Principles of potentiometric titrations

of the cell containing the initial solution is determined, and relatively large increments (15 mL) of the titrant solution are added until the equivalence point is approached the e.m.f. is determined after each addition. The approach of the e.p. is indicated by a somewhat more rapid change of the e.m.f. In the vicinity of the equivalence point, equal increments (e.g. 0.1 or 0.05 mL) should be added the equal additions in the region of the e.p. are particularly important when the equivalence point is to be determined by the analytical method described below. Sufficient time should be allowed after each addition for the 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

A number of commercial titrators are available in which the electrical measuring unit is coupled to a chart recorder to produce directly a titration curve, and by linking the delivery of titrant from the burette to the movement of the recorder chart, an auto-titrator is produced. It is possible to stop the delivery of the titrant when the indicator electrode attains the potential corresponding to the equivalence point of the particular titration this is a feature of some importance when a number of repetitive titrations have to be performed. Many such instruments are controlled by a microprocessor so that the whole titration procedure is, to a large extent, automated. In addition to the normal titration curve, such instruments will also plot the first-derivative curve (AE/AV), the second-derivative curve (A2 E/AV2), and will provide a Gran s plot (Section 15.18). 

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This reaction illustrates many principles of potentiometric titrations Mn04 + 5Fe2+ + 8H-" - Mn2+ + 5Fe3+ + 4H20 (A)... 

The titration of with KMn04 nicely illustrates principles of potentiometric titrations. 

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