Grans method Nernst equation

To appreciate that an ISE must be selective if its measurements are to be useful, and how the Nernst equation can be adapted to take account of the ISE selectivity by using the concept of a selectivity coefficient (or ratio ). To leam how the problems of selectivity can be overcome by using a standard-addition method, such as drawing a Gran plot - one of the more useful of such methods. 

In potentiometry with ISEs, however, the second Gran method [46] has found especially wide use, not only in titrations, but also in multiple addition methods in general. In these methods, the concentration of the test substance is plotted against the volume of the titrant or of the standard solution and thus the curve is linearized. The end-point in the titration or the determinand concentration in a multiple addition method is found as the intercept of the straight line with the volume axis. Linearization is attained by taking the antilogarithm of the Nernst equation ... 

The Gran plot (Fig. 13.2c2). This method consists of the mathematical transformation of the titration curve into straight lines via rearranged Nernst equations. Using a selective electrode that responds only to a... 

Because the equation describing the electrode potential in the presence of thiosulphate ions is more complicated than the simple Nernst equation the usual Gran method must be modified. The Gran function has the form ... 

The common procedure of finding the endpoint, which should correspond to the equivalence volume, is based on the Gran method. This procedure is especially useful for flat and vmsymmetrical titration curves. The titration data are transformed in such a way that the titration curve consists of two linear parts - before and after the equivalence point. The intersection of those lines or their intersection with the abscissa defines the equivalence volume of the titrant (Figure 3). The linearization is based on the transformation of the Nernst equation into the form... 

Gran s method is based on the idea that a linear titration curve is obtained if the concentration of the indicated ion is plotted rather than the corresponding electrode EMF. This is due to the simple relationship Titration with 10% of the equivalence volume = 10% less free ion in solution 50% equivalence volume titrated = 50% less 100% titrated = no free measured ion in solution. With titrations followed by conductance measurements a direct proportionality (not logarithmic, as with EMF measurements) is obtained between the measured ion concentration and the conductance, resulting in linear concentration curves which can be extrapolated. To get the concentration corresponding to a particular EMF one can either use a suitable calibration curve, or read the concentrations directly off of a logarithmically divided instrument scale. The empirical Nernst equation can also be mathematically manipulated into a suitable form ... 

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