Chemical Forums

Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: Link The Worlds on September 16, 2010, 08:10:19 PM

Title: Preparing Buffer Solutions
Post by: Link The Worlds on September 16, 2010, 08:10:19 PM

I'm really lost for this assignment and don't even know where to begin...

I'm supposed to describe the preparation of 15 liters of .07 M phosphate buffer, pH 7.00 starting from:

a) 1.00 M KH₂PO₄ and 1.00 M Na₂HPO₄


What I do know is that the pKa of H₂PO₄ is 6.82, and I was told this is the only pka necessary, but I don't know why the pka of HPO₄ is not necessary.

Title: Re: Preparing Buffer Solutions
Post by: Borek on September 17, 2010, 02:57:28 AM

Solution is dominated by the H₂PO₄^- and its conjugated base, other acids/bases are present in minute quantities.

Title: Re: Preparing Buffer Solutions
Post by: Link The Worlds on September 17, 2010, 08:15:19 AM

Okay, that makes sense. But what if instead of 1.00 M KH2PO4 and 1.00 M Na2HPO4, there is 1.2 M K₂HPO₄ and 2 M H₂SO₄.


When I plug H₂SO₄'s pka value (-3) into the Henderson-Hasselbech equation, I come out with A/HA = 1 x 10¹⁰

So I don't know what to do first.

Title: Re: Preparing Buffer Solutions
Post by: AWK on September 17, 2010, 11:18:14 AM

Sulfuric acid is much stronger than phosphoric one, so calculate reaction between salt and sulfuric acid first.