0
$\begingroup$

I've been trying to learn about indicators and titrations, but I keep on getting stuck on what "colour change" means in relation to the equivalence and endpoint, and I feel like I'm reading definitions which don't really support each other.

From libretext

The equivalence point is when the color changes most rapidly, not when the solution has changed color

And later

Indicators change color gradually at various pH...The long stretched color in the middle of the last line has equal intensity of BLUE and RED. If a solution has a color matching this, the pH would be the same as the pKai of the indicator enter image description here

Which I understand as the equivalence point not so much referring to when the indicator changes colour itself, but when the ongoing colour change is most rapid. But because there seems to be a gradient of colour change within the range of the indicator, I was wondering what the "colour change" present at the equivalence point and endpoint refer to from some other sources on google. For example, from Khan Academy

an indicator is a weak acid or base that is added to the analyte solution, and it changes color when the equivalence point is reached

But one sentence later:

Endpoint: refers to the point at which the indicator changes color in an acid-base titration.

Given that both of equivalence and end point are said to have colour changes, I was wondering if someone would be able to help point out to me exactly what this "colour change" is referring to as I find myself really confused as to what the indicator is meant to measure. To my knowledge it's meant to measure equivalence point(?) as that is when pKa=pH, but would that be the initial colour change, rate of fastest colour change, or when you have a 50-50 blend of the colours of the acid and base form?

Further, in terms of Endpoint, would that be when there is an overwhelming concentration of one form of the indicator over the other such that only one colour is present, or would it be the initial change in colour?

Thanks in advance

$\endgroup$

1 Answer 1

1
$\begingroup$

The titration equivalence point is the point at the titration curve when there has been provided the equivalent amount of reactant.

The titration inflection point is the point at the titration curve where the curve is the most steep. It is very close to the equivalence point but generally not identical.

The titration end point is the point at the titration curve where the titration is stopped and the spent volume is taken to calculation.

  • It is usually related to the conventionally chosen color change or color appearance of the used titration indicator. E.g. at titration of strong acids by $\ce{NaOH}$ solution with methyl orange indicator, it is when red color turned to yellow.
  • For instrumental titration, the second derivative of being recorded titration curve may be used, detecting the inflection point.
  • Depending on the method and the used indicator, there may be smaller or bigger bias, caused by the difference between the end point and equivalence point.

The general approach is to choose such titration indicators, where the equivalence point falls with the indicator color change range.

$\endgroup$
4
  • $\begingroup$ Thanks so much for your explanation. Would there be the case where the methyl orange indicator is instead orange instead of red or yellow? So if the PkA of methyl orange is around 3.5, if equivalence point is around there would the solution be orange? If it is somewhere in between red and yellow, would one still say that the colour has "changed", or is that terminology more for complete colour changes? $\endgroup$ Commented Feb 3, 2023 at 16:00
  • $\begingroup$ Since the answer upvoting and accepting has been invented, there is no need for explicit thanks. But, they are, of course, appreciated. :-) $\endgroup$
    – Poutnik
    Commented Feb 3, 2023 at 16:03
  • $\begingroup$ MO range is usually listed as 3.1(red) - 4.4(orange). Note that due different eye sensitivity for different colors, the subjective middle color does not match the indicator pKa. Mixing red and yellow does produce orange. It is not like switching, it is transition. $\endgroup$
    – Poutnik
    Commented Feb 3, 2023 at 16:05
  • $\begingroup$ End points are chosen in such a way that there is sharp visual edge between "not yet there" (MO not yet yellow, PhPh still clear) and "already there" (MO yellow, PhPh violet). $\endgroup$
    – Poutnik
    Commented Feb 3, 2023 at 16:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.