pH measurment question? - (Nov/12/2007 )

Hi

As I said mine is just a suggestive thought…i think “mdfenko” post clear off certain doubts…still to continue on the topic we can all pool in ( from our experience and logical evaluation ) the reagents and solution which must not be measured by pH electrode…

Thank you all

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never put a pH electrode into a solution containing tfa. i killed the pH electrode on my hplc after a very short exposure (had i read the booklet that came with the module i would have known that it would happen and would have taken precautions; stupid, stupid, stupid).

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I have meet this surprising yesterday.

I dilute 1mM EDTA with 0.5M EDTA stock solution(pH8.0) and deion water, and use pH electrode to measure pH to 8.0. Puting the pH electrode to 1mM EDTA, pH displays 10.2, it is unbelievable(because deion water is acidic, it should lower than 8). So I check pH with pH indicator paper, it's about 7.

I advised my colleague not use pH electrode to measure pH with EDTA containing solution, he said his stock solution was measured by pH electrode in the past. I check pH with pH indicator paper, and it definitely was 8.

why?

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Have you check the pH of the stock solution with a pH meter? Dont trust anyone else's stuff.

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'rick112'
Hi

I havent heard any thing about it because I use both EDTA and SDS for preparing running buffer and Extraction buffer respectively in DNA electrophoresis and DNA extraction. And I do measure the pH of the solution

I don't use b-mercaptoethanol in any sense therefore I can't tell you about it but definetly you can measure the pH of the solution containing SDS and EDTA.
sallie

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as i pointed out earlier, readings in the presence of sds are difficult to make. we do it by allowing all motion in the solution to stop so that we can make a stable reading. when the solution is stirring the apparent pH is different from the "resting" pH. this is time consuming but necessary (for the buffer we make with this method).

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Can I ask an odd question:
I have a Jenway GP probe and on a sister lab to my own I've been told that to get a true reading from the probe (actually, was told to stop from getting "crazy readings") I should open up the stoppered hole at the top of the probe (where you put in electrode buffer). This seems odd to me as you're running the risk of getting reagent in there (if you're sloppy) and I've never heard of such a thing before for this.

Anyone got an opinion on this?

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that is the proper way to use the probe. by opening the hole you permit free flow of the fill solution through the porous plug (assuming it isn't clogged). this should be stated in the information sheet that comes with the probe.

and don't worry about contamination. as long as you keep the probe filled the flow will be outward. you shouldn't get anything in through the hole because it should be near the clamp.

and you won't significantly contaminate the solution you are adjusting because the flow through the porous plug is minute.

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Cheers, mdfenko. What I should have said is that I was under the impression that aside from the entry point for electrode buffer I thought that there was commonly a smaller hole up at the top of the probe to allow the free flow through the porous plug (and make it easier to fill the probe with electrode buffer). In fact, everytime I've had a new probe I've checked and there always has been.

I've used these things for nigh on 25 years and this has been the case up til now, now I find a probe where it isn't the case...weird.

It makes obvious sense that there should be a hole but...well, it's just weird that this has never been an issue until now.

I don't know what to make of that at all.

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i thought it was a little strange that you asked that question. now i understand what you were asking.

my experience is just the opposite of yours. in more than 35 years of lab work i have never seen a pH electrode (combined or separate) with an opening other than the pluggable fill hole (although i have seen a few different ways of closing up the hole).

if there was another hole then the fill solution would evaporate faster and would lead to a lot more crystal formation in the electrode.

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