In Pharmacopoeia we could see mobile-phase preparation as follow:
“Prepare a solution consisting of a mixture of methanol, water, and glacial acetic acid (700:300:10), add 3.0 g of docusate sodium per liter of solution, and mix. Adjust with perchloric acid to a pH of 3.0, filter, and degass.”
As we know that the best way to prepare the mobile-phase is add an aliquot of adjusted aqueous portion to certain volume of solvent organic. In this example, which is the best of these proposals:
1. Both methanol and water contains 3.0 g docusate sodium and 1.0% glacial acetic acid per liter. pH adjustment just be done to aqueous portion. Add 300 ml of this adjusted solution to 700 ml of methanol.
2. Dissolve 3.0 g docusate sodium and 10 ml of glacial acetic acid to 300 ml of water. Adjust to a pH of 3.0 with perchloric acid (?) or using its basic because the solution has pH below 3. Add this solution to 700 ml of methanol.

Neither one will get you to the same place. Follow the procedure, even if the procedure is junk!

The second option is the same method with BP 2002:
Dissolve 3.0 g docusate sodium in the mixture of 10 ml of glacial acetic acid and 300 ml of water, adjust pH with perchloric acid and add the solution with 700 ml of methanol. The mixture of docusate sodium, glacial acetic acid and water in that proportion results solution with pH of 2.51 - 2.54. How can we adjust this solution with acid to get solution with higher pH?

Hi Syx

It is always right to adjust the pH with aqueous.
Organic solvent never give right ph even your pH meter is calibrated. pH adjustment with aqueous is reproducible so there won't be any Rt change.

This is why I said that you should follow the procedure. The procedure measures the pH after the solvent has been added. This is a different pH value than the pH measured in water.

To anonymous
I agree that the better way to do this is to measure the pH in water. But this has not been done in the original procedure. Therefore the pH specified has nothing to do with the pH in water. If you adjust the pH in water to the value that is specified, your chromatographic conditions are different, and you will get different results.

There have been discussions about this subject here before.

The bottom line is:
You should stick to the procedure, if you want to get the same results, and the procedure says that you should measure the pH after the methanol has been added.

A.Mouse, I would like to choose BP version of buffer preparation of this case. But the problem is: the pH of solution before adjusted by perchloric acid is lower than target pH. How can we adjust this solution with acid?

The first question is, if you are testing the pH as prescribed, with the organic solvent - 70% methanol - in place. If indeed you are doing this correctly, and you get a pH that is lower than 3, I am very surprised, but it is not impossible. The next question is then, how the pH meter was calibrated - in water or with the organic solvent. I would assume that the BP procedure calibrated the pH meter in a completely aqueous solution. The third thing that is possible is the quality of the ion-pair reagent that you are using compared to the ion-pair reagent that was used originally, but this is more far fetched.

As I understood it from a training session give by Waters, the addition of a miscible solvent (methanol, acetonitrile) to a solution has the effect of causing apparent pH (as read by a pH meter calibrated in aqueous buffers) to be more neutral.

The procedure detailed by me above would appear to be similar to the USP buffer preparation method you gave at the start of this thread. This is not the way many people on this board, you and myself included, would make an HPLC eluent. But, we can't discard this procedure out of hand, for no reason except that is how many USP buffer recipies are written.

If you follow the USP method, you may see that you would need to add acid to get to the correct pH. You should not be surprised that changing part of the USP buffer preparation method causes the another part to become invalid.

You should follow the USP method exactly if you are required to. If not, then you can adjust the pH in your buffer by any method you want, because you aren't following the USP method anyway. I can't guarantee the analysis will run properly in the latter case, but I can't say for sure that it won't either.

I always measure the pH in aqueous solution (without organic solvent).

To Syx Interbat:

Since you are not measuring the pH as described, i.e. after the addition of the organic solvent, you get the higher pH. If you would do it as prescribed, measuring it after the addition of the organic solvent, you would measure a lower pH. This lower pH may need to be adjusted with perchloric acid to get you to exactly the same spot as prescribed by the procedure. Any other measurement gets you to a different pH, and you are not following the procedure. If you need to follow the procedure you need to measure the pH after the methanol has been added, and you will need to make the pH adjustment with perchloric acid in this final solution.

Did you actually try to dissolve DOSS in water + acetic acid? Because mine “didn’t want to” :).
If you are working on minoxidil follow the USP method, works good for me (a bit of the tailing, but nothing dramatic), RT repeatability is good even on my quite old and simple HPLC unit.

Good luck

You are right, A.Mouse. I have tried it, follow the instruction and prove it.
For A.Mouse, R.C. and Anonymous, thank you for your help.