Litmus

Lackmus

7-hydroxyphenoxazone

Parmelia sulcata

Litmus paper

Litmus is a water-soluble mixture of different dyes extracted from lichens, primarily Roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity or alkalinity.

History

The use of litmus is thought to date back to the 1300s, although the exact origins are unclear. It was first used in Europe after being introduced by the Arabs, who had discovered its properties through their interest in alchemy and chemistry. The name "litmus" comes from an Old Norse word meaning "to dye" or "to color."

Production

Litmus is made through a complex process that involves fermenting the lichens with potassium carbonate, ammonia, and various other compounds over several weeks. This fermentation process leads to the formation of the litmus mixture, which contains around 10-15 different dyes. These dyes are primarily of the anthraquinone family, which are known for their pH-sensitive color changes.

Chemical Properties

Litmus is a pH indicator that changes color in response to the acidity or alkalinity of a solution. When added to an acidic solution (pH < 7), litmus turns red, and when added to an alkaline solution (pH > 7), it turns blue. The transition range of litmus is approximately between pH 4.5 and 8.3. However, it is most accurate around the neutral range (pH 7).

Uses

Litmus paper, which is paper treated with litmus dye, is commonly used in educational settings and laboratories to quickly assess the pH level of a solution. It is particularly useful in chemistry and biology labs for performing simple acid-base titrations or for testing the pH of various substances. Despite the development of more precise methods of pH measurement, such as pH meters, litmus paper remains popular due to its simplicity and ease of use.

Limitations

While litmus paper is a convenient tool for indicating pH, it has limitations. It does not provide an exact pH value, only a rough indication of whether a solution is acidic or alkaline. Additionally, the color change can be subjective and may be influenced by the lighting conditions or the presence of other colored substances in the solution.

Conclusion

Litmus remains an essential tool in the field of chemistry and education for the simple and rapid assessment of the acidity or alkalinity of a solution. Its ease of use, affordability, and the immediate visual feedback it provides make it an enduring choice for basic pH testing.

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