Complex sentence: When oxidised it forms haematein, a compound with rich blue-purple color, and is used, together with a suitable mordant (most commonly Fe (III) or Al (III) salts), to stain cell nuclei prior to examination under a microscope.
Split into simpler sentences: When oxidised it forms haematein, a compound that forms strongly coloured complexes with certain metal ions, notably Fe (III) and Al (III) salts. Metal-haematein complexes are used to stain cell nuclei prior to examination under a microscope.