Question

It is always good to have an assay where something happens. For example red pH paper that just remains red in acid is unsatisfactory. How can you make Ph paper which is green and then turns red when put in acid solution? how can you make red pH paper?

Answer 1

pH papers are small strips of papers coated with a chemical substances, they undergo a reaction when comes in contact with the liquid, to be tested. These papers usually come in two colors green and red, Red paper turns green when it comes in contact with a base, similarly green paper turns red when it comes in contact with an acid. neutral solutions do not change the color of the strip.

Litmus is a mixture of dyes usually extracted from lichens from the species "roccella tinctoria", it changes color according to the pH of the solution, which is to be tested.

To prepare pH paper, the lichens are crushed and fermented in a mixture of ammonia and potash. The fermented mixture is blue in color. This mixture is mixed with chalk to make a paste, the paste is dried and powdered. This mixture is absorbed onto a filter paper to make it an absorbent litmus paper.

To make Green litmus paper, the white dye-infused paper is fermented in the presence of Ammonia and/or potassium hydrogen carbonate solution.

To make Red litmus paper, the fermentation process is done in presence of a dilute solution of a strong acid, preferably Hydrochloric Acid or Sulfuric Acid.

The dye used have large molecules in them, called chromophores. Chromophores are a group of atoms which controls the color of the dye. The acidic or basic nature of a chemical solution breaks the bonds of the chromophores and alter the range of wavelengths the new structure of chromophores can absorb.By this the color of the paper changes.