Acid Base & Salt - Class 10th Science
Indicators
Acid and Base in Laboratory
In laboratory acid and base are detected using indicators.
Indicators
Indicators are substances which tell us that whether a given substance is an acid or a base.
Types of Indicators:
On the basis of occurrence indicators are categorized into two types:
- Natural Indicators
- Synthetic Indicators
Natural Indicators:
Indicators which are obtained naturally are called natural indicators, for example: Litmus, red cabbage, turmeric, onion, vanilla, clove, etc.
Litmus:
Litmus is a natural indicator. Litmus is soluble in water. Litmus is a type of dye and extracted from an organism called Lichen. Lichens are composite organism fungus and generally green alga, which shows symbiotic relationship.
Litmus papers are prepared by dipping filter paper into the extract of litmus. The original color of litmus is purple generally called red.
In laboratories litmus papers are frequently used to detect the acid or base.
Litmus papers come into two colours: Blue and Red.
Acid turns blue litmus paper into red.
Base turns red litmus paper blue.
Turmeric:
Turmeric is another natural indicator. The colour of turmeric is yellow.
Acid has no effect on the colour of turmeric. This means acid does not change the color of turmeric.
Base changes yellow color of turmeric to reddish brown or can be called orange.
You will notice that cloth with stain of curry turns reddish brown or orange after applying of soap when taken for washing. This happens because soaps are made of bases (Sodium bicarbonate), this base turns stains of curry, which is actually turmeric on clothes from yellow to reddish brown. This reddish brown color again turns yellow when clothes are dried.
Turmeric papers can also be made by dipping and drying the blotting, filter or toilet paper in the solution of turmeric. Turmeric solution can be made by using water.
Red Cabbage:
The juice of red cabbage is used as another natural indicator. Red cabbage is of purple color and consequently juice is purple in color.
When red cabbages grown on acidic soil, their leaves are reddish but while grown on alkaline soil, their leaves are of yellowing green color.
That’s why juice of leaves of red cabbage is used as natural indicator.
Red cabbage juice is originally purple in colour.
This juice turns reddish when acid is added.
Juice of red cabbage runs greenish when base is added.
Thus, juice of red cabbage is used as natural indicator to detect acid or base.
Olfactory Indicators:
Nature gives us many substances which change their order with acid and base. Vanilla, cloves, onion, etc. are such substances. In other words, substances which change their order with acid and base are called olfactory indicators.
Generally vanilla, cloves and onion are used as olfactory indicators to detect acid or base. Olfactory indicators are useful for visually impaired people. Use of olfactory indicators insures the participation of visually impaired students in lab activities equally.
Onion:
Onion does not lose its smell when acid is added to it.
But, onion loses its smell when base is added to it.
Keeping this character of onion, it is used as olfactory indicator.
Vanilla:
When base is added, vanilla loses its smell.
But, when acid is added, vanilla does not lose its smell.
Clove:
The smell of clove oil vanishes with base.
But, clove does not lose its smell with acid.
Synthetic Indicators:
Indicators which are made in laboratories are called synthetic indicators.
Phenolphthalein and methyl orange are two most common synthetic indicators used in laboratory.
Phenolphthalein:
Phenolphthalein is colourless solution in original.
Solution of phenolphthalein turns pink with basic solution.
But, phenolphthalein solution remains colourless with acedic solution.
Methyl Orange:
Methyl orange is an orange liquid.
Methyl orange turns yellow with basic solution.
While methyl orange turns red with acidic solution.