[desmondo84]

Hi
I've a bit of a problem here, I followed a buffer recipe to make a phophate buffer and would now like to calcuate its ionic strength.
Basically I dissolved 12g of Sodium  Dihydrogen Phoshate in water and adjusted to pH 7.4 to give a 0.1M phoshate buffer.  No salt was added.

Can someone please assist me to calculate the ionic strength of the buffer.  I have tried using the equation, but don;t think my answer is correct!

Thanks
D

[Borek]

What is ionic strength definition? What ions are present in the solution? What are their concentrations?

[AWK]

Calculate molar concentrattions of both salts in solution.
I = molarity of KH2PO4 + 3 times molarity of Na2HPO4
Note, for precise calculation you should take into account activity coefficients and problem can be solved in iterative mode

[desmondo84]

I used an online buffer calculator to obtain this recipe.  It gave me 2 methods - either to use KH2PO3 and Na2HPO4 or just to use NaH2PO4 and dissolve it up in water.  I went with the latter as it was easier to prepare.
But I'm not sure how NaH2PO4 dissociates in solution - just to Na+ and H2PO4-?  Doesn't sound correct to me.
thanks
d

[AWK]

Salts are strong electrolytes, phosphoric acid a weak one. Then Na₂HPO₄ produces 2Na⁺ + HPO₄^2-
NaH₂PO₄ produces Na⁺ + H₂PO₄^-

[desmondo84]

so for a 0.1 M soln of NaH2PO4, the ionic strength is 0.1?

[AWK]

E

so for a 0.1 M soln of NaH2PO4, the ionic strength is 0.1?

xactly.
And 0.1 M Na2HPO4 has ionic strength 0.3

[Borek]

Note: after adjusting pH to 7.4 ionic strength is somewhere between these numbers. You should calculate concentrations of H₂PO₄^- and HPO₄^2- using Henderson-Hasselbalch equation and than calcuate ionic stength of the soultion.

In fact you should then adjust your calculations to account for ionic strength and activity coefficients, but that's another story.