Emerson Process Management 3081 pH/ORP Glass Electrode Slope, Buffers and Calibration, 117

MODEL 3081 pH/ORP	SECTION 13.0
	pH MEASUREMENTS

4. the liquid junction potential.

The second term, 0.1984 T pH, is the potential (in mV) at the outside surface of the pH glass. This potential depends on temperature and on the pH of the sample. Assuming temperature remains constant, any change in cell voltage is caused solely by a change in the pH of the sample. Therefore, the cell voltage is a measure of the sample pH.

Note that a graph of equation 1, E(T) plotted against pH, is a straight line having a y-intercept of E°(T) and a slope of 0.1984 T.

13.6 GLASS ELECTRODE SLOPE

For reasons beyond the scope of this discussion, equation 1 is commonly rewritten to remove the temperature dependence in the intercept and to shift the origin of the axes to pH 7. The result is plotted in Figure 13-6. Two lines appear on the graph. One line shows how cell voltage changes with pH at 25°C, and the other line shows the rela- tionship at 50°C. The lines, which are commonly called isotherms, intersect at the point (pH 7, 0 mV). An entire family of curves, each having a slope determined by the temperature and all passing through the point (pH 7, 0 mV) can be drawn on the graph.

meters, including the Model 3081pH/ORP transmitter, have automatic temperature compensation.

The slope of the isotherm is often called the glass electrode or sensor slope. The slope can be calculated from the equation: slope = 0.1984 (t + 273.15), where t is tempera- ture in °C. The slope has units of mV per unit change in pH. The table lists slopes for different temperatures.

Temp (°C)	Slope (mV/unit pH)
15	-57.2
20	-58.2
25	-59.2
30	-60.1
35	-61.1

As the graph in Figure 13-6 suggests, the closer the pH is to 7, the less important is temperature compensation. For example, if the pH is 8 and the temperature is 30°C, a 10°C error in temperature introduces a pH error of ±0.03. At pH 10, the error in the measured pH is ±0.10.

FIGURE 13-6. Glass Electrode Slope.

The voltage of a pH measurement cell depends on pH and temperature. A given pH produces different voltages depending on the temperature. The further from pH 7, the greater the influence of temperature on the relationship between pH and cell voltage.

Figure 13-6 shows why temperature is important in making pH measurements. When temperature changes, the slope of the isotherm changes. Therefore, a given cell voltage corresponds to a different pH value, depending on the temperature. For example, assume the cell voltage is -150 mV. At 25°C the pH is 9.54, and at 50°C the pH is 9.35. The process of select- ing the correct isotherm for converting voltage to pH is called temperature compensation. All modern process pH

13.7 BUFFERS AND CALIBRATION

Figure 13-6 shows an ideal cell: one in which the voltage is zero when the pH is 7, and the slope is 0.1984 T over the entire pH range. In a real cell the voltage at pH 7 is rarely zero, but it is usually between -30 mV and +30 mV. The slope is also seldom 0.1984 T over the entire range of pH. However, over a range of two or three pH units, the slope is usually close to ideal.

Calibration compensates for non-ideal behavior. Calibration involves the use of solutions having exactly know pH, called calibration buffers or simply buffers. Assigning a pH value to a buffer is not a simple process. The laboratory work is demanding, and extensive theoretical work is needed to support certain assumptions that must be made. Normally, establishing pH scales is a task best left to national stan- dards laboratories. pH scales developed by the United States National Institute of Standards and Technology (NIST), the British Standards Institute (BSI), the Japan Standards Institute (JSI), and the German Deutsche Institute für Normung (DIN) are in common use. Although there are some minor differences, for practical purposes the scales are identical. Commercial buffers are usually trace- able to a recognized standard scale. Generally, commercial buffers are less accurate than standard buffers. Typical accuracy is ±0.01 pH units. Commercial buffers, sometimes called technical buffers, do have greater buffer capacity. They are less susceptible to accidental contamination and dilution than standard buffers.

Figure 13-7 shows graphically what happens during cali- bration. The example assumes calibration is being done at

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