[_jess_]

Hi, everyone,

I need to make a solution for an oral fluid with a final concentration of 25 ng/mL of morphine. The morphine that I will use is sold as 1 mg/mL in methanol. Is there anyone that can help me? My lecturer told me to look at this paper (look at appendix) but I can't understand it.

Jessica

[Borek]

Sounds like a standard dilution problem, do you know how to deal with these?

[_jess_]

Not really, I'm trying to figure out the number of moles contained in the ample, and I got 3.5 x 10^-6,but I don't know what to do after that.

[_jess_]

And is it correct to assume that the molar concentration is 0.1 M?

[Borek]

In this particular case you don't need to follow the moles, both starting and final point are mass concentration (mass per volume). What is important is that the amount of the substance (here measured as mass) doesn't change. So, if you start with 1 mL you have 1 mg, no matter in what volume it will be diluted, it will be still 1 mg. If you dilute it to 1L you will have 1 mg in 1 L, or 1 mg in 1000 mL, or 1 μg/mL. Do you see where these numbers come from?

No, 1 mg of morphine per 1mL is in no way related to 0.1 M.

[_jess_]

I see. Could you tell me if my reasoning is right?
To make a final concentration (V2) of 25ng/ml, I would pipette 2.5ul (0.0025 ml) from the stock of 1mg/ml and mix it with 100 mL of methanol. Correct?

And even if I don't need it, the molar concentration of the stock solution is 3.5 mM, correct?

[Borek]

To make a final concentration (V2) of 25ng/ml, I would pipette 2.5ul (0.0025 ml) from the stock of 1mg/ml and mix it with 100 mL of methanol. Correct?

Looks OK, although the correct procedure is that you should not add to 100 mL, but fill up to 100 mL, addind 1 mL to 100 mL typically ends with 101 mL of the mixture which is not what you need (concentration error is in the 1% range and easy to avoid).

And even if I don't need it, the molar concentration of the stock solution is 3.5 mM, correct?

Yes.

[_jess_]

Awesome, and thank you very much for all the clarification and for the stupid questions.