PHAR 4634 - Chapter 22 Page 3    ## Extraction of Salicylic Acid

AIM:

1) To measure the extraction of salicylic acid from aqueous buffer solution into two different organic solvent mixtures, hexane/ethyl acetate (10 % ethyl acetate in hexane) and hexane/propanol (10 % isopropanol in hexane).

2) To measure the extraction of salicylic acid (0.02%) from aqueous buffer of various pH's (2.5, 2.8, 3, 3.5, 3.8, and 4.0) and thereby determine the pKa for salicylic acid.

BACKGROUND:

Salicylic acid is a relatively polar, poorly aqueous soluble material. The salt form however is quite water soluble. By changing the pH of the aqueous buffer you are able to alter the ratio between the ionized and unionized form of the acid. Since it is the unionized form which is extracted into the organic phase, the fraction extracted will vary with pH of the aqueous solution. The definition of Ka can become useful.

In aqueous buffer The partition between organic and aqueous buffer can be described by a 'true' partition coefficient, PC, as or

[HA - organic] = PC * [HA - aqueous]

In lab you will be measuring an apparent partition coefficient, PC', which will vary with pH or [H+]. This apparent partition coefficient is given by: substituting for HA-organic (PC equation) and [A-] (Ka equation) gives or This gives us an equation with PC' as function of [H+] with two unknown parameters, PC and Ka.

This can be converted into a straight line equation by taking the reciprocal of both sides of the equation.

Thus:- OR Therefore plotting 1/PC' versus 1/[H+] should give a straight line with a slope of Ka/PC and an intercept of 1/PC

METHOD:

PC' as a function pH

Take 4 ml of the salicylic acid - buffer pH 2.5 stock solution and add 1 ml of the ferric nitrate solution. Allow the color to form and measure the absorbance at 540 nm. This is absorbance ONE. Next take 5 ml of the salicylic acid - buffer pH 2.5 stock solution and add 5 ml of the organic solvent hexane/ethyl acetate. Stopper and shake the test-tube for 5 minutes to complete the extraction. Allow the two phases to settle, remove 4 ml of the aqueous phase, add 1 ml of ferric nitrate solution (0.55% Ferric Nitrate in 0.4 M Nitric Acid), allow the color to form, and measure the absorbance at 540 nm. This is absorbance TWO.

Using the two absorbance readings, calculate the apparent partition coefficient, PC', as with [total in organic] = First absorbance - Second absorbance and [total in aqueous] = Second absorbance

Thus Repeat the experiment with the buffers of pH 2.8, 3.0, 3.5, 3.8, and 4.0.

Plot 1/PC' versus 1/[H+] and calculate PC, Ka, and pKa. {Remember [H+] = 10(-pH)}.

PC' with different organic solvents

Repeat the experiment above at pH 3 only, except use hexane/isopropanol as the organic solvent. As you will be only doing this experiment at one pH, you will only need to measure the apparent partition coefficient at the one pH used.

WRITE-UP:

Clearly present the aim, theory, methods, results, analysis, and conclusion to the experiments. Compare your results with theoretical results (for Ka). Compare PC' in the two organic solvents. Your report is due by the start of the next lab session.    