This thesis reports on 4 projects undertaken as part of the experimental work for the degree. Participation in a study to determine the feasibility of a new calcium cell, the Covington reference cell or CRM, as the basis for a reference method for ionised calcium measurement in blood products, was carried out. This involved the testing of "blind" samples, both aqueous and protein containing, to investigate the precision and accuracy of the method. In our laboratory, the cell was found to have comparable precision with commercial analysers used in clinical laboratories and results tallied well with the assigned values of protein solutions. This cell was used to investigate the effect of increasing protein on measured ionised calcium and showed that there was an apparent rise in measured ionised calcium with increasing protein levels. Use of an isotonic salt bridge in the reference electrode cell resulted in reduced measurements of ionised calcium compared that using the normal hypertonic salt bridge junction. A novel KCI-doped resin, RepHex, was evaluated as a reference electrode material. When used as the reference electrode with a pH electrode, it was found to give comparable pH measurements in unstirred solution but much improved stability and precision in stirred solutions compared to a conventional frit-restricted calomel electrode. The leakage from this junction was found to be much less than that of the conventional electrode, particularly when surface areas were normalised. Preliminary studies on the ionophoric capabilities of five novel calixarene compound for potassium ions were carried out. Compounds la and lb were found to have a linear range of 10'3 to 10_1M K+ and Compounds II,III and IV had a linear range of 10'4 to 10_1 M K+. Limited lifetime studies were carried out and compounds lb and II were still functioning after 10 days. The selectivities for these compounds were such that it is unlikely that they could be incorporated into electrodes for general use.