Determination of activity coefficients of aqueous solutions of urea in the ultracentrifuge

Apparent molecular weights of aqueous urea solutions have been determined by equilibrium sedimentation over the concentration range 1-9M. At the speeds used in the ultracentrifuge (39460 to 67770 r.p.m.) there was evidence of the effect of pressure on the sedimentation of urea. Apparent molecular weights of urea were therefore calculated at the solution-air meniscus where the solution is at l atm. The derived activity coefficients showed a similar type of oncentration dependence to published data for urea but there were significant quantitative differences. The significance of the results from the point of view of calculating theoretical distributions in the ultracentrifuge is discussed.