solution is undesirable, or for non-aqueous titrations where a calomel or silver–silver chloride reference
electrode may become blocked by precipitation of the salt at the liquid junction.
Null-point Potentiometry
This is a method involving a two-compartment cell with a salt bridge connection and having two
identical indicator electrodes. The sample solution is placed in one compartment and a blank solution
having the same total ionic strength in the other. Increments of a standard solution of the species to be
determined are added to the blank compartment until the cell potential is zero. At this point, the
activities of the species of interest in each compartment are equal and that of the sample solution can
therefore be calculated. A concentrated standard solution should be used to minimize dilution errors.
This method is particularly useful for the determination of trace amounts or where no suitable titrant
can be found.
Applications of Potentiometry
The measurement of pH using a glass electrode is a necessary part of a large number of analytical
procedures. In addition, the measurement and control of acidity is important in process streams and in
such areas as soil science, water treatment technology and clinical diagnosis. A specific example of the
latter is the measurement of CO 2 in blood and respiratory gases during operations using a gas-sensing
electrode. In general, ion-selective electrodes are finding increasing and widespread use for monitoring
purposes in laboratories, in plant and for on-site analyses such as those related to oceanographic and
river studies. Because of their activity rather than concentration dependence they are also valuable in
fundamental studies of solubility and complexation. One of the most successful and widely used is the
fluoride electrode which is now the basis of routine methods for the determination of fluoride in
drinking water, toothpastes, soils, plant tissue and biological fluids.
Potentiometric titrations are readily automated by using a motor-driven syringe or an automatic burette
coupled to a chart recorder or digital printout system. This is described in more detail in Chapter 12. A
micro-processor-controlled titrator is discussed in Chapter 13.
6.2—
Polarography, Stripping Voltammetry and Amperometric Techniques
Summary
Principles
Measurement of the diffusion-controlled current flowing in an electrolysis cell in which one electrode is
polarizable. The current is directly proportional to the concentration of an electroactive species.