Duration of study: 32 weeks, in two phases. Randomised to 16 weeks of hemodialysis with acetate followed by 16 weeks of citrate or 16 weeks of hemodialysis with citrate followed by 16 weeks of acetate. Each patient will serve as control of itself and there will be no changes in the pattern of dialysis during the study with the exception of the dialysate, following the usual pattern of work