ANALYTICAL SCIENCES AUGUST 1985, VOL.

1 297

Letters to the Editor

A Color Reaction of Cadmium(II) with Dithizone in

Methyltrioctylammonium Thiocyanate Medium
Hideo AKAIWA,Hiroshi KAWAMOTO
and EtsuoHIYAMUTA
Departmentof Chemistry,Facultyof Engineering,GunmaUniversity,Kiryu,Gunma376

KeywordsDithizone,
methyltrioctylammonium
chloride,
thiocyanate,
cadmium(II
), spectrophotometry

The extraction of cadmium(II) with dithizone(H2dz) were of guaranteed grade.

from strongly alkaline solution has been employed for The absorbance measurements were conducted with
the purpose of separation of the element from zinc(II). a Hitachi 200-10 type spectrophotometer and 1.00 cm
In this case, cadmium(II) is extracted as dithizo- glass cells. The pH of aqueous phase after the
natocadmium(II), while zinc(II) remains in the aqueous extraction was measured with a Hitachi-Horiba F-7ss
phase as zincate ion.' Although this method is rather type pH meter. Cadmium(II) and zinc(II) remaining in
popular, it is generally hard to obtain a successful the aqueous phase were determined with a Hitachi 170-
result probably due to the difficulty in purifying 30 type atomic absorption spectrophotometer.
sodium hydroxide. Recently, the authors reported that
the above purpose could be achieved by employing the Procedure
synergistic extraction of cadmium(II) with a mixture of The pH of the aqueous solution containing cadmium-
dithizone and 1,10-phenanthroline.2 On the other (II) or zinc(II) and potassium thiocyanate was
hand, Pribil and Veseley reported the mutual separa- preliminarily adjusted by adding sodium acetate and
tion of cadmium(II) and zinc(II) using methyltrioctyl- acetic acid. The chloroform phase was prepared by
ammonium chloride(MTOA=R3R'N.CI).3 This system mixing 5 cm3 of 0.002% dithizone solution with an
was reexamined into a new spectrophotometric method equal volume of 10% MTOA. Each 10.0 cm3 of the
of determination of a trace amount of cadmium(II) by aqueous and the organic phase was placed together in a
the authors.4 The separation procedure in the above 100 cm3 separatory funnel, and the mixture was shaken
method, however, is somewhat complicated. vigorously for 15 min at room temperature(ca. 293K).
In the work described below, we found an interesting During the extraction process, MTOA was converted
phenomenon i. e., upon extraction of cadmium(II)and to thiocyanate type. After the phases were allowed to
zinc(II) with a mixture of dithizone and MTOA from a separate, the absorption spectrum of the organic phase
thiocyanate medium, only cadmium(II) forms an and the pH of the aqueous phase were measured.
orange colored species which may be useful for the Cadmium(II) and zinc(II) remaining in the aqueous
spectrophotometric determination of cadmium(II) in phase were also determined.
the presence of a large amount of zinc(II).

Results and Discussion

Experimental
Both cadmium(II) and zinc(II) could be extracted
Reagents and apparatus from thiocyanate medium with MTOA. The extracted
Standard solution of cadmium(II): A stock solution species were estimated by the conventional slope
(Cd: 1000 mg dm-3) was prepared by dissolving a analysis to be [R3R'N+]2[Cd(SCN)4-] and [R3R'N+]2[Zn-
weighed amount of cadmium (II) acetate, Cd(CH3C00)2• (SCN)2-], respectively. The former was found to react
2H20 in deminerallized water. with dithizone causing a change in color of the organic
Dithizone solution: Dithizone stock solution(0.01 phase, whereas the latter did not react with dithizone.
w/ v%) was prepared by dissolving the reagent in In order to estimate the composition of the colored
chloroform and stored in a refrigerator. complex of cadmium(II), the molar ratio method was
MTOA solution: Twenty five grams of MTOA(Capri- applied and the result indicates that the species
quat) was dissolved in chloroform and the volume of contains Cd(Hdz)2 in its composition. However, the
the solution was adjusted to 250 cm3 by adding extracted species is, at least, different from the well-
chloroform. known red Cd(Hdz)2 in color, suggesting that the
All reagents obtained from Wako Pure Chemicals species might be formed by further coordination of
 298 ANALYTICAL SCI ENCES AUGUST 1985, VOL. 1

Table 1 Determination of cadmium(II) in the presence

of zinc(II)

Cd(II) taken: 10.0 µg

The above extraction mechanism is also supported

by the spectra shown in Fig. 1. The spectrum obtained
by the extraction of zinc(II) with a mixture of dithizone
and MTOA agreed exactly with that of the reagent
blank, indicating that zinc(II) does not react with
dithizone in the present condition. On the contrary,
Fig. 1 Absorption spectra. The aqueous phase-KSCN, cadmium(II) forms the orange colored species, i.e.,
5.0%; pH 5.5. The organic phase-H2dz, 0.001%;
MTOA, 5.0%. Curve 1, reagent blank; 2, Zn(II) 100 [R3R'N+] [Cd(Hdz)2(SCN)(H20)-] having an absorp-
Lg; 3, Cd(II) 100 .cg. tion maximum at 480 nm.
A brief study on the extraction system was done
aiming at the spectrophotometric determination of
cadmium(II). The calibration curve prepared at pH 5.5
was linear in the cadmium concentration range of zero
thiocyanoto ligand to Cd(Hdz)2 and the color change to 1.5 µg cm-3, the sensitivity being 0.0033 µgCd cm_2
in the organic phase may occur according to the at 480 nm. Effect of zinc(II) on the determination of
following reaction: cadmium(II) was examined and the results are
summarized in Table 1. The coexistence of 500-fold
amount of zinc(II) was tolerated within a relative error
[R3R'N+]2[Cd(SCN)42-]org + 2H2dz,org ~- of 13%.
[R3R'N+][Cd(Hdz)2(SCN-)org + R3R'NSCNorg + From the above results, the present extraction
2SCN- + 2H+ system is concluded to be applicable to the spectro-
photometric determination of cadmium(II) without
where the subscript org denotes the organic phase. separating zinc(II) preliminarily.
From the slope analysis conducted by assuming the
distribution ratio,
References

D= {[[R3R'N+]2[Cd(SCN)42-]]org 1. E. B. Sandell: "Colorimetric Determination of Traces of

+[[R3R'N+][Cd(Hdz)2(SCN)-]]org}J~;[Cd(SCN)x2-x], Metals", p. 350, Interscience Publishers, New York
(1959).
2. H. Akaiwa, H. Kawamoto and E. Yoshimatsu, Bull.
the colored species was concluded to be [R3R'N+] Chem. Soc. Jpn., 52, 3718 (1979).
3. R. Pribil and V. Vesely, Collection Czechoslov. Chem.
[Cd(Hdz)2(SCN)-]. In general, cadmium(II) tends to Commun., 37, 13 (1972).
form stable octahedral complexes, while tetrahedral 4. H. Akaiwa, H. Kawamoto and T. Takenouchi, Bunseki
structure is favorable to zinc(II) complexes. This Kagaku, 27, 449 (1978).
structural difference probably enables the ligand
exchange in thiocyanatocadmium(II) and the orange
colored [R3R'N+] [Cd(Hdz)2(SCN)(H20)-] results. On (Received June 20, 1985)
the other hand, tetrahedral Zn(SCN)- may be stable (Accepted June 25, 1985)
and inactive toward dithizone.