I am studying anesthetics and came across thiopental which is considered a weak acid. However it has a pH of 10.5. I always learned that greater than 7 is alkaline. I’m looking for some guru to help me outs thanks!


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    $\begingroup$ related: chemistry.stackexchange.com/q/7851/69977 $\endgroup$ – William R. Ebenezer Jan 27 at 2:30
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    $\begingroup$ Pure compounds such as thiopental don’t have a pH. The pH is a property of aqueous solutions. $\endgroup$ – Loong Jan 27 at 9:06
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    $\begingroup$ What is the source of your statement that it has a pH of 10.5? $\endgroup$ – Buck Thorn Jan 27 at 11:01
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    $\begingroup$ Are you talking about thiopental or thiopental sodium? $\endgroup$ – Karsten Theis Jan 27 at 14:40

No, acids can't have pH greater than 7. By definition, at 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic.

In your question you say thiopental and pH = 10.5. Actually this is sodium thiopental with pH range of 10.2-11.2 for a 8% solution SOURCE. The solution is alkaline because of hydrolysis.

The pH value however can be less than 0 or greater than 14 for very strong acids and bases respectively. (Lim, Kieran F. (2006). "Negative pH Does Exist". Journal of Chemical Education. 83 (10): 1465.)


"Acid" is an ambiguous term. It can mean:

  • an acidic solution is an aqueous[a] solution with a concentration of H+ higher than pure water, i.e. [H+] > 10-7 mol/L, i.e. pH < 7, or
  • an acidic compound[b] is a compound that turns water into an acidic solution when the compound is dissolved, i.e. a compound that contains a hydrogen atom that can "come off"[c] as H+.

Thiopental is an acidic compound. Hydrogen ions can come off of the nitrogen atoms, leaving their electrons behind, like this. (You'd be left with H+ of course, not Na+) "Weak acid" means that some but not all of those H+ ions come off.

Once thiopental is dissolved in water you get an acidic solution. You can then add a base, say NaOH, until the pH is where you want it, say 10.5. At that point pedants will point out that your solution no longer contains thiopental but its conjugate base (=thiopental minus the H+). Everyone else will know what you mean when you say "thiopental solution, pH 10.5." This will be the same thing as sodium thiopental dissolved in water (see the link in Jakub Muda's answer).

[a]almost always
[b]or molecule


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