Overview In this experiment, you will either perform an experiment or watch a video of the experiment to determine the alkalinity of two samples of water. Each sample has been prepared by adding sodium carbonate (Na2CO3) to it. When added to water, sodium carbonate undergoes the following reaction:

The OH– ions make the solution basic. When you add a phenolphthalein indicator to a basic solution it turns pink. It will lose its pink color if an acid is added to reduce the pH below 8.3. The phenolphthalein acts as a pH indicator, and its color changes at pH 8.3. (Other pH indicators will change color at different levels of pH, some more generalized across the pH range, and others specific as phenolphthalein is.)

After you add the phenolphthalein, you will add a dilute solution of sulfuric acid drop by drop to the sample until the pink color disappears. By knowing how much acid you added you can determine how much acid it takes to reduce the pH to 8.3. You will then add a second indicator to the solution and continue to add drops of acid until the pH is 4.5. A second color change, this time from green to red, will indicate that a pH of 4.5 has been reached.

The procedure just described—the drop-by-drop addition of an acid or a base to determine pH—is called a titration. Titrations just like the one you are doing are used by environmental professionals at ponds, rivers, and lakes to test water quality. They use this exact kit, so what you have done is on-the-job training. The technique is useful when, for example, you need to know if acid rain is affecting a lake.

There is a difference, however. These kits were set up using water samples provided for you. In real-life situations, workers collect water samples in a lake or stream instead of using Samples A and B that you tested. For this lab to work, sodium carbonate was added to make the water samples alkaline. In lakes and streams it is calcium carbonate (CaCO3) that causes alkalinity.

Calcium carbonate acts as a buffer, a substance that serves to resist small changes in acidity or alkalinity in a solution. When acid rain enters lake water, it can be neutralized by the bases present and thus the lake does not become too acidic. Since the key ion is the hydroxide ion, the specific cation doesn't matter in this case. All over the world, right now, pollution workers are measuring calcium carbonate in lakes and streams.

Objective

Apply the technique of titration to determine the acidity or alkalinity of a sample of water.

Question
How can the process of titration be used to measure some aspects of water quality?

Observe
You have probably heard the term acid rain. When certain industrial practices release pollutants into the air, those pollutants can travel long distances before they fall to earth. If the acidic chemicals then find their way into streams and lakes, the acids can change the chemistry of the lake and affect the living things that are in it. Most lakes are basic enough to buffer the acids from rain for a while, but lakes will suffer if they become too acidic over time. In an optional activity at the end of this lab exercise, you will have the opportunity to research the effects of acid rain on lands and lakes near you.

Question 1 (10 points)
A pH indicator will when it is mixed with an acid or base.

Question 1 options:

Change the pH.

Produce bubbles.

Change color.

Form a new substance.