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A comparison of the free hydrogen ion concentration and the total hydrogen ion concentration of rain samples shows that rain is a weak acid. The weak acid nature of rain casts doubt on the concepts that the acidity of rain is increasing and that these increases are due to strong acids such as sulfuric acid.

Ionization constants of the following carboxylic acids have been determined by a spectrophotometric method: MenPh3 -"MC02H, where M = C, Si and Ge, = 0-3, in ethanol-water media and where M = C and Ge, in dimethyl sulfoxide; (XCeH4)3GeC02H, where X = H, p-Me, m-Me, p-OMe, m-OMe, p-F, m-F, and p-CFs, in ethanol-water media and X = H, p-Me, m-Me, p-OMe, in dimethyl sulfoxide. The relative order of acidity, R3Si-C02H » RsGeCOaH > R3CC02H, is explained in terms of ( -d) bonding between the orbitals of the C02_ group and the vacant nd orbitals of the metalloid with steric hindrance to anion solvation playing a smaller role. The transmission of substituent effects through the phenyl group and germanium also are discussed.