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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: IK on December 06, 2016, 10:24:40 AM

Title: Adjusting pH with Tris buffer
Post by: IK on December 06, 2016, 10:24:40 AM

I want to make a solution of 0.1M KH2PO4 and 8mM TBAHS (tetra-n-butylammonium hydrogen sulphate) and adjust its pH to pH 6 using tris buffer (pH8). Please can I have advice on how to achieve this.

Best regards

Title: Re: Adjusting pH with Tris buffer
Post by: Babcock_Hall on December 06, 2016, 10:39:38 AM

Is this a homework problem?  If not, what is the buffer being used for?

Title: Re: Adjusting pH with Tris buffer
Post by: IK on December 06, 2016, 10:46:20 AM

Hi

Thanks for replying. It is for HPLC analysis. I am following a HPLC method reported in literature where mobile phase A is 0.1M KH2PO4 with 8mM tetrabutyl ammonium hydrogen sulphate that is adjusted to pH6 with Tris buffer.

Title: Re: Adjusting pH with Tris buffer
Post by: Babcock_Hall on December 06, 2016, 11:28:30 AM

If it were me, I would simply dissolve the two salts in a slightly lower volume of water than the final volume and stir the solution in the presence of a pH electrode.  I would then add small portions of solid Tris base until I had obtained the pH that I desired.  Then I would add water to achieve the final volume.

Title: Re: Adjusting pH with Tris buffer
Post by: AWK on December 06, 2016, 12:31:15 PM

Since concnetration of Tris buffer is unknown, the only possibility is adjusting pH experimentally using pH-meter (as Babcock_Hall suggested). Calculations, even when the concentration of Tris buffer is known, are quite difficult without computer program.

Title: Re: Adjusting pH with Tris buffer
Post by: Arkcon on December 06, 2016, 09:08:43 PM

Adding solid TRIS is going to be extremely impractical.  You can add small mounts of strong, say 1 or 2 M TRIS seems like a fair idea, but not the best one either, as TRIS is slow to dissolve, is a weak base, and its pKa is more temperature dependent than buffers.  Go ahead and do this once, but try to find a better method later.