Potentiometric determination of silver

D

Dave

New Member
#1
Hi

I am having intermittent problems with my titrations that I can't put my finger on.

My gut feeling is that it is the electrode I'm using in some way but I am trying to rule out other possibilities just in case.

I think it is the electrode because when I put a new electrode on I stop having these problems for a while and then it reoccurs.

I am using a Metrohm auto-titration kit and a thermo scientific Orion silver/sulfide combination electrode.

The problem I am having is the end points get very noisy giving erratic results and I don't know why.

My technical manager thinks it may be down to the fact that I am not boiling off the nitrogen oxides properly but I don't think this is the case. I am not even sure if this is actual an interference with this type of method. I am using KBr as my titrant. Reading on the internet it specifically states that oxides of nitrogen interfere with thiosulphate titrations and this is a colour change end point but I cannot confirm if this will effect a potentiometric titration.

I use an in-house filling solution of 10% KNO3. Maybe this could be the problem. Orion do a filling solutions for precise silver results but this is basically 10% KNO3 but this contains, according to the spec sheet, <0.01% AgNO3. Do you think this could be the problem?

I am getting all kinds of samples thrown at me so there maybe mercury in the samples and according to the electrode manual that poisons the electrode. Could it be this? Who knows?

The manual says to store the electrode in 4M KCl solution. This seems to keep the silver sulfide sensing element nice and shiny but doesn't stop the problem.

Finally in the manual, in so many words, it says to rejuvenate the electrode to soak it in 1ppm Ag standard, which does improve things but it never gets back to the way it was before the problem occurred.

Please please help because this is stressing me out.
 
fireguy

fireguy

Supermoderator
#2
Dave, I think contacting the instrument vendor may be the best way to proceed on this.

Sorry I couldn't be more help.
 
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