What is the equivalence point on this graph?

[Tianna]

I am attempting to do a lab that requires the determination of an unknown diprotic acid using a titration curve. I have graphed my data but I am unsure what point to use as my eq. point. In my lab manual it says I may use the second or first eq. point but the second is often clearer. I am not given a specific equation to determine the point.

In your opinion, which point should i use to continue my calculations? There was a very large jump between 20 and 22ml NaOH leading to the pH jumping from 6.7 to 10.47.

Should I use 10.47 as my eq point?

 

[Tianna]

I am also thinking that maybe if I add up pH 6.7 and pH 10.47 and divide by 2, I get a value of 8.5 that when graphed seems to line up to what I believe is the equivalence point?

 

[Borek]

Best approach would be to to read the end point from both and to find the average (that is, not a direct average, but the 2nd should be exactly twice the 1st).

Second alone should be a little bit better, as the jump is higher.

Another approach would be to use Gran's method for the end point determination.