Mass transport of phosphoric acid in water:: A 1H and 31P pulsed gradient spin-echo nuclear magnetic resonance study

Aqueous solutions of phosphoric acid (H3PO4) of varying concentrations have ionic conductivities as high as 0.25 Scm(-1) at ambient temperatures which cannot be accounted for on the basis of regular hydrodynamic movement of mobile ions. We report careful measurements of self-diffusion coefficients (D) of mobile species for the 85 wt% (14.6 M) phosphoric acid solution over a range of temperature from 293 to 353 K, using H-1 (I=1/2) and P-31 (I=1/2) pulsed gradient Hahn spin-echo (PGSE) techniques. The experimental D values are interpreted together with previously published viscosity (eta) and conductivity (sigma) data. The data show that protons diffuse faster than the phosphorus carrying species. The diffusion data for both nuclear species are found to be linear on an Arrhenius plot with activation energies of 25 and 36 kJ mol(-1) for H-1 and P-31 species, respectively. Analysis on the basis of the Nernst-Einstein relation yields a proton transference number of t approximate to 0.99 and a proton charge carrier number density of n approximate to 1.6x10(28) m(-3). A plot of the product D eta as a function of temperature suggests that the proton and phosphorus species undergo significantly different mass transport mechanisms. In particular the P-31 data show a nonlinear increase in D eta with temperature, while the H-1 data exhibit a decrease in D eta with increasing temperature which is not expected for hydrodynamically simple fluids. The latter behavior is attributed to the water mediated transfer of protons between the phosphate groups. Room temperature P-31 diffusion coefficients of condensed phosphates are also reported, and as expected the diffusion coefficient is dependent on the size of the moving species. (C) 2000 American Institute of Physics. [S0021-9606(00)51319-1].