Good old litmus paper

Phenolphthalein (pronounced fe-nul-tha-Leeri) is another commonly used indicator. Until a few years ago, phenolphthalein was used as the active ingredient in a popular laxative. In fact, I used to extract the phenolphthalein from the laxative by soaking it in either rubbing alcohol or gin (being careful not to drink it). I d then use this solution as an indicator. 

Phenolphthalein is clear and colorless in an acid solution and pink in a basic solution. It s commonly used in a procedure called a titration, where 

Suppose, for example, that you want to determine the molar concentration of an HCl solution. Rrst, you place a known volume (say, 25.00 milliliters measured accurately with a pipette) in an Erlenmeyer flask (that s just a flat-bottomed, conical-shaped flask) and add a couple drops of phenolphthalein solution. Because you re adding the indicator to an acidic solution, the solution in the flask remains clear and colorless. You then add small amounts of a standardized sodium hydroxide solution of known molarity (for example, 0.100 kQ with a buret. (A baret is a graduated glass tube with a small opening and a stopcock, which helps you measure precise volumes of solution.) 

Suppose that it takes 35.50 milliliters of the 0.100 M NaOH to reach the endpoint of the titration of the 25.00 milliliters of the HCI solution. Here s the reaction  

From the balanced equation, you can see that the acid and base react in a 1 1 mole ratio. So if you can calculate the moles of bases added, you ll also know the number of moles of HCI present. Knowing the volume of the acid solution then allows you to calculate the molarity (note that you convert the milliliters to liters so that your units cancel nicely)  

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