My anesthesiology preceptor posed this question to me last week. He asked, "What do think the pH of normal saline is?" Thinking that it's a reaction between a strong acid and strong base, i.e. HCl + NaOH <--> NaCl + HOH, I said it must be neutral or pH 7. When we actually obtained a bag of normal saline, the label said pH 4.7-5.5 (or something like that), basically acidic, right?
I could not figure out how NS could be acidic and for once, Google failed! Some online sources cited the plastic packaging, which they claimed leeched acidic compounds into the saline. Or, they explained the acidity through dissolved CO2.
The answer turned out to be deceptively simple, one I never could have reached because of course, I've forgotten all my basic chemistry. Think about the definition of pH. pH = -log [H+] How does something become pH 7, for strong acids/bases? It needs to have a measured H+ concentration of 10^-7. Turns out, the salt in NS alters the dissociation constant of water such that there is more measured H+ dissociated than OH-. It is still "neutral," but the pH is calculated to be lower.
Hello, I am a junior doctor in the UK who has bee aware for a while that the pH quoted for normal saline is between 5.0 and 6.0.
ReplyDeleteAn explanation on titration curves is given in the following website, treasured by students of chemistry across the UK:
http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html
This source says that, when adding hydrochloric acid to sodium hydroxide, at the equivalence point the pH will be 7.0 (at room temperature, I believe).
Therefore I am still puzzled as to why the pH of normal saline (sodium chloride) is 5-6. If you are correct in saying that sodium chloride changes the dissociation constant of water, then it must have to be a large change, as from pH 7 to pH 6, the concentration of hydrogen ions increases 10-fold, and from 7 to 5, 100-fold.
We you say "neutral", do you mean to say that the *calculated* pH is 5 to 6, but that the measured pH is 7? Or by neutral, are you saying that hydrochloric acid and sodium hydroxide were added in equivalent mole amounts to complete the reaction?
I think that the only way to solve this will be to get some pH paper or a pH meter, and actually test a bag of saline! Watch this space.
Forgot to mention:
ReplyDeletehttp://www.frca.co.uk/article.aspx?articleid=100924
attributes the pH of normal saline being less than 7 to something called the Grotthus mechanism and also due to dissolved CO_2.
Was doing some research on this topic and found a good write-up on this very topic:
ReplyDeletehttp://www.medsci.org/v10p0747.htm