Some useful top tips for tip top pH performance
15th June 2021
Jon Penn, Global Product Line Manager - CWA
Measurement & Analytics
The acidity or alkalinity of a liquid or solution – its pH value – is critical to many industrial processes. Here we provide some top tips to help you get the best performance from your pH measurement equipment.
Whilst the principles of pH measurement might be straightforward, actually making the measurements themselves can be much more difficult. Many pH measurement applications are very aggressive, making it vital not only to ensure that you've got the right equipment in place from the outset, but also that you check and maintain it on a regular basis if it is to remain accurate.
But what do you need to know to maximise the performance of your pH measuring system?
The following are some helpful tips based on over 65 years of developing, manufacturing and servicing pH equipment for industrial applications.
Get the right sensor
Just as there’s no such thing as a standard industrial process, neither is there any such thing as a ‘one size fits all’ pH measurement solution.
Instead, a range of different options are available, each suited to a specific set of conditions. To find the best one for your application, the first step should be to start with the characteristics of your process. A high temperature application, for example, will call for a device made from high temperature glass, suitable for temperatures in excess of 90 degrees, whilst an application that handles cooler mediums or is located in a cold area, may need one that can operate from 15 degrees to down below zero.
The nature of your process can also influence the design of the pH sensor.
Flat profile glass sensors have a flat probe tip, enabling them to offer a self-cleansing solution for applications where there is the risk of fouling of the pH sensor, while bulb glass sensors are the prime choice for any application up to 140˚C and 10 bar g.
There is also the reference type to take into account. Solid reference electrodes offer excellent low maintenance by preventing the ingress of ‘poisons’ in the sample process liquid that could attack and destroy the sensor reference electrode.
Solid electrode sensors are ideal for most industrial and municipal waste water applications, where high levels of sulphides are present.
Finally, flowing reference electrodes are the best choice where you need to monitor the pH value of high purity water.
Make sure of easy access
Install your pH sensor where it can be easily accessed – you’ll be glad you did when it comes time to calibrate, check or replace it.
Keep it wet
Don’t let the sensor dry out, as it can dramatically reduce the sensor’s service life. Always mount it where it is constantly wetted, perhaps in a u-bend.
To calibrate or not to calibrate?
It really depends on whether there is any need for adjustments. They are often unnecessary if there is a difference of less than 0.2 pH between a sample measurement and the process pH meter.
Always calibrate before use
All pH systems should be calibrated before use. This means the pH measurement cell should be calibrated with a solution with a known pH value using two different pH standards or buffers. However, just because the buffer bottle says 9.18 pH doesn’t mean it actually is – 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.
Consider a grab sample
While in most applications, buffer calibration is fine and will present no difficulties, in some processes, relying solely on buffers can result in incorrect readings.
Consequently, buffer calibration should only be a starting point, followed by one-point grab sample calibrations. In this type of calibration, the sensor is allowed to acclimatise to the process and a sample is measured with a high quality lab sensor, with the resulting value being used to calibrate the process pH meter.
Not all lab measurements are equal
Beware of variations in laboratory samples when comparing with inline measurements from 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. Transport these samples in a sealed polyethylene container, or make the measurement as near as possible to the process.
The pH of laboratory grab samples can also be affected by the sample cooling on the way to the laboratory. Also, if a sample is taken early in the process, its pH could differ from the value of an in-line sample taken later on.
Account for discrepancies
As processes change, we often see discrepancies in pH values from the real process compared to laboratory and historic sample values. Trace the cause of such variations by making a note of the sample and process temperatures. The errors can then be accounted for via the meter’s solution temperature compensation facility.
Keep it clean
Clean sensors regularly to avoid excessive fouling, reduced accuracy and a shortened service life. If a chalky film is on the sensor glass, the sensor should be wiped with a clean cloth and distilled water. If the film remains, try isopropyl alcohol.
Although pH sensors and monitoring systems themselves are not complex, their successful use requires their performance to be monitored, as well as a commitment to proper and regular maintenance. Following these easy guidelines should help you measure pH accurately and keep your sensors in good working order.