The Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2. / With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.

by Julius Stieglitz

The concentration of hydrosulphide-ion is, therefore, inversely proportional to the concentration of hydrogen-ion. It is clear that the addition of a strong acid, readily yielding concentrations of hydrogen-ion very much greater than 0.95E−4 (the value of [H⁺] in a saturated aqueous solution of hydrogen sulphide) will, as the result of the greatly increased total hydrogen-ion concentration, reduce the concentration of hydrosulphide-ion to correspondingly low values. For instance, the presence of 0.1 molar hydrochloric acid will increase the concentration of hydrogen-ion close to a thousandfold and will reduce the concentration of hydrosulphide-ion to 0.9E−7.

For the secondary ionization (see p. [101]) of hydrogen sulphide, HS⁻ ⇄ H⁺ + S²⁻, we have

[H⁺] × [S²⁻] / [HS⁻] = K₂.

(III)

The value of this constant has recently been determined[397] and found to be 1.2E−15. Recalling the fact that the concentrations [H⁺] and [HS⁻] of hydrogen-ion and hydrosulphide-ion, [p201] respectively, resulting from the primary ionization, are each[398] 0.95E−4, we have for the concentration of the sulphide-ion, in aqueous solution saturated with hydrogen sulphide at atmospheric pressure and 25°, [S²⁻] = 1.2E−15.

Combining equations (II) and (III), we have, further:[399]

[H⁺]² × [S²⁻] = k × K₂ = k₂ = 1.1E−23,

(IV)

which shows, directly, the relation between the concentration of the sulphide-ion and that of the hydrogen-ion, the relation of primary importance in considering the precipitation of metal sulphides in acid solutions. The concentration of the sulphide-ion is, thus, inversely proportional to the square of the concentration of the hydrogen-ion. A thousandfold increase in the concentration of the latter, which is very nearly the effect produced by the presence of 0.1 molar hydrochloric acid ([H⁺] = 0.091), reduces the concentration of sulphide-ion in the saturated aqueous solution a millionfold: If we call [S²⁻]_Ac. the concentration of the sulphide-ion in the acid solution, [S²⁻]_Ac. = (1.1E−23) / (0.091)² = 1.3E−21, whereas, in the absence of acid, as found above, [S²⁻] = 1.2E−15.

On the other hand, the addition of alkali to hydrogen sulphide, by neutralizing and suppressing the hydrogen-ion, and by forming the salts MeSH and Me₂S, will very greatly increase the concentrations of the hydrosulphide-ion and of the sulphide-ion. Since the constant for the secondary ionization of hydrogen sulphide shows that HS⁻ is an exceedingly weak acid, its salts, Me₂S, are very largely hydrolyzed, the constant for water being somewhat greater[400] than its own. According to Knox, in a 0.1 molar solution of Na₂S about 99% of the sulphide is hydrolyzed: Na₂S + H₂O ⥂ NaSH + NaOH. In spite of this almost complete hydrolysis, sufficient sodium sulphide remains in a solution of this [p202] substance, to yield a concentration of the sulphide-ion that is far greater than that obtained from a solution of hydrogen sulphide. In a 0.1 molar solution of sodium sulphide the concentration of the sulphide-ion is, approximately, [S²⁻]_alk. = 0.9E−3, as compared with [S²⁻] = 1.2E−15 in a saturated solution of hydrogen sulphide (25°, 760 mm.), and with 1.3E−21 in the same solution in the presence of 0.1 molar hydrochloric acid.