What is reasonable variation of pH measurements?

[Mattias]

Hi,

We are producing a liquid product that needs to under quite tight pH control. For this reason, the production adjust the pH and measure the pH in several in process controls. The product contains a buffer with appropriate pKa.

When the bulk is ready the pH is measured again, and a sample is sent to QC for verification (100 meters away). The problem is that there seem to be a high variation between measurements in production and in QC, about 0.1-0.2 pH units. Shouldn't it be better than that?

The pH meters are calibrated just before each measurement, and pH is checked with a known buffer.

[Markus Laeubli, Metrohm]

Changes in measured pH in the same product but in different labs are quite common.

If your pH needs to be in a very narrow range you need to make sure that the pH measurement takes place
- at a very stable temperature
- use temperature compensation
- with a propper calibration (fresh buffer solutions)
- with a propperly mainained pH electrode (clean diaphragm, correct inner solution etc.)

A difference in measurements of 0.1 to 0.2 pH units is not very big.

[Mattias]

Thanks Markus!

Neither QC nor the production used any temperature control, so we are installing that now!