Chromatography Forum

I have to prepare 500mM Acetic Acid / Triethylamine (pH 7.3) for HPLC mobile phase. I need to know whether TEA is only used for adjusting the pH of the solution or as a part of the composition. Please someone help me...

triethylammonium acetate (equimolar acetic acid and triethylamine) is supposed to give a pH of 7 (http://en.wikipedia.org/wiki/Triethylammonium_acetate), so you will have to add a bit of additional TEA to bring the pH up to 7.3.

Unfortunatly, this Wikipedia entry is somewhat sloppy.
There is a bit of info on triethylamine here:
http://www.frankmckinnon.com/triethylamine.htm
for instance,

Dissociation Constants:

pKb= 3.25
[Clayton, G.D., F.E. Clayton (eds.) Patty's Industrial Hygiene and Toxicology. Volumes 2A, 2B, 2C, 2D, 2E, 2F: Toxicology. 4th ed. New York, NY: John Wiley & Sons Inc., 1993-1994. 1089]**PEER REVIEWED**


pKa = 10.78 @ 25 deg C
[Riddick, J.A., W.B. Bunger, Sakano T.K. Techniques of Chemistry 4th ed., Volume II. Organic Solvents. New York, NY: John Wiley and Sons., 1985.]

Thus:
Triethylammonium acetate at a pH of 7.3 is not a buffer, so it doesn´t matter how you prepare it, just state how you did it. Since ammonia has a pKa of ~9.26, and since ammonium acetate has a pH of 7, the statement that triethylammoniumacetate has a pH ~ 7 is incorrect.

(Also note that the Wikipedia is wrong on the color. From my link one gets:
Color/Form:

Colorless liquid
[Lewis, R.J., Sr (Ed.). Hawley's Condensed Chemical Dictionary. 13th ed. New York, NY: John Wiley & Sons, Inc. 1997. 1129].

Since acetic acid is colorless as well, and the protonation does not introduce color, the triethylammonium acetate should be colorless).)

Triethylammonium acetate at a pH of 7.3 is not a buffer, so it doesn´t matter how you prepare it, just state how you did it. Since ammonia has a pKa of ~9.26, and since ammonium acetate has a pH of 7, the statement that triethylammoniumacetate has a pH ~ 7 is incorrect.

I would agree that it isn't a *good* buffer, but it *is* sold as a 1M buffer with a nominal pH of 7 in the Sigma-Aldrich catalog:
http://tinyurl.com/2fex7hs

I saw that, must be due to saving on competent people.

I have to prepare 500mM Acetic Acid / Triethylamine (pH 7.3) for HPLC mobile phase. I need to know whether TEA is only used for adjusting the pH of the solution or as a part of the composition. Please someone help me...

Yes, TEA is necessary in order to prepare the TEAA buffer. TEAA has two pKa value, 4.76 (pH 3.8 to 5. and 11.01 (pH 10.0 to 12.0). you can find more information in this mobile phase chart:
http://www.younglin.com/brochure_pdf/wa ... 0Chart.pdf