Spectrophotometric Determination of Trace Amounts of Mercury(l1) by Extraction with Bindschedler’s Green Masahiro Tsubouchi Faculty of Education, Tottori University, Tottori, Japan MANYINVESTIGATIONS have been made regarding the spectrophotometric determination of mercury(I1) (I). The standard method based on the reaction with dithizone has been considered unsatisfactory because of its extreme sensitivity to variations in laboratory conditions, while the diphenylcarbazone method has serious drawbacks because of its extreme sensitivity to changes in acidity. Recently, rhodamine B (2), phthalein complexone (3), and xylenol orange (4) were described as spectrophotometric reagents for mercury(I1). These methods suffer from a number of interferences and are not sufficiently sensitive. Another method based on the complex formation between mercury(I1) ions and iodide ions has been described by Pappas and Powell (5). A molar absorptivity of about 2.34 X l o 4 is claimed. 2,2 ’-Dipyridyl iron(l1) chelate was proposed for the spectrophotometric determination of mercury(I1) by solvent extraction (6, 7). Although this method is applicable over a wide pH range, it is not sufficiently sensitive. Bindschedler’s green leuco base (BG) [4,4 ’-bis(dimethy1amino)diphenylamine] is known to be oxidized to form the green dyestuff cation (BG+) (8-11). Mercury(I1) was converted into a bromo-complex anion and extracted into 1,2dichloroethane with the BG+. This system was studied to establish a method for the determination of mercury (11). EXPERIMENTAL
Apparatus and Reagents. Absorbance measurements were made using a Shimazu Beckman Model QR-50 spectrophotometer with matched 1-cm quartz cells. A Toa Denpa Model HM-SA pH meter was used for pH measurements. The shaking for extraction was carried out with an Iwaki Model K M shaker. A stock solution of 5 X 10-3M mercury(I1) was prepared from mercuric chloride. The concentration of mercury(I1) was standardized by EDTA titration. The mercury solution for the experiment was prepared by diluting an aliquot of this stock solution with water. A lO-3M BG solution was prepared by dissolving Bindschedler’s green leuco base in 0.01Nsulfuric acid solution. The pH 2.0 buffer solution was prepared from 0.5M sodium citrate solution with several drops of dilute sulfuric acid. All solutions were prepared from analytical reagent grade chemicals and deionized water. Recommended Procedure. Ten milliliters or less of a sample solution containing less than 10-6M of mercury(II), 1 ml of 0.05M potassium bromide solution, 5 ml of the citrate (1) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” Third ed., Interscience, New York, N. Y., 1965, p 621. (2) H. Imai, Nippon Kagaku Zasshi (J. Chem. Soc. Japan, Pure Chem. Sect.), 90, 275 (1969). (3) S. Komatsu and T. Nomura, ibid., 88, 542 (1967). (4) S. Komatsu, T. Nomura, and M. Saito, ibid., p 1124. ( 5 ) A. J. Pappas and H. B. Powell, ANAL.CHEM.,39, 579 (1967). (6) K. Kotsuji, Bull. Chem. SOC.Japan, 38, 402 (1965). (7) Y. Yamamoto, S. Kikuchi, Y. Hayashi, and T. Kumamaru, Bunseki Kagaku (Japan Analyst), 16, 931 (1967). (8) P. Wehber,Z. Anal. Chem., 149, 161 (1956). (9) Zbid., p 241. (10) Zbid., 150, 186 (1956). (11) Zbid.,151, 276 (1956).
buffer solution, 1 ml of the BG solution, and 2 ml of 0.01M ammonium persulfate solution were placed in a 100-ml separatory funnel. This solution was brought to 25 ml with water. After adding 10 ml of 1,2-dichloroethane to the separatory funnel, the contents were mixed thoroughly by the shaker for about 1 minute. The separation of the two layers took place after standing for about 5 minutes. The dichloroethane layer was drawn off into a glass tube with a glass stopper containing about 1 gram of anhydrous sodium sulfate, and shaken to remove droplets of water. The absorbance of the extract was measured at 740 mp against a reagent blank. RESULTS AND DISCUSSION
Absorption Spectra. In the absence of mercury(II), no significant color was extracted into dichloroethane, while in its presence a green color was observed in the organic layer. The absorbance maximum of the extracts was at 740 mp. The absorbance was proportional to the mercury (11) concentration at this wavelength. Effect of pH. It was observed that the green color was formed in the dichloroethane layer from pH 1 to 6. Quantitative extraction was obtained in the pH range 1.6-2.6. In more acidic or more alkaline solutions, the extraction decreased presumably because of the decomposition of bromomercury(I1)-dyestuff complex. Sodium citrate buffer was effective in keeping the solution at pH 2.0. When the citrate concentration was less than 0.01M in the aqueous layer, it took too long to separate the two layers. Citrate (0.050.2M) had no influence on the absorbance of the extract. At pH 2.0 no citrate-mercury(I1) complex formation was apparent from this experiment. The citrate was thus useful not only as a buffering agent but also as a salting-out agent. Solvent for Extraction. Several organic solvents have been examined for their ability to extract the bromo-mercury (11)-dyestuff ternary complex. The solvents investigated were benzene, butyl acetate, carbon tetrachloride, chloroform, 1,Zdichloroethane, ether, ethyl acetate, n-hexane, isoamyl alcohol, methyl isobutyl ketone, monochlorobenzene, nitrobenzene, and toluene. Among solvents tested, nitrobenzene gave a large absorbance (1.40) of the reagent blank, and only 1,2-dichloroethane was capable of extracting the complex. In the other cases, a slight green precipitate was observed at the interface. Effect of Reagents. The absorbance of the extract from a series of 2 X 10+M mercury(I1) solution remained unchanged when the potassium bromide concentration was varied from 8 x to 4 X 10-aM. Several kinds of oxidizing agents were tested to obtain the colored dyestuff cation. They were ferric sulfate, hydrogen peroxide, ammonium persulfate, ceric sulfate, potassium dichromate, iodine, manganic sulfate, potassium bromate, potassium iodate, calcium hypochlorite, and chloramine T. Of these, ammonium persulfate was chosen for the present work because it is the most effective in oxidizing the BG and gives a lower absorbancy of the reagent blank. Maximum and constant color development required about a 5-fold
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Table I. Effect of Diverse Ions Tolerance, M Ion Tolerance, M
Ion
1 x 10-4 5 x 10-8
Ag+ AP+
Baa+ Bea+ Bi a+
Cas+
Cd2+ coat Cr a+
cu
Fez+ Fe a+ Mgz+ Mna+ Ni Pb2+
Sb8+
Sn2+
x 10-3 x 10-3 5 x 10-4 5 x 10-3 1 x 10-8 5 x 10-3 5 x 10-3 2 x 10-3 2 x 10-4 2 x 10-8 5 x 10-8 2 x 10-3 5 x 10-8 5 x 10-3 5
Sn4+
5
Th4+ TI+ Zn2+ CHaCOOc1CNFHCOaH2m4-
I-
NOa-
SCNSora-
2 x 10-4 8 X 10-8
x 10-7 x 10-4 x 10-4 x 10-3 x 10-3 x 10-3 x 10-7 5 x 10-3 5 x 10-8 5 x 10-3 1 x 10-6 1 x 10-4 2 x 10-7 5 x 10-3 1 2 2 2 5 5 2
Table II. Application of Method Mercury content in waste water (mg/20 ml) Sample No. Found Certified 1 2
0.412 0.428
0.431 0.440
molar excess of the ammonium persulfate to the BG concentration. The 2 ml 0.01M ammonium persulfate solution used in the recommended procedure supplies a ratio of 20 to 1 against the 4 X 10-6MBG. Samples containing 2 X 10-6Mof mercury(1I) were treated by the recommended procedure, except that the concentration of BG was varied. With smaller amounts of BG, low results were produced. Maximum color development in the dichloroethane layer required about a 8-fold molar excess of BG to mercury(I1). The 1 ml of 10-*M BG solution used in the recommended procedure supplies a ratio of 20 to 1, if the sample contains 2 X 10-6Mof mercury(I1). Other Variables. To check the completeness of the extraction with dichloroethane, a sample containing 2 X 10-6M of mercury(I1) was treated according to the recommended procedure. The first extraction yielded an absorbance of 0.340 against the reagent blank. The aqueous solution was than extracted with another 10 ml of dichloroethane. The measured absorbance of this organic layer was 0.002 using a reagent blank as a reference. It seems that the extraction rate using 10 ml of dichloroethane is not less than 95 from this experiment. The stability of the developed color was tested under normal laboratory conditions. The color intensity of extracts remains constant for 2 hours. The shaking time for the extraction was varied from 5 seconds to 10 minutes, while the other variables were kept constant. Quantitative extraction of mercury(I1) was found with 10 seconds of shaking. Continued shaking up to 10 minutes produced no further change in absorbance.
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The extraction of mercury(I1) was carried out at various temperatures over the range 10-26 OC. Normal room temperature fluctuations were without measurable effect. Calibration and Reproducibility. The system followed Beer's law over the range of 8 X lO+-4 X 10-6Mof mercury(11). The molar absorptivity can be calculated to be 1.7 x 105 mole-' cm-1 liter. Ten identical samples, each with a final mercury(I1) concentration of 2 X 10-6M, were treated according to the recommended procedure, and their absorbances were measured. The mean absorbance was 0.340, with a standard deviation of 0.005 absorbance unit. Method of Continuous Variations. In order to investigate the composition of the extracted species, continuous variations plots were employed. The total concentration of mercury(I1) plus BG was 1.6 X 10-6M. The continuous variations plots at 700 mp and 740 mp have a maximum at 0.5 mole fraction of mercury(II), indicating a 1 to 1 mercury (11)-BG ratio. The extracted species, therefore, can be suggested as BG+ IHgBrsl-. Effect of Diverse Ions. The effect of diverse ions was studied for a sample containing 2 X 10-6M of mercury(I1) by the recommended procedure. The tolerance to a given ion was defined as the maximum concentration which could be present without causing a deviation of 0.020 for the absorbance of the sample. Tolerances for diverse ions tested are listed in Table I. Anions were added as solutions of their sodium or potassium salts. Cations were added as solutions of their chlorides, sulfates, or acetates. The citrate used as a buffering agent is also useful as a masking agent for many cations. Antimony formed insoluble precipitates under this condition, and only tin(II), (IV) among the cations tested showed serious interferences. Application of Method. The method was applied on a waste water containing ethylmercuric phosphate which had been used as an antiseptic solution for a tulip bulb. A 20-ml portion of the waste water was treated with 10 ml of concentrated sulfuric acid and 10 ml of 30% hydrogen peroxide in a digestion flask fitted with a cold-finger condenser, then heated gently over a small flame until foaming ceased, and allowed to cool. The solution was adjusted to a pH of about 2 with a sodium hydroxide solution, transferred to a 500-ml volumetric flask, and diluted to mark with deionized water. The sample was then treated according to the recommended procedure. The absorbance measurement was made against a reagent blank. Mercury content in the waste water was certified by the dithizone method ( I ) . The results obtained are summarized in Table 11. ACKNOWLEDGMENT
Appreciation is expressed to Professor Y. Yamamoto of Hiroshima University for valuable advice. RECEIVED for review February 16, 1970. Accepted May 18, 1970.
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