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.