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.
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