I need to prepare 10ml of 1000ppm(ug/ml) of streptomycin stock solution. So i need to weigh out 10mg of pure streptomycin and dissolve in 10ml of water. But I have with me a Sigma Streptomycin sulfate with formula C21H39N7O12. 1.5H2SO4 and MW=728.69, moisture content of 6% and potency 749units streptomycin base per mg on dried basis. How to calculate how much of this streptomycin sulfate to weigh which is equivalent to 10mg of pure streptomycin? Do i have to convert unit/mg into ug/mg? Anyone please help?
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Re: convert units/mg to ug/mg (potency)
Originally posted by xiaolingI need to prepare 10ml of 1000ppm(ug/ml) of streptomycin stock solution. So i need to weigh out 10mg of pure streptomycin and dissolve in 10ml of water. But I have with me a Sigma Streptomycin sulfate with formula C21H39N7O12. 1.5H2SO4 and MW=728.69, moisture content of 6% and potency 749units streptomycin base per mg on dried basis. How to calculate how much of this streptomycin sulfate to weigh which is equivalent to 10mg of pure streptomycin? Do i have to convert unit/mg into ug/mg? Anyone please help?
Pure streptomycin = MW 728.69  147.12 = 581.57 ????? (check this).
Therefore weigh out 10mg x 100/94 x 728.69/581.57 = 13.3mg plus any allowance for different activity factors.
Also, it is important to understand that you need to make this up to 10mL, for example in a volumetric flask, rather than adding 10mL of water, as the material will have a small but significant volume.  This is the difference between having a known amount in a known volume, as opposed to having a known amount in an unknown volume.
Hope this helps, sometimes it is worth sitting down and spending a few hours really coming to grips with these dilution questions, otherwise you can struggle with them for years, and never be confident of your results.
Good luck with your studies.

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Re: convert units/mg to ug/mg (potency)
Thanks a lot Mrs X. But can you please tell me what is activity factor? Is that the potency?
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Re: convert units/mg to ug/mg (potency)
Originally posted by xiaolingThanks a lot Mrs X. But can you please tell me what is activity factor? Is that the potency?
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Re: convert units/mg to ug/mg (potency)
Hi mrs X, thanks for your advice. I will try to clarify it. Thanks again
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Re: convert units/mg to ug/mg (potency)
I am having the same problem!!
So if you have a sample of streptomycin sulfate that has activity units of 730 units per mg; then how do I calculate the molar concentration so I can then determine the activity against a bacterial strain?
Is it as simple as 730/1000 for the 'real' mass you are using?
Or a division of the real mwt including sulphate by the 730??
Heeeellllppp
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Re: convert units/mg to ug/mg (potency)
Hello, also I have encountered a problem concerning streptomycin sulfate. I have to calculate a solution of 7 % streptomycin sulfate (dissolved in ionized water) into ion strength, that is mM. I have used streptomycin sulfate from Sigma with product description similar to the above mentioned (in this thread): Mw of 1457.38 g/mol; H2O content: 6,5 mol/mol; 764 units streptomycin base per mg (not sure what that means). I am dumbstruck!
If would appreciate if a kind soul would give some help!
/Jim
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Re: convert units/mg to ug/mg (potency)
In addition to adjusting for the H2O content and sulfate, you may also need to apply a conversion for IU to ug : theoretically, 100% pure anhydrous streptomycin sulfate contains 798ug streptomycin base, and the Third International Standard for used for determining potency is 785 units per mg sulfate. Therefore, you need to multiply the I.U. value for your material by the ratio 798ug/785IU to obtain ug streptomycin base per mg streptomycin sulfate weighed. In the above example,
749IU x 798/785 = 761ug/mg
or 76.1% streptomycin base. Adjusting for moisture, 1000ug  6% = 940ug anhydrous sulfate x 761/1000 = 715ug pure streptomycin base for each mg of sulfate weighed.
Hope this helps...
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Re: convert units/mg to ug/mg (potency)
i have maxillase solution contains 20000 units of alpha amylase per 100 ml solution and i want to prepare 50 mg per ml solution how can i calculate thank you for help
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