from The ABB Guide to Fast pH Measurement (click on the title to view the complete article)
Used for a host of applications across a variety of industries, getting the best from pH equipment requires consideration of a range of factors to achieve optimum efficiency and cost effectiveness.
The measurement and control of pH – the degree of alkalinity or acidity of a liquid or solution – is instrumental in many processes throughout industry.
In basic terms, pH is a measurement of the relative amount of hydrogen and hydroxyl ions in an aqueous solution using measuring and reference electrodes with an analysis
and display unit for calculating and displaying pH readings. These systems may be stand alone or form part of a more sophisticated control system to ensure that pH is maintained at a certain level.
The aggressive nature of many pH measurement applications means that periodic maintenance and checking are required as a matter of good practice to ensure continued accuracy.
Maximizing pH performance
- Choose the right equipment to meet the application
Process conditions dictate that you select the proper electrode and reference types to ensure proper measurement.
- Install for easy access
Installing your pH sensor where it can be easily accessed will reduce the effort required whenever calibration, checking or occasional replacement is needed.
- Watch out for air
Exposure to air can dry out pH glass and form crystalline deposits at the reference junction, dramatically reducing the sensor’s service life. For this reason, sensors should
never be installed at the top of a pipe, as a half-empty pipe will not permit direct contact with the process.
- Do you really need to calibrate?
The frequency of calibration really depends on whether you think there is any need for adjustments.
- Configure buffer tables
All pH systems should always be calibrated before use. This requires the pH measurement cell to be calibrated with a solution with a traceable, known pH value. However,
calibration does have its own peculiarities, being affected by a range of different factors, of which temperature is the most important. Remember, unless the buffer is
maintained at an ambient temperature of 25°C, its pH will vary. At 0°C, for example, its pH will rise to 9.46.
- Be wary of lab measurements
Beware of variations in laboratory samples when comparing with the process. Neutral or mild alkali, high-purity waters, for instance, will dissolve CO2 from the air on the way to the lab, resulting in a drop in pH.
- Make sure the sensor is adjusted for temperature
In-line sensors measure at up to 140°C so you may need time to cool to calibration temperature. This could take quite a while unless using a fast acting temperature sensor with balanced pH and reference electrodes offering similar temperature responses,
- Make a sample and process log sheet
Over recent years, changes have occurred in chemical usage. Factors such as the introduction of new process techniques, environmental legislation and a general trend
towards increased process temperatures have resulted in some users seeing discrepancies in pH values from the real process compared to laboratory values.
- Clean the sensor regularly
Up to half of industrial pH applications benefit from some sort of cleaning regime.