Stock Standard Preparation

[KMP]

Dear All,
Please give me kind suggestions, step by step.
Want to prepare stock standard solution in acetonitrile (For stock std which one is suitable ? 1000 ppm or 2000 ppm?, The volume 10 ml is Ok?) :
My standards are as bellow
1) PCB8
Mol.wt = 223.10
I have 25 mg (powder)


2) For PCB138
Mol. wt = 360.88
I have 10 mg (powder)

3) For Tris (2-chloroisopropyl) phosphate
Mol wt = 327.6
I have 0.25 g (liquid)

4) And for the 5 % formic acid solution in acetonitrile, (vol/vol),How should I do?
So can I prepare 5 ml of formic + 95 ml of acetonitrile? But my formic acid bottle write 85%. Need to care this 85%?
I will be happy if you could help in this matter.

[tom jupille]

First of all (as has been mentioned in other threads here) "ppm" is *horrible* nomenclature. You must determine whether you are referring to wt/vol or wt/wt.

If it is wt/vol, then 1000 ppm = 1 ppt (part per thousand) = 1 mg/mL
If it is wt/wt, then 1000 ppm = 1mg analyte per gram of solution. You have to divide by the density of acetonitrile (0.78 g/mL), which gives 1.3 mg/mL. Molecular weight is irrelevant, since you are defining your solutions in terms of mass, not moles.

1000 vs. 2000 ppm? I have no idea; its a matter of convenience when you do your final dilutions (assuming there are no solubility problems).

What volume to prepare is up to you. It depends on how much do you expect to use over the lifetime of the stock solution.

To prepare a 5% formic acid solution starting with 85% formic acid, transfer 5.9 ml to a 100 mL volumetric and make up to the mark with acetonitrile. The 5.9 mL accounts for the 85% purity of your formic acid, and by making up to volume you minimize any volume-change-on mixing issues.

[KMP]

Thank you for all the help.