Sulfide in qualitative analysis

Thioacetamide is used as a replacement for gaseous hydrogen sulfide in qualitative analysis. 

Mercury(n) ion is often precipitated as mercury(II) sulfide in qualitative analysis. Using the solubility product constant (Table 18.1), calculate the molar solubility of mercury(II) sulfide in water, assuming that no other reactions occur. 

The affinity of arsenic for sulfur is seen in the list of common minerals. Table 1. Readers may recall that arsenic is usually precipitated as the sulfide in qualitative analysis schemes. Many proteins contain -SH groups and these readily react with arsenic compounds in both oxidation states. As(V) is reduced to As(III), which then binds to the protein and, as a consequence, interferes with its normal function. 

When solutions of soluble ionic compounds are mixed, an insoluble compound will precipitate if the ion product (IP) for the insoluble compound exceeds its fCsp. The IP is defined in the same way as /equilibrium concentrations. Certain metal cations can be separated by selective precipitation of metal sulfides. Selective precipitation is important in qualitative analysis, a procedure for identifying the ions present in an unknown solution. 

Hydrogen sulfide is a strong reducing agent that will react with many substances to yield insoluble sulfides. It is frequently used as a step in qualitative analysis. 

Nowadays, hydrogen sulfide used in qualitative analysis (see Section 16.10) is prepared by the hydrolysis of thioacetamide ... 

The effect is especially pronounced with sulfides because S is the conjugate base of the very weak acid HS , and this case is of interest because of its importance in qualitative analysis. If any two of the three concentrations [H ], [S ], and (H S] are given, the third can be calculated from the equation (Example 19, page 284),... 

The preceding example illustrates an important step in qualitative analysis the separation of metals according to the solubility of their sulfides. 

Example 18-14 illustrates the use of in the type of calculation needed to sort the sulfides into two qualitative analysis groups. Pb is in qualitative analysis cation group 2, and Fe is in group 3. The conditions cited in the example are those generally used. 

Many of the most important naturally occurring minerals and ores of the metallic elements are sulfides (p. 648), and the recovery of metals from these ores is of major importance. Other metal sulfides, though they do not occur in nature, can be synthesized by a variety of preparative methods, and many have important physical or chemical properties which have led to their industrial production. Again, the solubility relations of metal sulfides in aqueous solution form the basis of the most widely used scheme of elementary qualitative analysis. These various more general considerations will be briefly discussed before the systematic structural chemistry of metal sulfides is summarized. 

FIGURE 11.20 Part of a simple qualitative analysis scheme used to separate certain cations. In the first step, three cations are separated as insoluble chlorides. In the second step, cations that form highly insoluble sulfides are removed by precipitation at a low pH and, in the third step, the remaining cations are precipitated as the sulfides at a higher pH. 

Before analysis, it is often useful to determine qualitatively the presence of sulfide in the sample and the concentration range at which it is present. The following tests may be performed ... 

The student should remember from qualitative analysis that white zinc sulfide dissolves readily in dilute HC1, that yellow cadmium sulfide may be dissolved in concentrated HC1, but that black mercuric sulfide (one of the least soluble salts known) requires the action of a trifunctional reagent such as aqua regia (Exercise 1). 

Structural data are available (Table 30) for a range of binary, ternary and quaternary sulfides of manganese, almost invariably Mn", and these set the scene for the structures to be expected in the compounds with the more discrete polyhedra.319 Indeed, the structural pattern is established in the simple binary compound MnS. Whereas, the stable modification of this (a-MnS) is green and has the cubic rock salt structure with [MnS6] octahedra, the well-known flesh-coloured precipitates of the qualitative analysis system are metastable / - and y-modifications, which have [MnS4] tetrahedra with respectively the zinc blende or diamond (cubic) and wurtzite (hexagonal) structures. And so, in the rest of the known solids, there are almost equal numbers of four-coordinate tetrahedra and six-coordinate octahedra with no other polyhedra having been detected. 

Cd + can be detected by the insolubility of its yeUow sulfide (see Analytical Chemistry of the Transition Elements). Several reaction and spot tests allow the identification of Cd +. Quantitative determinations are based on gravimetric (CdS or /3-naphthylquinoltne complex) or titrimetric (EDTA) methods. Several physical techniques can be used in quantitative and qualitative analysis polarography (or related techniques, even in the presence of Zn, Cu, Bi and Pb), electrodeposition, colorimetric methods, flamephotometric methods, neutron activation, atomic absorption, and ICP spectrometry and ion selective electrodes. 

The sulfides of the alkali and alkaline-earth metals are colorless substances easily soluble in water. The sulfides of most other metals are insoluble or only very slightly soluble in water, and their precipitation under varying conditions is an important part of the usual scheme of qualitative analysis for the metallic ions. Many metallic sulfides occur in nature important sulfide ores include FeS, CugS, CuS, ZnS, AggS, HgS, and PbS. 

An alkaline solution of sodium sulfide and sodium disulfide (or of the ammonium sulfides) is used in the usual systems of qualitative analysis as a means of separating the precipitated sulfides of certain metals and metalloids. This separation depends upon the ability of certain sulfides (HgS, AS2S3, AsoSg, Sb2S3, Sb2Sj5, SnS, SnSg) to form thioanions... 

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