Page 205 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OETERMINATION OF COPPER AS THE OIETHYLOITHIOCARBAMATE COMPLEX 6.10

to obtain the absorbance, A, ,. The absorbance, A, to be used in the calculation
of the boron concentration is obtained from the following equation:

is
A(blank~o determined by repeating procedure (a), i.e. zero-minutes, using 100 mL
of distilled water in place of the sample solution.
Calculate the amount of boron present by reference to a calibration graph
of absorbance against boron concentration (mg L-'). Multiply the result
obtained by the appropriate volume correction factor arising from neutralisation
of the sample.
Calibration. Take 5, 10, 25, 50, 75 and 100 mL of the standard boric acid
solution (2.5 x 10-4M) and make each up to lOOmL with distilled water;
this yields a boron concentration range up to 2.70mg L-'. Continue with
each solution as described under procedure (b), i.e. one-hour reaction time,
except that the initial neutralisation of the boron solution to pH 5.5 is not
necessary. Construct a calibration graph of absorbance at 516 nm against boron
concentration, mg L-'. For maximum accuracy, the calibration should be
carried out immediately prior to the analysis of samples.

6.10 DETERMINATION OF COPPER AS THE DIETHYLDITHIOCARBAMATE COMPLEX
Discussion. Sodium diethyldithiocarbamate (B) reacts with a weakly acidic or
ammoniacal solution of copper(I1) in low concentration to produce a brown
colloidal suspension of the copper(I1) diethyldithiocarbamate. The suspension
may be extracted with an organic solvent (chloroform, carbon tetrachloride or
butyl acetate) and the coloured extract analysed spectrophotometrically at
560 nm (butyl acetate) or 435 nm (chloroform or carbon tetrachloride).

Many of the heavy metals give slightly soluble products (some white, some
coloured) with the reagent, most of which are soluble in the organic solvents
mentioned. The selectivity of the reagent may be improved by the use of masking
agents, particularly EDTA.
The reagent decomposes rapidly in solutions of low pH.
Procedure. Dissolve 0.0393 g of pure copper(I1) sulphate pentahydrate in 1 L
of water in a graduated flask. Pipette 10.0 mL of this solution (containing about
100 pg Cu) into a beaker, add 5.0 mL of 25 per cent aqueous citric acid solution,
render slightly alkaline with dilute ammonia solution and boil off the excess of
ammonia; alternatively, adjust to pH 8.5 using a pH meter. Add 15.0mL of
4 per cent EDTA solution and cool to room temperature. Transfer to a
separatory funnel, add 10mL of 0.2 per cent aqueous sodium diethyldithio-
carbamate solution, and shake for 45 seconds. A yellow-brown colour develops
in the solution. Pipette 20 mL of butyl acetate (ethanoate) into the funnel and
shake for 30 seconds. The organic layer acquires a yellow colour. Cool, shake
for 15 seconds and allow the phases to separate. Remove the lower aqueous