pH, pKa

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pH, pKa

Post by Miss_Me » Tue Feb 27, 2007 7:24 pm


Could someone help me out here? I've been trying to answer this question for the past week or two and I don't seem to be getting very far. As it is related to chemistry (biochemistry module!), it makes it twice as difficult for me as I'm not very good at chemistry!

Thanks in advance :)

This is the information I have:
I have plotted a graph with absorbance, also known as optical density(OD), on the y-axis against pH on the x-axis. The pH values range from 4 to 10. This is of o-nitrophenol
A positive linear relationship can be seen.

Q) Calculate the pKa

The information which was given and is required to calculate pKa are:
1) Change in absorbance with pH is directly proportional to change in ionisation of o-nitrophenol with pH.
2) pKa of o-nitrophenol is the pH at which the change in ionisation is half complete (NB [base]=[acid]).
3) O-nitrophenol can be considered unionised at pH 4 despite exhibiting a absorbance at this pH.

Thanks again!

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Post by mith » Wed Feb 28, 2007 12:44 am

Figure out when the ionization is complete, then divide that value by 2 to get the half-complete point. Note that your "zero" value is probably calibrated at 4pH.
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