Sodium interference with glass electrode

Possible applications of enzyme electrodes include the determination of urea in blood (Equation (20)) and for which the optimum pH is about 7 at which 99.7 per cent of the ammonia product exists as NH4 cation, the only form among the species generated that elicits a potential response from the Beckman cation-selective glass electrode. Any sodium or potassium ion interference introduced with the enzyme (or substrate) can easily be detected because of the large positive potential reading that it causes. In such instances Dowex 50 cation exchanger may be used to remove the interferences beforehand [372]. Average differences between the urea values in blood and urine as determined by the urea-urease electrode and a spectrophotometric technique were 2.8 and 2.3 per cent m/m, respectively [373]. 

This chapter is concerned with electrodes. Section 3.1 provides information about the structure and characteristics of glass electrodes along with possible sources of error, such as sodium ion interference. This should assist in optimum selection and use of electrodes. In Section 3.2, the structure and functioning of reference electrodes are discussed. The liquid junction between the reference electrode electrolyte filling solution and the sample can introduce a junction potential, which is a major source of error in pH measurement. This subject is discussed in detail. The combination electrode is compared with an electrode pair and its advantages in small-volume samples or flat-surface measurements are discussed in Section 3.3. 

The first ion-selective electrode was the hydrogen-responsive glass electrode (q.v.). No electrode is entirely specific towards a particular ion the presence of other ions can seriously interfere with electrode performance. Thus, electrodes may show a mixed response the normal glass electrode at pH > 9 shows errors in solutions containing sodium ions, attributed to sodium ion response. Variation of the composition of the glass permits this sodium ion response to be extended to lower pH values, eventually showing interference to the hydrogen ion at low pH values. For such electrodes the empirical equation is... 

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