Potentiometry by ISE—Units of Measure and Reporting for Clinical Applications

Direct Potentiometry by ISE—Units of Measure AND Reporting for Clinical Applications 

Classical analytical methods such as dame photometry for the measurement of electrolytes provide the total concentration (c) of a given ion in the sample, usually expressed in units of millimoles of ion per liter of sample (mmol/L). Molality (m) is a. measure of the moles of ion per mass of water (mmol/kg) in the sample. Using the sodium ion as an example, the relationship between concentration and molality is given by  

In addition to the difference between molality and concentration, measurement of ions by direct potentiometry provides yet another unit of measurement known as activity (a), the concentration of free, unbound ion in solution. Unlike methods sensitive to ion concentration, ISEs do not sense the presence of complexed or electrostatically hindered ions in the sample. The relationship between activity and concentration using, again, sodium ion as an example, is expressed as  

and = 0.31. Referring to equation 15, activity and concentration will differ greatly in samples of physiological ionic strength, especially for divalent ions. 

A typical set of solutions for multi-ISE calibration in an analyzer is shown in Table 4-2. Two points are used to calibrate each ISE. The difference in the cell potential generated 

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