When extracting diphenhydramine from hexane and calculating the concentration of diphenhydramine using the Beer-Lambert law, the ratios of NH3/NH4+  don't match the expected ratios based on the Henderson-Hasselbalch equation. Why?

Any number of reasons. What factors do you know of that might affect equilibrium? How did you do the extraction?

Step 1: Creating pH 4, 7, 10 buffered stock solutions. Step 2: Obtain absorbance by spectroscopy of buffered solutions. Step 3: Obtain absorbance of stock solutions after extraction with hexanes. Step 4: calculate concentration with Beer's Law 

Question: Why don't these values match the expected ratios based on the Henderson-Hasselbalch equation?

The difference is due to the buffer solutions are able to withstand most changes in pH and maintain a reasonably stable pH?

 

Edited October 5, 20178 yr by gammagirl
addendum

I have another question. Even though a buffered solution at pH 7 should have a 100:1 ratio of ammonium ion: amine, why does the absorbance of the aqueous solution after extraction with hexane have such a  large difference when compared to the unextracted stock solution of the amine a ph 7?

Edited October 5, 20178 yr by gammagirl
at ph 7, amine moved to organic layer out of aqueous layer

Hold up, I still don't understand what you're doing exactly. Can you paste the exact procedure and any preamble? 

To clarify: I understand the concepts, but what you've relayed in terms of procedure is a bit unclear to me. 

 

Part 1: (1) Made an unbuffered stock solution of DPH-diphenhydramine (100mg/100ml) in water (2) Made ph4 buffered stock solution of DPH      (3) Made ph7 buffered stock solution of DPH (4) Made a ph10 buffered stock solution of DPH                                                                                                Part 2: Obtain absorbance at 252 of solutions in Part 1                                                                                                                                                                      Part 3: Add 5ml of DPH stock solutions to 5 ml hexanes and obtain absorbances.                                                                                                                          question: even though a buffered solution at pH7 should have a 100:1 ratio of ammonium: amine, why does the absorbance of the aqueous solution after extraction have such a large difference when compared to the unextracted stock solution of DPH of ph7?                                                      question: Assuming molar absorptivity of DPH is 388L/mole cm (a) calculate the approximate conc of DPH in each stock solution, before and after extraction with hexane. (did it) (b) Why don't these values match the expected ratios based on Henderson-Hasselbach equation? (my answer: HH is valid when it contains equilibrium concentrations of an acid and conjugate base. In this lab, changing pH increases the amount of DPH moving to the organic layer from the aqueous layer, changing the expected ratios of conjugated base and acid.)

The answers mentioned above are correct, but I thoroughly understand the experiment at a higher level. Therefore, the Henderson-Hasselbach case is closed. 

Great! Sorry I didn't reply to your last post, I must have missed seeing it. Glad you're understanding things better. 

Thank-you