3 Important Factors for Buying a pH Data Logger
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CLEVELAND - OhioPen -- Common Issues and Recommended Solutions

pH Measurement is very common in industrial processes, plating, wastewater treatment, and environmental monitoring. Using a data logger to record pH provides information on long-term trends and the impact of various factors along with providing information for quality control, compliance reporting, and alarming.

Basics of pH Measurement

The pH value represents 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). The most common pH measurement method utilizes a special Hydrogen ion selective electrode (pH probe) immersed in the solution along with a second reference electrode.

Considerations for Accurate pH Measurements

Using a simple voltage input data logger to record the potential between the two electrodes creates several challenges that must be taken into account to obtain accurate pH measurements.

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 data logger that provides a full-scale input range of ± 1 to 2 volts will ensure adequate accuracy and resolution to detect changes in pH of 0.1 or less.

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 are very small. A standard voltage data logger may have an input impedance of 1 Megohm which is fine for typical voltage measurements.

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3. The last consideration when measuring pH is the effect of temperature on the output voltage of the sensor. The error in the output voltage becomes worse as you move away from a pH value of 7 and as the temperature deviates from 25°C. For example, without taking temperature into account, simply measuring the voltage output of a probe immersed in a solution of pH of 2 at a temperature of 85°C can lead to errors as high as .9 pH.

Conclusion

Using an intelligent data logger with the appropriate input range and input impedance that also allows a temperature measurement and application of a correction factor allows accurate reliable pH measurements.

For more information on dataTaker and Grant Instruments data loggers, or to find the ideal solution for your application-specific needs, contact a CAS Data Logger Applications Specialist at (800) 956-4437 or visit us at https://www.dataloggerinc.com.

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