Reading this article, I came across a passage which got me to think of this question:

"Normal saline contains approximately $\pu{154mM}$ of each $\ce{Na+}$ and $\ce{Cl-}$ with average pH of 5.0 and osmolarity of $\pu{308mOsm/L}$. LR solution has electrolytes $\pu{130mM}$ of $\ce{Na+}$, $\pu{109mM}$ of $\ce{Cl-}$, and $\pu{28mM}$ of lactate. The Average pH of LR is 6.5 and is slightly hypo-osmolar with $\pu{272mOsm/L}$. 6,7"

Regular $\ce{NaCl}$ solutions should have 7 as a pH, yet this solution is 5. A lactate ringer which has a similar composition to regular $\ce{NaCl}$ is 6.5.

I thought that the pH of 5 doesn't have anything to do with the $\ce{NaCl}$ but with the reaction of blood $\ce{CO2}$ with the water in the $\ce{NaCl}$ solution lowering the pH, but clearly that was wrong.

  • 1
    $\begingroup$ Note that pH of water in equilibrium with air is expected to be about 5.6. NaCl itself does not cause pH lowering, but CO2 and possible impurities do. $\endgroup$ – Poutnik Jun 15 '19 at 6:03
  • $\begingroup$ Note that presence of lactate as salt of weak acid breaks the similarity from acid-base equilibrium point of view. $\endgroup$ – Poutnik Jun 16 '19 at 14:45

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