Page 212 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS
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6   SOLVENT  EXTRACTION

       separatory funnel, allow them to settle and separate the aqueous and chloroform
       layers. Remove excess dithizone from the chloroform layer by  back-extraction
       with 10 mL of 0.1 M sodium hydroxide: vigorous shaking for about one minute
       is sufficient. Again separate the chloroform  layer and measure its absorbance
       in  a  1 cm  ce11  at  520 nm  against  a  similarly  treated  blank.  Construct  a
       calibration curve (which  should  be  a  straight line  through  the  origin) using
       standard nickel(I1) solutions containing 2, 4, 6, 8 and  10 pg in  10 mL.

       Notes.  (1)  The  reagent  solution  should  be  freshly  prepared  using  analytical-grade
       dithizone and 1,lO-phenanthroline, preferably taken from new or recently opened reagent
       bottles.
         (2) Glassware should be rinsed with dilute acid and then several times with de-ionised
       water. Al1  aqueous solutions must be made up using de-ionised water.

       6.18  EXTRACTION AND  DETERMINATION OF  LEAD, CADMIUM,  AND  COPPER  USlNG
       AMMONIUM  PYROLLIDINE DITHIOCARBAMATE
       Discussion.  Because of  the specific nature of  atomic absorption spectroscopy
       (AAS) as  a  measuring  technique,  non-selective  reagents  such  as  ammonium
       pyrollidine  dithiocarbamate  (APDC) may  be  used  for  the  liquid-liquid
       extraction of metal ions. Complexes formed with APDC are soluble in a number
       of ketones such as methyl isobutyl ketone which is a recommended solvent for
       use in  atomic absorption  and allows a concentration factor of  ten  times. The
       experiment described illustrates the use of APDC as a general extracting reagent
       for heavy metal ions.

       Reagents and solutions.  APDC solution. Dissolve 1.0 g of APDC in water, dilute
       to 100 mL and filter (Note 1).
       Standard  solutions.  Prepare,  by  appropriate  dilution  of  standardised  stock
       solutions of  lead, cadmium  and  copper(I1) ions, mixed  aqueous standards of
       Pb2+, Cd2+ and Cu2+ containing the following concentrations of each metal
       ion: 0.4, 0.6, 0.8 and  1.0 mg L-'  (Note 2).

       Procedure.  To  lOOmL  of  sample  solution,  containing  0.5  to  l.Omg L-'  of
       Pb2+, Cd2+ and Cu2+ ions, add 10 mL of APDC solution and adjust to pH 5
       with dilute acetic (ethanoic) acid  or sodium hydroxide. Transfer  the solution
       to a  separatory funnel  and extract the complex into 8 mL of methyl isobutyl
       ketone  (MIBK) by  vigorously  shaking the phases  for  30  seconds. Allow  the
       mixture to stand for about two minutes, transfer the aqueous phase to another
       separatory funnel and  repeat the extraction with  2mL of  MIBK. Discard  the
       aqueous phase,  which should now  be  colourless,  combine  the extracts  in  the
       first funnel, mix, and filter through a cotton-wool plug into a small dry beaker.
       Standard extracts should be prepared from the mixed aqueous standards using
       the same procedure. Aspirate the standard extracts successively into the flame,
       followed  by  the sample  extract.  In each-case  read  the separate  absorbances
       obtained using lead, cadmium and copper hollow cathode lamps (Note 3). Plot
       a calibration curve for each metal ion and use this to determine the concentrations
       of  Pb2+, Cd2+, and Cu2+ in the sample solution.
         When the AAS measurements have been completed, aspirate de-ionised water
       for several minutes to ensure thorough cleaning of the nebuliser-burner  system.
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