Colorimetric analysis

Colorimetric analysis is a method of determining the concentration of an element or compound in a solution with the aid of a color reagent. It is applicable to both organic compounds and inorganic compounds and may be used with or without an enzymatic stage. The method is widely used in medical laboratories.

Equipment

The equipment required is a colorimeter, some cuvettes and a suitable color reagent.

Non-enzymatic methods

Examples

Calcium

Calcium + o-cresolphthalein complexone ----> colored complex

Copper

Copper + bathocuproin disulfonate ----> colored complex

Creatinine

Creatinine + picrate ----> colored complex

Iron

Iron + bathophenanthroline disulfonate ---> colored complex

Phosphate (inorganic)

Phosphate + ammonium molybdate + ammonium metavanadate ----> colored complex

Enzymatic methods

In enzymatic analysis (which is widely used in medical laboratories) the color reaction is preceded by a reaction catalyzed by an enzyme. As the enzyme is specific to a particular substrate, more accurate results can be obtained. Enzymatic analysis is always carried out in a buffer solution at a specified temperature (usually 37°C) to provide the optimum conditions for the enzymes to act. Examples follow.

Examples

Glucose

1. Glucose + oxygen + water --(enzyme glucose oxidase)--> gluconate + hydrogen peroxide
2. Hydrogen peroxide + ABTS --(enzyme peroxidase)--> colored complex + water

In this case, both stages of the reaction are catalyzed by enzymes.

Urea

1. Urea + water --(enzyme urease)--> ammonium carbonate
2. Ammonium carbonate + phenol + hypochlorite ----> colored complex

In this case, only the first stage of the reaction is catalyzed by an enzyme. The second stage is non-enzymatic.

See also

• Blood sugar