Showing True Colors with Acid-Base Indicators - Acids and Bases - Chemistry Essentials for Dummies

Chemistry Essentials for Dummies

Chapter 11. Acids and Bases

Showing True Colors with Acid-Base Indicators

Indicators are substances (organic dyes) that change color in the presence of an acid or base. You may be familiar with an acid-base indicator plant — the hydrangea. If it’s grown in acidic soil, it turns pink; if it’s grown in alkaline soil, it turns blue.

In chemistry, indicators are used to indicate the presence of an acid or base. Chemists have many indicators that change at slightly different pHs, but the two indicators used most often are litmus paper and phenolphthalein. I discuss both in this section.

Doing a quick color test with litmus paper

Litmus is a substance that is extracted from a type of lichen and absorbed into porous paper. There are three different types of litmus — red, blue, and neutral. Red litmus is used to test for bases, blue litmus is used to test for acids, and neutral litmus can be used to test for both. Here’s how the paper reacts to acids and bases:

If a solution is acidic, both blue and neutral litmus will turn red.

If a solution is basic, both red and neutral litmus will turn blue.

Litmus paper is a good, quick test for acids and bases.

Phenolphthalein: Finding concentration with titration

Phenolphthalein (pronounced fe-nul-tha-Leen) is a commonly used indicator. Phenolphthalein is

Clear and colorless in an acid solution

Pink in a basic solution

Chemists use phenolphthalein in a procedure called a titration, in which they determine the concentration of an acid or base by its reaction with a base or acid of known concentration (see Chapter 10 for info on molarity and other solution concentration units). Here’s how to evaluate an acid solution using titration:

1. Add a couple drops of phenolphthalein to a known volume of the acid solution you want to test.

Because you’re adding the indicator to an acidic solution, the solution in the flask remains clear and colorless.

Suppose, for example, that you want to determine the molar concentration of an HCl solution. First, 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.

2. Add small, measured amounts of a base of known molarity (concentration) until the solution turns light pink.

Add small amounts of a standardized sodium hydroxide (NaOH) solution of known molarity (for example, 0.100 M) with a buret. (A buret is a graduated glass tube with a small opening and a stopcock valve, which helps you measure precise volumes of solution.) Keep adding base until the solution turns the faintest shade of pink detectable. I call this the endpoint of the titration, the point at which the acid has been exactly neutralized by the base.

3. Write the balanced equation for the reaction.

Here’s the reaction:

4. Calculate the molarity of the acidic solution.

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 HCl present. 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 HCl solution. 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):

You can calculate the titration of a base with a standard acid solution (one of known concentration) in exactly the same way, except the endpoint is the first disappearance of the pink color.