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

  • $\begingroup$ 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

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 $\pu{0.01 M}\ \ce{NaOH}$.

  • $\begingroup$ 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

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