Volumetric Analysis Involving Acid/Base reactions?

Question

In practical experiment,we find the Molarity of NaOH(aq) by reating with HCl(aq).We use 2 different indicators (methyl orange(change from orange to rad) and phenolphthalein(change from pink to colourless))to realise when the solution is neutralized.The question is "Was there any difference in the volumn of acid added using the 2 different indicators?Explain why?
My result was the volumn of acid for methyl orange indicator is greater than phenolphthalein.

Answer 1

You are quite right in that methyl orange apparently requires a bit more acid than when using phenolphthalein. The difference should not have been very large, though - much less than 1cm3.

The reason is that the operating range for methyl orange (3 - 5) is at a much lower pH than for phenolphthalein, which changes colour at a much migher pH (9 - 11). So you need a bit more acid past the normal equivalence point to make methyl orange change colour. In truth, phenolphthalein gives results that are ever so slightly too low for the same reason.