Page 206 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS
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6 SOLVENT EXTRACTION
layer; add 20 mL of 5 per cent sulphuric acid (v/v), shake for 15 seconds, cool,
and separate the organic phase. Determine the absorbance at 560 nm in 1.0 cm
absorption cells against a blank. Al1 the copper is removed in one extraction.
Repeat the experiment in the presence of 1 mg of iron(II1); no interference
can be detected.
6.11 DETERMINATION OF COPPER AS THE 'NEO-CUPROIN' COMPLEX
Discussion. 'Neo-cuproin' (2,9-dimethyl-1,lO-phenanthroline) can, under certain
conditions, behave as an almost specific reagent for copper(1). The complex is
soluble in chloroform and absorbs at 457nm. It may be applied to the
determination of copper in cast iron, alloy steels, lead-tin solder, and various
metals.
Procedure. To 10.0 mL of the solution containing up to 200 pg of copper in a
separatory funnel, add 5.0mL of 10 per cent hydroxylammonium chloride
solution to reduce Cu(I1) to Cu(I), and 10 mL of a 30 per cent sodium citrate
solution to complex any other metals which may be present. Add ammonia
solution until the pH is about 4 (Congo red paper), followed by 10mL of a
0.1 per cent solution of 'neo-cuproin' in absolute ethanol. Shake for about
30 seconds with 10 mL of chloroform and allow the layers to separate. Repeat
the extraction with a further 5 mL of chloroform. Measure the absorbance at
457 nm against a blank on the reagents which have been treated similarly to
the sample.
6.12 DETERMINATION OF IRON AS THE û-HYDROXYQUINOLATE
Discussion. Iron(II1) (50-200 pg) 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(II1) oxinate is
dark-coloured in chloroform and absorbs at 470 nm.
Procedure. Weigh out 0.0226 g of hydrated ammonium iron(II1) sulphate and
dissolve it in 1 L of water in a graduated flask; 50 mL of this solution contain
100 pg of iron. Place 50.0 mL of the solution in a 100 mL separatory funnel,
add 10 mL of a 1 per cent oxine (analytical grade) solution in chloroform and
shake for 1 minute. Separate the chloroform layer. Transfer a portion of the
latter to a l.Ocm absorption cell. Determine the absorbance at 470nm in a
spectrophotometer, using the solvent as a blank or reference. Repeat the
extraction with a further 10 mL of 1 per cent oxine solution in chloroform, and
measure the absorbance to confirm that al1 the iron was extracted.
Repeat the experiment using 50.0 mL of the iron(II1) solution in the presence
of 100 pg of aluminium ion and 100 pg of nickel ion at pH 2.0 (use a pH meter
to adjust the acidity) and measure the absorbance. Confirm that an effective
separation has been achieved.
Note. Some typical results are given below. Absorbance after first extraction 0.605; after
second extraction 0.004; in pres&ce of 100 pg Al and 100pg Ni the absorbance obt'ained
is 0.602.