Acid Base Indicators
The colors of certain pigments present in fruits, flowers and vegetables are influenced by pH or acidity. Examples are extracts from red apple skin, red cabbage, beets, blueberries, cherries, red onion, yellow onion, peach skin, pear skin, plum skin, tomato, turnip skin, violets, roses, etc. Since the colors of these substances depend on pH, they can be used to determine pH. They are natural acid‑base indicators. The purpose of this experiment is three‑fold: (1) to prepare acid‑base indicators from certain fruits and vegetables, (2) to determine the colors of these indicators at various pH values, and (3) to use them to determine the pH values of several common commercial products.
Part 1. Indicator preparation: In the case of fruit juices, the only preparation which may be necessary is the addition of deionized water to lighten the color of the juice.
If the starting material is a solid, e.g., radish skins, cabbage leaves, place the chopped starting material in a beaker and add enough water to cover it. Heat, but do not boil, for about 30 min., and pour off the solution from the solid material.
Your instructor will tell you how many different indicator solutions to prepare.
Part 2. The dependence of the indicator colors on pH: The second purpose of this experiment is to determine how the colors of your indicators change with pH.
Place 2 ml of one of your indicator solutions into a test tube. Adjust its pH to about 2 by adding vinegar with a dropper. You can measure the pH of the solution by dipping a stirring rod
into the solution and then touching it to a piece of pH paper. Record the pH and solution color. Increase the pH to 3 by adding (1 M NaOH) sodium hydroxide, dropwise, and again record the pH and color. Repeat the procedure at each pH value until a pH value of about 12 is reached.
Repeat this experiment with at least two other indicators, and summarize your results in a chart of color versus pH, as shown in the example below:
2 3 4 5 6 7 8 9 10 11 12 13
Example: pH _______________________________________________________
green blue green blue yellow brown
Smurf berry juice colors
Part 3. The pH of common substances: In this part you will use one of the indicators calibrated in the previous section to measure the pH values of several common commercial products. You should exercise some care in choosing the indicator. Pick an indicator that exhibits several different colors over the pH range studied.
If the substance whose pH is to be measured is a liquid, add about 1 ml of the substance to a test tube and then add to it several drops of the indicator. Record the color.
If the substance to be tested is a solid, add enough water to dissolve about 0.5 grams in a test tube, again add a few drops of indicator and record the color.
The Results section of your report should contain the following:
A description of the indicator solutions (color, etc.)
A table of solution pH versus indicator color for each of the indicators used.
1. The indicator used to test the commercial products, and your reasons for choosing this indicator over all of the others.
2. The names of the commercial products tested, and the color of the indicator in each product.
Your conclusion section should contain:
1) Color charts, as described on the previous page, based on your results from Part 2.
2) The pH values, obtained from your color charts, of the commercial products studied. State whether each substance is acidic or basic.
1) Repeat these experiments with other fruits and vegetables.
2) The pigments present in some flowers are also influenced by pH and could be studied.