreasonable to expect that other metal sulphides and selenides (possibly also tellurides, although tellurium tends to be much less soluble, if at all, in such solvents) may be deposited as films in this manner. 

These processes have been described for rapid precipitation reactions. However, they should also be valid in general for slow precipitation—i.e., for CD— with possible differences due to the very different kinetics involved. Thus, if free sulphide is involved, since it is always present in very low concentration, the lower-solubility product metal sulphide is more likely to deposit first, compared to rapid precipitation. Solid-state diffusion processes have much more time to occur in CD (although they may occur in rapid precipitation after the precipitation itself), increasing the probability of solid solution formation. 

Preparation of Metal Sulphides by an Exchange Decomposition Reaction. Precipitation with Ammonium Sulphide. Pour 2 ml each of solutions of iron(ll), manganese(II), zinc, cadmium, lead, antimony, and copper salts into separate test tubes. Add 2 ml of an ammonium sulphide solution to each tube. Note the colour of the formed precipitates. Write the equations of the reactions and the values of the solubility products of the sulphides of these metals (see Appendix 1, Table 12). Using the concept of the solubility product, explain the process of precipitation of sulphides under these conditions. 

By submitting various metallic sulphides, e.g. those of bismuth, silver, cadmium or zinc, to the action of a solution of sulphur chloride in benzene or toluene, a greenish-blue precipitate of sulphur is obtainable, but the product invariably contains several units per cent, of mineral impurity.2 The suggestion that Ultramarine owes its colour to the presence of a blue variety of sulphur appeal s to have little probability, especially in view of the stability of this substance towards heat,4 and indeed the true nature of the blue- or green-coloured precipitates of sulphur, obtained by any of the afore-mentioned methods, requires much more experimental investigation before the existence of a blue or green modification of sulphur can be accepted. 

Various other processes can be made to yield sulphur monoehloride. The distillation of sulphur with stannous chloride or mercuric, chloride yields sulphur monochloride and, indeed, may be regarded as a modification of the method first given. The action of phosphorus pentacliloridc on sulphur or on metallic sulphides and the action of chlorine on metallic sulphides form closely analogous processes, especially in view of the formation of chlorine as a dissociation product from phosphorus penta-chloride. With phosphorus pentachloride tire phosphorus is found finally as sulphochloride. Of other methods there may be mentioned the interaction of sulphur or of phosphorus sulphide with thionyl chloride,6 and the action of dry chlorine on a hot or boiling solution of... 

All sulphates undergo reduction when heated with carbon, the product being the metal, metallic sulphide or metallic carbide, according to the salt in question and the conditions of the treatment.2 Magnesium sulphate, however, when heated with carbon at 750° C., yields the oxide and free sulphur, the primary reaction being 3... 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

The solubility of all the heavy metal sulphides is very small they are all insoluble in water, but some dissolve in hydrochloric acid. The solubility product of these is not quite so small because it is not reached when the ionization of H2S is driven back by the strong acid. 

Thiophosphites.—These salts may be regarded as derived from mono-, H3PS02, di-, H3PS20, and tri-thiophosphorous acids, H3PS3. They were prepared, with other products, by heating metals with a mixture of sulphur and phosphorus, e.g. AgsPS3, or metallic sulphides... 

Precipitation of sulphides Hydrogen sulphide gas is a frequently used reagent in qualitative inorganic analysis. When hydrogen sulphide gas is passed into a solution, metal sulphides are precipitated. For this precipitation the rule mentioned above can be applied precipitation may take place only if the product of concentrations of metal ions and sulphide ions (taken at proper powers) exceed the value of the solubility product. While the concentration of metal ions usually does fall into the range of 1-10 3 mol 1, the concentration of sulphide ion may vary considerably, and can easily be selected by the adjustment of the pH of the solution to a suitable value. 

Metal sulphides Sulphide produced in bacterial sulphate respiration can precipitate heavy metal ions from solution when the sulphide concentration is in excess of that demanded by the solubility product of the... 

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