Question

We had an experiment in class: Soda Ash Titration using Double Indicator Method

How would you know the composition of the sample based on the comparison volume of titrant (HCl) used? i.e. if Vmo > Vph, the sample is made up of NaHCO3 and Na2CO3. why is it like that?

Answer 1

Na2CO3 + HCl ==> NaHCO3 + H2O so the Na2CO3 is titrated half-way (one H has been added as opposed two) at the phenolphthalein end point. So if it takes 20 mL to add one H, it follows that it will take 20 more (for a total of 40) to reach the methyl orange end point. Thus a PURE sample of Na2CO3 (with no NaHCO3) will have the same volume for each indicator BUT if the sample is made up of Na2CO3 as well as some NaHCO3, then the phenolphthalein end point tell you that one H has been added to the Na2CO3 (nothing happens to the NaHCO3 at that point)and the m.o. end point titrates the second H to the Na2CO3 PLUS the added H to the NaHCO3 there originally. Hope this helps.