Hydrogenion Activity pH

The effective concentration of hydrogen ion in solution is expressed in terms of pH, which is the negative logarithm of the hydrogen-ion activity:

The relationship between activity and concentration is a = yc (2)

where the activity coefficient y is a function of the ionic strength of the solution and approaches unity as the ionic pH DETERMINATION

Two methods are used to measure pH: electrometric and chemical indicator (1-6). The most common is electro-metric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instrument measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal-metal oxides (eg, antimony-antimony oxide) have also found use as pH sensors, especially for industrial applications where their superior mechanical stability is needed. However, because of the presence of the metallic element, these electrodes suffer from interferences by reduction-oxidation (redox) systems in the test solution. Nonglass pH electrodes have also been described in which synthetic organic ionophores, selective for hydrogen ions, entrapped in plasticized polymeric membranes have shown excellent pH-response behavior. More recently, the pH ion-sensitive field-effect transistor (ISFET) has shown utility for pH measurements, especially where solid-state ruggedness is desired (7). These devices are based on replacing the metal gate of an ordinary field-effect transistor by a pH-sensitive layer such as silicon nitride. These sensors exhibit ideal Nernstian response over pH ranges comparable to conventional glass pH electrodes.

The second method, which has more limited applications, is the indicator method. The success of this procedure depends on matching the color that is produced by the addition of a suitable pH-sensitive indicator dye to a portion of the unknown solution with the color produced by adding the same quantity of the same dye to a series of standard solutions of known pH. Alternatively, the color is matched against a color comparison chart for the particular dye. Because of the limited color resolution, the results obtained by the indicator method are less accurate relative to those obtained using a pH meter and also suffer from errors when used in highly colored solutions or those containing reactive substances such as bleaches. The indicator method, however, is simple to apply and inexpensive. In addition to being used by direct addition to the test solution, indicator dyes can be immobilized onto paper strips (eg, litmus paper) or, more recently, onto the distal end of fiber-optic probes that, when combined with spectropho-tometric readout, provide more quantitative indicator-dye pH determinations.

Reference Buffer Solutions

The uncertainties introduced by the reference electrode liquid junction that exist in conventional electrochemical cells can be avoided by using a cell without transference, for example,

Potassium chloride, KC1, of molality m is added to each reference solution. If the standard potential, E°, of the cell and the molality of the chloride ion, ma are known, emf measurements yield values of the acidity function p(aHycl), as shown by the following equation:

P^H}'ci) = - log(mH>'H>'ci) = 2 303RT + logmci (5)

To eliminate the effect of the added KC1 on the acidity of the buffer solution, p(aHycl) is determined for three or more portions of the buffer solution that contain different amounts of added chloride. The limiting value of the acidity function is obtained by extrapolation to zero molality of chloride ion. If the single-ion activity coefficient of chloride ion in the buffer solution could be obtained, the activity of hydrogen ion would be readily accessible.

To establish a conventional scale of hydrogen-ion activity, it has been suggested (8) that the activity coefficient of chloride ion in selected reference buffer solutions having ionic strengths of S0.1 be defined as

where A is a constant of the Debye-Hiickel theory and I is the ionic strength. The paH for these selected reference solutions is identified with pH(S) in the operational definition:

The pH(S) values at 25°C of the primary and secondary reference buffer solutions certified by the U.S. National Institute of Standards and Technology (NIST) are listed in Table 1 (2). Of particular note, the pH 6.86 and 7.41 phosphate buffers have long been accepted as the primary reference standards for blood pH measurements, with the knowledge that the ionic strength of these buffers is significantly different from that of blood, thus biasing (however reproducibly) the pH measurement in blood due to the residual liquid-junction potential. Recently, NIST certified two concentrations each of two zwitterionic buffer systems (HEPES/HEPESate and MOPSO/MOPSOate) as pH buffers. These secondary standards have been certified at ionic strengths comparable to that of blood and conse-

Table 1. pH Standards, Molality Scale
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