Using a Data Logger to Measure pH
Background
pH is a measure of Hydrogen ion activity of a solution. The range of pH varies from 0 which is highly acidic with a high concentration of H + ions to 14 which is highly alkaline with a high concentration of OH - ions. To electrically measure the pH of a solution, a special Hydrogen ion selective electrode (pH probe) is immersed in the solution along with a reference electrode. This special electrode provides a voltage output that varies with the concentration ratio of Hydrogen ions inside the electrode to those outside the electrode. The reference electrode output is independent of the ion ratio. By measuring the voltage between these 2 electrodes, the pH of the solution can be determined.
Measuring pH with a Data Logger
Using a common voltage input data logger to measure pH provides several challenges which must be taken into account to get accurate results. First, the range of voltages provided by a standard pH sensor is in the range of +400 mV to -400 mV corresponding to a pH range of 0 to 14 at room temperature. A data logger that will be used for pH measurements must be capable of accepting positive and negative voltages. Also, it must be sensitive enough to be able to accurately measure fairly small changes in voltage. A logger that provides a full scale input range of ± 1 to 2 volts will provide adequate accuracy and resolution to see changes in pH of 0.1 or better.
A second major issue is that the impedance of the pH electrode is very high. Effectively, the pH electrode produces a voltage, but because this voltage is developed across an ion selective glass membrane the amount of current that the electrode can supply to the measuring circuit is very small. A typical data logger may have an input impedance of 1 Megohm which is fine for normal voltage measurements but in the case of pH measurements the amount of current that is drawn by this resistance will cause loading effects that will produce large errors in the measurement. Dedicated pH meters have a high impedance input amplifier, typically on the order of 10^12 ohms or higher, that allows the measurement of the voltage from the pH electrode with negligible current draw.
To enable pH measurements with standard data logging equipment an external amplifier or pH probe with an internal amplifier can be used to provide an adequate signal and avoid the effects of input loading. The most common amplifier is a small battery powered unit that simply provides buffering with no voltage amplification. Because the currents are very small, these amplifiers can run for months on a single battery. There are also amplifiers that can operate from an external DC power supply to provide continuous operation for extended times. If there will be a long distance between the pH electrode and the data logger, it’s better to use an amplifier with a 4-20 mA output. This allows the signal to be transmitted hundreds of feet with very little loss of accuracy or increase noise. Battery powered preamplified pH electrodes provide a convenient solution for portable or temporary systems; battery life for these systems can be up to a year.
The last issue is the effect of temperature on the pH measurement. The effect of temperature becomes worse as you move away from a pH value of 7 and as the temperature moves away from 25 °C. For example, at a pH of 2 and temperature of 85 °C the error could be as much as .9 pH. A data logger which allows the measurement of temperature as well as the voltage for pH can be used to manually correct for temperature. Intelligent, universal input data loggers such as those from dataTaker or Grant Instruments can be programmed to apply the correction factor of .003 pH/pH for each change of 10°C from 25°C. If the logger can not measure temperature, it is also possible to get a preamplifier as previously described that incorporates an automatic internal temperature compensation circuit. These devices use an external RTD or thermistor sensor to measure the temperature of the solution and provide the appropriate correction to the output voltage.
Summary
In conclusion, measurement of the pH of a solution is not as simple as it may first appear. There are 3 important factors that need to be considered, the small output voltage of the pH sensor, the effects of input impedance loading of the data logger on the sensor output and the effect of temperature on the measurement itself. By using an intelligent data logger with the appropriate input range that also allows a temperature measurement and application of a correction factor along with a preamplified sensor or separate amplifier, accurate pH measurements can be obtained.
