Chromatography Forum

Hello,
I have a corning ph meter 220 and I perform my calibration using 2 points, ph 4 and 7, now slope is 99%. The pH holds very well when the electrodes are in these buffers (which are not expired). When I put the electrode in deionized water though the pH reading goes down from 5.8 to around 2.5 in a matter of 1-2 minutes. Does my electrode needs to be replaced. When I check a pH of ammonium acetate buffer for example it is stable... not deionized water....
Let me know what you think!
F

is it ok to leave the electrode in ph7 buffer solution for storage?

Dear:
I looked at the group page of Prof. Janusz Pawliszyn (see URL:
http://www.science.uwaterloo.ca/chemist ... ty/jp.html).
If you are a PhD student in this group, you should have known by now how to use & maintain the pH meter.
Give your professor/advisor a call. If he does not know, pls. post on this board again.
I should not blame you, because I have interviewed university graduates (Chemistry-related, BS degree holders in California) who couldn’t sketch a van Deemter plot, and couldn’t plot a titration curve for a weak acid (despite being a tutor of Chemistry in school!)
Alfred.

Francois,

Saturated KCl for electrode storage... About your earlier post, if the pH meter gives the right readings for titrated buffered solutions then it should be OK. The problem with deionized water is that it does not have any buffer capacity so small amounts of acid or base will change the pH dramatically. In your case, maybe the pH meter is very close to some acid source (I guess it can even be CO2) that chages the pH like this... tough to say...

As for Alfred88, I wouldn't make any judgement calls like this or provide contitional advices... People that ask questions can be just summer students or engineers with not very good background in chemistry or they feel that their question is too basic to bother the professor with or whatever... If for any reason I do not like the post I just do not reply... saying that you should know that by now, is just not very polite and defies the purpose of the present forum...

We sometimes have a similar problem.. If the deionised water source isn't regularly serviced our water pH goes from ca. 8 up to 10 or even 12!! We have also wondered if it's the electrode or maybe our chemicals that were ruined or contaminated but 9 times out of 10.. it's our water source. Luckily it's a few years ago since it happened last so hopefully it under control now..

Deionized water is deprived of the entities (ions) which sustain conductance, the [H+] and [OH-] being too low to allow enough current to be "seen" by a pH meter. (It seems to have been in pre-college days when I saw the experiment with a cicuit of a battery, light bulp, water or salt solution. The bulp lit only if there was salt in the water).


A strange feeling is that both Alfred and Kostas make a legitimate point here.

pH 2.5 in the DI water but OK in the Buffer?

That is rather strange.

The above suggestions are good, but I would like to add one more.

Test the water with pH paper to check the pH (or even taste it to see if it is acidic). A pH of 2.5 is very low for DI water (even acid rain is not that low)

I got by BSc in Chemistry at Waterloo (born in Owen Sound) and a was there when Janusz Pawliszyn first joined the faculty - I took one of his courses in Instrumentation concepts (fourier tranforms and stuff like). At the time I had difficulty understanding his accent but I would have no problem now as I went on to marry a Polish girl.

I was a TA in a analytic chem lab for one term at Waterloo and I never ran across your problem. But you should actually be happy since problem solving is at the core of Analytical chemistry.

Have fun!


david.woods@stiefel.com

Good Luck!

Hello,
Thanks for some of your answers, I had in fact checked the pH with pH paper and it indicates a pH round 6-7, not 2.5. I have tried running the calibration with the same standards and same water on a separate, newer, pH meter and it gives me a pH 5.8. Furthermore it is stable at 5.8 and does not drift. I have had a couple instances with the older instrument where the reading stabilized but it does so at different pH everytime.
As for the use/maintaining of the pH meter, I have read the manual. They don't talk about their instrument pH reading going all over the map only in deionized water.
I have found no chemical close to the pH meter that could make the pH drop so fast or CO2 source.
I guess i will try to recondition and clean the electrode just to make sure this doesnt start to occur even in buffered solutions.
Thanks for your help, thanks kostas for showing alfred some humility

Francois,

Playing it double advocate, it would have been helpful if you have stated what you have already tried (i.e. checked the water pH with a pH paper and tried another new pH meter). It would have shown that you took actions towards the right direction and probably Alferd wouldn't have written what he wrote...

The average water deionizing system puts out water with a lot of dissolved CO2, hence the pH is acidic and not very stable.

pH measurement in low conductivity water is a rather difficult task. But pH 2.5 corresponds to a milimolar solution of acid. And dissolved CO2 cannot be the reason for such a reading.
For precise pH measurements the measuring conditions like electrode system, bridge electrolyte, diaphragm and so on need to be optimized.
For more information read the information supplied in the leaflet to the Metrohm Aquatrode plus (6.0253.100): http://www.metrohm.com/products/downloa ... plus_e.pdf

Taking Markus´, Kostas´ 2nd, and my statements into account and seeing the rest of the discussion it seems that Alfred88 was very kind, indeed.

also my phmeter is not stable with deionized water...it's obvious becouse of lack of ions...but pH 2.5 in 1-2 minutes is strange...probably there's something wrong...

Of course there is something wrong: The electrode in deionized water.

I don't know if this is helpful, but if you're testing pH to determine
the quality of your water system - have you tried the USP test
for water? Here's a link that does a nice job of explaining it:

http://www.usfilter.com/NR/rdonlyres/11 ... BR0305.pdf