pH Scale - red cabbage Chemistry 5.5.3

5.5.3 pH SCALE

Concepts to Investigate: pH scale, acids, bases, indicators.

Materials: Part 1: Head of red cabbage, transparent plastic cups or glass 250 mL beakers, hot plate, knife and cutting board, large beaker or similar container; items listed in Table 1.  You may also wish to test other colorless household solutions such as aspirin, shampoo, dish soap, sugar, milk, bleach, Epsom salts, colorless soft drinks, Alka-Seltzer®, and pickle juice;  Part 2:  Red cabbage, beaker, filter paper, materials in part 1;  Part 3:  Grape juice, beets or red onion; materials in part 1.  Part 3:   Grape juice, beets or red onions.

Safety: This activity uses houswhold chemicals such as drain cleaner and pool acid that are potentially dangerous.  Read manufacturer's warnings.  Wear goggles, lab coat and gloves.

Principles and Procedures: Each year millions of tourists travel to the woods of New England, upstate New York, Wisconsin, and Canada to witness the appearance of brilliant autumn colors.  Sugar maples, red oaks, sumac, birch and other trees and shrubs turn from green to bright red, orange, and yellow.  The short, cool days of autumn bring an end to the production of chlorophyll (the green light-gathering pigment).  As chlorophyll gradually breaks down, the colors of the more stable carotenoid (yellow/orange) and anthocyanin (red/blue/purple) pigments become visible.  Some years the leaves of a particular species are bright yellow, while other years they may appear redder or even purple.  Although no one can accurately predict the timing or coloration of each year's show, we do know that there is a chemical basis for it.

Anthocyanins (Figure H) are responsible not only for the red and purplish colors of autumn, but also for similar colors in various summer leaves (red cabbage, red lettuce, red plum tree), flowers (roses, hydrangeas, geraniums, bachelor buttons, dark pansies ), fruit (cherries, red apples, grapes, tomato, blackberry, blueberry, plum), roots (beets, radishes), bulbs (red onions), and petioles (rhubarb).  Anthocyanins are water-soluble and are dissolved in the cell sap rather than bound to the membranes as chlorophyll is.  If cell sap in a leaf is quite acidic the anthocyanins impart a bright red color, but if it is less acidic the color may appear purple.  

The color of anthocyanin depends on acidity, and thus it may serve as a pH (acid/base) indicator (Figure I).  pH is a quantitative measure of the acidity or basicity of solutions.   Numerically, the pH of a solution is defined as the negative logarithm (base 10) of the hydronium ion (H3O+)concentration.  If the pH is less than 7 the solution is acidic, if it is greater than 7 the solution is basic, and if it is 7 the solution is neutral.  The higher the pH, the more basic the solution, and the lower the pH, the more acidic the solution.  Figure I illustrates the relationship between color and pH for anthocyanin pigments extracted from red cabbage or other sources.  In this activity you will determine the pH of numerous household products using this pH scale.

Part 1:  Testing the pH of household chemicals using red cabbage juice:  Put on safety goggles and lab coat.  Fill beakers with common household chemicals to be tested according to the specifications in Table 1. Dissolve all solids by stirring until the solution is relatively colorless.   Using a knife and cutting board, carefully dice a quarter head of red cabbage and boil for ten minuets in a pan or beaker containing 1000 mL of water (Figure J).   Allow the mixture to cool and then pour the contents through a strainer or filter paper and collect in a beaker.  The red cabbage extract appears dark purple, the color of anthocyanin at pH 7.  Test each of the household chemicals by placing 20 mL of the anthocyanin indicator solution in each beaker (Figure K).  Record the color and estimate the pH according to the scale listed in Figure I.    Which appears to be the most acidic?  Which is the most basic?  Do any of the chemicals appear to produce neutral solutions?