Best Practices for pH Measurement in Process Control

Of all the common inline process measurements pH tends to present the greatest challenges in terms of obtaining a reliable and representative process measurement. This article will explore the main considerations when it comes to choosing and maintaining pH measurement equipment.

Choosing the Right pH Probe

Analogue vs Digital

There are a number of critical requirements for a successful pH measurement:

• A clean, non-fouled probe.
• A calibration that takes into consideration calibration buffer temperature.
• Correct buffers used for calibration.
• Adequate submersion of probe tip in measured media.
• Sufficient time allowed for measurement to settle.

Whilst more expensive, smart digital probes automate all the above informing the user of the state of the probe, stability of the measurement, automatically compensating for temperature during calibration as well as recognising that the calibration buffer pH.

Unlike analogue pH probes they provide real time diagnostics and are not susceptible to ground noise. They are typically more accurate & tend to require less frequent calibration than analogue pH probes. Their additional cost very quickly pays for itself in terms of error reduction, increased performance and reduced maintenance.

Glass Refillable vs Gel

When maintained properly glass refillable probes can be a little more accurate and provide a longer useful working lifespan than their disposable gel counterparts. However, correct maintenance is of critical importance and needs to be carried out by a skilled technician as even a minor lapse on maintenance can result in significant pH errors.

Measurement Application

Temperature Consideration

PH changes with temperature, unlike conductivity where there is a universal relationship with temperature that is the same for all solutions, the relationship between pH and temperature varies with each solution.

A change of 0.03ph/^OC is not uncommon and in situations where process tolerances are very tight, normal HVAC temperature fluctuations may take the pH measurement outside of its process tolerance if the measurements are not compensated for temperature.

“Of all the common inline process measurements pH tends to present the greatest challenges in terms of obtaining a reliable and representative process measurement.”

Compliance Requirements

USP 791 pH is the required standard in the Pharmaceutical industry but often applied in other GMP industries (especially Medical Devices). This standard is often mis-interpreted!

Despite the following lines toward the start of the standard:

“Because of variations in the nature and operation of the available pH measurement systems, it is not practical to provide universal directions for the calibration of the measurement system. However, the general principles to be followed are set forth in the following paragraphs.”

The example that follows it is often taken as a requirement. This example is given of a calibration using 3 different buffers, 2 to calibrate and 1 to verify,  the verification buffer is in between the values of the calibration buffers. The following tolerances are applied:

• 90% – 105% Slope (the slope of the line between the 2 calibration points).
• +/- 30mV Offset (the mV reading at 7pH should be 0+/-30mV).
• +/- 0.05 pH Measurement (difference between the probe and the buffer).

Note that these tolerances are very specific and the overall requirement is to follow the general principle which is to include slope, offset and measurement tolerances as part of calibration.  

“Where analogue probes are used its worth adding a response time check and using a specific response time to determine when a probe should be retired.”

Another area where this gets misinterpreted is to require having a verification buffer between two calibration buffers rather than above or below the calibration buffers. Technically it doesn’t matter as a calibration buffer check and a verification buffer check require the calibration point to be within a specified tolerance.

The only difference is the verification buffer check is at a point where the pH meter calibration is not adjusted and the calibration buffer checks are at points where the pH meter is adjusted as part of calibration. Note it is normal to adjust a pH probes entire calibration curve at two points especially on older pH measurement equipment.

Preventative Maintenance

Calibration Considerations

As mentioned previously in this article calibration should include checks for measurement accuracy, slope % and offset. The majority (if not all) of available pH measurement equipment (both analogue and digital) will report these values.

Whereas older analogue pH equipment can only calibrate using 2 calibration points (one of them typically 7pH), newer digital pH equipment can calibrate at many more points. It’s worth bearing in mind that every calibration point introduces the uncertainty of the calibration buffer and its possible to achieve a more accurate calibration curve by limiting calibration points to no more than 3.

Where very tight pH process tolerances are used, especially with older pH measurement equipment a more accurate result can be obtained by calibrating across a shorter measurement range (this may require multiple pH meters to be used to cover the entire process range).

Response Time

Aside from the standard calibration parameters (accuracy, slope, offset), response time is a good indicator of pH probe health. Typically, a new healthy probe can stabilise in 10 – 30 seconds whereas older probes can take 1 – 2 minutes.

Where analogue probes are used its worth adding a response time check and using a specific response time to determine when a probe should be retired (in addition to the usual calibration parameters). Digital probes typically have this feature built in.

“If digital probes are being used it is likely they will alert the user to a probe fouling issue, but analogue probes will not.”

Cleaning

Where probes are used inline without removal over long durations or where probes are exposed to solutions high in protein concentration there is a risk of fouling which is not necessarily visible. If digital probes are being used it is likely they will alert the user to a probe fouling issue, but analogue probes will not.

The result of probe fouling is the probe will behave like an old probe taking longer to respond and sitting closer (or past) calibration tolerance immediately following calibration. Where fouling isn’t an issue purified water can be sufficient for probe cleaning, in more challenging applications more aggressive purpose-built cleaning solutions are required.  

Hopefully this article has shown that there is a lot more to pH measurement than most other process measurements making it easier to run into complications. Whilst this article isn’t an exhaustive list it does cover the main considerations and should help to navigate the murky waters of pH measurement.

FAQs: pH Measurement in Process Control

Why is pH measurement more difficult than other process measurements?
It’s sensitive to temperature probe condition calibration accuracy and process variability making consistent reliable readings more complex

What’s the difference between analogue and digital pH probes?
Digital probes offer real-time diagnostics temperature compensation and less calibration drift while analogue probes are simpler but more prone to error

How often should pH probes be calibrated?
Calibration frequency depends on probe type process conditions and regulatory requirements but regular checks on slope offset and accuracy are essential

Why is temperature compensation important in pH measurement?
pH readings vary with temperature and since each solution reacts differently compensation ensures measurements remain within tolerance

What are slope and offset in pH calibration?
Slope measures electrode response and offset indicates deviation at pH 7 both are key indicators of probe health and calibration validity

What does USP 791 pH compliance mean?
It refers to pH measurement standards used in pharma and GMP requiring calibration verification and tight tolerances for slope offset and measurement

How do I know when a pH probe should be replaced?
Poor response time reduced slope and calibration failures suggest it’s time to replace the probe especially for analogue models

What causes pH probes to foul and how is it prevented?
High protein media and inline usage cause fouling requiring routine cleaning with purified water or aggressive cleaning agents in harsh conditions

Can calibration accuracy be improved with more calibration points?
Not always fewer well-chosen points within a narrow range often provide more accurate calibration especially with digital probes

Are digital probes worth the investment?
Yes they reduce errors offer longer life require less maintenance and provide advanced features that support better process control