Aluminium-oxinate

The use of an organic precipitant may be exemplified by reference to the employment of 8-hydroxyquinoline (Oxine), to determine aluminium. Aluminium oxinate, Al(C9H6NO)3, can be quantitatively precipitated from... 

The resulting precipitate of aluminium-oxine complex is crystalline in nature and hence can be filtered conveniently, washed with water and finally dried at 130-150°C to constant weight. 

Between pH 4.5 and 10, 8-hydroxyquinoline (oxine) forms the chelate AKCgHeON), which is sparingly soluble in water but dissolves readily in CHCI3. The yellow extract of aluminium oxinate is the basis for the determination method [2,14]. Carbon tetrachloride and trichloroethylene are also used as extraction solvents. The absorption maximum of the chloroform extract is at 390 nm (e= 7.310, a = 0.27). The absorption of oxine in CHCI3 increases rapidly below 390 nm. 

Discussion. Iron(III) (50-200 fig) can be extracted from aqueous solution with a 1 per cent solution of 8-hydroxyquinoline in chloroform by double extraction when the pH of the aqueous solution is between 2 and 10. At a pH of 2-2.5 nickel, cobalt, cerium(III), and aluminium do not interfere. Iron(III) oxinate is dark-coloured in chloroform and absorbs at 470 nm. 

Discussion. 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) , where n is the charge on the metal M ion [see, however, Section 11.11(c)]. 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 ... 

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. 

Transfer 25 mL of the solution to be analysed, containing about 0.02 g of aluminium, to a conical flask, add 125 mL 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 Al(C9H6ON)3 will be formed. Complete the precipitation by the addition of a solution of 4.0 g of ammonium acetate in the... 

Discussion. Some of the details of this method have already been given in Section 11.11(C), This procedure separates aluminium from beryllium, the alkaline earths, magnesium, and phosphate. For the gravimetric determination a 2 per cent or 5 per cent solution of oxine in 2M acetic add may be used 1 mL of the latter solution is suffident to predpitate 3 mg of aluminium. For practice in this determination, use about 0.40 g, accurately weighed, of aluminium ammonium sulphate. Dissolve it in 100 mL of water, heat to 70-80 °C, add the appropriate volume of the oxine reagent, and (if a precipitate has not already formed) slowly introduce 2M ammonium acetate solution until a precipitate just appears, heat to boiling, and then add 25 mL of 2M ammonium acetate solution dropwise and with constant stirring (to ensure complete predpitation). 

Beryllium is sometimes precipitated together with aluminium hydroxide, which it resembles in many respects. Separation from aluminium (and also from iron) may be effected by means of oxine. An acetic (ethanoic) acid solution containing ammonium acetate is used the aluminium and iron are precipitated as oxinates, and the beryllium in the filtrate is then precipitated with ammonia solution. Phosphate must be absent in the initial precipitation of beryllium and aluminium hydroxides. 

Fluorimetry is generally used if there is no colorimetric method sufficiently sensitive or selective for the substance to be determined. In inorganic analysis the most frequent applications are for the determination of metal ions as fluorescent organic complexes. Many of the complexes of oxine fluoresce strongly aluminium, zinc, magnesium, and gallium are sometimes determined at low concentrations by this method. Aluminium forms fluorescent complexes with the dyestuff eriochrome blue black RC (pontachrome blue black R), whilst beryllium forms a fluorescent complex with quinizarin. 

Acetic acid - 8-hydroxyquinoline reagent - dissolve 10 g 8-hydroxy-quinoline in a solution of 2.5 % (v/v) acetic acid and make up to 1 I. (The 8-hydroxyquinoline blocks the readsorption or precipitation of phosphate by active iron and aluminium during acetic acid extraction. Synonyms hydroxybenzopyridine oxine phenopyridine 8-quinolinol. Not carcinogenic, but may be harmful if swallowed, and causes irritation to eyes, respiratory tract and skin safety data sheet at http //www. jtbaker.com/msds/q7250. htm.)... 

Yamane, Y., Miyazaki, M., Iwase, H. Analysis of metals in water II. Analysis of aluminium, beryllium and chromium by oxine, 2-methyloxine extration and by thin-layer chromatography. Eisei Kagaku 14, 106 (1968) C. A. 45 939 d (1968)... 

The most stable oxo-complexes are chelate compounds. Gallium and indium form tris- -diketone complexes, soluble in alcohol and benzene, and structurally similar to those of aluminium (Fig. 155). Gallium and thallium form trioxalato compounds like the oxalato-aluminates MJ[Ga ( 304)3]HgO. All the metals form tris-complexes with oxine (Fig. 156). 

Small amounts of aluminium may be precipitated as the oxinate. If fluoride is present, it is masked as the stable, soluble beryllium complex [2]. Fe(UI), Ti, and Zr are separated from aluminium by precipitation with cupferron [3]. 

Extraction is used mainly for the preliminary separation of macro- and microquantities of metals which interfere in the determination of aluminium. After Fe(III), Ti, Zr, and Cu cupferronates have been extracted from dilute HCl into chloroform, aluminium cupferronate is extracted at pH 3.5 [5]. Some interfering metals are separated from Al by extraction with trifluoroacetylacetone in CHCI3 [6], and as chloride- or thiocyanate complexes in the presence of DAM [7]. Aluminium has been separated from various elements by extraction as chelates with BPHA [8], oxine [4,9], 8-hydroxyquinaldine [10], and acetylacetone [11]. 

Although oxine is a group reagent and reacts with many metals, the use of appropriate masking agents makes the method specific for aluminium. There are many variants of the oxine method, depending on the kinds of metals that accompany aluminium in the analyte... 

Hydrogen peroxide prevents oxine from reacting with Ti, V, Nb, U, and Ce. Cyanide masks Ni, Co, Cu, Zn, Cd, Ag, and Fe(II). EDTA or tartrate keeps aluminium in solution at higher pH values, at which it normally hydrolyses. The presence of EDTA does not interfere in the extraction of aluminium with oxine when the pH of the solution is above 8. 

Small amounts of heavy metals can be separated from aluminium by extraction as dithiocarbamates and dithizonates. Ti and Zr can be stripped from chloroform extracts of Al, Ti, and Zr oxinates by shaking with ammoniacal solution at pH 9.2 [9]. Vanadium and titanium can be separated from Al by extracting the cupferronates from a 1 M H2SO4 medium with CHCI3. 

Transfer the solution to a separating funnel, and extract the aluminium with two portions of the chloroform-oxine solution. Wash the combined extracts with two portions of water and make the solution up to the mark with the oxine solution in a 25 ml standard flask. Measure the absorbance of the extract at 390 nm, using as reference a reagent blank solution. 

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