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What is litmus paper?

The litmus paper is a material used in the pH tests to determine whether a solution is acidic or basic. Measuring pH is crucial when it comes to recognizing the properties of a chemical compound , and it can help us conduct 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 electron pairs, 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 of measuring this property. It is still used today, primarily in educational classrooms.

Source

The active part of litmus paper, called litmus from the Nordic word meaning “to color” or “paint”, comes from a natural material formed by the lichens Lecanora tartarea  and  Roccella tinctorum , which have been used to dye fabrics 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 is itself made from wood cellulose 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 passing 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 a blue or red litmus paper is to be produced.

It is known that the first use of this paper was recorded in the 14th 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.

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

Litmus paper functions

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

The litmus test owes its use to its interaction with liquid and gaseous samples, where each 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 by inserting a section of the paper into an aliquot of the sample (avoid completely inserting the paper into the entire container of sample for reliable results).

Gaseous samples

For gaseous samples, the paper should be moistened with distilled water and let the gas sample come into contact with it.

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

Limitations

The limitations of this test arise when wanting to quantify the pH of a substance, since it will not provide any measure 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 show acidity or basicity.

For these last reasons litmus paper is considered useful, but its competitors (such as the 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 during its preparation has been applied, making it acidic.

This paper will turn blue after 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 (either 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 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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