Chromatography Forum

Hellow

I am praveen . I want to know that It was found some time that the peak hight is more when we usese KH2PO4 as a buffer as compair to NaH2PO4. Probably it is due to high buffering capacity of KH2PO4 or some thing else.

Please send me if any suitable litrature.

Regards \
Praveen

That is difficult to answer without more details:
- what pH?
- what concentration of buffer?
- what is the chemistry of the analyte (acid, base, both, pKa value(s)?)
- how are you detecting (UV, if so, what wavelength; RI . . .?)

The buffering capacity should be equal since the Na+ and K+ ion has no buffering effect whatsoever.

Slightly off-topic: does anyone know the reason why USP standard phosphate buffer contains a mixture of sodium and potassium phosphate?

To Toms question I would like to add: Did the area change when the peak hight increased?

Mattias, The presence of both K+ and Na+ might go back to a Krebs-Ringer buffer, which contains both ions in an attempt to mimick the aqueous part of blood. Just a guess.
K+ can have a different ionic effect than Na+. This can also be reflected in a different solubility of K vs Na buffers.

[quote="tom jupille"]That is difficult to answer without more details:
- what pH : Near to 4.2-4.5
- what concentration of buffer = 0.01M
- what is the chemistry of the analyte = Acidic name is Hydroxy Nitro Biphenyl
- how are you detecting : UV detection was takes plase with 205nm
Basically I bserved several time that responce in KH2PO4 buffer is high as compair to NaH2PO4 . As my Concern of thinking that in acidic media, the movability of K+ ion is more as compair to Na+ ion . This is because of High charge dencity over Na+ ion . The high charge dencity is due to smaller size of Na+ ion as compair to K+ ion.
If any other logic and litrature please send us.

Thanksn and regards

Praveen

At this pH, phosphate is not a buffer, and your pH may wiggle, depending on the salt used. If your analyte's spectrum changes with the pH, you will get different responses.

if the substance is acid I think it is better if you use lower pH, ie 2.0 - 2.5. phosphate buffer is suitable to use in this pH range.

Hydorxy biphenol is acidic right now I change the buffer I took 0.1% H3PO4 in water. I ot good retention.
My next product is 3-Amino 4-methoxy biphenyl. This also gave us same result at the pH near to 4.5 the sodium buffer gave us slight lower responce as compair to pottassium buffer.

The difference in responce is not very high but it was observe thet KH2PO4 when use as a buffer the peak hight is slight higher than NaH2PO4

as Uwe pointed out already: phosphate (sodium or potassium) has no significant buffer capacity at pH=4.2-4.5! If you want to run at that pH use sodium acetate as buffer salt.
If you use phosphoric acid to adjust the pH of your mobile phase make sure that small changes in concentration don't significantly alter your retention behavior or efficiency of your separation.

--
Robert Haefele

The variation in peak heights (Absorbances) of the sod. & Pot. buffers may be due to some impurities present in one of these buffers, that may be absorbing in the 205 nm region.

To avoid this:
1. Use a Sod or Pot buffer from a reputed manufacturer such as J.T.Baker.
2. Impurities in the buffer absorbing is not a new issue. We had a similar problem, and had to discontinue with one supplier.
3. When preparing buffers, make sure to filter the aqueous part to avoid any spikes or flow disturbances in the column or detector.
4. Make sure, your mobile phase composition is compatible with buffer formulation i.e., no excess buffer or buffer precipitation, which could occur with a higher organic modifier content (> 50% ACN). Your buffer strength is only 10 mM (0.01M) and hence, I do not foresee this problem.
And, I am assuming your %ACN is not over 60%.