Page 91 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Electrochemistry 169

circumstances, none of these provides the reliability and sure of metal-ion activity. If one is concerned purely with
precision that is afforded by the glass electrode. In the ab- the actual activity of free metal ion, these measurements
sence of interfering substances, the quinhydrone electrode are meaningful. However, if the measurement is taken to
(an equimolar combination of quinone and hydroquinone represent the total metal-ion content of the solution, both
with a gold-foil electrode) provides a simple monitoring as a free solvated ion and as its various complexes, then
system for measurements up to pH 8. However, the pres- highly erroneous conclusions can be made. Thus, the abil-
ence of oxidizing or strongly reducing ions in the sample ity to monitor the concentration of ligands should be rec-
system will interfere, as is true for the hydrogen gas elec- ognized as a pitfall if one does not take account of this in
trode and most other systems that are an alternative to the the use of potential measurements to monitor metal-ion
glass electrode. concentrations.
Potentiometric redox measurements are often per- A recent and rapidly developing extension of poten-
formed in nonaqueous or mixed-solvent media. For such tiometry is in the area of membrane-type indicator elec-
solvents, various potentiometric sensors have been devel- trodes. These include (1) specialized glass electrodes that
oped, which, under rigorously controlled conditions, give respond to ions other than the hydronium ion, (2) ion-
a Nernstian response over a wide ranges of activities (par- exchange membranes, and (3) single-inorganic-crystal
ticularly in buffered solutions). There are some experi- membranes. Each year the selectivity and reliability are
mental limitations, e.g., solvent purification and handling improved for this important class of electrode. In partic-
or use of a reference electrode without salt bridges, but ular, the development of ion-exchange membranes that
there are important advantages. Solutes may be more sol- provide selective response for a number of anions has
uble in such media, and redox properties of the species made new areas of analysis amenable to potentiometric
may be altered in comparison with aqueous solutions. measurements. This has been particularly important for
Measurements of pH in nonaqueous solvents almost with- the biomedical field where nondestructive, highly specific
out exception use the glass electrode in combination poteniometric measurements are desirable. Furthermore,
with an appropriate reference electrode, frequently the the potentiometric method, because of its continuous na-
silver/silver-chloride electrode. In general, the response ture, is particularly attractive to those concerned with in
of the glass electrode follows the Nernst expression in vivo monitoring of biological substances.
nonaqueous solvents and is an accurate representation of Because potentiometry (through the Nernst equation)
the changes in activity of hydronium ion. Unfortunately, gives a response that is proportional to the logarithm of
few, if any, standard buffers are available to calibrate pH the activity of the electroactive ion, the accuracy and pre-
meters for nonaqueous measurements. Thus, nonaqueous cision are more limited than for many methods that give
pH measurements are only meaningful for monitoring the a direct proportional response. Thus, for a one-electron
course of an acid-base titration or relative to some refer- redox process, an order of magnitude change in activity
ence measurement made within the individual laboratory. givesapotentialchangeof59.1mV(atroomtemperature),
Little, if any, confidence can be attached to absolute pH a 10% change in activity gives a change of 2.5 mV, and a
measurements in nonaqueous systems. The application of l% change in activity gives only a change of 0.25 mV.
ion-selective electrodes in nonaqueous media has been Potentiometry has found extensive application over the
limited, but the response for several cations and anions past 50 years as a means to evaluate various thermody-
is usable, especially when 10–20% of water is added to namic parameters. Although this is not the major appli-
purenonaqueous solvent. Limited use ofliquid-membrane cation of the technique today, it still provides one of the
electrodes in such media arises from the solubility of most convenient and reliable approaches to the evalua-
electrode-system components in organic media. tion of thermodynamic quantities. In particular, the ac-
Second-class electrodes, that is those whose response tivity coefficients of electroactive species can be evalu-
is dependent on the change in concentration of an anion ated directly through the use of the Nernst equation (for
which gives an insoluble salt with the metal ion of the species that give a reversible electrochemical response).
indicator electrode, provide a general means for moni- Thus, if an electrochemical system is used without a junc-
toring the concentrations of anions. Some of those half tion potential and with a reference electrode that has a
reactions, which are well behaved electrochemically and well-established potential, then potentiometric measure-
provide means for the potentiometric monitoring of anion ment of the constituent species at a known concentration
species,aresummarizedinTableI.Thistablealsoincludes provides a direct measure of its activity. This provides a
a tabulation of redox reactions that are useful to moni- direct means to evaluate the activity coefficient (assuming
tor the concentration of ligands that can complex metal the standard potential is known accurately for the con-
ions. Consideration of these indicates one of the difficul- stituent half reaction). If the standard half-reaction poten-
ties with absolute potentiometric measurements as a mea- tial is not available, it must be evaluated under conditions