Chemical indicators: Ranges and the Science behind Color changes

Chemical indicators are organic substances that are used to determine the endpoint. Indicators are generally weak acids or weak bases. Indicators change their colors at a certain pH range due to ionization. Unionized forms essentially have a different color than ionized forms. Most of the pH indicators work at a pH range from 3 to 11. Commonly used indicators are phenolphthalein, litmus, methyl oranges, etc.

Titrations and its types
Indicators in chemistry

Types of indicators

  1. Universal indicators
  2. Acid-base indicators
  3. Complexometric or metallochromic indicators
  4. Adsorption or precipitation indicators
  5. Redox (Reduction-oxidation) indicators
  6. Indicators with luminescent property
  7. Nonaqueous indicators
  8. Chemical moisture indicators
  9. Chemical indicators of sterilization

Universal indicators (Multiple Range Indicators)

A universal indicator is a mixture of several indicators that show a color change in solutions. This information could be used to interpret how acidic or basic a chemical is. The main components of the universal indicator mixture are thymol blue, methyl red, bromothymol blue, and phenolphthalein, etc.

Universal indicators change colors at wide ranges of pH. They are not used for titration purposes because the endpoint detection is very difficult as it changes colors with changing pH.

Universal indicators are available in the form of paper strips or solution forms.

Paper strips

The universal indicator paper strip is actually just a paper, soaked with a universal indicator solution and then dried. It changes its colors with varying pH. For dark colored solutions, paper strips are preferred.

Universal indicator solution

The main components of universal indicator solution are phenolphthalein, methyl orange, thymol blue, methyl red, bromothymol blue, ethyl alcohol, sodium hydroxide, and deionized water. These components are ionized depending upon the acidity or alkalinity of a solution. Universal indicator solutions are usually used for colorless solutions.

pH range Description Colour
Less than 3 Strong acid Red
3 to 6 Weak acid Orange or Yellow
7 Neutral Green
8 to 11 Weak alkali Blue
Greater than 11 Strong alkali Indigo or Violet

After the addition of a universal indicator, the color of the solution tells all about the chemical nature of that solution. Yellow, orange, and red colors indicate acidity whereas purple, blue, and violet colors indicate the solution is basic. The green color indicates neutrality.

Ph scale for universal indicators 1 to 14

Acid-base indicators

Acid-base indicators are weak acids or bases commonly used to find out the endpoint in acid/base neutralization titration. Indicators change their color with the change of pH. Acid-base indicators are also named pH indicators.

Theories of acid-base indicators

The working mechanism of acid-base indicators can be well explained by two theories.

1. Ostwald theory of indicator

An indicator is an organic weak acid i.e Phenolphthalein or weak base i.e methyl orange or methyl red, etc. Now according to Ostwald’s theory of indicators, a chemical possesses different colors in ionized and unionized forms. In other words, the ionization of indicators causes a color change.

Indicators are feebly ionizable but the addition of strong acid or strong base considerably increases their ionization potential. Weak acidic indicators tend to get ionized in alkaline environments e.g. phenolphthalein, etc whereas weak alkaline indicators ionize in acidic environments e.g. methyl orange, etc.

Phenolphthalein as an acidic indicator possesses colored anions and colorless unionized molecules i.e.

HPh (colorless) H+ + Ph (violet-pink colored)

Methyl orange being a basic indicator possesses colored cations and different colored unionized forms i.e.

MeOH (Yellow) ⇌ Me+ (orange) + OH (colorless)

When does phenolphthalein turn pink gives quite the perspective of why are these theories are important.

2. Quinonoid theory of indicators

Acid-base indicators are usually organic aromatic compounds. They exist in at least two interconvertible tautomeric forms, i.e benzenoid form, and quinonoid form. There exists an equilibrium between these tautomeric forms.

As a general rule, one of the tautomeric forms exists in acidic medium and the other alkaline one. As different tautomeric forms must exhibit different colors, the colors of indicators change with changing pH of the solution.

Quinonoid forms generally have darker colors than benzenoid forms.

Indicators pH ranges In acidic medium In basic medium
Methyl violet 0.5 ↔ 1.6 Yellow Blue
Thymol blue 1.2 ↔ 2.8 Yellow Blue
Methyl orange 3.2 ↔ 4.4 Red Yellow
Bromocresol green 3.8 ↔ 5.4 Yellow Blue
Methyl Red 4.8 ↔ 6.0 Red Yellow
Chlorophenol red 5.2 ↔ 6.8 Yellow Red
Bromothymol blue 6.0 ↔ 7.6 Yellow Blue
Phenol red 6.6 ↔ 8.0 Yellow Red
Neutral red 6.8 ↔ 8.0 Red Yellow/Orange
Phenolphthalein 8.2 ↔ 10 Colorless Magenta
Thymolphthalein 9.4 ↔ 10.6 Colorless Blue
Alizarin yellow 10.1 ↔ 12.0 Yellow Red

Complexometric (Metallochromic) indicators

Complexometric indicators are also named metal or metallochromic indicators. They are used to determine the endpoint in the complexometric titrations. They change their specific colors in the presence of specific metal ions.

Complexometric indicators form chelates (complex) with metal atoms. These indicators act as Lewis bases where metal atoms act as Lewis acids. They are mostly water-soluble organic compounds.

Metaln+ + Indicator ⇌ Metal-Indicator (complex)

The reaction between the indicator and metal atoms is reversible. They have different colors in free form than complex compound colors. Some examples of such cases are shown:

Indicator Metal ions Individual colors Metallic complex
Pyrocatechol violet
Bi+3, Cd+2, Co+2, In+3, Mn+2, Zn+2
Yellow Blue
Green/Violet Red
EBT (Eriochrome black T)
Al+3, Cd+2, Zn+2, Ca+2, Mg+2, Pb+2
Blue Red
Calconcarboxylic acid]
Ca+2 in the presence of significant Mg+2
Blue Red
Hydroxynapthol blue
Ca+2, Mg+2
Blue Red
Ca+2, Cu+2, Co+2, Ni+2
Violet Yellow
PAN (Pyridyl azo naphthol)
Cu+2, Cd+2, Zn+2
Yellow Red
Phthalein purple
Ba+2, Ca+2, Sr+2
Colorless Purple
Fe+3, SO4-2
Yellow Blue
Xylenol orange
Al+3, Bi+3, La+3, Sc+3, Th+3, Zr+3
Yellow Red
Ca+2, Mg+2
Orange Yellowish green
(Ferrate) Fe3+
Colorless salts Blood red
Curcumin Boron Bright yellow Red

Adsorption or precipitation indicators

Adsorption or precipitation indicators are primarily used in titration reactions with species that can disturb the entire table balance if they exceed the specific concentration limit against one another. The formation of colored precipitates indicates the endpoint of that reaction. Although endpoint and equivalence can be quite confusing, Endpoint vs Equivalence point (PSIBERG) will help in better understanding.

Adsorption indicators change their color due to the adsorption of substances on their surfaces. Precipitation titrimetry, the evaluation process of such reactions is the correct determination of endpoint provided that there is a significant difference in solubilities of precipitates and solute.

Adsorption/precipitation indicators are used to determine slight excess of reactant or ion in a precipitation reaction. For example,

  • Potassium chromate (K2CrO4) is an indicator in the titration of chlorides with standard silver nitrate (AgNO3) solution.
  • Eosin Y (yellow), soluble in both alcohol and water.
  • Ferric alum (Ammonium Iron (III) sulfate), a saturated chemical solution.
  • Tartrazine, a trisodium salt of tartrazine acid.
  • Phenosaframine, simple red safranine dye.
  • Dichlorofluroscein, a dye from the fluorescein family used for monitoring ROS (Reactive oxygen species), etc.

Redox indicators

Redox indicators are used to determine the endpoints in redox titrations. They essentially have different colors in oxidized and reduced forms. They undergo reversible changes in oxidation or reduction environments and work at specific electrode potentials. Redox indicators require a quick establishment of reaction equilibrium and their color changes take place with changing electrode potentials.

It is essential that the reduction potential or oxidation potential of the redox indicator is quite close, so a sharp color change and good results are obtained.

Most of the redox indicators are organic compounds so there exist two main classes:

1. Organic redox indicator systems such as Methylene blue, etc.

Methylene Blue indicator cation

2. Metallic complexes of phenanthroline and bipyridine.

Bipyridine and phenanthroline indicators

Working of Potassium permanganate, a Redox indicator

KMnO4 (pink) to MnO2 (colorless) i.e. oxidized form to reduced form is a reversible reaction depending on the solution pH.

Potassium permanganate is a strong oxidizing agent so it gets reduced even in mild conditions. In acidic conditions, it forms MnO2 (colorless) while in basic environments, it forms KMnO4 (pink colored). It is a self indicator.

Reduced form ⇌ Oxidized form

Indicator+ (colorless) + ne⇌ Indicator (colored)

There are two different types of indicators for redox titrations. The examples are as follows.

1. pH-dependent redox indicators

The pH-dependent redox indicators include:

  • Sodium-o-cresol, thionine
  • Indigotrisulfonic acid
  • Indigotetrasulfonic acid
  • Phenosafranin
  • Methylene blue
  • Sodium-2,6-dichlorophenol-indophenol
  • Indigo carmine
  • Neutral red, etc

2. pH-independent redox indicators

The pH-independent redox indicators include:

  • o-dianisidine
  • Diphenylamine
  • Ethoxy chrysoidine
  • Diphenyl benzidine
  • Sodium diphenylamine sulphonate
  • Viologen
  • Dimethyl phenanthroline
  • Bipyridine (Fe) complex, etc

Indicators with luminescent property

According to recent researches, indicators with luminescent properties exhibit enhanced selectivity and a threshold better than other detecting techniques. Near-Infrared fluorescence indicators called NIR indicators are primarily the focus of the latest studies.

For example, a genetically encoded near-infrared fluorescent calcium ion indicator enables enhanced studies on cells of mammalian and even other origins.

Nonaqueous titration indicator

Nonaqueous titration indicators are used to detect endpoints in nonaqueous titrations. These indicators usually change three different colors in acidic, basic, and neutral mediums. These indicators are made in glacial acetic acid or alcohol, and other organic solvents.

Examples of nonaqueous titration indicators

  • Crystal Violet

It is prepared as a 0.5% solution in glacial acetic acid that gives violet color in basic, yellowish-green color in acidic, and bluish-green in neutral medium. It is used for the titration of pyridine with perchloric acid.

Crystal violet a non aqueous titration indicator

  • Oracet Blue B indicator

It is also prepared as a 0.5% solution in glacial acetic acid. However, it is blue in basic, pink in acidic, and purple in neutral medium.

Oracet Blue B indicator - a non aqueous titration indicator

  • Alpha naphtholbenzein

Alphanaptholbenzene is used as 0.2% solution in glacial acetic acid. It is blue in basic, dark green in acidic, and orange in neutral conditions.

Alpha naphtholbenzein - a non aqueous titration indicator

  • Quinaldine red

Quinaldine red is used as a 0.1% solution in acetic acid. It is magenta colored in basic and colorless in an acidic medium.

Quinaldine red - a non aqueous titration indicator

Chemical Moisture indicators

Chemical moisture or humidity indicators are moisture sensitive chemicals with the property of changing color when the level of humidity exceeds the limit.

For example,

  • Cobalt (II) chloride [CoCl2] turns its color from blue to pink when the level of humidity exceeds the specified limit.
  • Copper (II) chloride [CuCl2] does the same except it changes its color from yellow to blue.

Such indicators are being used on electrical appliances like mobile phones to detect the level of humidity when it gets dangerous for the electrical circuit systems.

Chemical indicators of sterilization

Sterilization is a process of removing, killing, and even deactivating life forms on equipment and foods, etc. Physical monitors coupled with chemical indicators are used to check whether the whole sterilization cycle is completed or not. The autoclave process specifically requires an indication of completion of the sterilization cycle for which tape indicators come into play.

Bowie dick test is a commonly used indicator test in which a color change from yellow to blue on an indicator sheet indicates complete sterilization.

Properties of good indicators

A good indicator indicates a very sharp change in color such that the turning point could be specified. The indicators should be cheap i.e. easily available. The color change must be stable and notable.

Specificity of the indicators is also an important term to pick a good indicator.

Uses of chemical indicators

Chemical indicators are used for

  • Determination of endpoints in different types of volumetric analysis.
  • The determination of the acidity and basicity of a solution.
  • They are used to determine the completion of a reaction.
  • Detection of metals is an important use of chemical indicators.
  • Indicators are used for the standardization of volumetric solutions.

Concepts Berg

How are chemical indicators used for sterilization?

Bowie dick test and several other chemicals dipped strips are used for sterilization checking. The color changes represent the completion of the sterilization cycle.

What are indications of a chemical change?

Some indications for chemical changes are

  • Color change
  • Precipitate formation
  • Temperature change
  • Formation of gas bubble
  • Change in melting and boiling point
  • Change of taste
  • Change of smell
  • Change in volume
  • Production of light
  • Change of conductivity

What is the difference between an indicator and a universal indicator

A universal indicator cannot be used to determine the endpoint of several titrations whereas other low-range chemical indicators serve the purpose. In short universal indicators are not specific and are wide-ranged whereas other chemical indicators are specific and short-ranged.

Why does an indicator change its color?

The indicator changes its color due to ionization. They have different colors in ionized and unionized forms according to the Quinonoid theory. When the pH of a medium having indicators changes, the indicator also changes its color because of essential ionization.

What is the definition of self indicator?

We any reagent (titrant) changes its color during titration without the need of an external indicator then this is called a self indicator. KMnO4 acts as a self indicator in redox titrations. It decolorizes at the endpoint of redox titration.

KMnO4 turns pink to colorless when going from acidic to basic medium.

What are the features of a good indicator?

A distinguishable endpoint, easily available, a very sharp color change, stable and specificity are the properties of a good indicator.

What is a household acid-base indicator?

Litmus paper is a household acid-base indicator. It works at pH ranges from 4.5 to 8.3. Under acidic conditions, the blue litmus paper turns red while under basic conditions, it turns red litmus to blue.

What is a suitable indicator for sulfuric acid and sodium carbonate?

As sulphuric acid being added to sodium carbonate is an acid-base titration, acid-base indicators or neutralization indicators must be used.

Sulfuric acid and Sodium carbonates are strong acids and strong bases so phenolphthalein or methyl orange should be used.

Which indicators are suitable to be used in the titration of HCl and NaOH?

Acid-base indicators phenolphthalein, methyl orange, methyl red, etc can be used for HCl and NaOH titration. Phenolphthalein is most commonly used in acid-base titrations because of its sharp and clear endpoint.

Why is potassium ferricyanide used as an external indicator?

Potassium ferricyanide is used as an external indicator because it interferes with the internal environment (species) of the solution.

What is litmus paper?

Litmus is a vegetable-based dye impregnated on paper. Litmus paper is available in two different colors and is used for the detection of acidity or the basicity of a solution. A blue litmus paper turns red under acidic conditions whereas red litmus paper turns blue under basic conditions.

What are the chemical formulae of litmus?

The chemical formulae of litmus are C9H10O5N and C13H22O6.

What is an indicator give some examples of indicators?

Indicators are organic compounds that are used to determine endpoints of titrations. These are usually weak acids or weak bases. They change colors according to the pH of the solution. For example phenolphthalein, Methyl oranges, methyl red, dithizone, crystal violet, etc.

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