The adsorption of various types of organic surface-active substances in neutral and in alkaline solutions is investigated by measuring the differential capacity at a dropping mercury electrode. It is pointed out that the change of adsorptional behaviour of these substances This communication reports the use of m-nitrobenzoic acid, which has been known as a thorium precipitant, as a reagent for scandium determination and as a means for its separation from thorium. This can be realized by regulating the acidity of test solutions, as thorium precipitates at pH 1.7, and scandium at pH 2.8-3.0 (see Table 1^*). By double precipitation of thorium, good separation can be achieved from solutions containing thorium. and scandium oxides in the weight ratio of 1.66:1 to 19.6:1. Absolute errors are -0.7 to +0.5 mg of ThO₂ and -0.2 to +0.3 mg of Sc₂O₃ for 50-200 mg of ThO₂ and 10- 15 mg of Sc₂O₃ mixtures (cf. Table 7 in the Chinese text). A detailed procedure for the separation and determinations of thorium and scandium has been worked out and described. The optimum conditions for using m-nitrobenzoic acid as precipitant for scandium have been studied. Excess precipitant should be one to four times of the theoretical value (see Table 2^*). For 1-51 mg of scandium oxide, satisfactory results have been obtained (see Table 3^*). The thermal decomposition curve of scandium m-nitrobenzoate (see Fig.1^*) has been recorded with a torsion-type thermobalance (Shimadzu Co., Japan). Between 75-450℃, the compound exists as an anhydrous salt, (m-NO₂·C₆H₄·COO)₃Sc. Then it decomposes with increasing temperature, and becomes scandium oxide at 885-1000℃. According to the analysis of the dried salt, its formula is deduced as (m-NO₂·C₆H₄·COO)₃Sc (Found: C 46.45%，H 2.64% and Sc₂O₃ 12.80%;theoretical: C 46.42%，H 2.23% and Sc₂O₃ 12.69%). After drying to constant weight at 120℃, the anhydrous salt may serve as the weighing form, with a gravimetric conversion factor of 0.12693[Sc₂O₃/(m-NO₂·C₆H₄·COO)₃· Sc] (see Table 4^*). The effect of ammonium ion is negligib1e, while alkali metals cause slightly positive errors. However, their effects can be remedied by double precipitation. Owing to the formation of scandium complexes with sulfate and acetate ions, they interfere appreciably (see Table 5^*). With double precipitation of scandium m-nitrobenzoate, the presence of up to two hundred times of light lanthanons or ten times of yttrium does not interfere with the scandium determination (see Table 6^*).