Acids and Bases: Buffers

GOAL(s):

• Understand how a buffer resists changes in pH even when acids and bases are added
• Choose a conjugate acid-base pair that will make a buffer at a given pH
• Calculate the pH of a buffer solution
• Understand buffer capacity

How hard is your water?

Have you ever found a white residue on a pan after boiling water? Where does it come from? You may have been told this occurs because you have "hard" water. What exactly is "hard" water?

Water is said to be "hard" when metal ions (specifically Ca²⁺ and Mg²⁺) are dissolved in it. If calcium ions are present, CaCO₃ will precipitate when the water is heated. This precipitate builds up not only on cookware, but also in pipes, eventually clogging the pipes.

So, what do you do if you suspect you have hard water? Kits can be purchased that allow you to test your water at home and measure its "hardness". How exactly do these kits work?

The basic idea behind these kits is rather simple. A few drops of an indicator are added to a water sample. Then, a compound called EDTA (short for ethylenediaminetetraacetic acid) is slowly added to the sample. EDTA forms complexes with the calcium and magnesium ions that are red. Because of the presence of the indicator, when all of the metal ions have formed complexes with the EDTA, the solution will change color from red to blue. If you know how much EDTA you have added and the stoichiometry of the chemical reaction, you can calculate how many metal ions were initially present in the water sample.

In order for this analysis to work well, the water sample must be kept at a basic pH. Since both EDTA and the indicator are themselves weak acids, a buffer solution , which is able to maintain a fairly constant pH even when acids and bases are added, is used.

This may not sound simple at all. You may have many questions:

• How does the indicator cause the solution to change color?
• What exactly is EDTA?
• Why must the water sample be kept at a basic pH?
• What is in the buffer solution that allows weak acids to be added without affecting the pH?

This module will answer these questions by first describing each component of the water hardness test in more detail. Buffer solutions will be explored to a greater extent, including their preparaton and limitations.