Mass transport of phosphoric acid in water: A H-1 and P-31 pulsed gradientspin-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 m obile 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 valu es are interpreted together with previously published viscosity (eta) and c onductivity (sigma) data. The data show that protons diffuse faster than th e 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 b asis 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 appr oximate to 1.6x10(28) m(-3). A plot of the product D eta as a function of t emperature suggests that the proton and phosphorus species undergo signific antly 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 wa ter mediated transfer of protons between the phosphate groups. Room tempera ture 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)5131 9-1].