Solving Titration Problems with Acid Rain

[louischaman]

Firstly his is my first post on this forum so hello.


Okay, well i encountered a major problem while doing my titration 2 days ago and I am unsure of why.

the titration was to find out the concentration of sulphuric acid in a sample of "acid rain" (i recon it was just sulphuric acid dissolved in water to form a weak concentration, not actual acid rain) using sodium hydroxide to nuetrelise it and thymolphthalein as an indicator.

Volume of SH2O4 = 25cm3
Concentration of NaOH = 0.01M

After adding about 26 cm3 of NaOH the solution as excpected turned purple due to the pH change and the thymolphthalein. However after stirring swilling round the solution in the conical flask for 30-60 seconds the mixture retuned to its clear state. I then would add more NaOH till the solution turned purple again and then repeated the process. this continued till way over the expected result. What happened??

One possibility is that the CO2 in the air caused extra acidification of the solution but i wouldn't have thought it would have such a large effect.

my chemistry teacher said that we should have found the concentration of SH204 to be 0.005M.

If C02 does change my results what can i do?

 

[symbolipoint]

You want to titrate until you have a color persistent for at least about a minute. You have a low concentration of acid, but in any case, near the end point your solution may still absorb a little carbon dioxide from the air and pH drift downward.

 

[Eshi]

would that fact that H2SO4 is a polyprotic acid do anything?

 

[symbolipoint]

would that fact that H2SO4 is a polyprotic acid do anything?

No. In fact, the second dissociation is fairly strong.