Thio-salts

A large number of salts are known which are collectively designated as thio or sulfo salts. These may be classified in various ways, and several types will be described in the following exercises. 

A and B are usually those of the more electropositive elements of the earlier A subgroups or of certain B subgroup elements (e.g. In Bi In thio-salts A may be an alkali metal, Ag, Cu(i), NH4, or TI(i), and B a non-metal or metalloid (Si, As, Sb) or a transition metal in a high oxidation state (V, Mo ). In compounds of class (c) both metals are typically from the B subgroups (Cu, Ag, Hg, Sn, Pb, As, Sb, Bi) but include some transition elements such as Fe. 

We have noted one difference between complex oxides and sulphides, namely, the compounds of class (c) have no counterpart among oxy-compounds. A second difference is that sulphides other than those of the most electropositive elements show more resemblance to metals than do oxides. Metal-metal bonding occurs only rarely in simple oxides whereas it is more evident in many transition-metal sulphides. In many complex sulphides of class (c), as indeed in simple sulphides such as those of Cu, it is not possible to interpret the atomic arrangements and bond lengths in terms of normal valeilce states of the metals, suggesting a partial transition to metallic bonding, as is also indicated by the physical properties of many of these compounds. 

A considerable number of thio-salts containing alkali metals have been prepared in one of two ways  

(a) Arrangement of Fe atoms (small circles) in chains of FeSa tetrahedra in KFeS2. (b) Projection of the structure of KFeSj along the direction of the chains. 

Sulphides structurally related to zinc-blende or wurtzite 

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Schwefel-natrium, -natron, n. sodium sulfide, -nickel, m. nickel sulfide, -niederschlag, m. precipitate of sulfur, precipitated sulfur, -ofen, m. sulfur burner, -oxyd, n. (any) sulfur oxide, -phosphor, m. (any) phosphorus sulfide, -probe, /. sulfur sample mercury sulfide, -quelle, /. sulfur spring, -rducherung, /. sulfur fumigation, -rubin, m. ruby sulfur, realgar, -salz, n. sulfur salt, thio salt, sulfo salt sulfate. 

Sulfo-olsaure, /. sulfodleic acid, -persaure, /. peroxysulfiiric acid, persulfuric acid, -salz, n. thio salt, sulfo salt. 

Thio-salz, n. thio salt, -saure, /. thio acid. 

Mercury(ll) sulfide dissolves in concentrated solutions of alkali or alkaline-earth metal sulfides forming thiosalts, such as Na2[HgS2] XFI2O. Such thio-salts are stable in solution only when alkaline hydroxides are present in excess. These salts also are obtained as bright and deliquescent needles when HgS is heated with sulfur and alkaline hydroxides. 

Salts of Thionic Acids.—A third class of thio salts are those derived from the various thionic acids of which sodium thiosulfate and barium dithionate may be... 

With sulphur, arsenic forms three stable sulphides of composition As2S2, As2S3 and As2S5. The two latter possess acidic properties and with metallic sulphides form series of thio-salts, analogous to the arsenites and arsenates. Intermediate oxythio-salts are also known. 

Copper Thioarsenates.—Cupric Orthothioarsenate, Cu3(AsS4)2, is formed as a dark brown precipitate when sodium orthothioarsenate is added to a solution of a cupric salt.2 The reaction, however, is complex, sulphides of copper and arsenic also being formed.3 A similar precipitate is formed when ammonium hydrosulphide or hydrogen sulphide is added to a solution of arsenic acid and a copper salt,4 and the proportion of sulphide and thio-salt in the precipitate varies with the concentration of the reactants. Copper hydroxide reacts with alkali thioarsenates to form copper sulphide and alkali arsenate, but some copper orthothioarsenate is formed and remains in solution in excess of alkali thioarsenate.5... 

Although thermodynamically possible, the slower rate of substitution of the tungsten(VI) species results in the formation of only a small amount of the corresponding thiotungstate ion, provided that the conditioning period with sulfide ion is controlled. Molybdenum trisulfide is then quantitatively precipitated by the decomposition of the thio salt by the slow addition of sulfuric acid to a pH value of between 2 and 3 ... 

Thio-Salts of Tin. Perform Experiment 1 under Preparation 43. Stannous sulphide does not dissolve in Na2S solution. Addition of sulphur causes it to dissolve. Addition of HC1 to the solution produces a yellow precipitate and an evolution of hydrogen sulphide. 

Sulphur and oxygen are interchangeable in sulphides and oxides. Metal oxides (basic) and non-metal oxides (acidic) combine to form salts. Likewise metal sulphides may combine with sulphides of weakly metallic or non-metallic elements to form salts, the so-called thio-salts, or sulpho-salts. Thio-salts of a few of the elements, notably tin, are very well defined. Stannous sulphide does not form a thio-salt but addition of sulphur converts it to stannic sulphide which does form a soluble thio-salt with the sulphide of an alkali metal. 

What is a thio-salt Describe how a thio-salt of tin can be formed, and discuss its properties and its relation to the corresponding oxy-salt. 

Sulphides and Thio-Salts. Pass hydrogen sulphide into hot dilute solutions of arsenic, antimony, and bismuth trichlorides in separate test tubes. Note that yellow, orange, and black precipitates respectively are formed. Let the precipitates settle to the bottom of the tubes, pour off the liquid, and treat the solid with sodium polysulphide (Na2S solution in which sulphur is dissolved) in each case. The yellow and orange precipitates dissolve the black one does not. To the two solutions add 6AT HC1 in excess and note that yellow and orange precipitates respectively are again thrown out. 

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

Hydrogen sulphide orange-red precipitate of antimony pentasulphide, Sb2S5, in moderately acid solutions. The precipitate is soluble in ammonium sulphide solution (yielding a thioantimonate) in alkali hydroxide solutions, and is also dissolved by concentrated hydrochloric acid with the formation of antimony trichloride and the separation of sulphur. The thio-salt is decomposed by acids, the pentasulphide being precipitated. 

Hydrogen sulphide black (or dark brown) precipitate of the disulphide, PtS2 (possibly containing a little elementary platinum), slowly formed in the cold, but rapidly on warming. The precipitate is insoluble in concentrated acids, but dissolves in aqua regia and also in ammonium polysulphide solution it is reprecipitated from the last-named solution of thio salt by dilute acids. 

May contain HgS, PbS, Bi2S3, CuS, CdS, PdS together with Au, Pt, Mo (trace), Sn (trace) as sulphides. Group IIA present. May contain solutions of the thio-salts of As, Sb, and Sn together with Mo, Au, Pt, Se, and Te. Just acidify by adding concentrated HC1 drop by drop (test with litmus paper) and warm gently. A coloured precipitate indicates Group IIB present. 

Separation. Vanadates are not precipitated by H2S in acid solution reduction to the quadrivalent state occurs. With ammonium sulphide solution, the soluble thio-salt is formed from which brown V2S5 is precipitated by pouring into 3m H2S04. The precipitate may be dissolved in concentrated HC1, and reactions 4 and 11 applied. 

Various sulphides of the chromium elements exist, and these, particularly in the case of molybdenum and tungsten, combine with alkalies and alkali sulphides to produce thio-salts thus, thio-tungstates of the types R jW03S, R WOjSj, R WOSg, and R WS, have been prepared. 

Only two sulphides of tungsten, WSj and WS3, are known, but these dissolve in alkali solutions with formation of a series of soluble thio-salts (see p, 247). 

We saw in Chapter 12 that from the structural standpoint many transition metal-oxygen systems are surprisingly complex. This is also true of many metal-sulphur systems, as we shall show later for the sulphides of Cr, Ti, V, Nb, and Ta. Before doing this we shall note some of the simpler binary sulphide structures, taking them in the order M2S, MS, MS2, M2S3 and M3S4. The chapter concludes with a short account of thio-salts and complex sulphides. 

In (a) and (c) there would be no great difference between the characters of the A-S and B—S bonds in a particular compound, while in (b) the B and S atoms form a covalent complex which may be finite or infinite in one, two, or three dimensions. By analogy with oxides we should describe (a) and (c) as complex sulphides and (b) as thio-salts. Compounds of type (c) are not found in oxy-compounds, and moreover the criterion for isomorphous replacement is different from that applicable to complex oxides because of the more ionic character of the bonding in the latter. In ionic compounds the possibility of isomorphous replacement depends largely on ionic radius, and the chemical properties of a particular ion are of minor importance. So we find the following ions replacing one another in oxide structures Fe, Mg , Mn , Zn, in positions of octahedral coordination, while Na" " more often replaces Ca (which has approximately the same size) than K , to which it is more closely related chemically. In sulphides, on the other hand, the criterion is the formation of the same number of directed bonds, and we find atoms such as Cu, Fe, Mo, Sn, Ag, and Hg replacing Zn in zinc-blende and closely related structures. 

Crystal structures of some complex sulphides and thio-salts... 

Arsenic (As, at. wt. 74.92) occurs in its compounds in the oxidation states -III (arsine, AsHa), III (arsenite), and V (arsenate). Arsenic (IE and V) are amphoteric, but with much more acidic than basic character. The sulphides are characteristically capable of yielding soluble complexes (thio-salts). Arsenic(V) forms heteropoly acids. 

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