Halochromism
Encyclopedia
A halochromic material is a material
which changes colour when pH
changes occur. The term ‘chromic
’ is defined as materials that can change colour reversibly with the presence of a factor. In this case, the factor is pH. The pH indicator
s have this property.
Halochromic substances
are suited for use in environments where pH changes occur frequently, or places where changes in pH are extreme. Halochromic substances detect alterations in the acid
ity of substances, like detection of corrosion in metals.
Halochromic substances may be used as indicators to determine the pH of solutions of unknown pH. The colour obtained is compared with the colour obtained when the indicator is mixed with solutions of known pH. The pH of the unknown solution can then be estimated. Obvious disadvantages of this method include its dependency on the colour sensitivity of the human eye, and that unknown solutions that are already coloured cannot be used.
The colour change of halochromic substances occur when the chemical binds to existing hydrogen and hydroxide ions in solution. Such bonds result in changes in the conjugate systems of the molecule, or the range of electron flow. This alters the amount of light absorbed, which in turn results in a visible change of colour. Halochromic substances do not display a full range of colour for a full range of pH because, after certain acidities, the conjugate system will not change. The various shades result from different concentrations of halochromic molecules with different conjugate systems.
Material
Material is anything made of matter, constituted of one or more substances. Wood, cement, hydrogen, air and water are all examples of materials. Sometimes the term "material" is used more narrowly to refer to substances or components with certain physical properties that are used as inputs to...
which changes colour when pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
changes occur. The term ‘chromic
Chromism
In chemistry, chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter...
’ is defined as materials that can change colour reversibly with the presence of a factor. In this case, the factor is pH. The pH indicator
PH indicator
A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions or hydrogen ions in the Arrhenius model. Normally, the indicator causes the...
s have this property.
Halochromic substances
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
are suited for use in environments where pH changes occur frequently, or places where changes in pH are extreme. Halochromic substances detect alterations in the acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ity of substances, like detection of corrosion in metals.
Halochromic substances may be used as indicators to determine the pH of solutions of unknown pH. The colour obtained is compared with the colour obtained when the indicator is mixed with solutions of known pH. The pH of the unknown solution can then be estimated. Obvious disadvantages of this method include its dependency on the colour sensitivity of the human eye, and that unknown solutions that are already coloured cannot be used.
The colour change of halochromic substances occur when the chemical binds to existing hydrogen and hydroxide ions in solution. Such bonds result in changes in the conjugate systems of the molecule, or the range of electron flow. This alters the amount of light absorbed, which in turn results in a visible change of colour. Halochromic substances do not display a full range of colour for a full range of pH because, after certain acidities, the conjugate system will not change. The various shades result from different concentrations of halochromic molecules with different conjugate systems.