Acids and Bases: Buffers


Table of Contents


Molecular Structure

Ionization Constants



Lewis Theory

What is in the water hardness test kit?

A home testing kit contains four components. Each one is described in detail below.

Vessel: This is simply a container that has been calibrated to hold the volume of water for which the test was designed.

Ethylenediaminetetraacetic Acid (EDTA): EDTA forms complex ions with the calcium and magnesium ions present in hard water. Use the figure below to learn more about EDTA:

Click on the circles in the figure below to learn more about EDTA. Clicking the mouse will reset the figure.

The chemical reactions of EDTA and each metal ion are shown below:

As EDTA is added to hard water, which metal ion will it react with first?

Ca2+ Mg2+

Good! Since the reaction of calcium ions with EDTA has a larger equilibrium constant, it will be a more favored reaction.

Remember that the larger the equilibrium constant, the more product-favored the reaction.

Indicator: The indicator for this reaction is Eriochrome Black T (structure is shown at right; click on structure to see the three forms of the indicator). It is a weak polyprotic acid. The fully deprotonated form forms a red complex with most metal ions, including Ca2+ and Mg2+. The indicator works like this:

Step 1: A small amount of indicator is added to the hard water sample and forms a complex with some of the metal ions. Since metal-indicator complexes are red, the solution appears red.

Step 2: EDTA is added to the solution and reacts with the free metal ions (those not bonded to the indicator). Since the metal-EDTA complexes are colorless, the solution still appears red.

Step 3: EDTA continues to be added. After all the free metal ions are bound to the EDTA, the EDTA displaces the indicator, complexing the remaining metal ions until all metal ions are bound to EDTA. The red metal-indicator complex is no longer present and the indicator is free. The indicator is a weak acid and its color is dependent on how it is protonated:

Calculate what the following ratios would be in a solution with a pH of 5: [HIn2-] to [H2In-], [In3-] to [HIn2-], [H2In-] to [HIn2-], and [HIn2-] to [In3-]. Click on the color of an indicator solution at this pH on the color strip on the right.

How do I calculate these ratios and how do they affect the color of the solution?

[HIn2-] / [H2In-] = [In3-] / [HIn2-] =
[H2In-] / [HIn2-] = [HIn2-] / [In3-] =

Some of your calculated ratios are not correct. Did you read the help available by clicking the test tube above? After you have entered your revised answer, click on the color of the solution again to check your calculation.

The correct answers have been entered for you. The next question involves the same type of calculation so try and work through the problem and see how these values are obtained before moving on.

The ratios you have calculated are correct but you have not correctly chosen the color of the solution. Which species is most prevelent in the solution? Is it so prevelent that its color will overpower the color of any other species present (if you add 100 drops of blue food coloring to water, adding two drops of yellow food coloring will not change the color of the water)?


Perform the same calculations at pH's seven, nine, and eleven.
  Key: Correct Answer  Incorrect Answer, Review your calculations above The correct answer has been entered for you.
If your calculations are incorrect, clicking on the color of the solution after you have reentered your calculation will recheck your answer.

pH = 7

[HIn2-] / [H2In-] = [In3-] / [HIn2-] =
[H2In-] / [HIn2-] = [HIn2-] / [In3-] =

pH = 9

[HIn2-] / [H2In-] = [In3-] / [HIn2-] =
[H2In-] / [HIn2-] = [HIn2-] / [In3-] =

pH = 11

[HIn2-] / [H2In-] = [In3-] / [HIn2-] =
[H2In-] / [HIn2-] = [HIn2-] / [In3-] =

Good! In order for this analysis to work, there needs to be a distinct color difference between the indicator-metal complex (which is red) and the free indicator so it will be visible when the EDTA has replaced the indicator and complexed all of the metal ions. As you just determined, the free indicator will have significant red color at pH's below seven. When the pH reaches 11, the free indicator is significantly orange in color. A color change from red to orange may be difficult to detect visually. So the key to this analysis is to maintain a pH above seven and below eleven.

Buffer: A buffer is a solution that resists changes in pH even when acids and bases are added. In this analysis, it has been determined that the analysis will work best when the pH is between seven and eleven. One possibility would be to add a base (sodium hydroxide, NaOH for example) to the water sample until the desired pH is reached. Remember though that EDTA and the indicator are both weak acids. After these have been added, it is likely that the pH of the solution will no longer be between seven and eleven. In order for the analysis to work, the pH of the solution needs to remain in this range throughout the test.

The buffer used in this test kit is able to maintain a pH close to 10 throughout the test. How does this buffer maintain a pH of 10 and will it always do so? The rest of this module will explore these two questions in detail.