"Red cabbage indicator" experiment
How to make a pH indicator at home
Here’s another experiment to add to your bag of lifehacks. In this colorful experiment, we learn to make a universal pH indicator.
Reagents and equipment:
- concentrated sulfuric acid;
- 9% acetic acid;
- fizzy water;
- distilled water;
- a saturated solution of sodium carbonate;
- 10% ammonium;
- saturated solution of sodium hydroxide;
- 150 ml glasses (x70;
- 50 ml glasses (x6);
- 1-liter glass;
- 1-liter conic flask;
- cotton wool;
- red cabbage.
Cut the red cabbage, put it in the 1-liter glass, pour hot water over it and let it cool slightly. Filter the mixture in a funnel through cotton wool. Pour the obtained purple solution into 7 150 ml glasses, and gradually add solutions with different pH to our indicator for calibration: concentrated sulfuric acid, acetic acid, fizzy water (carbonic acid), ammonium carbonate solution, ammonium solution and sodium hydroxide solution. Observe the change in color from red to yellow.
Aqueous solutions can be neutral, alkaline or acidic. This means that they have potential of hydrogen, or pH. The potential of hydrogen, or pH, is the measure of activity (or concentration) of hydrogen ions in a solution. If pH is < 7, the solution is acidic, but if рН = 7, then you can relax, it’s neutral. But this gradation is only applicable to aqueous solution; if you take any other solvent, everything can change drastically. Potential of hydrogen can be measured by two methods: with a special pH measuring device, or with indicator paper or solution. Today, we’ve made an indicator from red cabbage, and calibrated it. Colored vegetables and fruits contain various colorants with the general name of anthocyans. Depending on the pH, they can have different colors. When we pour solutions with different pH values into our purple solution, we receive a gradation of color from red to yellow. Sulfuric acid, as a strong mineral acid, gives us the value of pH ≈ 0-1, acetic acid gives рН ≈ 3-4, fizzy water (carbonic acid solution) gives рН ≈ 5-6, a solution of sodium carbonate gives рН ≈ 8-9, solution of ammonia gives рН ≈ 10, and a solution of sodium hydroxide gives рН ≈ 14. We can also evaporate the solution incompletely and apply the remaining mixture to paper, creating indicator paper which can be stored for a long time.
Wear protective glasses and gloves. Concentrated solutions of acids and bases are very caustic substances.
Warning! Substances of this experiment are toxic and highly dangerous for your health. Do not try this at home. Only under professional supervision.