~Determination of Dioxide in a Manganese Ore.~--Weigh up 0.25 to 0.3 gram of the powdered ore; place in a flask, cover with 10 c.c. of hydrochloric acid, and close the flask with a paraffined cork, and bulbs (as shown in fig. 60), having previously charged the bulb with 5 grams of potassium iodide in strong solution. Heat the flask, and boil cautiously for about fifteen minutes. Wash the contents of the bulbs into a large beaker, nearly (but not quite) neutralise with dilute ammonia, and titrate with the standard "hypo." As an example, 0.2675 gram of pyrolusite was taken, and required 60.3 c.c. of standard "hypo" (100 c.c. equal 1.185 gram iodine, or 0.4042 gram MnO_{2}), which equals 0.2437 gram of the dioxide or 91.1 per cent. COLORIMETRIC METHOD. When compounds of manganese free from chlorides are boiled with nitric acid and dioxide of lead,[87] the manganese is converted into permanganic acid, which is soluble and tints the solution violet. The depth of colour depends on the amount of manganese present, and this should not much exceed 10 milligrams. A quantity of substance containing not more than this amount of manganese should be boiled for a few minutes with 25 c.c. of a solution containing 5 c.c. of nitric acid, and 10 or 20 c.c. of dilute sulphuric acid, with 2 or 3 grams of lead dioxide. Filter through asbestos, wash by decantation with dilute sulphuric acid, make up with distilled water[88] to a definite bulk, and take a measured portion for the colorimetric determination. The standard solution of manganese is made by dissolving 0.1435 gram of permanganate of potash (KMnO_{4}) in a little water acidulated with nitric acid, and diluting to 1 litre. One c.c. will contain 0.05 milligram of manganese. PRACTICAL EXERCISES. 1. What percentage of manganese (Mn) is contained in permanganate of potash (KMnO_{4})? 2. Ten c.c. of a solution of permanganate of potash is found to oxidise 10 c.c. of an acid solution of ferrous sulphate. The manganese is determined in the titrated solution by precipitation as dioxide and titrating. How much of the ferrous solution will be oxidised in the second titration? 3. What weight of potassium iodide would be just sufficient to absorb the chlorine evolved by 0.5 gram of pure dioxide of manganese? 4. What weight of iron must be dissolved up so as to have an excess of 0.25 gram after oxidation by 1 gram of pure dioxide? 5. What weight of the brown oxide, Mn_{2}O_{4} will be left on igniting 1 gram of the pure dioxide? CHROMIUM. Chromium occurs in nature chiefly as chromite or chrome iron ore (FeO_{2}Cr_{2}O_{3}, with more or less MgO and Al_{2}O_{3}), which is the chief ore. It is a constituent of some silicates, and is frequently met with in very small quantities in iron ores. It occurs as chromate in crocoisite (PbCrO_{4}), and some other rare minerals.