In chemistry

Chemistry

Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a color reaction or colour reaction is a chemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 that is used to transform colorless chemical compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s into colored derivatives

Derivative (chemistry)

In chemistry, a derivative is a compound that is derived from a similar compound by some chemical or physical process. In the past it was also used to mean a compound that can be imagined to arise from another compound, if one atom is replaced with another atom or group of atoms, but modern...

 which can be detected visually or with the aid of a colorimeter

Colorimeter (chemistry)

Not to be confused with a calorimeter.A colorimeter is a device used in colorimetry. In scientific fields the word generally refers to the device that measures the absorbance of particular wavelengths of light by a specific solution...

.

The concentration of a colorless solution cannot normally be determined with a colorimeter. The addition of a color reagent leads to a color reaction and the absorbance of the colored product can then be measured with a colorimeter.

A change in absorbance in the ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 range cannot be detected by eye but can be measured by a suitably equipped colorimeter. A special colorimeter is required because standard colorimeters cannot operate below a wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of 400 nanometers. It is also necessary to use fused quartz

Fused quartz

Fused quartz and fused silica are types of glass containing primarily silica in amorphous form. They are manufactured using several different processes...

 cuvette

Cuvette

A cuvette is a small tube of circular or square cross section, sealed at one end, made of plastic, glass, or fused quartz and designed to hold samples for spectroscopic experiments. The best cuvettes are as clear as possible, without impurities that might affect a spectroscopic reading...

s because glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 is opaque to ultraviolet.

Color reagents

Many different color reagents have been developed for determining the concentrations of different substances. For example, Nessler's reagent

Nessler's reagent

Potassium tetraiodomercurate is an inorganic compound containing potassium cations and the tetraiodomercurate complex anion. It is mainly used as Neßler's reagent, a 0.09 mol/L solution of potassium tetraiodomercurate in 2.5 mol/L potassium hydroxide, used to detect ammonia.-Nessler's...

 can be used used to determine the concentration of a solution of ammonia

Ammonia

Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

.

Thin layer chromatography

In thin layer chromatography

Thin layer chromatography

Thin layer chromatography is a chromatography technique used to separate mixtures. Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose...

(TLC) color reactions are frequently used to detect compound spots by dipping the plate into the reagent or by spraying the reagent onto the plates.