Problems with EDTA - pH measurements?

[Mattias]

Hi,

You think that pH measurements should be very easy, but I see a clear trend that samples that contain EDTA gives too high values for pH (using a Ag+/AgCl glass electrode).

It is typically not the first sample that gives too high values, but it happens after a while. Has anyone else seen problems with EDTA and pH measurements? Can EDTA bind the silverions and create a bias?

[krickos]

Hi

Have not encountered your problem myself, but Ag can interfer with EDTA titrations. In titrations you typically control the pH to reduce interfering ions and you can add masking agents (nasty ones) such as CN- (cyanide) or F-.
See page 7 for instance in this link: http://www.bhsu.edu/Portals/91/Analytic ... pter13.pdf

Technically speaking, if your pH electrode leach Ag ions or inner eletrolye from a sleeve or similar it may interfer during a longer pH measurement, at least in theory.

If you are opreating on very low pH values close to 1 you may have the "acidic error" that cases a drift upwards.

For "sleeve" saltbridge/junctions issues make sure that the electrode is properly maintained and calibrated link: http://downloads.vertmarkets.com/files/ ... ph_wol.pdf
Usually you allow some electrolyte to flow out of the junction to prevent "attack on electrolyte or the Ag/AgCl wire", loww on electrolyte and you will have issues. Normally you have high purity KCl as electrolyte, usually can only interfere at very high pH, if its not so pure it may of course have interfering contaminats.

[DSP007]

Hello. Well. A form of EDTA => sodium salt?

[Markus Laeubli, Metrohm]

EDTA is a compund with 4 carboxylic and 2 amine groups. It therefore has 6 pKa values (between 0 amd 10). This means that EDTA is possible to act as a buffer salt catching or relaesing H+ depending on the pH of the sample.

Ag+ to my knowledge does not build complexes with EDTA. Therefore I do not excpect to see pH changes with Ag+ flowing out of the reference system. Anyway Ag+ contamination of the sample will be diminished by corret use of the electrode, means frequent exchange of the bridge electrolyte.

[krickos]

Ag+ to my knowledge does not build complexes with EDTA. Therefore I do not excpect to see pH changes with Ag+ flowing out of the reference system. Anyway Ag+ contamination of the sample will be diminished by corret use of the electrode, means frequent exchange of the bridge electrolyte.

Indeed, I did not think that myself at first but there are actually formation constants for EDTA complexes listed in fundemental books that include Ag+, for instance Fundementals of Analytical chemistry 7th edition, Skoog-West-Holler. But as stated correct use of electrode should prevent any such risk.

[Mattias]

Very strange,

I changed the electrode from Mettler Inlab Micro to a standard (larger) Mettler Inlab electrode, and now all problems are gone.

I have tested to switch back to a brand new micro electrode, and the problems came back. Larger electrode was the key!

[DSP007]

EDTA is a compund with 4 carboxylic and 2 amine groups. It therefore has 6 pKa values (between 0 amd 10). This means that EDTA is possible to act as a buffer salt catching or relaesing H+ depending on the pH of the sample.

Ag+ to my knowledge does not build complexes with EDTA. Therefore I do not excpect to see pH changes with Ag+ flowing out of the reference system. Anyway Ag+ contamination of the sample will be diminished by corret use of the electrode, means frequent exchange of the bridge electrolyte.

100 %+ , EDTA is a buffer . pH of EDTA Na salt water solution about 8
PS
EDTA build compexes witch EDTA, but const of dissotiation more then AgCl dissotiation