Metanilic acid is an isomer of sulfanilic acid with molecular formula C6H7NO3S and molecular weight 173.18968 g/mol.

[2][3][4][5] It is a white powder that is slightly soluble in water.

The crystal structure called Form I is a disappearing polymorph that cannot be produced by researches anymore while Form II and III still exist.

[6] The reason for this is that Form I is converted to another polymorph upon contact with a seed crystal and most places are contaminated with tiny amounts of Form II or III that are enough to prevent any viable amounts of Form I to be produced.

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