A question states that 0.1 M solution of compound$$\ce{Na2CO3.NaHCO3.2H2O}$$ is titrated against 0.05M HCL, and that a different amount of HCL is used according to the indicators(phenolphthalein or methyl orange) used.

How does the indicator used affect the titration?


An indicator does not affect a particular reaction. But in your solution, you have two successive reactions when adding HCl to this solution : First $$\ce{CO_3^{2-} + H^+ -> HCO_3^-}$$ and a given indicator must be added to determine the end of this reaction. If you don't, all you see is a colorless solution being transformed into another colorless solution, even though its composition has changed. This added indicator may be phenolphthalein, which goes from red to colorless precisely at the end of this first titration. Then, by adding more HCl, you carry out the following reaction : $$\ce{HCO_3^- + H^+ -> H_2O + CO_2}$$ And you need is another indicator, maybe methyl orange, to determine the end of this second reaction. The color of the methyl orange in solution changes from yellow to reddish exactly after the end of this second reaction.

Phenolphtalein and Methyl orange are molecules which reacts with $\ce{H^+}$ exactly like $\ce{CO_3^{2-}}$ and $\ce{HCO_3^-}$. But they react before or after these two ions. To summarize, $\ce{H^+}$ reacts first with $\ce{CO_3^{2-}}$ and nothing else, and it takes a couple of milliliters to do it. Then it reacts with Phenolphtalein and nothing else, and it takes less than one drop to do it. Then it reacts with $\ce{HCO_3^-}$ and nothing else, and it also takes a couple of milliters to do it. And last but not least, it will react with methyl orange.

  • $\begingroup$ So is there an order of preference according to which H+ ions(and hydroxyls too, for that matter) react with conjugate bases and indicators? Can you tell me where I can read up more about that? $\endgroup$ – Harry Holmes Mar 30 '20 at 12:13
  • $\begingroup$ It depends on the pKa's. For the first titration, the pKa of $(HCO_3^-/CO_3^{2-})$ is 10.25. For the second titration, the pKa of $HCO_3^-/(CO_2+H_2O)$ is 6.35. A good indicator for the end of the first titration and the beginning of the second has a pKa between 10.25 and 6.35. The best choice would be an indictor with pKa = 8.35. But there is no such an indicator. We must choose an indicator whose pKa is near to 8.35. Phenolphtalein is acceptable, as it has pKa = 9. For the second titration, we must find an indicator of pKa smaller than 6.35. Methyl orange is OK, with a pKa = 3.9. $\endgroup$ – Maurice Mar 30 '20 at 13:36

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