Little wonder: pH experiments the microscale way Teach article

Author(s): Bob Worley and Adrian Allan

Drop by drop: Learn about pH chemistry and neutralization reactions, and produce wonderful colours using microscale methods that are cheap, quick, and easy.

The visible display made by the various colours of indicators is a wonderful way of introducing chemistry to students. The concept will be used continually throughout their studies. Indicators are used in titration studies; in the medical field, such as pH variations in urine caused by kidney infections; in water-supply analysis; and in the environmental analysis of water in streams, rivers, and aquarium tanks, as well as establishing the pH of soil. These activities are suitable for students aged 11–16.

Activity 1: Indicator colour changes

A template sheet for testing pH indicator colours.

The microscale approach saves valuable time for student activities and in clearing up afterwards. Students are seated, which adds to laboratory management.

This activity uses a template similar to that on the right, which can be found in the Worksheet Templates file.

There is also a Practical Notes document that provides tips on equipment and making templates, along with instructions for preparing different buffer solutions.

Safety note

The solutions used are of low hazard, but eye protection should be worn.

Materials

Reaction sheet, laminated or in a plastic wallet
Bromothymol blue indicator solution
Methyl orange indicator solution
Phenolphthalein solution
Litmus solution
pH buffer 7 (tap water may be suitable, but the procedure needs to be tested; for example, tap water in areas with soft water may have an acidic pH value)
Hydrochloric acid (0.02 M)
Sodium hydroxide (0.02 M)
Dropper bottles containing the above solutions or Pasteur pipettes and a beaker of water to rinse pipettes

Procedure

Insert the worksheet into the plastic folder.
Add drops of the reagent down the columns using transfer pipettes or dropper bottles.
Add the relevant indicators across the row.
Record/photograph the results.
Disposal: wipe the plastic surface with a paper towel and dispose of the towel in the waste.

Litmus and bromothymol blue change colour at pH 7. However, students must realize that not all indicators change colour at pH 7, even though this is a ‘neutral’ solution, where the concentration of hydrogen and hydroxide ions are the same.

The pH testing sheet shows coloured drops of indicator solution at different pH values. — Microscale pH indicator results on a laminated sheet
*Image courtesy of Beth Sutherland and Pixie Murray*

Variations

The method can be adjusted, depending on the chemicals available:

Instead of 0.02 M hydrochloric acid, use 0.05–0.1 M ethanoic acid (clear vinegar can also be used);
Instead of 0.02 M sodium hydroxide, use 0.1 M sodium carbonate.

Many other synthetic indicators can be used as well, such as bromophenol blue or thymolphthalein.

Extension: An example of a reversible reaction

Experiments can be repeated to focus on a different aspect of chemistry. Although colour changes are appealing to younger students, this change can also be used to explain the very difficult concept of reversible reactions.

A template sheet for demonstrating reversible reactions.

Indicators are weak acids, but the colour of the acid form (HIn(aq)) is different from that of the conjugate base (In⁻(aq)).

\[\color{red}{\text{HIn}\text{(aq)}}⇄\text{H}^+{ }\text{(aq)}+\color{blue}{\text{In}^{-}{ }\text{(aq)}}\]

Here is a method of switching between the two coloured forms by repeated addition of acid and alkali.

This means that the change can be explained by the Le Chatelier principle. However, it is better to explain these effects using the equation that defines the equilibrium constant, a value that is constant at the same temperature:

\[K_c = {[\text{H}^+ ]_\text{eq}\color{blue}{[\text{In}^- ]_\text{eq}}\over\color{red}{\text{[HIn]}_\text{eq }}}\]

If the concentration of hydrogen ions is increased, then for K_c to remain constant the value of [HIn]_eq has to increase. If hydroxide ions are added, then the value of [H⁺]_eq decreases, so the value of [In⁻]_eq has to increase for K_c to remain constant.

The use of dilute dyes, and especially natural dyes, to illustrate reversible reactions is an example of ‘green’ chemistry. A reversible reaction often quoted in textbooks involving dichromate/chromate exchange occurs as a result of changes to acidity. However, chromium(VI) compounds are very hazardous to both humans and the environment, so a ‘green’ replacement is welcome.

Drops of indicator solution change colour again after the pH is changed, which shows that the reaction is reversible. — Microscale reversible reactions of acids and bases
*Image courtesy of Beth Sutherland and Pixie Murray*

Activity 2: Investigating pH indicators

A template sheet for making a universal indicator.

A synthetic indicator, such as methyl orange, changes colour very dramatically over a narrow pH range.

A universal indicator shows a smooth colour change over a wide range of pH values. This type of indicator is made by mixing different indicators, so that there is a range of colours over a wide pH range. The commercial mixture is based on methyl red with thymol blue, bromothymol blue, and phenolphthalein. Students can make a version with methyl orange using the template provided. The colours have an immediate impact on students and can lead onto an investigation into natural indicators.

This activity would use over 20 test tubes in a traditional setup, but here some students could even work alone. This activity has the instructions in full view under the experiment, reduces the load on short-term memory, and hopefully some information will be retained in long-term memory.