Chromatography Forum

Hi, all -

I've had great success in this forum in the past with HPLC and GC topics ... here's a broad question about pH Meters.

Can anyone give me information about old pH meters - specifically a Corning 150? Or can anyone direct me to an analytical online forum that might have some experts in pH?

We are trying to figure out why our old Corning (30+ years old) gives pH values different from our new pH meter (off by 0.2 to 0.4 units). While the explanation "it's 30 freaking years old!!!" is kind of reasonable, it doesn't really have a scientific - or even technical - basis!

Thanks!

H_H

Provided that both meters pass their respective calibration routines, it is usually a difference in the probes' histories that is responsible for this sort of thing, though it could also be that one meter is fitted with a probe that is just not appropriate for the test solution.

Are both meters being calibrated with the same standard solutions for an adequate time before use with unknowns?
Is it just that one probe takes longer to reach the same number?
Are the unknowns being checked within the window of calibration solutions being used?

Thanks for the quick response, DR.

Both meters have identical probes, purchased at the same time, and appropriate for the solution (phosphate buffer) according to the manufacturer. Both meters are being calibrated immediately before use with the same NIST traceable pH meter standards, and both meters seem to have no trouble reaching their endpoint. We do not have temperature correction on either unit, but the meters are in labs that have a stable enough temperature for measuring phosphate buffer at neutral pH (according to literature).

On the new meters, the pH will read, say, 7.0. On the 30-year old meter, the same solution will read 7.2.

Someone who has been here a long time said they remember a Corning service guy saying that the machine would eventually reach a point at which it wouldn't be able to update the calibration slope (even though it would calculate and display a new one with each calibration) and it would continue to use the final slope it could "remember." He thought the guy said it was a RAM/ROM issue, or something.

If we've had an incorrect slope being repeatedly used for the past several years it could explain what is happening, but what I just typed above isn't very detailed for a CAPA justification ...

Corning doesn't support this instrument anymore, and in fact has sold the pH meter business to someone else.

Thanks again, DR!

H_H

A simple check for electronics :
Remove the probe from its socket.
Try to short circuit the probe connecting input with a tweezer or using a short cable , you must read near pH 7.00 , depending on the slope. ( or you must read 0.00 mV on mV scale )

If it reads very different than this , there is a problem on electronic board .

You may also apply the necessary voltages against pH values , if you have some electronic knowledge , and make better judgement.

pH meters are actually millivoltmeters , converting millivolts to pH values.

It is probably not the probe but for kicks can you see what happens when you change the probes between the two meters?