Kennedy Rainbow Cell

Initial Kennedy Rainbow Cell setup

Demonstrate electrolysis with an electrolytic cell in a petri dish.

Materials

• 1 × Large petri dish
• 1 × DC Power pack
• ~50 mL Distilled water dH₂O(l)
• ~3 g potassium nitrate powder KNO₃(s)
• 2 × Graphite electrodes
• 2 × Wires with crocodile clips
  
• 1 × Clamp and stand
  
• 1 × Very strong static magnet
  
• 1 × Roll of sticky tape (any type)
  
• ~10 drops of universal indicator
• ~50 mL dilute HNO₃(aq)
• ~50 mL dilute KOH(aq)
• 1 × Spatula
  

Method

1. Place petri dish on clean, light-coloured bench and add distilled water until it is two thirds full
2. Add ~10 drops of universal indicator and observe the colour.
   Q: What pH is the distilled water? (You’ll be surprised!)
   Q: Why is/isn’t the colour green?
3. Add ~3 g of potassium nitrate to the petri dish and stir using a spatula until completely dissolved
4. Adjust the pH of the distilled water carefully using the nitric acid and potassium hydroxide as required. Try to make the universal indicator colour green (as pictured) ~pH 7
5. Attach one electrode to each of two wires using crocodile clips
6. Dip each graphite electrode into the green solution at opposite sides of the petri dish. Hold these electrodes (and wires) in position by in position by sticky-taping each wire to the surface of the workbench
7. Demonstrate the strength of the magnet by attaching it to the clamp. Carefully, clamp the magnet into the clamp and position the magnet 2 mm above the surface of the green solution
8. Ensuring the power is turned off, very carefully, attach the wires to the DC power pack according to the manufacturer’s instructions
9. Turn the voltage to zero (or very low) and turn on the power pack
10. Turn the voltage up slowly (12 volts worked well) and observe any changes you might see in the Kennedy Rainbow Cell

Extensions

• Turn off the power pack and stir the solution. Explain why the colour goes back to being green. (If it’s not green, explain that, too!)
• Turn the magnet upside-down (TURN OFF THE POWER FIRST)
• Reverse the polarity of the wires
• Use AC current instead of DC
• Use different indicators
• Why would using NaCl(aq) be dangerous in this cell?
• How can you maximise the swirling?
• How can you make this experiment much more epic?

Click to download Kennedy Rainbow Cell worksheet (PDF)

Safety considerations

• Make your own risk assessment before carrying out this experiment
• The strong magnet is capable of attracting both wires to itself. Don’t be touching the exposed parts of the crocodile clips when this happens. If this does happen, immediately turn off the power pack and fix the problem. Secure the wires with more tape. Don’t touch the electrodes while the Cell is operating.
• Don’t use chloride salts or hydrochloric acid in this experiment. The voltages involved can cause the production of toxic chlorine gas if sodium chloride is used. Use nitric acid and potassium nitrate instead.
• Make sure the wires don’t touch each other.
• Again, make your own risk assessment before carrying out this experiment

Video

Disclaimer

This cell is potentially dangerous. I accept no responsibility for and loss, damage or injury caused by the operation of a Kennedy Rainbow Cell. If you’re under 18, always get adult permission before you make this type of cell.

Colourful Chemistry: Chemistry of UNIVERSAL INDICATOR

jameskennedymonash.wordpress.com

By definition, an indicator is a substance that changes colour in different pH environments. Universal indicator is a brown-coloured solution—containing a mixture of indicators—that can be added to any substance to determine its pH. Like all indicators, universal indicator changes colour in different pH environments. At low pH, it appears red, and at high pH, it appears blue or violet. At neutral pH, it appears green. Universal indicator can form a continuous spectrum of colours that give an approximate reading of the concentration of protons in a sample.

Water and propan-1-ol are used as solvents. They are both polar and dissolve all the other ingredients in the solution. Sodium hydroxide (NaOH) is an alkaline solution that adjusts the pH of the universal indicator to ensure that each colour is shown at the correct pH value. It is necessary to add NaOH to the universal indicator because some of the indicator compounds (e.g. methyl red) are acidic themselves, which would affect the colour of the other indicators present. NaOH is added to neutralise the solution.

Methyl red is red at pH <5 and yellow at pH >5. It provides orange and red hues to the universal indicator solution at low pH. The end point of an indicator compound is defined as the pH at which it changes colour. The end point of methyl red, therefore, is somewhere around pH 5.

Bromothymol blue is blue at pH >6 and yellow at pH <6. It gives blue and indigo hues at high pH. Its end point is therefore around pH 6.

Thymol blue has two end points: it is red below pH <2, blue at pH >8 and yellow in the middle. Thymol blue allows universal indicator to differentiate low and very low pH by providing another red hue below pH 2. Thymol blue is yellow at pH 7, which, when combined with bromothymol blue (which is blue at pH 7), give a green colour.

Finally, phenolphthalein gives universal indicator a deep violet colour at very high pH.

This 2-miunte BBC video is a great introduction to universal indicator: