«Analysis of the Aluminium Group.»—The precipitate of the aluminium group may contain aluminium, chromium and ferric hydroxides (possibly traces of manganic hydroxide) and their basic carbonates. A color test for ferric-ion has already been made (see p. 193) and chromium (and manganese) is readily found and identified by oxidation to the intensely colored salts of chromic (and manganic) acid (Part III, ‹q.v.›). In ascertaining whether aluminium hydroxide is present or not, advantage is taken of its ‹amphoteric character›. Chromium hydroxide, like aluminium hydroxide, is amphoteric; but, in agreement with the greater atomic weight of chromium, it is an even weaker acid than is aluminium hydroxide. Its sodium salt, sodium chromite, is completely decomposed by boiling water, chromium hydroxide being precipitated in a less hydrated, insoluble form. Ferric hydroxide, whose metal has the highest atomic weight of the three elements under consideration, has so little acid character, that it is not perceptibly soluble in solutions of potassium or sodium hydroxide. (That it has slight acidic properties is shown by its capacity to form ferrites, ‹e.g.› Me(FeO_{2})_{2}, which may best be obtained by dry methods, and of which ferrous ferrite or magnetic iron ore, Fe_{3}O_{4} or Fe(FeO_{2})_{2}, is the most important representative.) Of the three hydroxides, aluminium hydroxide is, therefore, the only one that will dissolve in boiling sodium hydroxide. In this solution we can best identify it, by converting the aluminate into an aluminium salt, by means of an excess of acid, [p196] and by a final precipitation of aluminium hydroxide with ammonium hydroxide. Aluminium hydroxide is too weak an acid to form a stable aluminate with so weak a base as ammonium hydroxide, when the latter is used only in slight excess (p. 186). If we attempt to prepare ammonium aluminate, by adding ammonium chloride to a solution of sodium aluminate, a precipitate of aluminium hydroxide is obtained (‹exp.›). For exact work, an excess of ammonium hydroxide is to be avoided and ‹its strength as a base should be weakened› by the addition of some ammonium chloride or nitrate (pp. 114, 169 and Lab. Manual, p. 9, § 6).