Anal. Chem. 1994,66,2152-2756
Micellar Colorimetric Determination of Dithizone Metal Chelates Rajesh P. Paradkar and Ron R. Williams' Department of Chemistry, Clemson Universi@, Ciemson, South Carolina 29634- 1905
Surfactantshave been used in the development of new methods for colorimetric analysis. Amphiphilic molecules (surfactants, detergents) on dissolution in water form organized molecular assemblies called micelleswhen an appropriate concentration, the critical micelle concentration (cmc), is exceeded. These micellesenhance the solubility of organic compounds in water by providing local nonpolar environments. A simple method for the determination of several metal ions has been developed using diphenylthiocarbazone (dithizone) and a nonionic surfactant,polyoxyethylene tert-octylphenol(Triton X-100).The metal-dithizone colored complexes are extracted into the nonpolar microenvironment of the surfactant, thereby eliminating tedious and time-consuming solvent extraction procedures using chlorinatedsolvents. The proposed method is faster, allows for the sequential determination of metal ions in the aqueous phase, and gives parts per million detection limits for the metal ions tested. Dithizone (diphenylthiocarbazone, H2Dz) is an organic colorimetric reagent that provides the basis of sensitivemethods 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).132Metal ions combine with dithizone to yield nonpolar colored complexes whose colors differ significantly from dithizone. These nonpolar complexesare 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. molecules (surfactants' detergents) consist of a hydrophobic chain joined to a hydrophilic head group. The hydrophobic part is generally a long chain hydrocarbon, typically 8-18 atoms. Surfactants are classified as either nonionic, cationic, anionic, or zwitterionic based on the nature of the polar head group.3 Surfactants,on dissolution in water, form organized molecular assemblies called micelles if the critical micelle concentration (cmc) is exceeded. The structure ofthemicelleissuchthatthe~olarheadgroupsareincontact (1) Marczenko, 2.Spectrophotometric Determination of Elements; Ramsay, C. G.,Ed.; John Wiley & Sons Inc.: New York, 1976; Chapters 2, 12, 19, 28,
31, and 60. (2) Cheng, K. L.; Ueno, K.; Imamura, T. Handbook of Orgunic Analytical Reagents; CRC Press: Boca Raton, FL, 1982; pp 363-375. (3) Georges, J. Spectrochim. Acta Rev. 1990, 13, 27.
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Am&ticalChemistry, Voi. 66, No. 17, September 1, 1994
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 complexesare 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 (4) Paramauro, E.; Bianca Prevot. A.; Peliuetti, E. Anal. Chim. Acru 1992,264,
303.
(5) Lopez Garcia, A,; Blanco Gonzalez, E.; Garcia Alonso, J. I.; Sanz Mcdel, A. AMI. Chim. Acta 1992, 264, 241. (7) Okada, (6) Pelizzetti. T, E.; Anal, Pramauro, Chem 1992, E. AMI. 64, 2138-2142. Chim. Acta 1985, 169, 1. (8) Cline Love, L. J.; Habarta, J. G.; Dorsey, J. 0.Anal. Chem. 1984,56.1132A. (9) McIntire, Watanabe,G.H.; C.Tanaka, Crir. Rev. H. Anal. Tuluntu Chem. 1978, 1990, 25, 585. 21 (4), 256279. (11) Chung-Gin. H.; Chao-Sheng, H.; Ji-Hong, J. Tulunru I W O , ~676. ~, (12) Watanabe H.; Junichiro. M. Bunseki K w k u 1977,26 (3), 196. (13) Watanabe, H.; Ohmori, H. Tuluntu 1979, 26, 959-961. (14) Goto, K.; Taguchi, s.;~ukue,Y.; ohta, K.; Watanabe, H. Tuluntu i m , 2 4 , 752-753. (15) Cline Love, L. J,; swlec,M.; H a b a d , J. G. Anal. ,-hem, 1980,52,754. (16) Skrilec, M.; Cline Love, L. J. AMI. Chem. 1980, 52, 1559. (17) Singh, H.N.; Hinze, W. L. Analyst 1982, 107, 1073. (18) Taketatsu, T.; Sato, A. Anal. Chim. Acra 1979, 108, 429. (19) Taketatsu, T. Tulantu 1982, 29, 397. (20) Medina-Escriche. J.; De la Guardia-Cirusede, M.; Hernandez-Hernandez,F. Analyst 1983, 108, 1386. (21) Diaz Garcia, M. E.; Sanz Mcdel, A. Talantu 1986,33, 255-264.
0003-2700/94/03662752t04.50IO
0 1994 American Chemical Sockty
~
T a b 1. Propardon of 8MIpk SoMknr lor D.1.lmlnrtkn ol Indlvlducrl W r l Ion8
H
rllm
S-
M = Hg(II), Cu(II), m(LI),Pb(II), Cd(II). Flgure 1. Structure of primary metal dithlzonate.
then the other, allowing extraction and concentration of the solubilized species. Several methods based on cloud point extraction have been reported in the literature.7JO We are currently investigating the the possibility of concentrating metal dithizonates using this technique. Structure of Dithizonates.1.2J2The metal ions used in this study form chelate complexes with dithizone (HzDz). These complexes called primary or normal dithizonates can be represented as M(HDz),, where M is a metal ion of charge n+. Primary dithizonates are formed when dithizone reacts with a metal as the anion of monobasic acid (HDz), e&, Hg(HDz)2, Zn(HDz)*, Cu(HDz)2, Pb(HDz)2, and Cd(HDz)~:
+
M”+ nHDz-
-
M(HDz),
Structural investigations of these complexes have shown that the metal is bonded to the sulfur atom and coordinately bonded to the nitrogen, as shown in Figure 1. Some metal ions (Cu, Hg, Ag, Pt, Au, Pd) form secondary dithizonates, which have found no real application in spectrophotometric analysis. Their stoichiometry corresponds to a meta1:ligand ratio of 2:n and are formed when dithizone reacts with a metal as the anion of a dibasic acid (Dz2-), e.g., CuDz and Ag2Dz.
EXPERIMENTAL SECTION Apparatus. A Hewlett Packard 8452A, single-beam diode array spectrometer with an integration time of 2 s was used for the study. The spectra of all sample solutionswere recorded using a quartz cell having a path length of 1 cm. The spectral range covered 190-820 nm with 2-nm resolution. Reagents and Chemicals. Standard Metal Ion Solutions. A standard solution of zinc(I1) (Matheson Coleman & Bell) was prepared by dissolving the metal in hydrochloric acid (Mallinckrodt). Other metal ion solutions were prepared from reagent-grade sulfates, nitrates, or chlorides (Baker & Adamson) and used without further purification. All solutions were prepared in distilled water. Acetate Buffer (pH5.0). Prepared by mixing 0.2 M sodium acetate (Fisher Scientific) and 0.2 M acetic acid (Mallinckrodt). Borate Buffer 0.3 M (pH 8.6). Prepared by dissolving 18.55 g of boric acid (Matheson Coleman & Bell) and 3.65 g of sodium hydroxide (Mallinckrodt) in 1 L of distilled water. TritonX- 100Solution (Aldrich). A 5% solution of Triton X-100 was prepared by mixing 5 mL of Triton X-100 with 85 mL of distilled water and 10 mL of 1 M hydrochloric acid. (22) Nowicka-Jankowska, T.; Irving, H. M. N. 489-496.
H.Anal. Chim.Acra
1971,51,
metal ion
wncn ran e (pg/mLf
vol used to adjust pH (mL)
vol of dithizone total vol soh (mL) (mL)
Cu(I1) Hg(I1) Zn(I1) Pb(I1) Cd(I1)
1-10 1&50 1-10 1040 1-10
5
7
5 5 10 3
7 7 10 6
13 13 13 21 10
DithizonelTriton X-100 Reagent. The reagent was prepared by dissolving a slight excess of AR-grade solid dithizone (Mallinckrodt) in 5% Triton X-100, prepared as mentioned above. The solution was allowed to equilibrate for 5 min and was then filtered through a Whatman No. 41 ashless filter paper to remove the undissolved excess dithizone. Reagents for SequentialD e t e h t i o n of Zinc and Mercury. Triton X - 100 Solution (10%). Prepared by mixing 10 mL of Triton X-100 (Aldrich) with 80 mL of distilled water and 10 mL of 1 M hydrochloric acid (Mallinckrodt). Acetate Buffer (pH 5). Prepared by mixing 0.6 M sodium acetate (Fisher scientific) and 0.6 M acetic acid (Mallinckrodt) DithizoneITriton X - 100 Reagent. Prepared as mentioned above, byequilibratingexcessdithizonewith 1mTriton X-100 solution for 5 min followed by filtration (Whatman No. 41) to remove the undissolved dithizone. Determinationof Solubility of Dithizonein 5%Triton X-100. To determine the solubility of dithizone in 5% Triton X-100, a standard solution of dithizonein ethyl alcohol (95%, Aldrich) was prepared by dissolving 30 mg of dithizone in 100 mL of ethyl alcohol. Aliquots of the standard solution to cover the range 0.06-0.24 mg of dithizone were transferred to a series of 50 mL standard flasks and diluted up to the mark using 5% Triton X-100. The total volume of ethanol present in each solution was kept constant and at 1.6% (v/v). The absorbance of these solutionsat 604 nm was used to construct a calibration curve. Next, a saturated solution of dithizone in Triton X-100 was prepared by equilibrating excess solid dithizone with 25 mL of 5% Triton X-100. The solution was filtered and diluted with more 5% Triton X-100, and the concentration of the dithizone was determined taking into account the dilution employed. The solubility of dithizone in 5% Triton X-100 was found to be 0.025 g/L (25 OC,pH 7), which is significantly higher then its solubility in water pH C 7). (5-7.2 X Determination of Metal Ions. Solutions used to generate calibration curves for the individual metal ions were prepared as shown in Table 1. Standard metal ion solutions were taken in small beakers, and their pH was adjusted to an optimum value for the formation of that ion’s dithizone complex. A saturated solutionof dithizonein Triton X- 100was then added to each solution, and the solutions were allowed to equilibrate for a few minutes before recording the spectra. In order to ensure that all the solutions had an excess of dithizone, the volume of dithizone required to give a mixed/stable green color with the solution containing maximum concentration of metal ion was used (e.g., 7 mL for Cu and 10 mL for Pb). However, it should be noted that this volume will vary depending on the concentration of Triton X-100 used and the
.
Ana~IcaIChemktry, Vol. 68, No. 17, September 1, 1994
2753
T a b 2. condnknr lor Formath and Dotoctlon ol MotaWhlrona Compkxe8 In Trltm X-100 and Chkrolorm measured A, measured ,A, metal
ion
PH"
c1
Cu(I1) Hg(II) Zn(I1) Pb(I1) Cd(I1)
2 5 7-10 >lob
Table 4. RoprodudMiny and Prornt R.cov.ry Rowib tor Soma Ropresontathrr Mota1 Ions
in TX-100(nm)
in CHCllC(nm)
554 492 520 510 542
534 486 534 518 514
Determination of Individual Metal Ions
From reference 1. Strongly alkaline medium (5-2096 NaOH). Refer to MSDS for complete information on handling, storage, and disposal. a
_
_
~
~
~
~
Tabla 3. Figure8 of M.rlt tor M k d a r CaHbratkn Curve8
metal ion
linear range (PPm)
regression equation
R2
Cu(I1) Zn(I1) Cd(I1) Pb(I1) Hg (11)
Determination of Individual Metal Ions y = 0.57105+ 2.2273 X 1t2x 0.08-0.80 0.08-0.80 y = 0.34988+ 6.0420X 1t2x y = 0.17494 7.9916X 1WZx 0.10-1.00 y = 0.50288 5.6211 X lt3x 0.48-1.90 y = 0.35753+ 1.5005 X lt3x 0.80-3.85
0.996 1 .Ooo 0.997 0.987 0.990
Hg(I1) Cd(I1) Zn(I1) Hg(I1)
Sequential Determination of Hg(I1) and Cd(I1) and Zn(I1) and Hg(I1) 0.1-1.0 y = 0.25316 + 2.7627 x io-zx y = 0.12841+ 4.2013X 1t2x 0.07-0.7 y = 0.57951 0.11425~ 0.1-0.8 y = 0.36772 2.2535 X ltzx 0.08-0.6
0.999 0.992 0.999 0.999
+ +
+ +
age of the reagent. As shown later, the amount of dithizone solubilized in Triton X-100increases with an increase in the concentration of the surfactant. Table 2 gives the conditions for the formation of various metal dithizonates in Triton X- 100 along with their wavelengths of maximum absorbance. All the metal dithizonates formed under these conditions were stable for at least 45 min. The wavelengths of maximum absorbance for the different metal dithizonates in chloroform are also shown in Table 2. The wavelengths of maximum absorbance differ with the nature of the solvent used to dissolve the metal-dithizone complexes. As mentioned earlier, this method eliminates the use of chlorinated solventssuch as chloroform. This is an advantage as chloroform is highly toxic and listed as a potential carcinogen in the NTP fourth annual report on carcinogens and in the IARC monographs. Cronic effects of prolonged or repeated overexposure include delayed liver and/or kidney damage and increased risk of cancer. Detailed safety considerations for the handling, disposal, and storage of chloroform can be found in the Material Safety Data Sheets (MSDS) and must be followed when handling this potential carcinogen. Proper eye and respiratory protection (a NIOSH-approved self-contained breathing apparatus) along with protective clothing (including poly(viny1alcohol) boots, gloves, and apron) are recommended for adequate personal protection when handling chloroform and other chlorinated solvents. RESULTS AND DISCUSSION CalibrationCurves, Sensitivity, and Precision. Calibration curves were constructed for all the metal ions according to the general procedure described above. Table 3 indicates the range over which Beer's law is obeyed by the various metal ions as well as the regression equations and their r2 values. The calibration curve for lead was plotted using four solutions. All 2754
ion % mean% SD CV added calibration (ppm) absorbance absorbance recovery recovery (ppm) (96)
Analytlcel Chembtry, Vol. 66, No. 17, September 1, 1994
Hg(W 2.3 1 2.31 2.31 2.31 2.31 Cu(I1) 0.62 0.62 0.62
0.907
0.898 0.919 0.915 0.909 0.926
99.1 101.3 100.9 100.2 102.1
100.6
0.027 1.16
1.001
1.009 0.987 0.996
100.8 98.1 99.5
99.7
0.007 1.14
Determination of Metal Ions in the Same Solution Hg(W 0.89 0.89 0.89 Cd(I1) 0.57 0.57 0.57
0.536
0.522 0.533 0.538
97.5 99.4 100.3
99.07
0.013 1.45
0.668
0.621 0.630 0.695
93 94.4 104.2
97.2
0.035 6.29
-1
I
A
\
I J/ 04 300
400
500
600
700
800
Wavelength (nm)
Flgure 2. Absorptlon spectra of metal dithkonates In chlorofrom.
the other calibration plots were obtained using five solutions. In the absence of other metal ions, the coefficient of variation for the determination of Cu(I1) in the range 0.08-0.80ppm is 1.14% and that for Hg(I1) in the range 0.80-3.85 is 1.16% (Table 4). Absorption Spectra. The absorption spectra of metal dithizonates in chloroform and 5% Triton X-100measured using chloroform and Triton X-100as blanks respectively are shown in Figures 2 and 3. The absorbance intensities of the complexes are different because different concentrations of metal ion solutions were used in each case. The exact solubilization site of the dithizone and the metal dithizonates is not known and is under investigation. The two solubilization sites available in nonionic micelles are the nonpolar hydrocarbon core and the surrounding poly(oxyethy1ene) mantel. It should also be noted that the wavelength of maximum absorbance for the complexes is different in the two solvents, i.e., Triton X-100and chloroform. The absorbance maxima of the cadmium, copper, and mercury complexesshift to longer wavelengths, and the absorbance maxima of the zinc and lead complexes shift to shorter wavelengths with respect to their value in chloroform.
01
2 m f
0.6 -
'
::
f
0.4
-
0.2 -
V."
,
300
400
500
600
Wavelength (nm)
700
800
Flgue 3. Absorptionspectra of metal dltMronatesIn 5 % Triton X-100.
A probable mechanism for metal-dithizone complex formation can be proposed by considering that solubilization like micelle formation is d y n a m i ~ ,i.e., ~ . ~the solubilizate is in dynamic equilibrium between the micelle and the bulk aqueous phase. Thus, dithizone although solubilized in the micelle can exchange and combine with the metal ions in the bulk aqueous phase. The nonpolar complex so formed is then extracted in the local nonpolar environment of the micelle. The residence time of the solute molecules in the micelles is generally 10-3-10-5s . ~Thus, the metal-dithizone complex formation is essentially instantaneous. This is an advantage over the liquid-liquid extraction proceduresfor the quantitative estimation of copper and zinc where prolonged shaking with the organic solution of dithizone is required as copper and zinc dithizonates are extracted slowly.1 Since an excess of dithizone is added, the methods for metal ion determination are essentially 'mixed color'. In alkaline medium, as in the estimation of lead and cadmium, the excess dithizone forms the yellow-orange dithizone anion (HDz). Stability of Dithizone. Watanabe and Junichiro,12and Ueno et al.23 have addressed the poor stability of dithizone by using Triton X-1 00-solubilized zinc dithizonate and copper dithizonate as a color-developingreagent in the estimation of copper and mercury, respectively. We have found that the addition of 1 M hydrochloric acid to the Triton X-100solution improves the stability of dithizone in Triton X-100. Figure 4 shows the change in the absorption spectrum of a saturated solution of dithizone in Triton X-100 with time. The solution seems to be reasonably stable over a period of 60 min, and the analysis can easily be completed in this time. The solution, however, is unstable when stored for prolonged periods of time and should be prepared fresh before each analysis. All the metal dithizonates formed are stable, and hence the instability of dithizone does not affect the analysis if an excess of dithizone is always added to all the sample solutions. SolubilityofDithizoneinTritonX-100.It has beenreported that the amount of solute solubilized is usually directly proportional to the number of micelles in s ~ l u t i o n . ~ The absorption spectra of saturated dithizone solutions in two different surfactant concentrations are shown in Figure 5 and (23) Ucno, K.; Shimishi, K.; Togo,T.; Yano, T.; Yoshida, I.; Kobayashi, H. AMI. Chim. Acta 1979, IOS,289.
200
300
400
500
600
700
so0
Wavelength (nm)
Flgure 4. Change In the absorption spectrum of a saturated solution of dithkone in 5 % Triton X-100, with time (1) a = 0, (2) b = 15, (3) c = 30, (4) d = 45, and (5) e = 00 mln.
300
400
500
600
700
Wavelength (nm)
Figure 5. Saturated solution of dithizone in (1) a and (2) b = 10% Triton X-100.
800
900
= 5 % Triton X-100
confirm this asumption as a 10% solution of Triton X-100 solubilizes nearly twice as much dithizone as a 5% solution. The absorption spectra of varying amounts of dithizone in 15 mL of 5% Triton X-100are shown in Figure 6. As seen from the spectra, Triton X-100 is saturated by the addition of 5 mg of solid dithizone. The addition of a large excess of dithizone (10mg) does not result in increased solubility of dithizone as all the micelles are saturated with dithizone. Addition of a very large excess of dithizone (15 mg) apparently results in higher absorbance because of the higher background. Thus, the amount of dithizone solubilized depends only on the concentration of the surfactant, and hence the exact amount of dithizone solubilized need not be known as long as a saturated solution is used and surfactant concentration is known.
SEQUENTIAL DETERMINATION OF METALS The method describedabove does not involve tedious liquidliquid extraction and can hence be easily applied to the sequential determination of two or more metal ions in the same solution. In order to demonstrate this, calibration curves for two pairs of metal ions [Hg(II) and Cd(I1); Zn(I1) and Hg(II)] were generated using standard mixtures of metal ions. Analytical ChemisW, Voi. 66, No. 17, September 1, 1994
2755
Table 5. Preparation ot Standard CaHkatlon M1dur.r for Sequential Determlnatlon of Hg( I I ) and Cd( I I ) and Zn( I I ) and Hg(I1)
I 300
400
500
600
700
so0
Wavelength (nm) Figure 6. Absorption spectra of (1) a = 1, (2) b = 5, (3) c = 10, and (4) d = 15 mg of dkhlzone In 5% Triton X-100.
Figures of merits for the calibration curves obtained in both cases are given in Table 3. Calibration mixtures 1-5, prepared as mentioned in Table 5, were taken in small beakers and used to generate calibration plots, first for mercury and then for cadmium. To each of the solutions (pH 2), a slight excess of a saturated solution of dithizone in 5% Triton X-100 was added. Mercury reacts with dithizone under these conditions to form its dithizonate while cadmium requires significantly higher pH. The solutions were allowed to equilibrate for a few minutes before measuring their absorbance at 500 nm as r e c ~ m m e n d e d . ~ ~ After estimating mercury, the solution from the cuvette was transferred back to the beaker, and the pH of the mixture was adjusted to >10 by the addition of 6% NaOH. This results in the decomposition of the mercury complex. The liberated dithizone combines with cadmium to form the cadmium complex, allowing the sequential estimation of cadmium. Beer’s law was obeyed over the concentration range (110) p g of Hg/9 mL and (1-10) p g of Cd/14 mL. However, the sample containing 10 pg of Cd showed a slight deviation from linearity. Sample analysis of some mixtures (Table 4) shows that the coefficient of variation for the determination of mercury in the presence of cadmium is 1.45%. The coefficient of variation for the determination of cadmium is however significantly higher then that of mercury. This is because the pH of the mixture cannot be adjusted in the cuvette after the estimation of mercury. Transferring the solution back into the beaker introduces error, because the transfer is not always quantitative. This method is thus more suitable for the analysis of individual metal ions. Zinc dithizonate is formed at pH 5 and is stable. It however dissociates rapidly and quantitatively below pH 2 to liberate free dithizone.” Thus, if a solution contains both zinc and mercury, the amount of zinc can be estimated first by preferentially complexing it with dithizone at pH 5. The zinc dithizone complex is then destroyed by adjusting the pH of
-
(24) Sandell, E. B.; Onishi, H. Photometric Determination of Trace Metals, 4th ed.;Sandell, E. B., Ed.; Wilcy-Intcrsciencc Publication: New York, 1978; Vol. 11, p 62.
2756
AnalyticaiChemistry, Voi. 66, No. 17, September 1, 1994
sample 3
metal ions
1
2
W I I ) (fig) C W ) (fig) vol of 1 M HC1 (mL)
1 1 3
2 2 3
Zn(W (fig) Hg(II) (fig) vol of buffer (mL)
1 1 3
2 2
5 5 3 5 5
3
3
4
5
8
10
8
10
3 8 8 3
3
the solution to - 2 by the addition of 2 M hydrochloric acid. The free dithizone so formed can now form a complex with mercury, allowing for its quantitative estimation. In principle, all five of these metal ions can be sequentially determined from the same solution as they form their dithizonates at different pH’s, but practical constraints (solution becomes too dilute) limit the determination to only two metal ions. In order to limit the volume of the final solution, all the solutions used in this estimation were more concentrated than the normal procedure. As in the estimation of Hg(I1) and Cd(II), calibration mixtures 1-4 (Table 5) were used to generate calibration plots for zinc and mercury. To each of the above solutions (pH 5), the required volume of saturated dithizone solution in 10% Triton X-100 was added. The solutions were allowed to equilibrate for a few minutes, and the absorbance of the pink zinc dithizonate was measured at 520 nm. A sample of 2 M HC1 was then added to all the solutions to adjust the pH of the final solution to approximately 2. The zinc dithizonate dissociates under these conditions, liberating free dithizone in solution which then complexes with the mercury ions. The absorbance of the solution was measured at 500 nm, and the amount of mercury was determined. Beer’s law is obeyed over the concentration range (1-008.00)pgofZn/lOmLand(1.00-8.00)pgofHg/13mL.The sample containing 10p g of zinc and mercury showed negative deviation from Beer’s law.
-
CONCLUSION A simple colorimetric method for the determination of five metal ions was presented. This method completely eliminates the need for tedious liquid-liquid extraction involving the use of toxic chlorinated solvents. The new method is much faster and can be applied for the sequential determination of metal ions. We are currently investigating the effects of foreign ion interferences and the potential application of this method for isolation and estimation of metal ions from an inorganic matrix of interfering ions. ACKNOWLEDGMENT The authors wish to thank Dr. B. R. Buchanan of Westinghouse Savanannha River Corp. for the loan of the diode array spectrometer. Received for review January 3, 1994. Accepted May 17, 1994.’
* Abstract published in Adoance ACS Abstrocts, July 1, 1994.
