Making pH sensors more durable

01 November 2007

Solving problems with pH measurement

Gilead Sciences, an international biopharmaceutical company, launched an effort in 2003 to solve its
greatest instrumentation problem: unreliable pH sensors.

‘Our pH sensors simply could not hold up to aggressive chemicals, such as hydrobromic acid,’ said Rob Pastushak, at Gilead’s facility in Alberta, Canada. The organic solvent constituent caused the probe's O-rings to degrade during the most critical stage of the process. In many cases, three probes, at approximately $600 per probe, would fail while processing just one batch.

Gilead was forced to confirm measurements on a benchtop meter in its lab. ‘When you process 3,000 to 5,000 litres and add 5 to 10 kilos of caustic solution at a time, it might take 20 to 40 lab tests to ensure the pH is right during adjustment,’ says Mr. Pastushak. ‘Going to the lab so often just killed production efficiency.’

Given the competitive nature of the biopharmaceutical manufacturing business, even a small variance in yield can have a huge impact on the bottom line. ‘A 1% or 2% increase in the target commercial yield
translates to 100% profit gain. Likewise, a consistent loss of 1% or 2% of the commercial target yield
translates as lost profit. You don't stay in business long with that type of performance,’ he says.

Rebuildable probes
The company decided to test the Foxboro 871PH Series sensor, from Invensys Process Systems. The 871 is a ‘rebuildable’ pH probe that incorporates patented technology from the Foxboro DolpHin sensor line.

Being rebuildable, the sensor includes a robust and continuously reusable sensor body with a field replaceable measuring electrode, reference junction and electrolyte. The measuring electrode is the
‘business’ area of the sensor and includes glass that comes in contact with the media being measured. The sensor features a patented glass formulation that improves measurement stability, accuracy, and service life in high temperature applications, up to 120°C.

The glass also increases response speed up to five times compared with conventional sensors and allows longer duty cycles. Gilead uses the Foxboro 871PH probe in conjunction with two 7,600-litre reactor vessels stationed sideby- side with a shared condenser.

To ensure that the product comes out of solution with the proper pH, Gilead typically dilutes the organic mixture with water. This mixture must then be measured for pH and adjusted until the right balance is achieved. To adjust the pH, Gilead pump-circulates the solution through the bottom of each vessel to the top where the sensor measures pH in a slurry loop. The probe provides reactive, real-time pH measurements, which are key to reducing cycle time.

‘We can now complete a pH adjustment in three hours rather than the 18 to 24 hours it previously took,’ says Mr. Pastushak. ‘And we no longer have to take 40 samples to the lab to confirm measurement accuracy—we only take one, as a matter of quality assurance protocol. Previously, every time we grabbed a lab sample, we had to put the process on hold until we got the results back.

‘The results have been consistent from batch-to-batch,’ says Mr. Pastushak. ‘As soon as we add a solution to adjust pH, the probe responds immediately and provides the new pH reading. We've found it to be accurate to ±0.03 pH units, which is well within our target limits.’

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