Litmus paper
29 September, 2020

Litmus paper

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Litmus paper concept:


Litmus paper is a water-soluble mixture of various pigments extracted from lichens.


Litmus Paper Function:


This mixture is usually absorbed by a filter paper which is then used as a pH indicator to measure the pH and alkalinity of the materials. A blue litmus leaf turns red when exposed to acid, and a red litmus leaf turns blue when exposed to an alkaline or basic substance.


When was litmus paper first used ?! Who used it?


It was first used in the 1300s by the Spanish chemist Arnolds de Vila Nova.


Litmus paper test:


The litmus test is a quick way to determine whether a liquid or gaseous solution is acidic or alkaline. The test can be performed using litmus paper and is either red or blue.


Limitations of the litmus test:


The test is quick and simple, but it has a few limitations.

  • Firstly it is not an accurate indicator of the pH. It does not give a numeric value, instead indicating whether the sample is an acid or a base.
  • Second, the paper can change colors for other reasons besides the acid-base reaction. For example, blue litmus paper turns white in chlorine gas. This color change is due to the bleaching of the dye from hypochlorite ions, not the acidity and alkalinity.


Alternatives to litmus paper:


Litmus paper is useful as a base indicator for general acid, but you can get more specific results if you use an indicator that has a more narrow test range. Orcein and Azolitmin yield similar results to those found in litmus paper.

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