The industrial method is electrolysis of a concentrated solution of ammonium sulfate, recovered by crystallization, and purified by recrystallization. The manufacture of persulfates, such as ammonium persulfate, is complicated by many factors. Two peracids of sulfur exist; viz., the permonosulfuric acid or Caro's acid and perdisulfuric acid commonly called persulfuric acid. The ammonium persulfate of commerce is the salt of the latter acid, having the formula (NH4)2S2O8. When an aqueous solution of fairly concentrated H2SO4 is electrolyzed between platinum electrodes using a high current density at the anode, H2S2O8 is first formed by the discharge of the HSO4 ions. The presence of H2SO4, however, this acid is not very stable and changes more or less quickly, depending upon the concentration of the H2SO4, into H2SO5, The H2SO5 can then undergo further decomposition with the production of H2O2
H2SO5 + H2O => H2SO4 + H2O2. The two acids are readily distinguished from H2O2 by the fact that they do not decolorize permanganate or give rise to the characteristic colorations produced by H2O2 with titanium salts, or with chromic acid; but it is more difficult to differentiate them from each other in aqueous solution.
Sulfuric acid of a density of 1.39 to 1.4 gives a maximum yield of H2S2O8, obtained with an acid density of 1.38 at temperatures of 8 to 10°C, the duration of electrolysis being 50 min.
The temperature at which the electrolysis is done is very important. A rise of temperature diminishes the yield which becomes practically zero at 60°, the H2S2O8 decomposing as fast as it is formed. Cooling the anode has a favorable effect as far as increasing the current efficiency is concerned. The presence of small quantities of many metallic sulfates, especially those of ammonium, potassium, nickel, and aluminum, favors the formation of H2S2O8. The addition of a drop of concentrated HCl increases the yield from 46 to 69 per cent.
In the preparation of (NH4)2S2O8, the electrolyte is either a saturated solution of purified (NH4)2SO4 or else an almost saturated solution of this salt made acid with H2SO4. The electrode materials are platinum anodes in the shape of either sheet, gauze, or wire, alone or held in a frame of a metal such as zinc; or sometimes tantalum plated or coated with platinum, often in the form of flattened wire, while cathodes are generally lead, either in the form of the tank lining or coils of pipe, or cast sections of lead which may be water-cooled, or in some cases aluminum. Current densities are of the order of 280 to 465 amp per sq ft (30 to 50 amp per dm2), with emf of 6 to 7 volts. It is general practice to add 0.2 per cent of K2Cr2O7 or K2CrO4 to the electrolyte to reduce cathodic reduction of the persulfate formed. Commercial cells vary considerably in size, from units as small as 250-amp capacity to as large as 1,500-amp. In larger units anode arrangements and mechanical difficulties in securing proper placement and rigidity of anode surface cause trouble. The tanks are of either cement or wood with lead linings or of chemical stoneware. Cooling of the electrolyte is done either in a system in which the electrolyte is continuously circulating and the (NH4)2S208 being produced is continuously removed (the liquor being cooled outside the cells in holding tanks) or the liquor is cooled by the use of lead cooling coils which are made part of the cathode surface in the tank.
It has been found practical in commercial operation to allow the temperatures to rise as high as 30°C without appreciable lowering of current efficiency. Temperatures higher than this markedly decrease the current efficiency, and lower temperatures increase it. Ammonium persulfate begins to decompose at 40°C.
Large cells are roughly 8 ft 6 in. long, 3 ft 2 in. to 4 ft 3 in. high, and 3 ft 2 in. to 4 ft 10 in. wide (2.6 m long, 1 to 1.3 m high, 1 to 1.5 m wide), operating at 1,500 amp. The energy requirements in medium-size cells-i.e., 500- to 750-amp capacity are of the order of 2.1 to 2.5 kwhr per kg (NH4)2S208 produced (0.95 to 1.1 kwhr per lb), with poorer operation and continuous running under average control tending toward the higher figure rather than the lower one.
Pure ammonium persulfate decomposes explosively at 180 °C, and if water or any organic matter is present, the reaction proceeds at a lower temperature. Pure ammonium persulfate can be dried at 100 °C without decomposition
