On the Basis of a Concentration Cell Set-Up

In contrast to all other techniques, which use an ion-selective electrode and a reference electrode, for this technique a concentration cell must be built using two identical ion-selective electrodes or one ion-selective membrane and two identical reference electrodes. Fig. 45 shows a few possible concentration cell constructions. The principle of this method rests on the fact that the EMF of a concentration cell is zero if the measured ion activity is the same in both electrode compartments (Null point poten-tiometry). One can start with a measured ion activity of zero in the reference compartment (or more precisely, of an activity corresponding to the solubility of the active phase in this solution), and gradually increase the concentration of indicated ion through the precisely measured addition of a solution of the indicated ion of known concentration. This process is continued until a cell EMF of zero is obtained. At this point the known concentration of the reference compartment is identical to that in the adjacent sample solution. With this procedure no calibration of the electrodes is necessary. The slope of the Nernst response is also unimportant it only affects the observed sensitivity of the EMF change per indicated ion addition. This technique is recommended if only a small amount of sample material is available, or with samples which cannot be contaminated. As Fig. 45B shows, even microliter samples can be measured accurately with this technique. 

If one wants to carry out precise concentration determinations, a constant ionic strength must be established in both electrode compartments so that the same activity coefficients apply to both solutions. This also reduces the liquid junction potential at the contact point of the two solutions, further increasing the accuracy. If it is not possible to introduce any foreign substances into the sample solution (as in physiological measurements or with samples which are to be used further), then a capillary connection between the two solutions is used, and the final reference solution is given a composition similar to that of the sample matrix. In doubtful cases a non-destmctive, nonpolluting conductance measurement can be made on the sample solution and the same conductance is established in the reference solution. In favorable cases a number of 

There are a great number of variations on this technique, but it is beyond the scope of this book to list them all here. Only the inverse procedure will be mentioned here, which starts with a reference solution having a hi er concentration than the sample solution. Here the dilution necessary to achieve a zero EMF reading is measured, thus determining the sample concentration. 

The zero EMF point can be ascertained graphically to achieve a more precise determination. For this the cell EMF after each incremental addition is plotted on semi-logarithmic paper (EMF vs. log Qeference)- A linear titration curve is obtained with a slope of 59/z mV per power of ten at 25°C. The concentration of the sample can be read off of such a plot at the zero EMF point. This variation reduces the chances of error due to stray values (indicator fluctuations due to any of the interferences discus- 

This null point. technique also makes possible accurate analyses of very dilute solutions which cannot be obtained using direct titration procedures in which the measured ion activity of the sample solution itself is altered. Durst [225] was able to determine fluoride and silver ions in only 5-100 /il sample volumes with this technique. 

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