Effects of thiolation on proton dissociation of inorganic and methylated arsenic(V) species

Environmental context Thiolated arsenic(V) compounds widely occur in natural environments. Their physicochemical properties, including pH-dependent charging, control their behaviour in terrestrial and aquatic environments. We show that the first thiolation of inorganic and methylated arsenic(V) compounds forming an arsinothioyl (As═S) moderately increases pKa values, whereas a second thiolation forming a thiol (–SH) strongly decreases pKa. In contrast to thiolation, consecutive methylations of arsenic(V) steadily increase pKa values. Abstract Thiolated arsenic compounds have been detected in an increasing number of environmental compartments. Experimental evidence on the effect of thiolation on the pH-dependent speciation (i.e. charging) of thiolated arsenic compounds is, however, very limited, or for some toxicologically highly relevant compounds, absent to date. Based on acid–base titration data recorded in 0.01 and 0.1 M of sodium chloride (NaCl) background electrolytes, we derive proton dissociation constants (pKa values) for some major environmentally relevant arsenic compounds, including dimethylmonothioarsinic acid (DMMTAV: pKa = 6.44). We further show that the calculated pKa estimates used so far to assess and predict the behaviour of thiolated arsenic compounds considerably deviate from experimental evidence.