Chromotropic acid has the formula is (HO)₂C₁₀H₄(SO₃H)₂.

It can be used for as a reagent in the quantitative determination of the systemic herbicide

Herbicide

Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...

, 2,4-Dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America...

.

The usefulness of this reagent in quantitative determination is the formation of a red colouration (peaking at 580 nm 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...

) when chromotropic acid in 75% sulfuric acid

Sulfuric acid

Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 reacts with formaldehyde

Formaldehyde

Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...

. The colouration is specific to this aldehyde and is not produced from other organic species such as ketones and carboxylic acids.