The Importance of pH Sensor Calibration

Any style of electrochemical sensor requires calibration to provide accurate measurements. The process of pH sensor calibration is straightforward, but many users are not aware of why.

A standard pH sensor outputs a millivolt (mV) signal, which corresponds to a pH value. An output of 0 mV is equal to pH 7. Moving one pH unit along the scale in any direction will correspond to a change in voltage of 59 mV. This 59 mV change is called the “slope” of the sensor (see figure below). There is no such thing as a sensor with perfect output. Even a brand new sensor will have slight variances in and this is where pH sensor calibration comes in.

 

 

 

 

 

 

Sources of pH Sensor Variability

There are a few main characteristics to consider related to calibration:

• Offset: Output at pH 7 will be slightly above or below 0mV, within a tolerance.
• Slope: May not be exactly 59 mV.
• Span: The slope may not be constant across the entire pH scale. For example, the sensor may be close to theoretical output at pH 7 but have a greater deviation at the extreme ends of the pH scale.

These variances do not mean the sensor is faulty. All manufacturers, including Sensorex, will specify and test for allowable tolerances in output from a new electrode.

How pH Sensor Calibration Works

The general process for calibration is to use at least two known data points to adjust the pH values associated with the mV output. It is common to use pH 7.00 as one buffer solution and then pH 4.01 or pH 10.00 as the other. Adding more data points will increase accuracy.

Adjusting for Temperature

Temperature must also be considered. See the figure below for the effects of temperature on pH readings. It is essential to calibrate the sensor in solution close to 25°C, at the same temperature as the sample or process, or to use a system with automatic temperature compensation.

How to Calibrate your PH Sensor

The majority of meters, controllers, and other types of instrumentation will make this process easy. The typical calibration procedure consists of the following steps:

1. Vigorously stir the electrode in a rinse solution.
2. Shake the electrode with a snap action to remove residual drops.
3. Vigorously stir the electrode in the buffer or sample and allow the reading to stabilize.
4. Take the reading and record the known pH value of the solution standard.
5. Repeat for as many points as desired.

Now your sensor is ready to use!

Recalibrating a pH Sensor

The typical question is, “how often does my electrode need to be calibrated?” Many factors affect adequate time between calibrations. Electrodes experience natural aging; offset and response time will increase, the span will decrease, and the meter will need to account for these changes. The sensor could become coated or dirty and should be calibrated after it is cleaned. The chemicals and environment around the sensor could affect output or accelerate aging. The best advice is to determine the level of accuracy required and adjust the calibration schedule based on experience. Eventually, the meter will no longer be able to calibrate the sensor and is no longer useable and should be replaced.