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

Hi

One of my colleague told me that you couldn’t measure pH if any of the following substance is added to your solution
(a) EDTA
( 2-mercaptoethanol
© surfactants such as SDS

can any one tell me the reason for this..? what happen if we take pH in presence of above agents???

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It sounds a bit strange.
a) EDTA can definitely be present in a solution for pH determination. Think about this: how do you prepare 0.5M stock at pH 8?

I've never heard of B-ME having any effect on the meter. Ditto for SDS.

If you're not sure, why not ask the sales rep for your pH meter? They should be able to give you the best information.

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I only know that it is a bad idea to pH loading buffer of the SDS-PAGE containing bromophenol blue... blue everywhere!!

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As long as you adjust the pH of tris-HCl to 6.8 before adding other components, it should be fine.

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ooh

Long time l prepare Running buffers and maintain pH in the presence of EDTA... with out this in the sence...?
l cann't uderstand....

Siva

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sds can cause a problem because the potassium from the electrode displaces the sodium. kds crystalizes and clogs the porous plug of the electrode.

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Did you mean the Na in the SDS? if so why are we using NaOH to adjust pH?

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the sodium from sds will be displaced by (exchanged with) potassium from the kcl in the electrode. kds is insoluble and will crystallize (precipitate) and clog (a technical term for "inhibit the flow through") the porous plug of the electrode. without free movement of the kcl solution from the electrode into the solution being measured you will not get reliable readings.

sodium will not have a negative effect on the readings so there is no problem with adjusting pH with naoh.

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hi

well let me elaborate little of previous topic…these are the reasons I thing we must not use EDTA, Surfactants (such as SDS) and 2-mercaptoethanol

(1) EDTA-as it’s a chelating agent it may bind to the potassium ions in the electrode and thus under longer exposure forming a layer over electrode. This may cause a variation/ fluctuation in pH reading
(2) Surfactants :-I think this may forum a stick layer over the electrode this preventing it from giving accurate reading
(3) 2-mercaptoethanol:- this is a strong reducing agent thus it may reduce the components in the solution or electrode and result in a faulty results

Above reasons are suggestive and I don’t have any proof to back them up..thats why I put up the question to obtain a more export view point in this matter……

Plz put forward your view point in this matter…also plz tell me if any of my reasons are wrong or biased

You can also enlist the chemicals or reagents which must not be measured using an pH electrode ( plz specify the reason)

Thank you all

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(these are observations based on experience)

1) edta has no significant effect on the chemistry of the electrode. if it binds to potassium from the electrode, it will bind outside of the electrode (the kcl flows out of the electrode through the porous plug).

2) surfactants, other than sds, are generally not in high enough concentration to coat the electrode and prevent ionic interactions.

3) reducing agents will not be in high enough concentration to affect the electrode. besides, what, inside the electrode, would be reduced?

if, however, you feel that your solutions could have any of these kinds of reactions with the electrode then you could switch to a solid state isfet electrode.

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