Understanding Acid-Base Indicators and Their Reversible Reactions
Dot-Point 2: conduct an investigation to demonstrate the preparation and use of indicators as illustrators of the characteristics and properties of acids and bases and their reversible reactions (ACSCH101)
Indicators are substances that change colour depending on the pH of the solution they are placed in. They help us distinguish between acids and bases and are essential tools in understanding the characteristics and properties of different substances. In this investigation, we will explore how indicators function, with a particular focus on their reversible reactions and the use of natural indicators like red cabbage juice.
What Are Acid-Base Indicators?
Indicators are compounds that change colour based on the concentration of hydrogen ions (H+) in a solution. The colour change occurs over a certain pH range, which is known as the transition range. The size of the transition range determines how easily the indicator can distinguish between acidic and basic solutions.
Key Indicators and Their pH Ranges
Indicators are compounds that change colour based on the concentration of hydrogen ions (H+) in a solution. The colour change occurs over a certain pH range, which is known as the transition range. The size of the transition range determines how easily the indicator can distinguish between acidic and basic solutions.
Each indicator has a specific pH range within which it changes colour:
Methyl Orange: pH 3.1–4.4
Red in acidic solutions
Yellow in basic solutions
Bromothymol Blue: pH 6.0–7.6
Yellow in acidic solutions
Blue in basic solutions
Litmus: pH 5.5–8.0
Red in acidic solutions
Blue in basic solutions
Phenolphthalein: pH 8.3–10.0
Colourless in acidic solutions
Pink in basic solutions
By observing these changes in colour, you can determine whether a solution is acidic, neutral, or basic. For example, a solution that appears yellow in methyl orange, blue in bromothymol blue, purple in litmus, and colourless in phenolphthalein suggests a neutral or slightly basic pH.
How Do Indicators Work?
Indicators function as weak acids or bases. The colour change is a result of a shift in equilibrium, as the indicator molecules either donate or accept protons (H+ ions) depending on the pH of the solution.
The reaction for an indicator can be summarized as follows:
Indicator (HA) ⇌ H+ + A-
HA represents the indicator molecule in its acidic form, which gives it one colour.
A- represents the deprotonated (basic) form of the indicator, which gives it a different colour.
When the pH of the solution changes, the equilibrium between HA and A- shifts to either the left (more acidic) or the right (more basic), causing the indicator to change colour.
Practical Investigation: Red Cabbage Indicator
In this experiment, we will use red cabbage as a natural pH indicator. Red cabbage contains anthocyanins, a group of weak acids that change colour depending on the pH of the solution they are placed in.
Materials
Red cabbage
1.00 M NaOH (sodium hydroxide)
1.00 M HCl (hydrochloric acid)
Spot plates
Measuring cylinder
Universal indicator
White paper or other white background
Colour charts
Method
Prepare Dilutions: Prepare various concentrations of HCl and NaOH by serial dilution. Start with 1.0 M solutions and dilute down to 1.0 x 10-6 M for both acid and base.
Extract Cabbage Juice: Finely chop half a red cabbage and boil it in water until the solution turns deep purple. Strain out the cabbage pieces to collect the liquid, then let it cool.
Testing Solutions: Place drops of each acid and base solution onto a spot plate. Add a drop of the cabbage juice to each drop of solution and observe the color change.
Use Universal Indicator: Also test the solutions using a universal indicator, comparing the colors with a pH colour chart to determine the pH.
What’s Happening in the Reaction?
The anthocyanins in the cabbage juice behave as weak acids. When an acid (like HCl) is added, the concentration of hydrogen ions (H+) increases, which shifts the equilibrium of the anthocyanin reaction to the left, producing a red colour. In contrast, when a base (like NaOH) is added, the OH- ions neutralize the H+ ions, shifting the equilibrium to the right and causing the solution to turn blue or green.
Thus, the cabbage juice changes colour based on the acidity or alkalinity of the solution. The colour range moves from red (acidic) to blue/green (basic).
Risk Assessment
NaOH and HCl are corrosive: Wear gloves and goggles to protect from skin and eye irritation. Always rinse exposed skin immediately with water if contact occurs.
Boiling water: Handle with care to avoid burns. Use tongs or appropriate equipment when handling hot glassware.
Understanding the Indicator's Behaviour
The behaviour of indicators can be explained using Le Chatelier’s Principle. As the concentration of hydrogen ions (H+) increases in an acidic solution, the equilibrium shifts to produce more of the protonated (acidic) form of the indicator, resulting in a red colour. In basic solutions, the concentration of H+ decreases, shifting the equilibrium toward the deprotonated (basic) form, causing the colour to change to blue or yellow.
For example, when methyl orange is added to a solution, the following equilibrium is established:
HIn (aq) ⇌ H+ (aq) + In- (aq)
In an acidic solution, the high concentration of H+ causes the equilibrium to shift to the left, and the solution remains red.
In a basic solution, the H+ concentration decreases, shifting the equilibrium to the right and causing the solution to turn yellow.
