Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

PKa of fatty acids

  1. Aug 3, 2011 #1
    Can someone help me understand this statement:

    The pKa of most fatty acids is around 4.5, so most fatty acids exist in their anion form in the cellular environment.

    I know what a pKa is... it is -log(Ka), where Ka is the equilibrium constant of the reaction. In this case the Ka would be 3 x 10^(-5).

    Also I understand that the larger the Ka, the stronger the acid strength (the more it would dissociate).

    How do we know that a Ka of 3 x 10-5 is a large Ka?
  2. jcsd
  3. Aug 13, 2011 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You can see from the Henderson-Hasslebach equation that if the pH of the solution is equal to the pKa, exactly half of the fatty acid must be deprotonated (-log(1)=0). Work it out for yourself. What is the physiological pH? How many orders of magnitude more basic is it?
  4. Aug 13, 2011 #3
    The same way you know that a person is tall or short or fat or skinny. You compare to other known things.

    Its actually not that big of a number if you compare carboxylic acids to HCl or HBr for example which have Ka's ~106+.

    Its a pretty strong acid if you compare it to ammonia or methane for instance (with pKa's of ~39+).

    If you want the Biochemistry to make sense you should do what chemisttree suggested. Sit down with a pen, paper and table of pKa values and use the Henderson-Hasselbach equation to see what the behavior of the acid/base pair is like at physiological pH's (~7).

    Or you can think of it qualitatively as such; if the pH of solution > the pKa of the acid the species will tend to exist in its anionic/deprotonated form. If the pH of solution if < pKa of acid, then the protonated form will predominate. At pH=pKa the protonated=deprotonated. The extend to which the acid will dissociate can be calculated using the H-H equation but you can get a rough idea that at pH=10 there will be much more carboxylate than carboxylic acid present in solution.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook