The Interaction between Silica and Electrolytes in its Relation to Theories of Soil Acidity

Abstract

IN a note to NATURE (Dec. 2, 1922) some experiments were described in support of a theoretical explanation advanced in an earlier paper (Phil. Mag., vi. 44. 338–45) regarding the nature of soil acidity. Joseph and Hancock (T. 1923, 123, 2022), however, state that “pure silica produces no effect on a solution of an acid,” and that the adsorption we reported “would not be observed if the silica were more highly purified.” We have since repeated our experiments with hydrated silica (obtained in three different ways), which was purified with the greatest care. We have been able to confirm our previous observations that hydrated silica adsorbs acids and that electro-osmotic experiments show that anions are preferentially adsorbed, but we have also found that the samples of silica we used before contained alkali, and gave a much higher value for the amount of acid adsorbed. The adsorption of oxalic acid can be very easily demonstrated in view of the simplicity with which it can be volumetrically estimated. 10 gm. of airdried hydrated silica obtained from the hydrolysis of pure silicon tetrachloride can retain, even after repeated washings, oxalic acid equivalent to 10 c.c. of N/100 permanganate solution.