You Should depend on the MIC

I am not sure, I understand the problem correctly. You name a quite specific concentration of 5120 ug/ml of streptomycin sulfate. So you already know you need this specific concentration and that this is sufficient potent for you?

If this is the case I would do he calculation as following:

5120 ug/ml streptomycin sulfate in 5 ml would be 25.6 mg streptomycin sulfate. But since the purity is 84.6%, you need to use 30.47 mg (25.6 mg/0.846) to have a sufficient amount of pure compound.

If the concentration you need would be 5120 ug/ml of streptomycin you calculate it as following:

5120 ug/ml for 5 mL = 25.6 mg streptomycin. 25.6 mg/(molecular weight of streptomycin) = mol of streptomycin

this multiplied by molecular weight of streptomycin sulfate = mg of streptomycin sulfate as pure compound. This divided by 0.846 to take the impurities into consideration = mg you need to dissolve in 5 ml.

I didnt calculate it with numbers, because you can find several streptomycin sulfates (but the molecular weight you posted doesnt seem to be correct, since streptomycin itself has a molecular weight of 581.6 g/mol)

I hope I could help you with this answer

following

Christina Lamers

so if I want 2mM final concentration in 1ml and I know that gentamicin sulphate has MW of 581 and the gentamicin alone is around 464, nd I know the potency is 59% that I should divide the MW/ potency? like 464/0.590? and the result will give me mg/ml (in this case 0.786mg/ml)?

If you want to have a concentration of 2 mM (2 mmol/L) in 1 mL, that corresponds to 0.002 mmol (or 2 umol). Now if you calculate with the MW of gentamicin, you need 0.928 mg gentamicin for the 1 mL of 2 mM solution. If you want to express the mass concentration, you can calculate directly from the molare concentration by multiplying the molar concentration mol/L by the MW (g/mol) = g/L. So 2 mM (=mmol/L) gentamicin is equal 0.928 g/L (or mg/mL).

Now if you work with gentamicin sulphate you need 1.162 mg/mL (= 2 mmol/L * 581 g/mol). With potency you mean for example purity? So you need to weigh in more: 1.162 mg/mL/0.59 = 1.97 mg/mL

Hope that helps