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preface to fifteenth edition

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ELECTROLYTES, EMF, AND CHEMICAL EQUILIBRIUM 8.115<br />

8.4.1 Electrometric Measurement of pH<br />

The pH value is defined for an aqueous solution in an operational (arbitrary but reproducible) manner<br />

according <strong>to</strong> the Bates-Guggenheim convention:<br />

pHx<br />

pHs<br />

<br />

Ex<br />

Es<br />

2.3026RT/F<br />

where R is the gas constant per mole, T is the temperature on the absolute scale, and F is the faraday.<br />

The pH x ofthe unknown medium is calculated from that ofan accepted standard (pH s ) and the<br />

measured difference in the emf (E) ofthe electrode combination when the standard solution is<br />

removed from the cell and replaced by the unknown. The double vertical line marks a liquid junction.<br />

Electrodes as fabricated exhibit variations in the reproducibility of the reference electrode, in the<br />

liquid-junction potential, and, with glass electrodes, in the asymmetry potential. These differences<br />

are all eliminated in the standardizing procedure with standard reference pH buffers. (See R. G.<br />

Bates, Determination of pH, Theory and Practice, Wiley, New York, 1964.)<br />

<br />

Electrode reversible Standard reference Salt bridge Reference<br />

<strong>to</strong> hydrogen ions buffer or unknown (KCl. 3.5M electrode<br />

solution or saturated)<br />

An electrometric pH-measurement system consists of(1) pH-responsive electrode, (2) reference<br />

electrode, and (3) potential-measuring device—some form of high-impedance electronic voltmeter<br />

for glass-electrode combinations and this or a potentiometer arrangement for other pH-responsive<br />

electrodes. Electronic pH meters are simply voltmeters with scale divisions in pH units which are<br />

equivalent <strong>to</strong> the values of2.3026RT/F (in mV) per pH unit. Values ofthis function at several<br />

temperatures are given in Table 8.22. There is no compensation incorporated in the meter for the<br />

changes in pH of the test solution as a function of temperature. Reliability of an indica<strong>to</strong>r–reference<br />

electrode combination must be ascertained by standardization ofthe pH meter with one standard<br />

buffer and checking the pH response by immersing the combination in a second and different reference<br />

buffer.<br />

The temperature compensa<strong>to</strong>r on a pH meter varies the instrument definition ofa pH unit from<br />

54.20 mV at 0C <strong>to</strong> perhaps 66.10 mV at 60C. This permits one <strong>to</strong> measure the pH ofthe sample<br />

(and reference buffer standard) at its actual temperature and thus avoid error due <strong>to</strong> dissociation<br />

equilibria and <strong>to</strong> junction potentials which have significant temperature coefficients.<br />

TABLE 8.22<br />

In millivolts.<br />

Values of2.3026RT/F at Several Temperatures<br />

t C Value t C Value t C Value t C Value<br />

0 54.197 25 59.157 50 64.118 80 70.070<br />

5 55.189 30 60.149 55 65.110 85 71.062<br />

10 56.181 35 61.141 60 66.102 90 72.054<br />

15 57.173 38 61.737 65 67.094 95 73.046<br />

18 57.767 40 62.133 70 68.086 100 74.038<br />

20 58.165 45 63.126 75 69.078<br />

Report ofthe National Academy ofSciences: National Research Council Committee ofFundamental Constants, 1963.

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