By Anonymous on Wednesday, April 7, 2004 - 05:58 am:
Dear all,
I usually prepared a phosphate buffer with K3PO4*3h2O 0.005M togheter with TBAHS 0.005M for C18 HPLC mth. Before use I adjust the pH at 6.5. Now K3PO4*3H2o is out of production of Merck, Fluka and other.
I've tried to use K2HPO4 instead of K3PO4 but I've obeserved a RT shift of 0.5 min. in my HPLC run, with the same pH.
Why ? Is it normal ?
Any suggestion is usefull.
Thank you
By bill tindall on Wednesday, April 7, 2004 - 03:24 pm:
The difference is probably related to how you adjust the pH in each case, in particular what is used to make the adjustment. What is used?
A faster and more precise preparation would be to decide how much of each component you need and simply prepare the buffer by weights and volumes.
By Anonymous on Thursday, April 8, 2004 - 04:40 am:
One of the reason I think , if you use K2HPO4 instead of K3PO4, so it means that when you adjust pH to 6.5 you have to use more or less amount of the pH-adjusting reagent (HCl, NaOH or whatever) ... is that right? ... so, the strength of the buffer (I mean the concentration of any ions in buffer) should be changed and this may affect the retention time.
By Uwe Neue on Friday, April 9, 2004 - 04:25 pm:
You should consider the concentrations of all compounds the potassium ion as well as the phosphate ion, as well as the compound with which you adjust the pH as mentioned above.
By HW Mueller on Tuesday, April 13, 2004 - 12:41 am:
If you use KH2PO4 at the concentration of K2HPO4 to adjust the pH....you always get the same result. Itīs hard to imagine that K3PO4 is not available, did you try Fisher, Baker....? With K3PO4 one should also use other phosphates to adjust pH, not HCl or NaOH, etc. Or calc. the required weights of these (to satisfy Bill). There are web pages on which you can calculate these.
By Anonymous on Tuesday, April 13, 2004 - 02:01 am:
Dear all,
thank you for you help. I add some details to my problem: with K3PO4 the pH is about 8 and I adjust it with few drops of H3PO4, now with K2HPO4 the pH is about 4.5 and I use some drops of NaOH s to bring value at 6.5.
So I can't use KH2PO4 to move up my pH
Thank you
By DR on Tuesday, April 13, 2004 - 06:16 am:
use KOH to move pH up.
Also, if the trihydrate is no longer available, figure out the weight of the phosphate needed on an anhydrous basis and substitute anh. K3PO4 - that should still be available.
By HW Mueller on Tuesday, April 13, 2004 - 11:53 pm:
Something is mixd up here. Here are some values I have seen many times:
0.1M NaH2PO4, pH~4.4
0.025M NaH2PO4, pH~4.6 (taken w. pHBoy, so up to .3 pH units off)
0.15M Na2HPO4, pH~9.3
The K version should be near that, since these are buffers the pH is normally not strongly concentration dependent, except maybe at extremely low concentrations.
By Anonymous on Friday, April 16, 2004 - 07:38 am:
Dear HW Mueller
I've got in my buffer tetrabutylammonium hydrogen sulphate 0.005m so I think that is the reason for the different pH.
By HW Mueller on Monday, April 19, 2004 - 03:05 am:
Sorry, didnīt register that. Arenīt diff. tetrabutylammonium phosphates available? Or tetrabutylammonium sulphate?
By Luca Corte on Wednesday, April 21, 2004 - 07:55 am:
I confirm 0.025 M NaH2PO4 (or KH2PO4) having a pH of 4.6, exactly, 4.64-4.65.
From experience, I'd rather prepare a phosphate buffer by using KH2PO4 and K2HP04 together.
usually, I weigh KH2PO4/K2HPO4 in a molar ratio of 3/5 to get an overall 0.2 or 0.02 M buffer at pH 7, with little adjustement (+/- 0.02 units)
Luca