ok.....i am trying to workout this in-house calcultaion, we use for generating potency and thus creating a secondary standard from a suppliers active. Why do i want to do this?.....cause there is no raw data of where the calulation was worked out!...so i want to see if it is correct.


We work out the potency of suppliers active mixture for 4-Hydroxmethlypenicillin & Phenoxmethlypenicillin. Both of these actives exsist as 4-Hydroxmethlypenicillin potassium & Phenoxmethlypenicillin potassium in the mixture.
We buy in an EP standard of (98.4% Purity). We weight approx 15mg 4-Hydroxmethlypenicillin potassium in 150mls.then further dilute 10mls up to 50mls. This is used to create bracketing standards which creates a calibration curve to calculate the % 4-Hydroxmethlypenicillin potassium in the active mixture.
For Phenoxmethlypenicillin potassium we use an EP standard (purity 99.0%)....we weight 100mg in 100ml and again use it as an bracket standard...same as the 4-Hydroxmethlypenicillin potassium .

Inhouse calculation:

(i) % Phenoxymethylpenicillin potassium calculated with reference to the Anhydrous substance =

AmountxStandard PurityX100
Dry matter x 0.902

Where Amount = Std (Weight) x Test (Area)
Test (weight) x Std (Area)

Standard Purity = % purity of phenoxymethylpenicillin in Phenoxymethylpenicillin potassium standard

Dry matter = 100% - %Karl Fischer result

(ii) 4 - Hydroxyphenoxymethylpenicillin potassium calculated with reference to the anhydrous substance =


Amount x Purity - 4 - Hydroxy x Correction Factor x 13.3333
Dry matter

Where Amount = Std (Weight) x Test (Area)
Test (weight) x Std (Area)



Dry matter = 100% - %Karl Fischer result

Correction Factor = 1.104 or that supplied with the EPCRS.


(iii) % Total assay calculated with reference to the anhydrous substance =

% Phenoxymethylpenicillin potassium + % 4 - Hydroxyphenoxymethylpenicillin potassium