The Concept of Litmus

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Litmus, what is it? And why is it important?

Litmus was first used around 1300 AD by Arnaldus de Villa Nova, a Spanish alchemist . The word Litmus, comes from the Norse word, meaning ‘to dye, or color’ . Litmus is a common acid-base indicator. Acid-base indicators are indicators used in chemistry as chemical detectors. They provide information about changes in the environment. They are widely used in chemistry due to the fact that they change color reversibly, indirectly indicating the concentration of hydrogen ions present in tested solutions or mixtures.

Being one of the most well known acid-base indicators, litmus is a water-soluble mixture of different dyes derived from natural substances, lichens, especially Roccella tinctoria, which is a kind of composite organism consisting of symbiotic association of a fungus with a photosynthetic partner, most commonly green algae . Litmus can be in a dark blue aqueous form, and it can also be absorbed onto filter paper to become the most common and oldest form of pH indicator. Neutral litmus paper is purple in color In the presence of acids, blue litmus paper turns pink; In the presence of alkalis, red litmus paper it turns blue. Hence, it is often used for testing for the presence of acids and alkalis. This color change occurs over the pH range of 4.5 – 8.3 at 25°C.

Litmus itself is a weak acid , when it is un-ionized it is red, when it is ionized, it is blue. If litmus is added to a solution where there is a presence of hydroxide ions, the hydroxide ions will react with the hydrogen ions from litmus, the equilibrium position moves to replace the lost hydrogen ions, hence litmus turns blue. Similarly, when it is added into a hydrogen ion containing solution, there will be extra hydrogen ions and so the equilibrium will shift in the other direction, to remove the extra hydrogen ions present, hence litmus turns red. When the concentration of hydrogen ions and unionized limus is the same, a position of...
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