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In adjusting the osmolarity of a solution of Benzalknonium Cl, Disodium Editate, and Disodium borate/boric acid buffer to be isotonic (about 290 mOsm), the addition of NaCl showed unexpected behavior, possibly non-linear.

I strongly suggest that the common ion effect plays a role here, but how to account for it quantitatively? in other words how to calculate the amount of molecules of say benzalkonium Cl shifted to undissociated state after addtion of NaCl ?

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  • $\begingroup$ How would you describe the unexpected behavior of NaCl in your mixture ? $\endgroup$
    – Maurice
    Commented Jan 16, 2020 at 17:32
  • $\begingroup$ NaCl was added to adjust osmolarity, but the calculated amount of which to acheive isotonicity turned out to be too low in first time, increseaed osmolarity by only 15 mOsm rather than the required 205 mOsm. In second time increment was too high = 260mOsm while the required was 190 mOsm. The amounts of NaCl were calculated by linear relationship with 0.9 NaCl aqueous solution for which osmolarity=300 mOsm $\endgroup$
    – M.ghorab
    Commented Jan 18, 2020 at 5:07

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