Ka for Phosphate Buffered Saline: 6.2E-8 or 1.59E-7?

[wils0645]

Anyone know the Ka for Phosphate Buffered Saline (pH 7.4). I have heard that it is 6.2E-8, but the formula says .008M Na2HPO4, .002M KH2PO4. Using Henderson Haselbalch, [H+] = Ka[KH2PO4]/[Na2HPO4] using pH of 7.4, Ka = 1.59E-7. Which one is correct?

 

[Borek]

pKas - 2.148, 7.199, 12.35. You should take solution ionic strength into account before calculations.