Classification of acids and acidities in I_h Ice II: Reversible photoacids as a probe for proton concentration

The proton concentration in acidic ice samples was determined by measuring the time-resolved fluorescence of two photoacids. We found that for a certain concentration of acid the overall proton reaction rate with the deprotonated form of the photoacid (RO^-*) to reversibly reform the excited-state photoacid ROH^*RO^-*+H⁺ a ROH^* depends on the kind of acid used. In strong acid-doped ice samples the rate was larger than in weak acid samples. The experimental results indicate that the acidity scale of commonly used acids in aqueous solutions is also applicable for Ih ice. The weak acids (pK_a > 2.5) are weaker in ice, and their pKa increases by bout 2 pKa units. Among the group of weak acids studied, HF (pK_a ) 3.2 in water) is a relatively stronger acid in Ih ice and its pK_a value is about the same as in water. This finding is in accord with previous electrical conductance measurements in ice.