Emerson Process Management 3081 pH/ORP instruction manual Junction Potential Mismatch, 118

MODEL 3081 pH/ORP	SECTION 13.0
	pH MEASUREMENTS

pH 7.00 and pH 10.00. When the electrodes are placed in pH 7 buffer the cell voltage is V7, and when the electrodes are placed in pH 10 buffer, the cell voltage is V10. Note that V7 is not 0 mV as would be expected in an ideal sensor, but is slightly different.

The microprocessor calculates the equation of the straight line connecting the points. The general form of the equation is:

E = A + B (t + 273.15) (pH - 7) (2)

The slope of the line is B (t + 273.15), where t is the tem- perature in °C, and the y-intercept is A. If pH 7 buffer is used for calibration, V7 equals A. If pH 7 buffer is not used, A is calculated from the calibration data.

t1

t2

(pH10, V10)

(pH7, V7)

FIGURE 13-7. Two-Point Buffer Calibration.

The graph shows a calibration using pH 7 and pH 10 buffers. The calibration equation is the straight line connecting the two points. If temperature changes, the slope changes by the ratio (t2 + 273.15)/(t1 + 273.15), where t1 is the calibration tempera- ture and t2 is the process temperature in °C. The calibration equations rotate about the point (pH 7, A).

The microprocessor then converts subsequent cell voltage measurements into pH using the calibration line.

13.8 ISOPOTENTIAL pH

Frequently, the calibration temperature and the process temperature are different. Therefore, the calibration slope is not appropriate for the sample. Figure 13-7 shows what the microprocessor does when buffer and sample temper- atures are different. Assume the sensor was calibrated at temperature t1 and the process temperature is t2. To meas- ure the pH of the process, the microprocessor rotates the calibration line about the point (pH 7, A) until the slope equals B (t2 + 273.15). The microprocessor then uses the new isotherm to convert voltage to pH. The point (pH 7, A) is called the isopotential pH. As Figure 13-7 shows, the isopotential pH is the pH at which the cell voltage does not change when the temperature changes.

The microprocessor makes assumptions when the meas- urement and calibration temperatures are different. It assumes the actual measurement cell isotherms rotate about the point (pH 7, A). The assumption may not be cor- rect, so the measurement will be in error. The size of the error depends on two things: the difference between the isopotential pH of the measurement cell and pH 7 and the difference between the calibration and measurement tem- peratures. For a 10°C temperature difference and a differ- ence in isopotential pH of 2, the error is about ±0.07 pH units. The factors that cause the isopotential pH of a real cell to differ from 7 are beyond the scope of this discussion and to a great extent are out of the control of the user as well.

Most pH cells do not have an isopotential pH point. Instead, the cell isopotential pH changes with temperature, and the cell isotherms rotate about a general area. Measuring the isopotential pH requires great care and patience.

One way to reduce the error caused by disagreement between the sensor and meter isopotential pH is to cali- brate the sensor at the same temperature as the process. However, great care must be exercised when the buffer temperature is significantly greater than ambient tempera- ture. First, the buffer solution must be protected from evap- oration. Evaporation changes the concentration of the buffer and its pH. Above 50°C, a reflux condenser may be necessary. Second, the pH of buffers is defined over a lim- ited temperature range. For example, if the buffer pH is defined only to 60°C, the buffer cannot be used for calibra- tion at 70°C. Finally, no matter what the temperature, it is important that the entire measurement cell, sensor and solution, be at constant temperature. This requirement is critical because lack of temperature uniformity in the cell is one reason the cell isopotential point moves when the tem- perature changes.

13.9 JUNCTION POTENTIAL MISMATCH

Although glass electrodes are always calibrated with buffers, the use of buffers causes a fundamental error in the measurement.

When the glass and reference electrodes are placed in a buffer, a liquid junction potential, Elj, develops at the inter- face between the buffer and the salt bridge. The liquid junc- tion potential is part of the overall cell voltage and is includ- ed in A in equation 2. Equation 2 can be modified to

show Elj, as a separate term:
E = A’ + Elj + B (t + 273.15) (pH - 7)	(3)
or
E = E’ (pH, t) + Elj	(4)
where E’ (pH, t) = A’ + B (t + 273.15) (pH - 7).

In Figure 13-8, calibration and measurement data are plot- ted in terms of equation 4. The cell voltage, E, is repre- sented by the dashed vertical line. The contribution of each term in equation 4 to the voltage is also shown. The liquid

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