Acids are compound that produce hydrogen ions (H+) when dissolved in water. An acid can be classified as weak or strong depending on the concentration of Hydrogen ions in solution. The strength of the acid is indicated by the scale which defined as the negative logarithm of the hydrogen ion concentration. The smaller number, the stronger the acid. The neutral point on this scale is seven at 25 °C . Solution with a pH value less than 7 are acid but those with a pH value of more than 7 are alkaline. Indicators are subtances wich show a range of colour changes according to the pH solution. They can thus be used indicate the strength of an acidic or alkaline solution. Many indicators can be easily extracted from vegetables, flower and fruits.
0,1 M hydrochloric acid
0,1 M sodium hydroxide
Preparing indicator from red cabbage
Boil some shreds of red cabbage in distilled water in a beaker. Boil for about 2 minutes to give a deeply coloured extract and then decant the solution, leaving all residue behind.
Preparation Of Standard solution pH 1 to 14
- Obtain about 5 ml of 0,1 M HCl in a test tube and label it pH 1
- Dilute 5 ml of 0,1 M HCl stock solution to 50 ml with distilled water, stir. The hydrogen ion concentration is 0,01 M and the pH is 2. Pour 5 ml of this solution in the a test tube labelled pH 2
- Using 5 ml of pH 2 solution, prepare 50 ml oh pH 3 solution following step 2
- Continue the dilution until pH 6. Distilled water water has pH of 7
5.Obtain about 5 ml of 1 M NaOH and label it pH 14
- Similarly, dilute the 1 M NaOH solution to obtain values pH values of 13,12,11,10,9,8
Using The Indicator
- Arrange test tube in increasing pH values in a test tube rack
- Add 1 ml of the red cabbage indicator into each test tube and record the colour changes in the table below
|Red cabbage Indicator||pH|
- Add a few drops of the indicator to saliva, urine, shampoo, detergents, baking powder, vinegar, smelling salts and lemon. What is their pH?
complete table below
Red cabbage is a plant that is most commonly used as a natural acid-base indicator, this is due to the red cabbage anthocyanin compounds that are able to change color according to the pH, below is a red cabbage in dropped with an acid solution, the color of red mange become increasingly red
In the next picture below is a red cabbage base solution drops, the color of the red cabbage into pale bluish
On the solution of different pH anthocyanins in red cabbage gives a different color from the experimental results obtained in accordance color of the solution as follows
- Prepare different types of indicators using rose petals (dark red), hisbiscus, clitoria,different types of tea, carrot, grapes,beetroot, cherries, pear skin, radish skin, turnip skin, apple skin and onion skin and record their colour changes at different pH. How effective are they as indicators? are they as good as the red cabbage indicator
- Leave the solutions to stand for some time and note the colour changes. Leave for another day or two. What do you notice? what disadvantages of using these natural indicators?
- Dissolve an aspirin tablet in 100 ml water. Filter if necessary. Test the pH of the filtrate. Compare the acidity of different brands of aspirin. If you brand of aspirin is too acidic, it may irritate the stomach
- Test the pH of toothpaste. Acids attack teeth, and your tooth paste should not be acidic
- Try testing different brand of washing powder (dissolved in water). Alkaline powder are not good for woolen fabrics and a powder can be harmful to nylon
- Extract fruit juice from various fruits. Rank fruits according to using the indicators prepared