determination of thioguanine

[tony83]

Hi!
I have a problem with the potenziometric assay of thioguanine:
I wheight 0.125 g of thioguanine in 15 ml of formic acid anhydrous and add 50 ml of MEK, I titrate with perchloric acid 0.1 N.
I don't understand why the assay is constantly very high (110 % and 108%). Consider that the potentiometer is working well and the perchloric acid factor is 0.9992.
I can not understand wich are the interferences,
is there someone that can help me???

Thanks a lot
Tony

[tom jupille]

The most likely explanation is that the perchloric acid is more dilute than you think it is.

[tony83]

Hi Tom
I standardized the perchloric acid solution.
I'm sure of this because also I thought it was the first case
Thanks
bye

[Don_Hilton]

How did you standardize the perchloric acid? How many replicates?

How pure is the thioguanine? (Any chance of oxidizable contaminants?)

[tony83]

I standardization perchloric acid with potassium hydrogen phthalate for three times.
Thioguanine assay is 98-101 %
Monday, I will test again the standardization of perchloric acid and I will try if need to work under the specific conditions of titrater for this assay

bye

[krickos]

Hmm

Have you done a blank titration? I would expect that 15 ml of formic acid anhydrous and 50 ml of MEK would add to the total. perchloric acid 0.1 N and glacial acetic acid is quite moister sensitive as well and standadirisation temp versus analytical temp should be includeed in assay.

The Japanese and FDA general requirement in general is to look at blank titrations and compensate as necessary.