Page 207 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

DETERMINATION OF LEAO BV THE OITHIZONE METHOO 6.13

6.13 DETERMINATION OF LEAD BY THE DITHIZONE METHOD*
Discussion. Diphenylthiocarbazone (dithizone) behaves in solution as a
tautomeric mixture of (C) and (D):
N-NHC6H5 - NH-NHC6Hs
// /
HS-C S=C

It functions as a monoprotic acid (pK, = 4.7) up to a pH of about 12; the acid
proton is that of the thiol group in (C). 'Primary' metal dithizonates are formed
according to the reaction:

Some metals, notably copper, silver, gold, mercury, bismuth, and palladium,
form a second complex (which we may term 'secondary' dithizonates) at a
higher pH range or with a deficiency of the reagent:

In general, the 'primary' dithizonates are of greater analytical utility than the
'secondary' dithizonates, which are less stable and less soluble in organic
solvents.
Dithizone is a violet-black solid which is insoluble in water, soluble in dilute
ammonia solution, and also soluble in chloroform and in carbon tetrachloride
to yield green solutions. It is an excellent reagent for the determination of small
(microgram) quantities of many metals, and can be made selective for certain
metals by resorting to one or more of the following devices.
(a) Adjusting the pH of the solution to be extracted. Thus from acid solution
(0.1 -0.5M) silver, mercury, copper, and palladium can be separated from
other metals; bismuth can be extracted from a weakly acidic medium; lead
and zinc from a neutral or faintly alkaline medium; cadmium from a strongly
basic solution containing citrate or tartrate.
(b) Adding a complex-forming agent or masking agent, e.g. cyanide, thiocyanate,
thiosulphate, or EDTA.
It must be emphasised that dithizone is an extremely sensitive reagent
and is applicable to quantities of metals of the order of micrograms. Only
the purest dithizone may be used, since the reagent tends to oxidise to
diphenylthiocarbadiazone, S=C(N=NC6H5),: the latter does not react with
metals, is insoluble in ammonia solution, and dissolves in organic solvents to
give yellow or brown solutions. Reagents for use in dithizone methods of analysis
must be of the highest purity. De-ionised water and redistilled acids are
recommended: ammonia solution should be prepared by passing ammonia gas
into water. Weakly basic and neutral solutions can frequently be freed from

* This experiment is not recommended for elementary students or students having little experience
of analytical work.