Dithizone (diphenylthiocarbazone, H2Dz) is an organic
colorimetric reagent that provides the basis of sensitive methods
for the determination of lead, zinc, cadmium, mercury, copper,
and other heavy metals.' It is a weak acid and is soluble in
ketones, alcohols, hydrocarbons, and chlorinated hydrocarbons
such as chloroform and carbon tetrachloride. It dissolves in
alkaline aqueous media (>20 g/L) but is practically insoluble
in water at pH < 7 (5.0 - 7.2 X 10-5).132 Metal ions combine
with dithizone to yield nonpolar colored complexes whose
colors differ significantly from dithizone. These nonpolar
complexes are generally extracted into solvents like chloroform
and carbon tetrachloride; this provides a convenient means of
concentrating the complexes and increases the sensitivity of
the analysis. Extraction spectrophotometry using dithizone
as a color-developing reagent is sensitive but suffers from
several disadvantages. It is time-consuming and tedious and
involves the use of chlorinated solvents.
with the bulk aqueous solution, while the hydrophobic chains
are directed inside the micelles and form the nonpolar core.
This results in the solution being heterogeneous on a
microscopic scale, but the whole system is macroscopically
homogenous and the macroscopic properties of this system
approximate those of a truly homogenous solution.3
Micelles enhance the solubility of organic compounds in
water by providing local nonpolar environments. This
phenomenon of micellar solubilization has been used in the
development of many new methods and in the modification
of existing methods of analysis."1° Although surfactantmediated
colorimetric methods have been rep0rted,l1-1~
micelles have primarily been used to enhance molecular
fluorescence and room temperature phosphorescence.3J5-~0
Micelles are convenient because they are relatively nontoxic,
readily available, optically transparent, and stable.21 They
are destroyed on dilution but can be re-formed if the surfactant
concentration is increased above the critical micelle concentration.
The purpose of this paper is to report a simple spectrophotometric
method for the estimation of some heavy metal
ions. Dithizonemetal complexes are formed in aqueous media
containing micelles, and the local nonpolar microenvironment
of the micelles extracts the nonpolar colored complexes. The
proposed method has several advantages over the liquid-liquid
extraction. It is convenient, selective, and less tedious and
does not involve the use of harmful chlorinated solvents.
Another potential advantage of this method is that it may be
possible to furthur concentrate the metal dithizonates by cloud
point extraction. This technique is based on the fact that a
micellar solution of a nonionic surfactant separates into two
phases above a certain temperature called the cloud point.
The surfactant concentration in one of these phases is higher