Measurements in Extracellular Fluids

In these bio-related fields, it is primarily the pH, pNa, pK, pCa, pCl or pF electrodes which are employed. Depending upon whether extra- or intracellular ion activities are to be determined, different electrode constructions are utilized. Measurements in extracellular fluids (whole blood, serum, etc.) are usually carried out with the conventional macroelectrodes. Anaerobic conditions are harder to maintain with this method.is expedient to thermostat the entire sample beaker and electrode pair set-up. For more accurate pH and pCa determinations the air space above the sample must also be equilibrated with an O2-CO2 mixture, to establish the same partial pressure of CO2 as in the original sample environment. This expense can be spared by working with microprobes under anaerobic conditions with small surface areas (low CO2 loss). In such cases the commercially available flow-thru cells or capillary cells are used. For ion activity measurements within the cell special microelectrodes with tip diameters about one micron are employed, so that the cell membrane is not damaged too much. To date, such electrodes are available only for pH and pNa measurements (Transidyne, U.S.A.). 

Here again two cases can be distinguished. With in vitro measurements there is no great difference in practical techniques as compared to other samples (see Chap. 5), aside from the complexity of the sample matrix. In contrast to other fields of application, however, in biological systems the danger always exists that the sampling process may influence the physiological equilibrium. The consequence of such a disturbance is that no matter how accurately an analysis is carried out, the actual variable of interest is not measured. This problem is reduced with in situ or in vivo measurements however, here problems of sterilization and electrode calibration come up. 

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