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

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SUMMARY

The correct acid dissociation constant (Ka) for Phosphate Buffered Saline (PBS) at pH 7.4 is 1.59E-7, as derived using the Henderson-Hasselbalch equation with the concentrations of 0.008M Na2HPO4 and 0.002M KH2PO4. While some sources cite a Ka of 6.2E-8, the calculations based on the provided pH and pKa values (2.148, 7.199, 12.35) confirm the higher value. It is essential to consider the ionic strength of the solution when performing these calculations to ensure accuracy.

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  • Understanding of the Henderson-Hasselbalch equation
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  • Familiarity with the composition of Phosphate Buffered Saline
  • Basic principles of ionic strength and its effect on pH calculations
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Biochemists, laboratory technicians, and researchers working with buffer solutions and pH calculations will benefit from this discussion.

wils0645
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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?
 
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pKas - 2.148, 7.199, 12.35. You should take solution ionic strength into account before calculations.
 

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