In the titration of sodium carbonate with hcl , isn't the first end point determined by phenol phthalene is equivalent to the volume of sodium carbonate

because my text book say thats the volume at the first end point is just half the sodium carbonate and the seconds end point is the whole sodium carbonate.

I know that at the first end point all the sodium carbonate all be converted to sodium bicarbonate but why shouldn't i stop here?!


1 Answer 1


The titration of hydrochloric acid with sodium carbonate is a common school practical, used to determine the concentration of the acid (standardisation).

In the first step, as you rightly point out, the sodium carbonate reacts with one equivalent of hydrochloric acid. The phenolphthalein shows the pH change associated with the formation of the sodium hydrogen carbonate (aka bicarb). You could use this value to calculate concentration, however, in order to ensure your answer was correct, you'd need to run this titration at least twice more, averaging your answers and discounting any anomalous titres.

$\ce{Na2CO3(aq) + HCl(aq) → NaHCO3(aq) + NaCl(aq)}$

As the bicarbonate is also reactive with hydrochloric acid, you can do something known as a 'double titration'. After the first titration, you can immediately do a second titration, this time using a different indicator, such as methyl orange. By doing this, you get a second value. If your titration is good, the values for the first and second titrations should be equivalent.

$\ce{NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) }$


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