Going back to your original post, you need to bear in mind that Bromophenol blue only shows green/yellow in a pH below about 3 and anything above 4.6 will show as blue. this means that your IPA can be acidic but still show as blue with the indicator. If you really wan to know how acidic it is you could titrate it with NaOH solution using either Phenol phthalein or tumeric as indicator. To use tumeric, dissolve 1/2 teaspoon of kitchen tumeric in 50ml of IPA and leave to settle, take a small amount of the liquid and use a few drops as indicator - it turns red/brown between pH 7.4 and 9.2. If you wan to use Bromohenol Blue as indicator you could back titrate - add a known quantity of NaOH and then titrate that with HCl.
The old IPA you have may not be as acidic as you expect (does the bromophenol blue still appear blue in it?). If you are using it as solvent for titration then blanking it should be adequate to enable its use. The acid formed in IPA is likely to be a weak organic acid and as such may not change the colour of bromophenol blue.
pH does not indicate the amount of acid present, except with strong acids, such as sulphuric, hydrochloric, nitric etc. For titrations you need to know how much acid (alkali) is present which is why you need to blank the solvent first. The problem is trying to blank acidic IPA using bromophenol blue is not realy ideal and you may be better off mesaure how much acid is present first by titrating that acid.
