What Is a Litmus test?

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Litmus test is a test done to find out whether a given compound is acidic , basic or amphoteric in nature. It is a natural chemical indicator that shows color change with pH(hydrogen ion concentration) change. While acid turns blue litmus red a base or alkali turns red litmus blue. Litmus test is done with the aid of a filter paper called Litmus paper, that has been pretreated with a specific indicator that shows color change in response to acidic or basic conditions. If the solution is neutral the color observed is purple.


How did the word Litmus Originate?

The term Litmus derives its name from old Norse word which means to dye or color. Litmus test was performed the first time in 1300 AD by Arnaldus de Villa Nova, an alchemist from Spain. Litmus paper was developed in around 1800 AD by French Chemist Gay Lussac. The idea or concept of performing Litmus test for identifying acidity or basicity of a compound was introduced by Robert Boyle.


How is litmus prepared?

Litmus is actually a die made from naturally occuring substances. Raw materials using for making litmus include wood cellulose, lichens and adhering compounds. The wood being used for making pulp is treated with solvents to remove  lignin or resinous material. Pretreatment of lichens with a solution of urine, potash and lime is followed by addition of plaster of paris. Moulding is done to get small cakes. Paper treated with litmus is known as litmus paper or litmus strips.The reason litmus paper shows color change is the infusion of lichens into paper.


What are the types of Litmus paper?

  • Red litmus paper: It is used to detect basicity. It turns to blue under basic conditions and is unchanged under acidic conditions. Red litmus paper is prepared by the treatment of  plain filter paper with a red dye. This red  dye is pre-treated with little sulphuric acid and dried in air.
  • Blue litmus paper: It is used to identify acids like sulphuric acid, nitric acid. It changes color from blue to red to indicate the  presence of aids. It is made by the treatment of plain paper with lichen blue dye.
  • Purple or neutral litmus paper: It can identify both acids and bases. In presence of acid its color changes from violet to red. It indicates basicity by turning blue.


How is a Litmus test performed?

While conducting a litmus test, liquid to be tested is applied to litmus paper by dipping one end of the paper in it. Alternatively some liquid is dropped on litmus paper using a pipette. The color change of red, blue, purple litmus strip is observed immediately before the paper dries. The results of the test directly predict the chemical acidity, alkalinity or neutrality of the liquid.The litmus paper should be free of any contaminants as they can change pH and this affect results. Litmus paper may be slightly hazardous and contact with eyes should be avoided.


What are the Applications of  Litmus paper?

Litmus paper is a simple, easy to use and suitable acid- base indicator. Results are observed or interpreted very soon. The concept of litmus test for determination of pH(Hydrogen ion concentration) finds multifarious applications in educational institutions and chemical laboratories. Litmus test is also used for molecular recognition using molecular membranes. Since many of the lichens used as raw materials in litmus paper making are becoming extinct, it may be desirable to use synthetic materials in the future. Litmus paper cannot predict the degree or strength of acidity or basicity of the liquid. Besides it does not work very well in non polar liquids.






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