Spectrophotometric Estimation of Magnesium

ESTIMATION OF MAGNESIUM

INTRODUCTION:

I) Thiazole Yellow:

Thiazole yellow and titan yellow dissolve in water giving a yellow coloration. In the presence of colloidal magnesium hydroxide they are adsorbed, with a pink colouration.

The method possesses the disadvantages associated with all adsorption techniques: coloration is affected by many variables such a time, temperature, concentration, operating procedure, pH etc. The results may also vary with the origin of the dye. The colloid may be stabilized by the addition of starch, hydroxylamine hydrochloride, polyvinyl alcohol, or sodium phosphate. And is then stable for 2 days in the absence of light.

 Sensitivity:

(Molar extinction coefficient) e =1500 at 535nm

The method is not, however, accurate (±2 to 5 %)

Interfering ions:

Interference is obtained from many ions and preliminary separations are often essential. P(V)  interferes above 100ppm, Ca(II) below 500ppm enhances the colour , but can be complexed with mannitol. Many ions (Cu (II), Ni (II) and Al (III), since Mg (OH) 2 adsorbs AlO₂^- , Also Sn(IV), Ag(I), Hg(I), (II), Cd(II), Co(II), Pb(II), Si(IV), Li(I), Fe(III), Zn(II).And La(III) precipitates, sometimes giving colorations by adsorbing the dye. Ti (IV) can be complexed with H₂O₂ to give a colorless solution at pH ³12. Mn (II) oxidizes in air but this may be prevented by adding hydroxylamine hydrochloride which in addition stabilizes the coloration due tp Mg (II). Sb (III), As (III), As (V) prevent the appearance of the coloration to some extent. NH₄⁺ present in large amounts (> 500ppm), interfere because of its buffering effect on the solution. Proteins interfere. C₂O₄^2- has no appreciable effect.

Reagents:

Caustic soda: 10N (400g per litre)

Hydroxylamine hydroxide, 5% in water

Polyvinyl alcohol, 2% in water.

Thiazole Yellow, 0.5% in 50% alcohol, kept in a brown bottle

Thiazole Yellow, 0.01%, 2ml of the preceding solution are mixed with 5ml of 0.5%polyvinyl alcohol and made upto100ml with water.

Operating procedure:

5ml of hydroxylamine hydrochloride, 4ml of polyvinyl alcohol, 5ml of 0.05% thiazole yellow, and 3.5ml of 10N caustic soda are added to 30ml of neutral solution containing 30-200mg of magnesium. The solution is allowed to stand fro 15min at 25±0.5°, and colorimetrically is carried out at 540 nm within the next half hour.

II) By Extraction of oxinate.

Hydrated magnesium oxinate is soluble in 5% butyl cello solve, from which it may be extracted by chloroform. Mg+2 can be extacted by chloroform at pH II ± 0.5 by a chloroform in the presence of n-butyl amine.

Sensitivity:

e =5,600 at 380nm

Interfering ions:

Fluorides and EDTA prevent the extraction. Oxalates, cyanides, sulphates, tartrates and citrates do not interfere.

Concentrations of Oxine and of the n-butyl amine are suck that the alkali metals and the alkali earth metals and Cr, Mo, W, As, Sb, B, Se, Te, Be do not interfere.

Sn (IV) prevent the extraction of Mg+ when present in amounts exceeding 5mg

Moderate amounts of Ti (IV), V (V), U (VI) can be complexed with H₂O₂; Zn (II), Cd(II), Ni(II), Co(II), Fe(III) can be complexed with cyanide .

15mg of Al (III) doesn’t interfere in the presence of triethlylene.

The following interfere and should be separated with oxine before the addition of n-Butyl amine: In (III), Ga (III), Tl (III), Sn (II), Pb (II), Zr (IV), Th (IV), Bi (III), Nb (V), Ta (V), Mn (II), and the rare earth metals.

The operating procedure describes permits Mg⁺² to be determined in the presence of small residues of interfering ions.

Reagents:

Oxine solution, 0.1% in chloroform.

Potassium sodium tartrate, 20%in water

Ammonia solution, M (1/12)

n-butyl amine

Potassium cyanide

Hydrogen peroxide, 30%(110-volume)

Operating procedure:

            To 30ml of solution containing 20 to 200 mg of magnesium, are added 5ml of tartrate and {V (V), Ti (IV), or U (VI) is present}, 2ml of hydrogen peroxide. 1 M ammonia solution is added to give a pH of about 9 and the compound is extracted with 20ml of the oxine solution; the extraction is repeated until the chloroform phase becomes colorless. 0.5 to 1 g of potassium cyanide and 1 ml of n-butyl amine are added to the aqueous solution, and the pH is adjusted to 11.0 ± 0.5 with concentrated ammonia; the mixture is shaken with 50ml of oxine for 1 minute and colorimetrically is performed at 380 nm automatically, the solution may be extracted twice with 20ml of the oxine solution, shaking each time for 30seconds. The extracts are transferred to a 50ml-measuring flask, 2ml of methanol are added, and the volume is made up to 50ml with chloroform.