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Homework Help: I have a titration question dealing with biochemistry!

  1. Sep 21, 2012 #1
    1. The problem statement, all variables and given/known data
    You are titrating a 75 mL volume of 33 mM lactate buffer (pKa 3.86) with 1.3 M HCl (starting fully deprotonated). What is the pH after you have added 1 mL of the HCl? Give 2 decimal places

    2. Relevant equations
    pH = pKa + log[A-/HA]

    3. The attempt at a solution
    I don't really understand the concept of deprotonated and fully protonated. Could someone explain the two? I also have no idea where to even start with this question...
  2. jcsd
  3. Sep 22, 2012 #2


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    Gold Member

    Deprotonated buffer - No H+ ions present in the buffer solution, meaning the buffer can no longer digest the base.

    Protonation - Addition of H+.

    If you add H+ in the buffer, you are protonating it, thus you are decreasing the pH of the buffer. If you want to attempt the question, ask yourself the following questions:-

    1. How many moles of buffer I've been given? How much moles of H+ I am adding?

    2. How much H+ can 1 mole of lactate buffer can assimilate?

    3. Can I use the relevant equation?
  4. Sep 22, 2012 #3


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    Staff: Mentor

    You are not "protonating a buffer". You are protonating the conjugate base. So if there is only acid from (HLactate) in the solution, lactate is fully protonated. That will happen at low pH. If there is a mixture of HLactate and Lactate- - it is protonated only partially, that happens when the pH is relatively close to pKa. Finally, in high pH all lactate is present in Lactate- form and it is not protonated at all.

    You can assume protonation is quantitative - all strong acid added reacts with Lactate- yielding HLactate. Use this information to calculate new concentrations of acid and conjugate form and plug them into the Henderson-Hasselbalch equation.

    Also compare this buffer pH change problem.
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