The most reliable method is that of v. Knorre.[233] The solution to be estimated is acidified with sulphuric acid, and oxidised by means of ammonium persulphate. The excess of the oxidising agent having been destroyed by boiling, the cooled solution is treated with a slight excess of hydrogen peroxide, which reduces the ceric salt according to the equation: 2Ce(SO₄)₂ + H₂O₂ = Ce₂(SO₄)₃ + H₂SO₄ + O₂ The excess of hydrogen peroxide is then estimated by means of a dilute permanganate solution. Permanganate is itself reduced by the cerous salt formed, but the action is so slow in acid solution at the ordinary temperature that the excess of peroxide can be accurately determined without unduly hurrying the titration. In this form the method is generally employed for the estimation of cerium in monazite sands, and in the incandescent mantle industry. The greatest difficulty is the adjustment of the concentration of the sulphuric acid required. If this be too low, basic ceric sulphate separates on boiling, and the estimation fails; if it be too high, oxidation to the ceric salt is hindered, and may even be inhibited. This difficulty disappears in the modified method of Waegner and Muller,[234] in which the oxidation to the ceric condition is effected by means of bismuth tetroxide in nitric acid solution. A similar method, in which reduction to the cerous state is effected by a ferrous salt, in place of hydrogen peroxide, has been employed by Metzger.[235] [233] _Ber._ 1900, ~33~, 1924. [234] _Ber._ 1903, ~36~, 282 and 1732. [235] _J. Amer. Chem. Soc._ 1909, ~31~, 523; see also Metzger and Heideberger, _ibid._ 1910, ~32~, 642. Many attempts have been made to estimate cerium compounds by means of permanganate, which in alkaline solution oxidises cerous salts to the ceric condition, but the autoxidation of cerous hydroxide in the air introduces errors, unless suitable precautions are taken. Meyer and Schweitzer[236] show that if the solution of the cerous salt be added, with constant shaking, to a known volume of a standard permanganate solution, in presence of excess of magnesia, the liquid being kept warm, this difficulty is overcome; the results are usually a little high, however, probably by reason of the oxidising action of the cerium dioxide on the other oxides present. [236] _Zeitsch. anorg. Chem._ 1907, ~54~, 104; see also Roberts, _ibid._ 1911, ~71~, 305. Good results have also been obtained by the use of potassium ferricyanide in alkaline solution,[237] oxidation taking place according to the equation: Ce₂O₃ + 2K₃Fe(CN)₆ + 2KOH = 2K₄Fe(CN)₆ + 2CeO₂ + H₂O The ceric hydroxide is filtered off, and the ferrocyanide formed estimated by means of permanganate in acid solution. [237] Browning and Palmer, _Zeitsch. anorg. Chem._ 1908, ~59~, 71. CHAPTER XII CERIUM GROUP (_continued_) LANTHANUM, PRASEODYMIUM, NEODYMIUM, AND SAMARIUM