chAPTeR | 24 Chemical Analysis: Electrochemical Techniques 393 Thus the fraction K of the total sodium activity will behave as though it were potassium. The smaller the value of K, the more selective that electrode is to potassium, that is, the better it is. To identify a particular selectivity coef- ficient, the data are best written in the form: K potassium + /sodium + = 2 . 6 # 10 ­ 3 This shows that the selectivity of potassium over sodium for the potassium electrode is about 385:1, that is, 1/(2.6 # 10 -3 ). it is important to note that selectivity coefficients are not con- stant but vary with the concentration of both primary and inter- ferent ions, and the coefficients are, therefore, often quoted for a particular ion concentration. They should be regarded as a guide to the effectiveness of an electrode in a particular measurement and not for use in precise calculations, particu- larly as quoted selectivity coefficients vary by a factor of 10 or more. For accurate work the analyst should determine the coefficient for himself for his own type of solution. direct potentiometric determination of ions by means of ion-selective electrodes has many applications. Exam- ples are determination of pH, sodium, and chloride in feed water, condensate, and boiler water in power stations; cya- nide, fluoride, sulfide, and chloride in effluents, rivers, and lakes; fluoride, calcium, and chloride in drinking water and