Spectrophotometric Estimation of Aluminium at 389nm

Estimation of Aluminium

Introduction:

Provided a suitable pH is employed, a large number of colorimetric agents, among which aluminon may be particularly distinguished, combine with Al (III) to give coloration which are due not only to definite compounds but often also to adsorption compounds (lakes). The corresponding colorimetric determinations are not very precise since the results vary with the time, the grain size, etc. The pH is a very important factor. Many other substances, in particular Fe (III), give analogous coloration and in view of this, they should be separated out or complexed.

Oxinate:

Aluminium oxinate is yellow when dissolved in chloroform, carbon tetrachloride or benzene. The coloration is not very appreciable to the eye, but the adsorption is high in violet and ultraviolet light.

Sensitivity:

Ε=6,700 at 390nm and 80,000 at 260nm

Interfering ions:

Separation by extraction of the oxinate:

Aluminium oxinate can be extracted quantitatively at pH 4.5-5.0 (acetic buffer). In this way, aluminium may be separated from a number of elements, including Be(II), Th(IV) etc.

More specific separation may be obtained by operating in an ammoniacal medium at pH

9 in the presence of tartrate , cyanide and hydrogen peroxide. Separation from the following may thus be achieved : Cu(II), Co(II), Ni(II), Zn(II), Cd(II), Fe(III), Ti(IV), V(V), V(IV), U(VI), Mn(II), Cr(III), Mo(VI), Sn(IV), and Ag(I). Less than 10mg of Zr(IV) or  Nb (V) does not interfere.

Reagents:

Oxine, 2g in100ml of chloroform

Potassium cyanide, 13% in water

Sodium sulphite, 20% in water

Tartaric acid, 10% inn water

Hydrogen peroxide, 10volume

Standard solution of Al(III): 1g of 99% aluminium dissolved in hydrochloric acid.

Operating procedure:

2 ml of tartaric acid and 1ml of hydrogen peroxide are added to 25ml of a weekly acidic solution 10-150µg of Al(III). The solution is allowed to stand for 5min, and 5ml of sulphite are then added. After standing for a further 3 minutes, 10ml of cyanide are added and the solution is heated about 70°C-80°C and then cooled to 25-30°C. Finally, 2g of ammonium nitrate are added, and pH is adjusted to 8.9 ±0.3 with ammonia or Hcl. The solution is then transferred to a separating funnel, 5ml of oxine are added  ,and the funnel is shaken for 2min. The solution is allowed to settle, and the organic phase is collected in a measuring flask. The extraction is repeated three times, the solution being made upto 50ml with chloroform.

Finally , a colorimetric estimation is performed at 389nm in comparison with a blank.