How to dilute and prepare standard solutions

There are three types of standard solutions that need to be prepared when analysing dissolution samples:

(I) Primary standard solution (PSS)
(II) Secondary standard solution (SSS)
(III) Working standard solution (WSS)

(I) Preparation of PSS:

Accurately weigh the amount of target drug substance into a volumetric flask and dissolve with organic solvent. If the drug substance is readily soluble and stable, methanol can be used. If not, DMSO or a proportion of DMSO can be used to increase drug solubility.


(II) Preparation of SSS:

SSS is required to improve the accuracy of drug concentration and has an intermediate drug concentration that is used for preparing WSS.
In most cases, if the drug substance is neutral, typically a mixture of methanol and water (e.g. 75:25 %v/v)  is suitable for our biorelevant media. For pH sensitive drugs, an appropriate buffer can be used instead of water to increase the solubility of drug in the SSS. Prepare SSS by diluting PSS with a potentially suitable diluent to the appropriate drug concentration.

(III) Preparation of WSS

Use the biorelevant medium to dilute SSS to prepare WSS.

Note: if diluent is required, use the same diluent that was used for the preparation of SSS. Typically, a diluent is needed unless dilution is being carried out with 1:2 dilution ratio.

Download our Biorelevant Dissolution Guide here.