Red Cabbage Extract as a pH Indicator

Aim Extracting a pH indicator from the red cabbage. Finding the color of the extract to a known pH solution. Determining the pH of the household items. Studying the behavior of a buffer solution.

Terms pH Acid – Base Indicator Buffer

pH Means p(otential of) H(ydrogen) pH scale can tell us whether a solution is acidic or basic. The pH values in aqueous media lie from 0 to 14. pH = -log [H + ]

pH Acids donate hydrogen ions in an aqueous solution and have a low pH. Bases accept protons and have a high pH. Each pH value below 7 is 10 times more acidic than the next higher value. Each pH value above 7 is 10 times more basic than the next higher value.

Acid – Base Indicators Acid – Base Indicators are the compounds usually change their colors if the pH of the solution is changed.

Acid – Base Indicator How do they work? Many indicators are weak acids. Their acidic form color is different from their conjugate bases. Phenolphthalein in Acid Phenolphthalein in Base

Acid – Base Indicator

Buffer Solution A buffer is a solution which resists to changes in pH when small amounts of an acid or an alkali are added to it. Two kinds: Acid buffer: (pH < 7); CH 3 COOH + CH 3 COONa Alkaline buffer: (pH > 7); NH 3 + NH 4 Cl. Equimolar mixtures are strongest buffers and therefore give approximately constant pH.

Red Cabbage Extract Red cabbage contains a pigment molecule called “ANTHOCYANIN”. Also found in Apple skin, plums, poppies, cornflowers and grapes. Soluble in water. It changes its color if the pH of the solution varied.

Extraction: Part - I Take 400 or 600 mL beaker. Fill half the beaker with cut red cabbage and add enough water. Boil it for 15 minutes. Keep ~ 75 mL of the extract. Divide into three portion

Making Standards: Part - II Making Color Standards for pH < 7: Take one of the beaker contain cabbage extract. Measure its pH (may be less than 7) Add 6 M HCl to the extract till you get pH = 5. Save little amount for comparison.

Making Standards: Part - II Continue the addition of 6 M HCl to get pH = 3 and 1. Save both the pH adjusted solution in a test tube for comparison.

Making Standards: Part - II Making Color Standards for pH > 7: Take another beaker contain cabbage extract. Measure its pH (may be less than 7) Add 6 M NaOH to the extract till you get pH = 7. Save little amount for comparison.

Making Standards: Part - II Continue the addition of 6 M NaOH to get pH = 9, 10, 11 and 13. Save all the pH adjusted solutions in a test tube for comparison.

Part - III Determining the pH of Household Items: Take ~ 2 mL of each household items in a separate, labeled test tubes. Add ~ mL of the Red Cabbage extract to all the test tube and stir it well. Compare the color of the extract with the std. colors which is from part II. Find its ~pH and note on the data sheet.

Behavior of buffer: Part - IV Behavior of a Buffer Solution: Take ~ 2 mL of distilled water, ~ 2 mL phosphate buffer in a separate test tube. Adjust the pH of the extract to 7 and then add ~ 2 mL of the extract to each test tube. Record the color change and its pH.

Behavior of buffer: Part - IV Now add 1 mL of 0.5 M HCl to each test tube. Note the color change after the addition of acid and its pH Do the same experiment with 0.5 M NaOH. Note the color change after the addition of base and its pH.