Tetrametaphosphimic Acid, P₄N₄H₈O₈

Tetrametaphosphimic Acid, P₄N₄H₈O₈, which was prepared by the hydrolysis of P₄N₄Cl₈, may be regarded as constituted in a similar manner, forming an eight-membered ring. It crystallised in needles containing two molecules of water, P₄N₄H₈O₈.2H₂O, and was only slightly soluble in water, still less so in acids. The solubility in 10 per cent, acetic acid was about 0.5 gram in 100 grams solvent.

This acid is more stable towards hydrolysing agents than the other metaphosphimic acids, and even resists the action of hot nitric acid and aqua regia. There are two series of salts derived from a di- and a tetra-basic acid respectively. Thus the dipotassium salt, K₂H₆P₄N₄O₈, which forms prismatic crystals, on treatment with excess of potassium hydroxide gives the tetrapotassium salt, K₄H₄P₄N₄O₈, which forms sparingly soluble tabular crystals. Ammonium and sodium salts have also been prepared. The tetra-argentic salt, Ag₄H₄P₄N₄O₈, was formed as a white precipitate on mixing silver nitrate with the acid, and the octo-salt, Ag₈P₄N₄O₈, as a yellow precipitate from ammoniacal silver nitrate and ammonium tetraphosphimate.