# Acid-Base Titration: Calculating % Purity of Sodium Carbonate

• danago
No, because the hydrogencarbonate ion will react with the water to form carbonic acid, but the reaction will keep going until the concentration of hydrogencarbonate ion is low enough that it no longer reacts.f

#### danago

Gold Member
Hi. In class, i am required to perform a titration where i am given a solution of sodium carbonate and a solution of hydrochloric acid of known concentration. I am to use the acid to calculate the percentage purity of the sodium carbonate.

Im not really sure how to go about doing this. The first thing i tried doing was coming up with an equation for the reaction, but I am a bit confused, since it it occurs in stages.

What I've got is:
$$CO_3 ^{2 - } + H_3 O^ + HCO_3 ^ - + H_2 O$$

Im really unsure though, since the hydrogencarbonate ion will probably then react to form carbonic acid or something.

Is somebody able to guide me in the right direction?

Thanks,
Dan.

Well, you can see the the stoichiometric ratio for sodium carbonate and hydrochloric acid is 1:2. After finding the volume of HCl, you time it with its concentration to find the total of mol of HCl used. Next, divide the answer by 2 to get the mol of sodium carbonate reacted. From there, times with the molecular weight of sodium carbonate to get the its weight. Then, you can get the percantage from there.

Alright, so i should be using the following equation then?:

$$Na_2 CO_3 + 2HCl \rightarrow H_2 O + CO_2 + 2NaCl$$

What indicator would i use? How do i know when enough HCl has been added?

You're adding the HCl (via a calibrated burette, presumably) to the sod. carbonate solution. The pH of sod. carbonate solution is neutral (7). The slightest excess of HCl will cause the pH to plummett (well below 7), which signifies the endpoint of the reaction. So, using this http://en.wikipedia.org/wiki/PH_indicator link as a guide, which indicator(s) do you think are suitable?

You will know the endpoint has been reached if a permanent color change remains despite adequate swirling. Make sure the conical flask or whatever you're using to hold the reaction mixture is not stoppered (in order to let the CO2 escape as it forms, otherwise the acidity might interfere with the reaction).

You know the number of moles (call it x) added to the reaction at endpoint. You can deduce from the stoichiometry that x/2 moles of sod. carbonate must have been present at the outset. You can now calculate the molar concentration of sod. carbonate solution that was present at the start. You really need more data (like the density of the impure solution of sod. carbonate, or you can work this out with an accurate weighing balance) to get the % purity as required.

You're adding the HCl (via a calibrated burette, presumably) to the sod. carbonate solution. The pH of sod. carbonate solution is neutral (7). The slightest excess of HCl will cause the pH to plummett (well below 7), which signifies the endpoint of the reaction. So, using this http://en.wikipedia.org/wiki/PH_indicator link as a guide, which indicator(s) do you think are suitable?

You will know the endpoint has been reached if a permanent color change remains despite adequate swirling. Make sure the conical flask or whatever you're using to hold the reaction mixture is not stoppered (in order to let the CO2 escape as it forms, otherwise the acidity might interfere with the reaction).

You know the number of moles (call it x) added to the reaction at endpoint. You can deduce from the stoichiometry that x/2 moles of sod. carbonate must have been present at the outset. You can now calculate the molar concentration of sod. carbonate solution that was present at the start. You really need more data (like the density of the impure solution of sod. carbonate, or you can work this out with an accurate weighing balance) to get the % purity as required.

Ah ok, that answers my question, thanks :) As for the density, i will be able to calculate that, as a set of scales will be avaliable.

Im a little unsure of the indicator though. The pH will be much lower than 7, but how much lower? Would something like methyl orange do the trick?

Thanks once again for the help,
Dan.

Ah ok, that answers my question, thanks :) As for the density, i will be able to calculate that, as a set of scales will be avaliable.

Im a little unsure of the indicator though. The pH will be much lower than 7, but how much lower? Would something like methyl orange do the trick?

Thanks once again for the help,
Dan.

Yeah, something like methyl orange or methyl red should do the trick. It takes a very small excess of HCl to make the solution highly acidic (drastically drop the pH).

Alright. Thanks a lot for the help :)

Just one question though: You mentioned that a solution of Na2CO3 will be neutral. Wouldnt the carbonate ion react with the water to form hydrogencarbonate ion and hydroxide ion to make the solution basic, or does that only occur to a very small extent?