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); CH3COOH + CH3COONa

• Alkaline buffer: (pH > 7); NH3 + NH4Cl.

• 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.