Hallo Karthik Ragavan ,

What you have is the Potency of standard Colistin sulfate in International Units/mg, stated to be 15,000 IU/mg.

This is similar to other cases where the potency of a substance can be given in terms of weight/weight or weight/volume or volume/volume etc.

Since you need to weigh it in mg, you have to convert the given potency to μg/mg. Thereafter you can use formulas to determine the weight of Colistin sulfate powder to be weighed. This will depend on the volume and concentration of the stock solution that you need for your experiments.

Colistin sulphate has a theoretical potency of 800 μg base activity/mg, which is equivalent to 24,000 IU/mg. This is in accordance to the EMEA summary report on Colistin (See the attached file page 1).

Therefore, the potency (X) of your standard in μg/mg can be calculated as:

800 μg/mg ≡ 24,000 IU/mg

X μg/mg ≡ 15,000 IU/mg

Where X will be = 500 μg/mg.

From there, you can determine the mass of Colistin sulfate to be weighed by using the above potency in μg/mg, your stated concentration 20 μg/mL and the volume you will need. You can find the formula in the CLSI Methods (see the attached file page 11).

Weight (mg)= [ Volume (mL) x Concentration (μg/mL)] / Potency (μg/mg)

For example, if you want to make a volume of 10 mL.

Weight (mg) = (10 mL X 20 μg/mL) / 500 μg/mg.

Weight (mg) = 0.4 mg

You can change the mass to be measured by altering the volume or concentration of your stock solution

Best wishes.

https://academic.oup.com/femspd/article-pdf/74/5/ftw036/24190509/ftw036.pdf

Nelson Masota T. H. Al-Noor thank you both for your answers.