I would like to know why sometimes we use PBS with potassium and other times, without this salt on ELISA assays.
Hello Paula K. Sato
There are several recipes to prepare PBS solution. The essential solution contains water, sodium hydrogen phosphate, and sodium chloride.
Some preparations contain potassium chloride and potassium dihydrogen phosphate. EDTA may also be added in cellular preparation to prevent clumping.
Some PBS solutions do contain calcium or magnesium.
PBS is not ideal for use in solutions that contain divalent cations (Fe2+, Zn2+) because precipitation may occur. Also, phosphate may inhibit enzymatic reactions. Phosphate buffer should not be used in assays where competition for phosphate groups or complex formation with a metal ion is essential for the enzyme activation. Phosphate ions will inhibit carboxypeptidase, carboxylase, urease, muscle deaminase, formase and phosphoglucomutase.
So, depending on the type of assay certain recipes will be much more suitable than others.
Best.
As the previous answer outlines, there are multiple formulations for ‘PBS’. It is hard to think of any reason why you would need potassium chloride for an ELISA. I’d use the formulation with just sodium phosphate and sodium chloride (for ease of preparation), unless the specifics of the interactions (or any unwanted interactions) demand the addition of other components e.g., more salt, EDTA, BSA, detergent, and so on.
Depends on what it's used for: If you just need a buffer, use e.g. 10mM of the cheapest phosphates you can get. If you require it isotonic, add 0.9% NaCl. If you are working with cells or animals, also take care of the Na/K balance.
Phosphate-buffered saline (PBS) buffer solutions are mixed Na/K monobasic and dibasic phosphate solutions with added Na/K chloride salts. The neutral salts (NaCl/KCl )do not contribute noticeably for the solution pH.
We can predict the pH of PBS buffer solutions after the following 'adapted' Henderson–Hasselbalch equation, within the effective buffer range pH = pKa2 ± 1 (6.2―8.2):
pH = pKa2 + log10{(CNa2HPO4 + CK2HPO4)/(CNaH2PO4 + CKH2PO4)}
Here C refers to nominal (formal) concentrations of the salts in the mixed solution. Ka2 is the second dissociation constant of phosphoric acid.
You may want to check my post at this related query:
https://www.researchgate.net/post/Why_do_I_struggle_to_adjust_lower_pH_of_my_phosphate_buffer_at_a_certain_pH_point_even_though_I_add_too_much_acidic_solution
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