A particular ISE the pH electrode

The terms and Erre are independent of the concentration Q of analyte i. They have no reason to vary during the measurement. Therefore the potential of the cell depends upon that of the membrane (Ej is minimized as explained above). 

The activity of the target ion being kept constant inside the electrode (flj.reference) and provided the membrane is only permeable to this single type of ion, the PD, 

In this Nernst-type equation, E is the standard potential of the measuring sequence used. It takes account of all other potentials. R is the ideal gas constant, F represents Faraday s constant (96485 C), T is the temperature, z is the charge carried by the target ion i to be measured indicating whether + or —) whose activity is O and 2.303 is the logarithmic conversion factor. 

The activity U of an ion i is linked to its concentration Q by the relationship flj = yjCj where is the activity coefficient which depends on the total ionic strength /, that is a measure of the quantity of charge of all ions present in the medium (/= O.SSCjZ ). For example, for an ion i, of charge z, expression 19.1 becomes  

For diluted solutions, the Debye-Hiickel law - log yj = —0.5z a// — indicates that for a given value of /, y is constant. This is why the same quantity of an inert electrolyte, called support electrolyte is added to the range of standard and sample solutions , in order to have a large excess of indifferent ions, which stabilize the ionic strength at a constant value. This ISAB (ionic strength adjustment buffer) or TISAB (for total ISAB), limits variations in yj. Under these conditions, the measured potential difference depends on the concentration of the ions to be analysed and is given by equation 19.3, which results from equation 19.2  

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