1
$\begingroup$

I read some information about phenolphthalein indicator, some websites suggested phenolphthalein indicator of 1%, others mentioned phenolphthalein indicator of 0.1%. What is the difference of both of these concentrations when we use them in titrations?

I assume that 1-2 drops of phenolphthalein indicator of 1% can be used as an indicator but for phenolphthalein indicator of 0.1%, it needs to drop more with 4-5 drops to obtain the same effeciency.

Look forward to hear from you all

$\endgroup$
3
  • $\begingroup$ In my university, all our indicators are available at concentrations 0.1%. The students know they may need 3 - 4 drops for their titrations. $\endgroup$ – Maurice Jan 18 at 15:35
  • $\begingroup$ I remember the same, even from labs in my analytical chemistry focused high school. 1% is an unusual high concentration. $\endgroup$ – Poutnik Jan 18 at 15:40
  • $\begingroup$ @Maurice Thank you for your answer. It is helpful to my experiments. $\endgroup$ – VanThanh Do Jan 24 at 14:46
1
$\begingroup$

During titration of small amounts of acids, the molar amount of the indicator in 1-2 drops of $\pu{1 \%}$ indicator solution may not be negligible compared to the acid molar amount, affecting the result.

So for that cases, $\pu{0.1 \%}$ solution is used, to be able to dose smaller indicator amounts.

As the phenolphalein molar mass is about $M=\pu{318 g/mol}$, 1 drop of $\pu{1 \%}$ solution is equivalent about 3 drops of $\pu{0.01 M}\ \ce{NaOH}$.

$\endgroup$
2
  • $\begingroup$ Thank you a lot for your answer, even i don't understand well your explanation. All statements from other answers support for using phenolphalein indicator 0.1%. I really appreciated your help Poutnik. $\endgroup$ – VanThanh Do Jan 24 at 14:44
  • $\begingroup$ @VanThanhDo And so does mine. unless perhaps for rare exceptions, pH indicators are not used at so high concentration as 1%. By adding pH indicator, you add an acid or a base to the one you want to determine. Your interest is this addition should be negligible. $\endgroup$ – Poutnik Jan 24 at 15:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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