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Chemical indicators: what are they for, types, examples

Chemical indicators are weak acids or bases that are used at very low concentrations; and therefore they do not interfere with the reactions in which they are present. They are generally organic compounds of complex formulas, best represented as HIn.

Test tubes with a methyl orange indicator showing its color changes. Source: LHcheM / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

There are several types of chemical indicators depending on the reactions in which they participate. There are acid-base (pH), oxidation-reduction reactions (redox), turbidity, precipitation, humidity, etc. The pH indicators are the most numerous and perhaps the best known.

These chemical pH indicators can be artificial or natural. Artificial ones are human creations; for example, methyl orange (top image). Meanwhile, the natural ones are found in vegetables; for example, the slime (litmus), or the juice of the red or purple cabbage.

What are chemical indicators for?

Indicators have two main applications:

-Estimation of parameters such as pH, humidity, oxidation-reduction potentials, solubility, etc.

-Determination of the end points of the degrees, regardless of what type they are.

Types of chemical indicators

Acid-Base Indicator

The color of the indicator varies depending on the concentration of H 3 O + or OH  in the solution; or rather, of the variation of the pH of the medium. Acid-base indicators are salts of weak acids or bases. These indicators are end-point, that is, the pH at which the color of the indicator varies.

The ionization process of an acid-base chemical indicator can be outlined as follows

HIn (color 1) ⇌ In  (color 2) + H 3 O +

As the concentration of H 3 O + (acid) increases, the reaction is oriented to the left and the acid form HIn predominates. Whereas if the concentration of H 3 O + decreases, the reaction is oriented to the right and the basic form In  predominates . In this way, colors 1 and 2 appear for HIn and In  , respectively.

However, there are indicators that have more than two colors.

Natural acid-base indicators

They are found in the leaves, flowers, fruits, and stems of numerous plant species. One can cite the example of the hydrangea, whose flowers are blue if it is planted in acid soil, rich in aluminum. But in alkaline soils, its flowers are pink to purple.

Indicators of oxidation-reduction reactions (redox)

Redox reactions generate oxidation potentials and reduction potentials, which can produce changes in certain chemical indicators. For example: Ferrous 1-10-phenanthroline is a highly sensitive chemical indicator.

Adsorption (precipitation) indicators

Fluorescein is a chemical indicator used to ensure completion of the binding process of silver and chloride ions in the precipitation of silver chloride.

Subsequently, an additional quantity of silver and fluorescein adhere to the silver chloride precipitate, changing the latter from green-yellow to red. This indicates that the formation of the silver chloride precipitate is completely complete.

Metallochromic or complexometric indicators

They are chemical indicators that change color in the presence of certain ions and can return to their original color once the ions are removed. These indicators are used to determine when all metal ions are chelated or sequestered, generally by EDTA.

There is specificity of metallochromic indicators for certain ions. For example: calcein is specific for calcium. Hematoxylin is specific for copper. And dithizone is specific for zinc and lead.

Chemical indicators with luminescence capacity

Luminol reacting with hemoglobin. Source: everyone’s idle from berlin, germany / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.5)

Luminol (5-amino-2,3-dihydrophthalazine-1,4-dione) is a chemical compound , which in the presence of an oxidizing agent, emits a blue light. Luminol is used in forensic practice to detect the presence of blood, although this is not evident, since luminol reacts with the iron present in hemoglobin.

Chemical moisture indicators

Silica gel bag. Source: Silver Spoon / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Silica gel is a chemical substance that has a great capacity to absorb water, being used to keep medicines, some foods, etc. dry. Silica gel is used in small bags that accompany the products to be dried.

To ensure that the desiccant is capable of fulfilling its function, it is accompanied by a chemical moisture indicator, for example: methyl violet. This indicator is orange when dry, and purple when wet.

Chemical sterilization indicators

They are chemical compounds that accompany materials subjected to an autoclave sterilization process. There are of several types. Some are solids that melt from heat, and others are substances that change color due to the sterilization process.

These indicators allow you to be confident that the material has completed the entire sterilization process.

Examples of chemical indicators

Base acid

Listed below are several acidic indicators along with the pH ranges where they change color:

-Methyl yellow 2.9 – 4.0

-Bromophenol Blue 3.0 – 4.6

-Congo red 3.0 – 5.0

-Methyl orange 3.1 – 4.4

-Bromocresol green 3.8 – 5.4

-Methyl Violet 10 B 0.0 – 2.0

-Phenol red 6.4 – 8.0

-Neutral red 6.8 – 8.0

Phenolphthalein (First change) 0.0 – 8.3 (colorless)

(Second change) 8.3 – 10.0 (purple)

(Third change) 12.0 – 13.0 (colorless)

Natural acid base

The anthocyanin pigment, which acts as an acid-base indicator, is found in the petals of various red flowers, including geranium, on stems such as rhubarb, etc.

Turmeric, a spice used in oriental cuisine, is yellow in acidic conditions, and brown in alkaline conditions.

Litmus

Litmus is an acid-base indicator that was used by medieval alchemists. It is a natural indicator made with a mixture of dyes extracted from various lichens.

Litmus is red at acidic pH and blue at alkaline pH, having a color change scale between 4.5 and 8.3.

Potassium permanganate

Potassium permanganate has a dual function: it acts as an oxidizing agent and as a chemical redox indicator. In reduced state it is pale pink in color, while in oxidized state it is deep purple.

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