Effective diffusion coefficients for methanol in sulfuric acid solutions measured by Raman spectroscopy

The diffusion of methanol into 0-96.5 wt % sulfuric acid solutions was followed using Raman spectroscopy. Because methanol reacts to form protonated methanol (CH 3OH 2 (+)) and methyl hydrogen sulfate in H 2SO 4 solutions, the reported diffusion coefficients, D, are effective diffusion coefficients that include all of the methyl species diffusing into H 2SO 4. The method was first verified by measuring D for methanol into water. The value obtained here, D = (1.4 +/- 0.6) x 10 (-5) cm (2)/s, agrees well with values found in the literature. The values of D in 39.2-96.5 wt % H 2SO 4 range from (0.11-0.3) x 10 (-5) cm (2)/s, with the maximum value of D occurring for 61.6 wt % H 2SO 4. The effective diffusion coefficients do not vary systematically with the viscosity of the solutions, suggesting that the speciation of both methanol and sulfuric acid may be important in determining these transport coefficients.