Ask A Scientist© Chemistry Archive

name        Travis H.
status      student
age         20s

Question -  I understand that salts--like NaCl--formed by
neutralization reactions
between a strong acid and a strong base give rise to neutral aqueous
solutions. Why then is the pH of normal saline, the fluid commonly
used intravenously in medicine--whose composition is 154 mM NaCl--
5.7 (as reported in several texts) instead of 7.0. (I suppose that
any preservatives or other additives could alter the pH as possibly
could interactions with the packaging material.) Is there some
chemical phenomenon (more advanced than that of general chemistry
courses) that accounts for this discrepancy? In the real world, does
Na+ (aq), in fact, hydrolize water leaving an excess of H+?
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Body fluids such as blood has a pH that is slightly acidic (probably about
5.7). One would not want to change that because the change could have
serious health consequences, so the saline is buffered to a slightly acid
pH.

Vince Calder
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Normal saline is actually more acidic 
than blood (pH 5.5 or so).  Normal blood pH is 7.35-7.45.  The reason 
that it is given is by tradition. Sometimes lactated ringers or 
plasmalyte is given which actually contains some HCO3 (a weak anion) that 
causes the pH to be somewhat more physiologic. There is actually a trend 
in medicine toward these fluids for resusitation fliuds for pH 
reasons.   The negative is that plasmalyte contains potassium which could 
elevate potassium further in the face of renal failure.

Jaime
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