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# How to find pKa using the absorbance of a solution at different pHs? watch

1. I've plotted the log function of the absorbance of A-/HA against the pH of the solutions, how can I use it to find the pKa?
2. help meh
3. Your equation for Ka is: [H+] [A-] / [HA]

We can calculate [H+] using pH and hence calculate Ka. From here, see if you can calculate pKa.
4. (Original post by S.H.Rahman)
Your equation for Ka is: [H+] [A-] / [HA]

We can calculate [H+] using pH and hence calculate Ka. From here, see if you can calculate pKa.
I know that, but how does the absorbance fit in?
5. (Original post by TheGreatPumpkin)
I know that, but how does the absorbance fit in?
hmm.. what are the units of this on the graph?
6. (Original post by S.H.Rahman)
hmm.. what are the units of this on the graph?
Both abstract units it's log(A-/HA) against pH
7. any ideas?
8. (Original post by TheGreatPumpkin)
I've plotted the log function of the absorbance of A-/HA against the pH of the solutions, how can I use it to find the pKa?
When you say A-/HA
do you mean [A-]/[HA]

When you use the word absorbance what experiment have you actually carried out/ how have you calculated the absorbance/ Do you know what absorbance is???

Perhaps try explaining your self more carefully/fully.

Anyways, ignoring the odd use of the concept of Absorbance

Ka = [H+][A-]/[HA]

can be very straightofrwardly rearranged to give

pKa = pH + log10([A-]/[HA])

so

pH= pKa - log10([A-]/[HA])

so a plot of log10([A-]/[HA]) on the X axis and pH on the y axis would yield a graph whose intercept would be equal to the pKa.

EDIT: have just read another of your recent posts on this same problem

Could you explain what kind of spectroscopy you were doing? what acid were you using? which species were you interested in quantifying the concentraton of?
9. (Original post by MexicanKeith)
When you say A-/HA
do you mean [A-]/[HA]

When you use the word absorbance what experiment have you actually carried out/ how have you calculated the absorbance/ Do you know what absorbance is???

Perhaps try explaining your self more carefully/fully.

Anyways, ignoring the odd use of the concept of Absorbance

Ka = [H+][A-]/[HA]

can be very straightofrwardly rearranged to give

pKa = pH + log10([A-]/[HA])

so

pH= pKa - log10([A-]/[HA])

so a plot of log10([A-]/[HA]) on the X axis and pH on the y axis would yield a graph whose intercept would be equal to the pKa.

EDIT: have just read another of your recent posts on this same problem

Could you explain what kind of spectroscopy you were doing? what acid were you using? which species were you interested in quantifying the concentraton of?
I had rearranged the formula but never thought to use the intercept . I was given UV/vis spectroscopy data on a solution UV/vis spectroscopy to interpret, which is why I was referring to absorbance, and asked to determine the pKa of it

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