The hydrated ion Al(H 2O) 6 3+ is colourless too. Most aluminium compounds are soluble in water (exceptions: its oxide and hydroxide).
Al oh 3 precipitate series#
All its simple salts are colourless (Al is not a member of the transition series of elements). In aqueous chemistry, the only important oxidation state of aluminium is the 3+ state. However, it does not mean that 10 electrons are involved in the bonding. The oxidation number in CrO 5 can be calculated to be 10. The following structure has been proposed for CrO 5. It is the basis for the confirmatory test for chromium. If H 2O 2 is added to a solution containing the dichromate ion, a transitory blue colour due to CrO 5 is observed. CrO 4 2- is bright yellow and if acid is added, the orange Cr 2O 7 2- ion is formed. Chromium is usually identified in the form of a Cr(VI) species in neutral or basic solutions. Chromium(III) can be oxidized to Cr(VI) with several oxidizing agents, such as ClO 3 – in 16M HNO 3, H 2O 2 in 6M NaOH, and ClO – in 6M NaOH. Chromium(III) hydroxide is amphoteric: it dissolves in excess base to form the green chromite Cr(OH) 4 – and in acid to form the hydrate chromium(III) ion.
The chromium-containing precipitate obtained in the group III precipitation scheme is the hydroxide Cr(OH) 3 rather than the sulphide. Cr(H 2O) 6 3+ is reddish violet in solution. Cr 3+ forms several complexes, all of which are coloured. The common oxidation states of chromium are 3+ and 6+. Nickel salts, like those of most of the other members of group III, are typically coloured: hydrates are green, Ni(NH 3) 6 2+ is blue and many other nickel complexes have characteristic colours, and nickel forms a very characteristic rose red precipitate with dimethylglyoxime (N 2C 4H 8O 2, structure shown below), an organic chelating agent. Under strongly oxidizing conditions, Co(II) can be converted to Co(III), which has a stability that is enhanced in a complex species like Co(NH 3) 6 3+ or an insoluble substance such as the yellow cobalt nitrite produced in the reaction above. Addition of KNO 2 to solutions of Co 2+ produces a characteristic yellow precipitate of K 3Co(NO 2) 6 in which the metal has been oxidized: Co(II) reacts with thiocyanate solutions to form a blue complex, Co(SCN) 4 2-, which is much more stable in ethanol than in water. Cobalt sulphide does not dissolve readily in 6M HCl even when heated. Cobalt chloride solution turns from pink to blue when heated to boiling because of a change in coordination number: the pink form arises from octahedrally coordinate Co(II), whereas the blue form is tetrahedrally coordinated Co(II). The 3+ state is stable only in presence of strong coordinating liquids such as NH 3 the general chemistry of cobalt schemes of qualitative analysis is usually that of Co 2+.Ĭobalt(II) salts in water solution are characteristically pink, the colour of the hydrated cobalt ion Co(H 2O) 6 2+, but the colour is too delicate to be used to characterise the ion. Like iron, cobalt can exist in two common oxidation states in aqueous solutions, Co 2+ and Co 3+. Conversion of Fe(II) to Fe(II) or the reverse is easily accomplished by common oxidizing agents (air of H 2O 2 in acid) or by reducing agents such as H 2S, Sn 2+ or I –.
Metallic iron is a good reducing agent, dissolving readily in 6M HCl with evolution of hydrogen. Cyanide complexes are perhaps the most stable iron complexes and have characteristic deep red colours. Both states form many complexes as Fe(CN) 6 4- or Fe(CN) 6 3. Iron(III) salts are also ordinarily obtained as hydrates are often yellow or orange. Iron(II) compounds are usually found as hydrates are light green. The latter is more common, since most ferrous compounds oxidize in the air, particularly if water is present. Iron in its compounds is ordinarily found in the 2+ (ferrous) or the 3+ (ferric) states. The hydroxides and sulphides of calcium, barium, magnesium, potassium and sodium are soluble and belong to the barium-magnesium group.ĭescription of the elements of the silver group Note that we used NH 4OH and (NH 4) 2S in several tests of the preceding groups to separate ions or to confirm their presence. Addition of NH 4Cl, NH 4OH and (NH 4) 2S to a solution containing all the cations not precipitated in the preceding groups results in the precipitation of aluminium, chromium and iron(II) as hydroxides, and manganese, nickel, cobalt, iron(II) and zinc as sulphides.
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