Armstrong's acid
# Armstrong's acid

Armstrong's acid (naphthalene-1,5-disulfonic acid) is a fluorescent organic compound with the formula $C_{10}H_6(SO_3H)_2.$ It is one of several isomers of naphthalenedisulfonic acid. It a colorless solid, typically obtained as the tetrahydrate. Like other sulfonic acids, it is a strong acid. It is named for British chemist Henry Edward Armstrong.

## Production and use

It is prepared by disulfonation of naphthalene with oleum:

    $C_{10}H_8$ + 2 $SO_3$ → $C_{10}H_6(SO_3H)_2$

Further sulfonation gives The 1,3,5-trisulfonic acid derivative.

## Reactions and uses

Fusion of Armstrong's acid in NaOH gives the disodium salt of 1,5-dihydroxynaphthalene, which can be acidified to give the diol. The intermediate in this hydrolysis, 1-hydroxynaphthalene-5-sulfonic acid, is also useful. Nitration gives nitrodisulfonic acids, which are precursors to amino derivatives.

The disodium salt is sometimes used as a divalent counterion for forming salts of basic drug compounds, as an alternative to the related mesylate or tosylate salts. When used in this way such a salt is called a naphthalenedisulfonate salt, as seen with the most common salt form of the stimulant drug CFT. The disodium salt is also used as an electrolyte in certain kinds of chromatography.

