Best pH meter for a QC lab

[John]

I know this is slightly off topic, but I was wondering if some of you might be able to give me the benefit of your experience and recommend a pH meter that is quick to calibrate and foolproof to use. These will be mostly used by low-level technicians pH-ing mobile phases and making buffers in a pharmaceutical QC lab.
Thanks
J

[rnelson]

Good luck finding anything foolproof! As a colleague told me once, "The more you idiot-proof something, the dumber idiots you find."

Seriously though, the meter itself is probably less important than the proper pH probe and proper maintenance on those probes. Most meters should be relatively easy to use, you hit a "cal" button, insert the probe into your first buffer until the meter stabilizes (often with an audible beep), adjust the given number to the certified value, then repeat with your second buffer.

I can't recall the policy of this board, but many do not want users to give recommendations or disparage any particular vendors, so you may not get real specific answers.

[DR]

They've undergone lots of "convergent evolution" over the past few decades. Once determining a minimum requirement for accuracy, I'd go by toughness and price, staying away from any model that uses nonstandard connectors for probes.

[Bruce Hamilton]

As it's for pharma QC, I'd go for one that has the easiest operation and compliance options ( such as printout ), along with good local support.

Avoid those that don't offer easy multi-buffer calibrations to the pH meter requirements of the regulatory compendia that your products comply with. Most major brands will offer such systems.

The trick is to simplify documentation and have good electrode rinsing and storage protocols, as every major change in calibration or electrode is going to trigger more painful documentation.

Ask each of the short-listed vendors for the name of a local user in your industry that you can talk to.

Please keep having fun,

Bruce Hamilton

[HW Mueller]

Looking back on some 40 years of using analytical instruments I would say that pH meters have been the most difficult to tame. I thought electronic sensors (Diodes or whatever) would finally end the difficulties, but what I have tried so far has been useless. Modern electrode instruments have given a false feeling of stability, etc., thus ease of handling. I have gone back to using an older analog "manual" adjustible version (Knick), and only to see whether I am in the "ball park". (I am a bit old fashined on this, I have finally been convinced that buffers are best adjusted to needed pH via weighing, but still use a pH meter to "check". In other words I am using a inferior method to check a superior one.)
The other experience I can relate is that the lower the level of technician (to use the expression of John) the less problems there seem to be with measuring pH, but only seemingly!!

In short, if one wants a simple method of pH measurement than one may use pH strips, that way one knows it´s approximate.

[tom jupille]

If you want to make it as "foolproof" as possible, ditch the pH meter entirely, and specify all you mobile phase prep by weight.

You have to make a 20% weighing error to be off by 0.1 pH unit. If a lab has people who can't weigh more accurately than that, then pH is the least of their problems!

[Victor]

HWM- I absolutely agree with you. I remember using an old Corning analog pH meter with a galvanometer swing dial. If you treated it carefully and allowed it to warm up before use, it was perfect and would hold its calibration for at least 24 hours. I have huge problems with modern digital pH meters with the calibration drifting even within 30 minutes. Many times I have carefully calibrated such meters, made a series of measurements and then checked the calibration again, only to find that it has drifted considerably. I am never sure if it is the electrodes or the meter. I paid about $600 dollars for the last electrode I bought (from a major manufacturer), hoping to solve these problems, but it was just as bad as all the others, even when used with a fairly expensive digital meter.

Tom- I am never certain how accurate it is to make a solution using a calculation. There are many calculators around which enable you to do this. Of course you can make up a solution very precisely this way, but I'm never sure how accurate it is. When my pH meter is working and I compare calculate and measured pH, I often see a difference of +/- 0.1 pH units. Even with these inferior pH measurement systems, I am not convinced they can be THAT wrong. Of course, calculation depends on accurate knowledge of the pKa of your buffer substance. I'm not convinced how accurately these are known. Just try looking up the pKa of phosphoric acid, and see how many different values you can find...even at the same temperature. And yes, usually a precise pH is often o.k., but it is not good enough in some cases.

[tom jupille]

And yes, usually a precise pH is often o.k., but it is not good enough in some cases.

The real issue is that we really don't care about the actual pH, we care about obtaining useful, consistent results.

To paraphrase a conversation I had with Bill Tindall:
(Bill) "How do you know the pH electrode/meter is correct?"
(me) "It was calibrated, of course!"
(Bill) "What did you calibrate it with?"
(me) "With a standard buffer."
(Bill) "And how do you know the standard buffer is correct?"
(me) "It was made up by weight."
(Bill) " So, 'cut out the middleman' and just make up your buffer by weight!"

It does require some effort to convert from meter to specified weight, but once the correct weights have been determined, the potential for error goes waaaaay down.

[Slammy1]

pH meters are definitely a weak point in the minds of pharmaceutical managers everywhere. It's not meant to be accurate, just precise. I remember reading McMillian (spelling?) who gave the example of a process line with 3 probes where if they were all within a 0.1 unit range it was assumed they were broken and were pulled. Anyway, buy the meter with the features you want but spend your money on the better probe to hold calibration, give you fast response, etc. I've mostly worked with Thermo/Orion and Corning meters, but the probe is the key irrespective of meter model (mostly).

[HW Mueller]

This just goes to show that precision is a matter of definition.

On weighing, one shouldn´t forget that some of the buffer salts are used as primary standards (those are the substances which may be used to test titrate a commercial liquid pH standard, etc.).

The literature differences in equilibrium constants of these common substances used in HPLC should be far less than ranges of pH obtained with a pH meter. I suspect that consistent discrepancies between calculated pH and pH-meter results are due to having used water which is saturated with CO2, or other mistakes.

[Victor]

Tom, if you are really trying to understand what is going on in an HPLC separation, you need to know the accurate pH. Clearly, most people are not interested in this, and a precise pH is fine.
HWM-you are right to bring up the question of primary standards.

pH meters are calibrated with substances which should generate a solution of accurate pH e.g. potassium hydrogen phthallate for a pH 4.0 buffer has been extensively studied. It is a primary standard. You can weight it out and there are few sources of error (ok -apart from drying it, etc).

Many HPLC methods use buffers which are not primary standards. e.g use of acid phosphate buffers at pH 3.0. You could take 25mM K2HPO4 and mix with 32 mM H3PO4 (ok-there are many other interpretations of a pH 3 phosphate buffer, but that is not the point here). Very simple? o.k.- you can weigh out the salt. But now check your phosphoric acid bottle- maybe it says >85% by weight. So now you have to standardise it by titration against a primary standard....hmmm, not difficult, but not so convenient. The problem is worse if your titrant is say, ammonia solution as this is in addition unstable.

HWM- if you are correct and the discrepancy between calculated measurements is due to CO2 saturation of the water you used, then it is better to use the "measured" pH than the calculated pH. However, I admit this would be a complex issue depending on the actual pH and the method of degassing used.

So I'd still like to be able to buy a precise and accurate pH meter....

[aceto_81]

Tom, if you are really trying to understand what is going on in an HPLC separation, you need to know the accurate pH. Clearly, most people are not interested in this, and a precise pH is fine.

I think this only holds when doing method development. If you are doing routine analysis with validated methods, you don't have to think about why a pH of 3.1 is better than 3.2.

I must agree with Tom that weighing is more accurate in this case.
So if you validate your method by preparing a buffer by weight instead of titration to a certain pH, there isn't the problem of reference buffers and measuring pH in organic phases.

By carefully choose your ingredients of your buffer, I think it should be easier to get more reproducible results.


Ace

[HW Mueller]

What I was trying to say is, that one can not be sloppy just because weighing is vastly more accurate.
There is another aspect that has, as ~ everything else here, been mentioned before: If one does pH adjustment via a pH meter one is highly prone to have an often unacceptable variation in ionic strength. The CO2 in H2O is an example of that.
Incidentally, I think Fisher has a HPLC grade phosphoric acid.

[DR]

^ re H3PO4 - The do have one, as do EMD, JT Baker, others...

[Victor]

I'm sure HPLC grade phosphoric acid is low in impurities, especially in UV absorbing impurities. But do you know its molarity accurately, or is it another one of these >85% solutions by weight? If so, you would still have to standardize it to make up your buffer accurately by weight.

Please let me know.