Hello,
I´d like to know where i could find information about kh2po4 solubility in organic phases; i have to study the influence of pH in my mobile phase and i suspect that 5mM kh2po4 is not totally soluble in 45%AcN;I´m not sure, because i can see like threads and is not the usual way of precipitation of phosfates..
I have to study from ph 3 to ph 10.It is possible that solubility decreases increasing ph?

Thanks

I'm not sure that there is any good source of tabulated information on this topic but I can give you a few bits of information from my experience.

1). The solubility of phosphate salts definitely decreases as the pH increases

2). Use of cations with improved solubility in organic phases will improve the solubility of phosphate salts somewhat (e.g. cesium or better still, triethyl amine).

3). As acetonitrile level increases solubility of phosphate salts decreases.

One other bit of advice: there's nothing magic about phosphate buffers so it would be advisable to consider alternative buffers. Organic based buffers such as amine based buffers or Goods buffers are considerably more soluble in organic solvents and much less corrosive to silica.

Dear Chris,

I´ve been trying all day to mix at 45%Acn kh2po4 5mM buffer of different ph , and i´ve found something rare: I could work at pH 3, and 4, but i got very small threads at pH 5, and Í´ve worked at ph 7 with a buffer 25 mM. I can´t understand it...


Thank you very much for your help. I know that it´s august, and most people is on holiday,(not me).

Celia

Be careful! Buffers are at their most effective at pH values near their own pKa. Phosphate is not very good from the low 3's to the low 6s.

Dear anonymous,
do you think i´m going to have problems if i prepare the buffer solution everyday?pH 3

You should be OK at pH 3. 10mM phosphate at pH 3 should not give you precipitation problems until you add approx 90% ACN.

thank you very much.
Celia

Celia,
Studies involving the effects of pH in partially aqueous solutions can be complicated by changes that occur when an organic solvent is added to aqueous buffers. Buffers prepared from neutral acids (for example phosphoric) as well as anionic acids (for example dihydrogen phosphate anion) typically get significantly more basic when diluted with acetonitrile or methanol. This is the reason that high pH buffers prepared from phosphate salts are partucularly aggressive in attacking silica. However, buffers prepared from cationic acids (for example ammoniium ion or amines) typically get more acidic upon dilution with organic solvent. For this and other reasons it is a complex problem to predict or interpret pH effects in partially aqueous solutions from aqueous data.

If you are going to study the effect of pH on something in acetonitrile/water mixtures it would probably be most reliable to make the pH measurements in the partially aqueous solution. With this approach reliable comparitive measurements can be made. However, there are many things to be concerned with using this approach as well.

Dear Bill, how could I meassure pH in an mixture? i can´t calibrate the electrode.

I was able of doing the study;and i found interesting things..one of the substances doesn´t elute at ph higher than 5.It´s benzoic acid( i´m warking on reversed phase : C18)

Thank you very much. I(´ve found interesting what you´ve told me..

Celia