About a good method to prepare the mobile phase

[Tequila]

Dear members of this forum,

I would like to ask a question about the best method to prepare a mobile phase. Always I adjust the pH in the buffer aqueous solution because the pH scale (0 - 14) is to aqueous solution.
The last week I had to do an hplc analysis where I had to adjust the pH in a mixture of methyl alcohol and phosphate buffer (pH: 5.5), the relation was 600:400. The technique was an official method, I disagree by two reasons, first at this pH the phosphate not work as buffer and second I adjust the pH of phosphate buffer to pH 5.5 and when prepared the mixture 600 ml methyl alcohol and 400 ml of buffer solution.
I adjusted the pH again according to the official tecnique and I would like to know other opinions about if it the preparation of this mobile phase is good or not.
Thanks in advance for your help,

Diego Delmonte

[John]

What you are achieving using this method is an apparent pH, not a true pH.

However, if the method you are following works and gives reproducible results, then the mp phase prep is good.
J

[Consumer Products Guy]

Agree with John; it's just very important that the preparation instructions are very specific, and not ambiguous. I have seen quite a few published procedures where things could be interpreted more than one way.

[Uwe Neue]

I actually think that the situation is even worse.

when you dissolve a monobasic phosphat salt in water, you get a pH of around 4.5. When you add organic to this (such as your 60% methanol)and measure again, you get a higher pH (prbably just around 5.5 with 60% methanol).

Conclusion: you don't have a buffer, and it is measured in a silly way.