[tiffany]

Hello.  I did a chemistry lab yesterday and we did a titration with KOH.  We did this with three solutions.  One was Glycine and the other two were unknown amino acids. We were given a list of pK values for a group of amino acids.  We are supposed to find the vblume of KOH needed and what the pK1 and pK2 values were and from there, determine what our unknowns were.  I plotted my graphs (pH vs. volume KOH added) but there is no clear point where the pK would be.  The lab manual says, "If you cannot see the positions of the pKa's you can calculate their position using M1V1=M2V2 and remembering that at the pKa half the protons have been titrated."  I am really confused on how to find the pK values from using this formula.  How do I determine what my unknown was?  My graph doesn't really flatten out, it's VERY curved.  There are 5 or 6 amino acids that it could be.  Thank you for all your help, I have been on the internet for 7 hours trying to figure this out.  Any feedback would be greatly appreciated!

[Borek]

You should use Henderson-Hasselbalch equation - at 50% titration pH=pK.