Wikipedia
Chromotropic acid
Chromotropic acid has the formula is (HO)CH(SOH).
It can be used for as a reagent in the quantitative determination of the systemic herbicide, 2,4-Dichlorophenoxyacetic acid.
Chromotropic acid is also used for testing of presence of formaldehyde.
The usefulness of this reagent in quantitative determination is the formation of a red colouration (peaking at 580 nm wavelength) when chromotropic acid in 75% sulfuric acid reacts with formaldehyde. The colouration is specific to this aldehyde and is not produced from other organic species such as ketones and carboxylic acids.