Common Issues and Recommended Solutions
Measurement of the pH of a solution is very common in industrial manufacturing processes, plating, wastewater treatment and environmental monitoring. Logging this data provides you with information on long-term trends and the impact of various external factors on changes in pH, and can also be used for control, compliance reporting and alarming. However, to capture accurate pH measurements with common data logging systems, you’ll need to take several factors into account which can directly affect measurement accuracy. CAS DataLoggers has put together this quick guide to show how to use a data logger to measure pH, discussing common problems and recommended solutions.
The pH value measures the Hydrogen ion activity of a solution. The pH scale varies from 0 (strongly acidic with a high concentration of H + ions) to 14 (strongly alkaline with a high concentration of OH – ions). To electrically measure a solution’s pH, 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, you can find the pH of the solution. However, measurement of the pH of a solution is not as simple as it may first appear. Using a simple voltage input data logger to measure pH provides several challenges which you have to consider before you can get accurate results:
1. The voltage range 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. Any data logger intended for pH measurements must be capable of accepting positive and negative voltages. Also, the device must be sensitive enough to be able to accurately measure small changes in voltage. A datalogger that provides a full scale input range of ± 1 to 2 volts will give you adequate accuracy and resolution to detect changes in pH of 0.1 or greater.
2. The pH electrode has a very high impedance–effectively, the pH electrode produces a voltage, but because this voltage develops across an ion-selective glass membrane, the amount of current that the electrode supplies to the measuring circuit is very small. An average data logger may have an input impedance of 1 Megohm (which is fine for typical voltage measurements) but when taking pH measurements, the amount of current drawn by this resistance causes loading effects which 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, allowing voltage measurement from the pH electrode with negligible current draw.
To take pH measurements with standard data loggers, you can use an external amplifier or pH probe with an internal amplifier to provide you with an adequate signal and avoid the negative effects of input loading. The most common amplifier is a small battery-powered unit that just provides buffering without voltage amplification. Because the currents are very small, these amplifiers can run for months on a single battery. Other amplifiers are available that can operate from an external DC power supply to provide continuous operation for extended periods. If your data logger and pH electrode are going to be spaced far apart, it’s best to use an amplifier with a 4-20 mA output to transmit the signal over hundreds of feet with very little loss of accuracy or increase noise. Battery-powered preamplified pH electrodes are convenient devices for portable or temporary systems; battery life for these systems extends up to a year.
3. The distorting effect of temperature on your pH measurement will become worse as you move away from a pH value of 7 and as the temperature deviates from 25°C. For example, at a pH of 2 and temperature of 85 °C, you can experience errors as high as .9 pH. In this case, you can select a data logger capable of temperature measurement as well as the voltage for pH 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. However, if your logger can’t measure temperature, you could also get a pre-amplifier as described above that incorporates an automatic internal temperature compensation circuit. These devices use an external RTD or thermistor sensor to measure the solution’s temperature and provide the appropriate correction to the output voltage.
In conclusion, there are 3 important factors you’ll need to consider before buying a pH data logger for your application: the pH sensor’s small output voltage, the negative effects of input impedance loading of the data logger on the sensor output, and the distorting effect of temperature on the measurement itself. However, 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, you can get reliably accurate pH measurements for your project.