Page 97 - Corrosion Engineering Principles and Practice

72 C h a p t e r 4 C o r r o s i o n T h e r m o d y n a m i c s 73

FIGURE 4.7
Experimental –405 mV Hi
Lo
set-up to
measure the
corrosion
potential of a
specimen.

In measuring and reporting corrosion potentials, it is necessary to
indicate the magnitude of the voltage and its sign. In the example
shown in Fig. 4.7, the corrosion potential of metal M is −0.405 V. The
minus sign indicates that the metal is negative with respect to the
reference electrode. However, if the metal was connected to the low
point (Lo) and the reference electrode to the high point (Hi) the
reading would be +0.405 V. It is customary to connect the reference
electrode to the low point or the instrumental ground to avoid any
confusion in reporting. Nonetheless, some manufacturers of
electrochemical equipment have done the opposite.

4.7 Measuring pH
Measuring pH involves either the use of indicators whose colors are
dependent on pH or the use of pH measuring electrodes. pH indicators
based on color changes are normally used in the form of pH papers.
The paper is wetted with the solution being measured and the resulting
color is compared with color standards to determine the pH.
A pH meter measures the difference in potential between a reference
electrode insensitive to changes in pH and an electrode sensitive to such
changes. A successful pH reading depends upon three components:
Electrodes: A pH electrode consists of two half-cells; an indicating
electrode and a reference electrode. Most applications today use
a combination electrode with both half-cells in one body. Over
90 percent of pH measurement problems are related to the
improper use, storage, or selection of electrodes.
Meters: A pH meter is in reality a high-precision and high-imped-
ance voltmeter capable of reading small millivolt changes from the
pH electrode system. The meter is seldom the source of problems