I am struggling a lot and have a very small amount of time left
to submit it. I will be very grateful if you answer these for me.
Please help.
1. In an HPLC experiment you are asked to prepare dilutions
scheme for the given sodium benzoate and caffeine stock solutions.
These stock solutions are required to prepare three standard
solutions and the stock solution of benzoate ion is 100 ppm and the
stock solution of caffeine is 200 ppm and both were produced in
deionized water. Hence, you are asked to dilute these in a buffer
that is provided. The three standard solutions should be 50 mL
solutions consisting of a) 20 ppm caffeine, b) 10 ppm benzoic acid,
and c) 20 ppm caffeine + 10 ppm benzoic acid mixture. These
dilutions all should be made in a pH 6 buffer.
2. Also, you are asked to calculate the amount of phosphoric
acid and phosphate salts (sodium phosphate, sodium hydrogen
phosphate, and sodium dihydrogen phosphate) you would to make the
solutions with pH values of nearly 2.0 and 6.0. Suppose you will
need approximately 500 mL of each buffer to complete your
experiment and the total buffer concentration is 0.010 M. This is
the fourth dilution.
3. Calculate the percent protonated benzoic acid you would have
in each of the buffers used. How might this impact the retention
times and separation of the analytes? Based on its pKa, is caffeine
ionized or not? Please explain your rationale.