Various metals (e.g. aluminium, iron, copper, zinc, cadmium, nickel, cobalt, manganese, and magnesium) under specified conditions of pH yield well-defined crystalline precipitates with 8-hydroxyquinoline. These precipitates have the general formula M(C9H6ON)n, where n is the charge on the metal M ion. Upon treatment of the oxinates with dilute hydrochloric acid, the oxine is liberated. One molecule of oxine reacts with two molecules of bromine to give 5,7-dibromo-8-hydroxyquinoline:
C9H7ON + 2Br2
= C9H5ONBr2 + 2H+ +2Br-
Hence 1 mole of the oxinate of a double-charged metal corresponds to 4 moles of bromine, whilst that of a triple-charged metal corresponds to 6 moles. The bromine is derived by the addition of standard 0.1N potassium bromate and excess of potassium bromide to the acid solution.
BrO3- + 5Br- +
6H+ = 3Br2 + 3H2O
Full details are given for the determination of aluminium by this method. Many other metals may be determined by this same procedure, but in many cases complexometric titration offers a simpler method of determination. In cases where the oxine method offers advantages, the experimental procedure may be readily adapted from the details given for aluminium.
Prepare a 2 per cent solution of 8-hydroxyquinoline in 2M acetic acid; add ammonia solution until a slight precipitate persists, then re-dissolve it by warming the solution.
Transfer 25 mL of the solution to be analysed, containing about 0.02 g of aluminium, to a conical flask, add 125mL of water and warm to 50-60 °C. Then add a 20 per cent excess of the oxine solution (1 mL will precipitate 0.001 g of Al), when the complex A1(C9H6ON)3 will be formed. Complete the precipitation by the addition of a solution of 4.0 g of ammonium acetate in the minimum quantity of water, stir the mixture, and allow to cool. Filter the granular precipitate through a sintered-glass crucible of porosity No. 4 and wash with warm water. Dissolve the complex in warm concentrated hydrochloric acid, collect the solution in a 250 mL reagent bottle, add a few drops of indicator (0.1 per cent solution of the sodium salt of methyl red or 0.1 per cent methyl orange solution), and 0.5-1 g of pure potassium bromide. Titrate slowly with standard 0.1N potassium bromate until the colour becomes pure yellow (with either indicator). The exact end point is not easy to detect, and the best procedure is to add an excess of potassium bromate solution, i.e., a further 2 mL beyond the estimated end point, so that the solution now contains free bromine. Dilute the solution considerably with 2M hydrochloric acid (to prevent the precipitation of 5,7-dibromo-8-hydroxyquinoline during the titration), then add (after 5 minutes) 10 mL of 10 per cent potassium iodide solution, and titrate the liberated iodine with standard 0.1M sodium thiosulphate, using starch as indicator (see Note) to determine the excess bromate.
From the above discussion, it is evident that Al = 12Br.
Note: A brown additive compound of iodine with the dibromo compound may separate during the titration; this compound usually dissolves during the subsequent titration with thiosulphate, yielding a yellow solution so that the end point with starch may be found in the usual manner. Occasionally, the dark-coloured compound, which contains adsorbed iodine, may not dissolve readily and thus introduces an uncertainty in the end-point: this difficulty may be avoided by adding 10 mL of carbon disulphide before introducing the potassium iodide solution.