How to make indicators at home
Colors and chemistry of indicators
Safety precautions
Wear protective gloves, eyewear, and a mask. Work in a well-ventilated area.
Reagents and equipment
- 0.1% alcohol solution of bromothymol blue;
- 10% sodium hydroxide solution;
- 10% hydrochloric acid solution;
- 1% methylene blue solution;
- 10 g glucose;
- 10% cobalt(II) chloride solution;
- filter paper;
- distilled water;
- 1% starch solution;
- 3% aqueous solution of iodine in potassium iodide;
- red cabbage;
- 9% vinegar solution;
- orange juice;
- 10% baking soda solution;
- drain cleaner (usually 10% sodium hydroxide solution);
- black tea;
- lemon;
- beakers;
- freshly-boiled water;
- conical flask with rubber stopper;
- cup.
Step-by-step instructions
- Add a few drops of the 0.1% alcohol solution of bromothymol blue to 30 mL of distilled water. Watch as the solution turns green. Add 1 mL of 10% hydrochloric acid solution – the liquid should turn yellow. Next, add 5 mL of 10% sodium hydroxide solution, and watch the solution turn blue.
- Pour 10 g of glucose into your conical flask. Add 50 mL of 10% sodium hydroxide solution, and a few drops of 1% methylene blue solution. The solution should turn colorless. When shaken, the solution turns blue.
- Apply some 10% cobalt(II) chloride solution to a filter paper. Use a blow dryer at maximum strength to dry the filter paper until it turns blue. Add a few drops of water and watch as it turns pink.
- Add a few drops of a 3% aqueous solution of iodine in potassium iodide to 70 mL of a 1% starch solution. Watch as the solution turns dark blue.
- Dice 300 g of red cabbage. Transfer the cabbage to your heat-resistant glass and add 250 mL of hot water. Let stand for 30 minutes. Drain the solution into the second glass through the funnel with cotton wool to filter out the cabbage slices. Use the pipette to apply the solution to a coffee filter, then let the filter dry at room temperature for 30 minutes. Cut into strips. Calibrate using reagents you likely have on hand: vinegar creates an acidic medium, which turns the strip red; water creates a neutral medium, so the strip stays purple; a 10% baking soda solution creates a basic environment, which causes the stip to turn blue; drain cleaner (usually a 10% solution of sodium hydroxide) creates a strongly basic medium, which turns the strip first green, then yellow.
- Brew a cup of strong black tea. Add a lemon slice and watch as the tea lightens in color.
Process description
- Bromothymol blue is an acid-base indicator. Its structure changes depending on the acidity of its environment, which causes its color to change too. It is green in a neutral environment, yellow in an acidic environment, and blue in a basic environment.
- Methylene blue is an oxidation-reduction indicator: it is easily reduced to a colorless state. In a basic environment, glucose is a good reducing agent, and it visibly reduces methylene blue to its colorless form. If the solution is shaken, the oxygen in the air oxidizes the colorless form back to blue.
- Anhydrous cobalt(II) chloride is a sky-blue salt, but turns pink when exposed to moisture, forming cobalt(II) chloride hexahydrate. This characteristic is often used in humidity indicators, such as those in smartphones.
- Starch can act as an indicator for molecular iodine. The iodine embeds in the molecular structure of the starch to form a dark-blue starch-iodine complex.
- Red cabbage contains pigments known as anthocyanins, which change colors in accordance with the acidity of their environment – a property that can help you determine the pH of various substances around you! They turn red in acidic mediums such as vinegar, purple in weakly acidic and neutral mediums such as water, blue in weakly basic mediums such as a solution of baking soda, and green, then yellow in strongly basic solutions such as drain cleaner.
- Tea leaves contain a pigment known as thearubigin, which influences the color of the drink. Adding a lemon slice to tea creates an acidic environment, which changes the structure of the thearubigin and causes the tea to lighten.
