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 im 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 wouldnt 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?