[HS2015]

I wanted to test a theory after a peculiar titration.

I was asked to titrate some Na₂CO₃.10H₂O against HCl.

Even though I was asked to use a 50ml burette my titre ended up at 72.20 cm³

After some thought I checked the solutions and Na₂CO₃.10H₂O was made to be 1M (286g in 1 litre) however after research into the solubility I don’t even think you could dissolve 286g of Na₂CO₃.10H₂O in 1 litre at a room temperature of approx. 20^oC.

I actually think this was intentional on the part of the examiners. The question spoke of an impurity and I believe they thought a saturated solution would ‘look the part’ with some undissolved Na₂CO₃.10H₂O in the bottom.
However the temperature in my sun baked lab yesterday rose to 27/8^oC and I think this is what lead to the rest of the Na₂CO₃.10H₂O  dissolving and me requiring +70ml of HCl to neutralise rather than the approximate 25ml I expect the examiners were after.

Could someone concur or otherwise with my conclusion!

Things which may affect my conclusion include the HCO₃^- -> CO₃^2- equilibrium in water which I do not know much about and also would it matter how fresh the solution was?

Any insights very welcome, thank you in advance!

[Hunter2]

The solubility of sodium carbonate dekahydrate is 217 g/l at 20 ° C. So 1 mol/l is not possible to dissolve.

Second question, which indicator or which pH  if using a pH-probe was used to get endpoint?

What was the molarity of HCl?

[HS2015]

Thank you for your reply.

Indicator was methyl orange which certainly did not give me a sharp end point as I am used to from that indicator.

I wondered whether somehow the indicator had become less effective in over 100ml of liquid as opposed to around 50ml as is usual.

[Borek]

End point detection when titrating carbonates is known to be tricky.

http://www.titrations.info/acid-base-titration-solution-standardization

[Hunter2]

Thank you for your reply.

Indicator was methyl orange which certainly did not give me a sharp end point as I am used to from that indicator.

I wondered whether somehow the indicator had become less effective in over 100ml of liquid as opposed to around 50ml as is usual.

Better is to use Phenolpthaleine and get end point at pH 8.2 first turning point of titration. Still hydrogen carbonate is there.

Methylorange  works at pH 4 what means a lot of carbon dioxide development takes place and disturbs the titration.

[HS2015]

Yes I prefer phenophthalein for that very reason.

Unfortunately this was an exam board setting this question and they dictated the indicator. I was just getting my facts correct before I wrote them a strongly worded letter.

Thank you again.

So the larger quantity in the conical flask wouldn't make it harder/easier to find and end point?