The invention relates to a sorbent for removing metabolic waste products from a dialysis liquid, the sorbent comprising a soluble source of sodium ions. The sorbent comprises an ion exchange system which converts urea to ammonium ions and which is configured to exchange ammonium ions for predominantly hydrogen ions and to exchange Ca, Mg, and K for predominantly sodium ions. The soluble source of sodium ions overcomes an initial drop in sodium concentration in regenerated dialysate. When used in conjunction with an infusion system configured to utilise exchange of Ca, Mg and K for sodium during dialysate regeneration a desired sodium ion concentration can be maintained.