coulometer

Basic questions from students; resources for projects and reports.

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Dears,

I am analyzing Water content in Glimepiride as per USP 35 with coulometer. The limit is NMT:0.5%. I am not getting reporducible results. The results of a same sample are from 0.3 to 1.5%. Please let me know what could be the reason?

Procedure; Dissolve 0.25g of sample in 5 mL DMF previously dried over moecular seives. From this solution add 1mL into the titration vessel.

Thanks
What make and model coulometer are you using and what coulometric reagents are you using? Have you made ond solution and taken several 1 mL replicates from it or have you made several solutions? How stable are your blanks? Have you run any kind of reference to check the functioning of the instrument? How are you measuring 1 mL? From the time of preparation to the time of measurement, how are you protecting the solution from exposure to moisture in the air?
Dear,

The Instrument Make :Mettler Toledo, Model: C30. I have prepared several samples of different batches and analyzed them.. all the results are abnormal. The blank is DMF, (To 100mL DMF iam adding molecular seives of 0.4mm for 30 minutes to 24 hours). The results are from 0.05 to 0.2%.
The instrument is without diaphram. The solution Hydernal AD.
The instrument was qualified 2 months back.
I have tried taking sample with micro pipet (1000µL), 1mL Pipet & 2 mL Syringe...
I am preparing the sample in 20mL head space vial, adding DMF & closing with cap & shaking the vial to dissolve the content.
Did you use a water standard to verify the instrument is working properly? Please provide actual results of blanks, standards, samples to help troubleshoot the problem.
We have done the qualifcation using water standards.
Blank value : 0.16%
Standard : 0.92%
Sample : 0.35% to 1.5%.
Have you put the blank or standard into a headspace vial and sampled from the vial in the same way as the samples? Assuming the variability you show for the samples reflects multiple samples from individual preparations, do the results show a trend, such as rising results from replicate to replicate from the vial? Is the sample fully dissolved?


Does the coulometer reach a sharp end point or does it slowly finish the titration? Is the Glimepiride the pure compund or has has it been compunded with other ingreedients?
The sample dissolves very slowly & Glimepiride is a pure compound.
A suggestion: I assume you shake for something like a few minutes or maybe half an hour? Prepare a sample vial but put it on an orgital shaker to shake overnight. Sample it the next day and see if the variability decreases.

One reason I was asking questions about the sample preparation and nature of the material and trend or lack of trend from samples taken from the same headapace vial was to see if I could get any detail that might suggest inhomogenaity in the sample.

Sharpness of the endpoint or noted trends in results would be matters of importance also, but could lead in other directions.
Why can you not add the solid straight into the cell? There are various reagents available to aid solubility of difficult materials.

Also, at low moisture levels, you may not be putting enough water in to determine. A typical coulometric KF won't give sensible results below around 100ug water, so with dry samples you may have to add a lot more than you think to make sure you are in its working range.
Where can I buy the kit they use in CSI?
mohammad wrote:
Dears,

I am analyzing Water content in Glimepiride as per USP 35 with coulometer. The limit is NMT:0.5%. I am not getting reporducible results. The results of a same sample are from 0.3 to 1.5%. Please let me know what could be the reason?

Procedure; Dissolve 0.25g of sample in 5 mL DMF previously dried over moecular seives. From this solution add 1mL into the titration vessel.

Thanks


Hi Mr. Mohammad,

I'm also facing the same issue with Glimepiride material.
Inconsistency between two replicate preparation.

Thanks
Vishnu Deore
Dear Mohammad

Please check with your representative of the supplier of your instrument. They should be able to help.
Dr. Markus Laeubli
Manager Marketing Support IC
Metrohm AG
9101 Herisau
Switzerland
Dear Mohamad and Dr.Markus

As we are facing the same issue with our laboratory and we have discussed the same with Metrohm engineers. They suggested to change the Coulometre electrode and use electrode with diaphragm. Same was also check with API supplier, they are also use the electrode with diaphragm.

Hope this will help to resolve your problem.

Thanks
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