Exercise 4

C HAPTER 4 | Back to Basics Understanding Acids, Bases, and pH 4 Procedure 2 Using pH Paper, pH Meters, and Purple Cabbage Extract The pH of a substance can be measured using a variety of methods. Using pH paper ( Fig 4.3 ) is an inexpensive and easy method to measure the pH of a substance. The pH paper has been treated with an indicator sensitive to cer- tain pH values. A color chart provided with the pH paper relates the color of the pH paper to a specific pH. Addi- tionally, many labs have a pH meter ( Fig. 4.4 ) that measures the pH. One of the most interest- ing properties of acids and bases is their ability to change the color of some plant ma- terials. Purple cabbage, hydrangea, and elderberry extracts respond in an amaz- ing manner to an acidic or basic solution. In this activity, you will also use purple (red) cabbage to develop an acid or a base scale and determine the approximate pH value of various substances. 1 Use pH paper to determine the pH of 2 to 4 mL of each substance for investigation. Record the pH in the first column of Table 4.1 . 2 If a pH meter is available, follow your instructor’s directions for its use in this activity. Record the pH in the second column in Table 4.1 . 3 To develop the acid or base scale using purple cabbage, divide into working groups of at least two students per group. 4 Procure a beaker of pre- pared purple cabbage extract, eight test tubes, a reaction well plate, pipettes, toothpicks, substances to test, and several paper towels. 5 To establish a standard for comparison, place seven test tubes in a rack, and label them: pH 2, pH 4, pH 6, pH 7, pH 8, pH 10, and pH 12. 6 Pipette 5 drops of each substance provided by the in- structor (aspirin, baking soda, milk, clear soda, etc.) into the test tubes and label them according to each substance used. 7 Pipette 8 drops of purple cabbage juice extract into each test tube, and gently swirl the test tube, mixing the substances. 8 Record the color of each test tube in the final column of Table 4.2 . Let the substance sit in the test tube labeled pH 12 for a few minutes. The color will shift because of the instability of the pigments at a high pH. Record the initial and final colors at pH 12 in Table 4.2 . Once completed, Table 4.2 will serve as the standard for comparison. 9 Place 5 drops of each of the 11 substances or the dry substance to be tested into an individual well on the reaction well plate, and record its position. If no well plates are available, test tubes can be used. Label the well or test tube with a wax pencil or Sharpie. 10 Place 5 drops of the purple cabbage extract into each reaction well, and stir it with a toothpick. Materials ❏ ❏ pH paper ❏ ❏ pH meter ❏ ❏ Purple cabbage extract ❏ ❏ Substances of a known pH (7), provided by instructor ❏ Substances for investigation (11), such as aspirin, baking soda, milk, clear soda, and other common and harmless substances provided by instructor ❏ ❏ Transfer pipettes ❏ ❏ Test tubes ❏ ❏ Reaction wells ❏ ❏ Toothpicks ❏ ❏ Wax pencil or Sharpie ❏ ❏ Colored pencils ❏ ❏ Beakers ❏ ❏ Paper towels F IGURE 4.3 pH paper and indicator strip. F IGURE 4.4 pH meter.