The pH of an aqueous solution can be determined by treating the filter paper with any of the aqueous solution. The first indicator that was used for such kind of purpose was litmus.
Litmus paper is the paper that has been treated with a specific kind of indicator- a mixture that contains 10-15 natural dyes which have been obtained from lichens (turns red in acidic conditions). The paper used for making litmus paper needs to be free of any contaminant that can hamper with the pH system of the substance it measures.
When the pH is neutral i.e. 7, the dye then turns purple in colour. Litmus was used in 1300AD for the first time by Spanish alchemist Arnaldus de Villa Nova. Litmus paper is known to be the most recognised chemical indicators. Like any other pH paper, the litmus paper changes colour in response to the exposure to the acidic or basic solution.
USES AND COLOUR OF LITMUS PAPER
According to the pH scale that ranges from 0-14, the 0 is the most acidic, 7 is neutral while 14 is the most basic or alkaline in nature. Litmus paper is used most commonly in chemistry classes. Even though litmus paper can indicate whether the particular substance is acidic or basic, it cannot tell or determine the exact numerical pH value.
The litmus paper is found in two colours- red and blue. Blue is the natural colour for the litmus paper. The blue litmus paper turns red when it is put in an acidic solution. The red litmus paper is mixed with an acid during its production hence the paper appears to be red. When the red litmus paper is put in the presence of a base, the paper then goes back to turn into its natural blue colour.
THE LITMUS TEST
For the test, a drop of sample liquid is placed on the litmus paper or the litmus paper is dipped in the small sample of solution. The sample taken is the one which has to be determined for its acidic or basic nature.
The litmus test very quickly tells whether the solution is acidic or basic. The blue paper changes its colour to red when it is placed in acidic solution. The red litmus paper turns blue when placed in an alkaline or basic solution. And the paper turns purple when the pH of the solution is neutral.
If the red paper does not change its colour, then it means that the sample is acid and if the blue paper does not change its colour, then the sample is alkaline or basic in nature.