phenolphthalein

An indicator commonly used in acid/base titrations which turns colourless in acidic solutions and is otherwise (in the concentrations commonly used in labs at least) pale pink. And I mean very pale, so it is f*cking useless without a colour chart and very good eyes. It also turns colourless in solutions of pH greater than 12. The Kastle-Meyer test shows the presence of blood; a dry swab or piece of filter paper is used to collect a sample, then a few drops of phenolphthalein dissolved in alcohol (due to its insolubility in water) is added, followed by a few drops of hydrogen peroxide - this mixture will turn pink in the presence of blood. If you've ever watched 5 seconds of CSI, you have seen someone doing this. In the past phenolphthalein has been used as a laxative, but is no longer used due to fears that it is carcinogenic.