7 junio, 2024

Litmus paper: what it is, origin, functions, blue and red

What is litmus paper?

He litmus paper It is a material used in pH tests to determine if a solution is acidic or basic. Measuring the pH is crucial when recognizing the properties of a chemical compound, and it can help us to carry out quality control studies in food, water and other species that we study.

In chemistry, all elements and compounds are considered to have properties of acidity and basicity; that is, they are classified as acidic, basic or neutral substances according to their pH. This property will decide how the substance will behave when it accepts or donates ions and/or pairs of electrons, as well as what kind of compounds an element forms.

Litmus paper has the interesting property that it will change color (thanks to the action of natural inks in its composition) depending on the pH of the substance in question, being one of the oldest ways to measure this property. It is still used today, mainly in educational classrooms.


The active part of litmus paper, called litmus from the Nordic word that means «color» or «paint», it comes from a natural material formed by lichens lecanora tartarea and Roccella tinctorumwhich have been used to dye cloth for several centuries.

In this way, this water-soluble mixture is absorbed on a special type of filter paper to produce the pH indicator, and then the solution in question is applied to it.

The paper used for this purpose itself is made from wood pulp and other compounds, and must be completely free of contaminants to avoid altering the measurement results.

On the other hand, the wood is processed with solvents to remove resinous material before going through a solution bath.

After this process, it is left to ferment and dry, in the presence of potassium carbonate and ammonia. The process will vary slightly depending on whether you are producing a blue or red litmus paper.

It is known that the first use of this paper was recorded in the fourteenth century, by a Spanish scientist named Arnau de Vilanova. In the same way, the French chemist JL Gay-Lussac also used this material for his chemical tests in the 18th century.

The dye itself can be said to have been extracted from lichens since the 16th century, so it is believed that the ancestors used other rudimentary types of litmus paper.

Functions of litmus paper

The litmus paper is responsible for indicating whether a chemical substance is acidic or basic, which is why it is called the litmus test at the moment when the paper is applied for this function.

The litmus test owes its use to its interaction with liquid and gaseous samples, where each one has a different process.

liquid samples

The test for liquid samples is done by dropping a drop of the aqueous solution sample onto a small strip of paper, as well as inserting a section of the paper into an aliquot of the sample (avoid fully inserting the paper into the entire container of sample to obtain reliable results).

gaseous samples

For gaseous samples, moisten the paper with distilled water and allow the gas sample to come into contact with it.

The gas should dissolve in the water, allowing the resulting solution to color the paper. In the case of neutral gases, such as oxygen and nitrogen, no change in the color of the pH paper is observed.


The limitations of this test arise when wanting to quantify the pH of a substance, since it will not provide any measurement or numerical value. Instead, it will only indicate whether a sample is acidic or basic in nature, and only vaguely.

Another problem with paper is that it can change color when certain types of substances are added to it, such as chlorine gas that turns it white. When this happens, the paper is useless and will not mark acidity or basicity.

For these latter reasons, litmus paper is considered useful, but its competitors (such as universal indicator and other dyes) have been replacing this material in recent decades.

Consequently, pH-meters are even better for this function, and are the most widely used equipment for calculations of this property.

blue litmus paper

Blue litmus paper is the original version of litmus paper, taking this color from the dyes used to make it.

The fermented mass of lichens in potassium carbonate and ammonia has a blue color, and the paper will be introduced into this mixture (regardless of whether it will be blue or red litmus paper).

This type of paper is basic in nature, and will turn red under acidic conditions at room temperature. Under basic conditions, no color change will occur and the result will be reported as “blue” instead of indicating “null”.

red litmus paper

Red litmus paper is litmus paper to which the additional step of adding a small amount of sulfuric or hydrochloric acid has been applied during preparation, causing it to take on an acid character.

This paper will turn blue after the application of the aqueous solution under basic conditions at room temperature, remaining red under acidic conditions, which is exemplified in the image below.

A curiosity about this paper (whether blue or red) is that it can be reused after drying once it has gone from blue to red (in the case of blue litmus paper) or red to blue (in the case of red litmus paper). ).

So it can be used again, but in reverse: now the red paper will be used as blue and the blue will be used as red.

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