Convert your DMM to a pH meter
Edited by Bill Travis
Bill Donofrio, Nu-Products, Cordova, TN -- EDN, October 17, 2002
It's often necessary to know the acidity of a solution to control a process. Even inexpensive pH meters can be relatively costly, and many of the inexpensive models have no output that you can readily connect to a computer interface. A simple solution to this problem is to attach a pH probe to a high-impedance input of an op amp and read the output with a digital voltmeter (Figure 1). Then, convert these readings to pH units using a calculator that can calculate the slope of a line. To calibrate the system, you can use pH standards. Generally, you would use three standards: 4-, 7-, and 10-pH units. These standards are inexpensive and available at any chemical-supply house. The calibration procedure is as follows:
-
Short the input leads together and adjust the offset potentiometer such that the output reads 0 mV.
-
Place the pH probe in each standard and record the output (in millivolts) for each standard.
-
Enter the values in your calculator and determine the slope of the line.
Figure 2 shows a typical plot of pH versus millivolt output. (Note that pH is temperature-dependent; you have to recalibrate if the temperature changes.)
If you want to measure directly in pH units, you can use the circuit in Figure 3. At approximately 24°C, the equation for the slope of the line is Y=–0.017X+7. To obtain readings in tenths of volts, you multiply the equation by 10. The new equation is 10Y=–0.17X+70. The circuit in Figure 3 comprises three sections. The voltage using the LM351 provides the high-impedance input. The inverting amplifier using one-half of the LM353 controls the slope. It multiplies the LM351's output by –0.17. The other half of the LM353 functions as a summing amplifier and controls the Y intercept by adding 70 mV to the input signal. When you build this circuit, solder the BNC directly to the op amp's input pin. This connection prevents slightly conductive pc boards from affecting the impedance levels of the probe. Another point to remember is to remove the pH probe from the unit when power is off.
To calibrate the circuit, first short the inputs together and adjust the offset potentiometer to obtain 0-mV output from the LM351. To calibrate the circuit for pH units, place the pH probe into a pH standard. Measure the voltage at the output of the LM351. Multiply this voltage by 0.17 and adjust the slope potentiometer until the output of the second op amp reads this inverted (negative) value. Then, connect the meter to the output of the circuit and adjust the Y-intercept potentiometer until the circuit yields the pH of the standard you use. (For example, a pH of 10.1 reads 0.101V.) To tweak the circuit, place the pH probe in other standards and adjust the Y-intercept potentiometer. Note again, if the temperature changes, you must recalibrate. The accuracy of this circuit is generally ±0.1 pH units. When you order pH probes, you should order low-impedance units. This circuit uses Cole-Parmer (www.colepalmer.com) U-59001-65 probes.
Is this the best Design Idea in this issue? Select at www.edn.com.
