Page 202 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

6 SOLVENT EXTRACTION
reagents, whereby the extractable complex is decomposed. The metal ions are
then quantitatively back-extracted into the aqueous phase.
Impurities present in the organic phase rnay sometimes be removed by
backwashing. The organic extract when shaken with one or more small portions
of a fresh aqueous phase containing the optimum reagent concentration and of
correct pH will result in the redistribution of the impurities in favour of the
aqueous phase, since their distribution ratios are low: most of the desired element
will remain in the organic layer.
Completion of the analysis. The technique of solvent extraction permits the
separation and often the pre-concentration of a particular element or substance
(or of a group of elements or substances). Following this separation procedure,
the final step of the analysis involves the quantitative determination of the
species of interest by an appropriate technique.
Spectrophotometric methods rnay often be applied directly to the solvent
extract utilising the absorption of the extracted species in the ultraviolet or
visible region. A typical example is the extraction and determination of nickel
as dimethylglyoximate in chloroform by measuring the absorption of the
complex at 366nm. Direct measurement of absorbance rnay also be made
with appropriate ion association complexes, e.g. the ferroin-anionic detergent
system, but improved results can sometimes be obtained by developing a chelate
complex after extraction. An example is the extraction of uranyl nitrate
from nitric acid into tributyl phosphate and the subsequent addition of
dibenzoylmethane to the solvent to form a soluble coloured chelate.
Further techniques which rnay be applied directly to the solvent extract are
flame spectrophotometry and atomic absorption spectrophotometry (AAS).13
The direct use of the solvent extract in AAS rnay be advantageous since the
presence of the organic solvent generally enhances the sensitivity of the method.
However, the two main reasons for including a chemical separation in the
preparation of a sample for AAS are:
(a) the concentration of the element to be determined is below the detection
limit after normal preparation of the sample solution; and
(b) it is necessary to separate the species of interest from an excessive
concentration of other solutes, affecting the nebuliser and burner system,
or from a very strong chemical interference effect.
Solvent extraction is probably the separation technique which is most widely
used in conjunction with AAS. It often allows the extraction of a number of
elements in one operation and, because of the specific nature of AAS,
non-selective reagents such as the thiocarbamate derivatives (e.g. APDC) rnay
be used for the liquid-liquid extraction (see Section 6.18).
Multi-element analyses involving solvent extraction and high performance
liquid chromatography (HPLC) have also been described. The extracts,
containing metal-chelate complexes with sulphur-containing reagents, such as
dithizone and diethyldithiocarbamate, were used directly for determination of
the metals by HPLC.14
Automation of solvent extraction. Although automatic methods of analysis do
not fail within the scope of the present text, it is appropriate to emphasise here
that solvent extraction methods offer considerable scope for automation. A fully
automated solvent extraction procedure, using APDC, for the determination of