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The reactor-separator membrane was prepared by molding a urease gel layer directly onto a commercial anion exchange membrane. When this two-layer membrane was used as a barrier between donor urea and acceptor buffer solutions, product ammonium ion was enriched in the acceptor solution up to a hundredfold over donor ammonium ion levels during a thirty-minute reaction period. The rate of product ammonium ion buildup was linear with time and was found to be proportional to initial donor substrate concentration over several orders of magnitude. A quantitative theoretical model of the transport behavior of this membrane was derived. The model predicted that under conditions of high enzyme concentration the overall reaction would be substrate diffusion-limited by the anion exchange layer. Experimental support for the model was obtained by comparing measurements of the urea diffusion coefficient in the reactor-separator membrane with the value obtained independently in the anion exchange membrane itself.

The amount of sodium ion binding in human sera and in dialyzed human sera was estimated from standard-addition titrations with an ion-selective electrode and from measurements of 23Na nuclear magnetic resonance (NMR) linewidth. For the untreated sera, maximum binding was 1% (1.4 mmol/L) as indicated by NMR; virtually no binding was found via the titration method. For dialyzed sera with low-sodium, normal-protein content, NMR indicated that sodium binding was less than 1.3% (0.14 mmol/L). The same dialyzed fluid analyzed with ion-selective electrodes shows no sodium binding, within the limits of experimental error (+/- 4%). Sodium ion binding to serum protein thus contributes only minimally to differences in sodium measurements observed between the direct (undiluted) ion-selective electrode and flame-photometric methods.