Thermometric Study of the Dissolution of Tin in Acid Solutions

Abstract

The behaviour of Sn in solutions of various concentrations of HCl, HNO₃, H₂SO₄ and H₃PO₄ has been studied by the thermometric method. Of these acids the metal dissolves only in HNO₃ and HCl, unless O₂ or other cathodic depolariser is present.

Starting from T_i, the temperature increases to reach a maximum value, T_m, t min after immqrsion. Both ∆T(=(T_m−T_i)) and the reaction number (R.N. =∆T/t) vary with the concentration. ∆T varies with molarity according to the relation, ∆T = k (M−M₀), where k and M₀ (the molarity at which there is no measurable rise in temperature) are constants. HNO₃ is more aggressive than HCl. The effect of varying additions of HCl, H₂SO₄ and H₃PO₄ on the R.N. of Sn in 4·0 M HNO₃ was examined. Addition of increasing concentrations of HCl produces an increase followed by a decrease in R.N., passing through a maximum at about 0·75 M HCl, whereas increasing additions of H₂SO₄ decrease the R.N. to a minimum value and then increase it again; H₃PO₄ additions decrease it.

The effect of HCl was explained in terms of peptis ation and reprecipitation of m-stannic acid as the HCl content in the mixture was increased. The effect of H₂SO₄ and H₃PO₄ additions was explained on the assumption of two counterbalanced effects, anion adsorption which retards dissolution and increasing acidity which promotes it. Theformer predominates in the case of H3P04 due to its relatively weak acidity.