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 Ti, the temperature increases to reach a maximum value, Tm, t min after immqrsion. Both ∆T(=(Tm−Ti)) 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.
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