In the van Ruymbeke process the spent lyes are allowed to settle, and then treated with "persulphate of iron," the exact composition of which is a trade secret, but it is possibly a mixture of ferric and ferrous sulphates.
Potassium persulphate oxidizes it in alkaline solution, the product on boiling with acids giving hydroquiirone carboxylic acid (German Patent 81,297).
Ammonium persulphate (NH4)2S208 has been prepared by H.
Dessaignes, ibid., 1860, 115, p. 120; by reducing fumaric and maleic acids with sodium amalgam; by heating bromacetic acid with silver to 130° C.; in small quantity by the oxidation of acetic acid with potassium persulphate (C. Moritz and R.
Chem., 1898, p. 845) who prepared monopersulphuric acid by the action of sulphuric acid on a persulphate.