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Titration curve


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There is a solution containing a mixture of carbonate ions and hydrogencarbonate ions.

A few drops of phenolphthalein indicator is added into the solution, ammonium solution is added drop by drop into the solution following.

When there's the sign of the permanent change of the colour of the indicator, my teacher calimed that all the hydrogencarbonate ions have been reacted through the reaction

(HCO3 - + H+ ---> CO2 + H2O)

My idea is that all carbonate ions did completely reacted instead.

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Congratulations! You have spotted your teacher's mistake.

If you add an acid (and ammonium salts are acids, albeit weak ones) to a mixture of bases like carbonate and hydrogencarbonate, the acid will react with the strongest base first.

For what it's worth if you did have carbonate and CO2 present they would react to give hydrogencarbonate

Na2CO3 + H2O +CO2 --> 2 NaHCO3


A good way to embarrass the teacher would be to ask why nobody saw the bubbles of CO2 they said had been produced, but I'll let you decide if that's a good idea.

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Thanks a lot. By the way, do you know why the curve(the pH against the volume of hydrochloric acid added into the mixture) is rather vertical at the point where the indicator changes its colour?

I thought it ought to be gradual instead.

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I am not sure what the point of inflection of the titration curve means.

IN the case of only acid and base titration, say NaOH and HCl,

I could perceive the term in the way that the point is when the dominating ion changes.( from OH- to H+)

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