Shigeya SnTO and SUIIllO UCHIKAWA. Faculty of Education, Kumamoto University, Kurokami, Kumamoto 860

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1 ANALYTICAL SCIENCES FEBRUARY 1986, VOL Extraction-Spectrophotometric Determination of Antimony(V) with 2-Hydroxyisocaproic Acid and Citrate, with Application to Differential Determination of Antimony(V) and Antimony(III) Shigeya SnTO and SUIIllO UCHIKAWA Faculty of Education, Kumamoto University, Kurokami, Kumamoto 860 A very simple and sensitive method for the extraction-spectrophotometric determination of antimony(v) and (III) was developed. It was found that antimony(v) reacts with 2-hydroxyisocaproic acid in weak acidic aqueous solution on heating for 15 min at 45 C and that the complex anion formed can be extracted into chlorobenzene with Malachite Green. Antimony( III) reacts quickly irrespective of the temperature. However, the Sb( III )-complex anion formed can not be extracted in the presence of citrate, whereas Sb(V)-complex anion can be extracted and determined under the same condition. This significant difference in reactivity between citrate and these two species was applied to the differential determination of antimony(iii) and antimony(v). The calibration graph was linear over the range µg for antimony(v), and µg for antimony(iii). Keywords Solvent extraction-spectrophotometry, differential spectrophotometry, antimony(i I ), antimony(v), Malachite Green, 2-hydroxyisocaproic acid, citrate A number of spectrophotometric methods exist for the determination of antimony.l-4 Most of them are based on the extraction of antimony as SbCl6-, which had been prepared by the addition of oxidizing agent, but they have several disadvantages.5'6 Few studies have been made of the differential determination of antimony(iii) and (V) by spectrophotometry. Very recently, it was found that antimony(iii) reacts quickly with mandelic acid to form a complex anion at room temperature, whereas antimony(v) reacts very slowly under the same condition, while it reacts rapidly on heating. We have reported a highly sensitive determination method of antimony(iii)7 and a differential determination method of antimony(iii) and (V) by spectrophotometry based on the difference between the rates of reaction of mandelic acid with antimony(iii) and (V).8 In further work, several a-hydroxy acids have been investigated as complexing agents for the sensitive determination of antimony(v). It was found that 2- hydroxyisocaproic acid (LA) reacted with antimony(v) in weak acidic medium like mandelic acid did; micro amounts of antimony(v) could readily be determined. Moreover, in the presence of an auxiliary complexing agent such as citrate or tartrate, antimony(iii)-la complex anion could not be extracted with Malachite Green at least for up to 100 µg of antimony(iii), although antimony(v)-la complex anion could be extracted. Consequently, not only the difference in reaction rate, but also the difference in the reactivity with citrate can be used for determination of both antimony(iii) and antimony(v). Experimental Apparatus Hitachi Model 181 and Model 624 digital spectrophotometers were used for absorbance measurements with 10-mm glass cells. An Iwaki Model V-DN Type KM shaker, a Hitachi centrifuge 03P and a Hitachi- Horiba M-8 ph meter were also used. Reagents Standard antimony(v) solution: A stock solution containing 1000 mg dm-3(ppm) of antimony(v) was prepared by dissolving potassium pyroantimonate (Wako Pure Chem. Ind. Ltd. GR grade) in deionized water; working solutions were prepared by suitable dilution. Standard antimony(iii) solution: A stock solution containing 1000 ppm of antimony(iii) was prepared by dissolving potassium antimony tartrate (Wako Pure Chem. Ind. Ltd. GR grade) in water; working solutions were prepared by dilution. Malachite Green solution: Malachite Green solution was prepared by dissolving guaranteed Malachite

48 ANALYTICAL SCIENCES FEBRUARY 1986, VOL. 2 Green (oxalate) (Katayama Chem. Co. Ltd.) in water to give a 1.OX 10-3mol dm 3 (M) solution. 2-Hydroxyisocaproic acid (LA) solution: A 1.

2 48 ANALYTICAL SCIENCES FEBRUARY 1986, VOL. 2 Green (oxalate) (Katayama Chem. Co. Ltd.) in water to give a 1.OX 10-3mol dm 3 (M) solution. 2-Hydroxyisocaproic acid (LA) solution: A 1.0X10' M solution was prepared by dissolving LA (Tokyo Kasei Kogyo Co. Ltd. GR grade) in water and adjusting to ph 3.0 with sodium hydroxide solution. Citrate solution: A 1.0X101 M solution was prepared by dissolving citric acid in water and adjusting to ph 3.0 with sodium hydroxide solution. Deionized water was used throughout. All the other reagents were of analytical-reagent grade and were used as received. Table 1 The reagent blank and the apparent molar absorptivity(~) with a-hydroxy acids under the optimal conditions Standard procedure (A) Transfer 1.0 ml of the sample solution containing up to 11.5.tg of antimony(v) to a stoppered 10-ml testtube, add 0.2 ml of the specified LA solution and heat the solution for 15 min at 45 C. After cooling, add 1.0 ml of Malachite Green solution and dilute to 4.0 ml with water. Shake the solution with 4.0 ml of chlorobenzene for 5 min. After phase separation, measure the absorbance of the organic phase at 628 nm, in a 10-mm glass cell, against a reagent blank as a reference. Table 2 tivity(e) Reagent blank(b) and with selected solvents the and apparent dyes molar absorp- Standard procedure (B) This procedure follows the extraction method given in procedure (A), except for the addition of 1.0 ml of the specified citrate solution adjusted to ph 3.0 after heating the solution for 15 min at 45 C. Results and Discussion Selection of the complexing agent, cationic dye and extraction solvent for antimony ( V) determination As described in the previous papers, the reaction rate of antimony(v) with a-hydroxy acid at room temperature is very slow compared with that of antimony(iii), and heating is needed to obtain maximum sensitivity. So, in order to obtain the most suitable determination method for antimony(v), various complexing agents, dyes and solvents were tested: 2-hydroxyisobutyric acid, 2-hydroxy-2-methylbutyric acid, 2-hydroxyisocaproic acid(la), mandelic acid and p-chloromandelic acid as complexing agents; Ethyl Violet, Methyl Violet, Crystal Violet, Brilliant Green, Malachite Green and Methylene Blue as dyes; dichloromethane, chloroform, chlorobenzene, benzene, toluene as solvents. For each of these acids, different combinations of extracting solvents and dyes were examined (Tables 1 and 2). From the apparent molar absorptivity(e) of the extracted complexes and the absorbance of the reagent blank, it was concluded that the method using Malachite Green and chlorobenzene should be the most suitable for the determination of antimony(v). On the other hand, when Methylene Blue was used as the cationic dye, the appropriate solvent was chloroform; the apparent molar absorptivity and the reagent blank obtained were dm3 mol' cm' and 0.30, respectively. When chlorobenzene, benzene or toluene was used, the ion-pair formed could not be extracted. Absorption spectra The absorption spectra of the reagent blank and the ion-pair formed between the antimony(v) complex and Malachite Green in chlorobenzene are shown in Fig. 1. Malachite Green itself was not extracted into chlorobenzene irrespective of the presence or absence of antimony(v), when LA solution was absent. The wavelength of maximum absorption of each spectrum occurs at 628 nm. Effects of experimental variables The effect of ph was examined by heating the mixture at various ph values, then adjusting the ph to 3.0 for the extraction. The optimal range for complex formation was found to be The effects of reaction temperature and time on the complex formation were examined. Figure 2 shows that use of higher reaction temperatures considerably shortens the reaction time needed, from more than 90 min at 20 C to only 10 min at 40 C. The complex formed was stable for at least two weeks. Accordingly,

ANALYTICAL SCIENCES FEBRUARY 1986, VOL. 2 49 Fig. 1 Absorption spectra. (I), reagent blank; (II), 5.0.tg Sb (V); LA, 5.0X103 M; MG, 2.5X10-4 M; ph, 3.0; reference, chlorobenzene. -3 Fig.

3 ANALYTICAL SCIENCES FEBRUARY 1986, VOL Fig. 1 Absorption spectra. (I), reagent blank; (II), 5.0.tg Sb (V); LA, 5.0X103 M; MG, 2.5X10-4 M; ph, 3.0; reference, chlorobenzene. -3 Fig. 2 Effect of standing time. Sb(V)=5.0 µg; LA, 5.0X10 M; ph, 3.0; MG, 2.5X10-4 M; reference, chlorobenzene;, at 20 ; O, at 30 ; e, at 40 ; 0, at 45. heating the mixture of antimony(v) and LA at 45 C for 15 min was chosen. The sensitivity of antimony(iii) is independent of the reaction temperature and time, because antimony(iii) reacts rapidly with LA at room temperature. Moreover, little prior reduction of antimony(v) to antimony(iii) caused by heating with LA was observed by the Brilliant Green method9 for antimony(v), and the degree of extraction of antimony(v) did not change even when the heating time was as long as 60 min. The effect of the LA concentration for 1.0 ml of sample solution containing 5.00 µg of antimony(v) was examined by adding 0.2 ml of various concentrations of LA. Increasing the LA concentration led to increased absorbance of both the reagent blank and sample extract, but maximum constant extraction was obtained with more than 0.2 ml of 0.1 M LA, when the reagent blank was used as reference. On the other hand, when 2.0 ml of sample solution containing 5.00 µg of antimony(v) was used, constant extraction was obtained with 0.2 ml of 0.2 M LA, but the reagent blank was high (Ab=0.22). Accordingly, 1.0 ml of sample solution and 0.2 ml of 0.1 M LA solution were used to keep the reagent blank as low as possible. When the standard procedure was applied to a constant concentration of antimony(v) (1.03X105 M), but with varying concentrations of Malachite Green and LA, the constant maximum absorbance against the reagent blank was achieved when the concentrations of Malachite Green and LA exceeded 2.0X10-4 M and 4.0X 10-3 M, respectively, although the absorbance of the reagent blank slightly increased. The effect of ph on the extraction was examined and the range found optimal. A ph of 3.0 is therefore convenient for both the complex formation and the extraction. An extraction time of about 2.5 min was found necessary for constant absorbance to be obtained. For routine work, it was fixed at 5 min for safety. The same results were obtained for the extraction of antimony(iii). Calibration curve The calibration curve at 628 nm obeyed Beer's law over the range µg of antimony(v). The apparent molar absorptivity calculated from the slope of the graph was 4.68X104 dm3 mol-' cm'. The absorbance of the reagent blank was against chlorobenzene as reference, and the coefficient of variation was 1.9% for ten runs with 5.00 µg of antimony(v). The absorbance of the organic phase did not vary during at least 60 min. If the volume of extracting solvent was halved, the apparent molar absorptivity was increased by a factor of 1.66, although the absorbance of the reagent blank was On the other hand, the apparent molar absorptivity for antimony(v) obtained by the method without heating process was 8.00±0.17X103 dm3 mol-' cm 1. Under the same conditions, the apparent molar absorptivity for antimony(iii) was 6.64X104 dm3 mol-' cm-' and the coefficient of variation was 1.8% for ten runs with 2.50 µg of antimony(iii). It is suggested that [Sb(LA)2(OH)2]- for antimony(v) or [Sb(LA)2]- for antimony(iii) is formed and extracted as its ion-pair with Malachite Green. Effect of other ions Table 3 shows the recovery of antimony(v) in a series of solutions containing 1.03X 10-5 M (5.00 µg) of antimony(v) and foreign ions. The borate gives a positive error because it reacts with LA to form an extractable complex anion. Iodide, perchlorate and thiocyanate, which are bulky and of low surface charge-density, cause positive errors in the same amounts (mole ratio) compared to antimony(v), and nitrate in 20-fold amounts. Chloride, sulfate and phosphate do not interfere even at very high concentrations. Most cations do not interfere when

4 50 ANALYTICAL SCIENCES FEBRUARY 1986, VOL. 2 Table 3 Effect of other ions antimony(v)1 on the determination of Fig. 3 Effect of shaking time in the presence of citrate (2.5X102 M). Sb(V)=5.0 µg; LA, 5.OX 10.3 M; ph, 3.0; MG, 2.5X104 M; reference, chlorobenzene. Table ple 4 Recovery of antimony(v) and antimony(iii) in samsolutions present in 500-fold amounts. Arsenic(III) and manganese(ii) interfere in 60-fold and 150-fold amounts, respectively. Iron(III) gives rise to a slightly negative error at 100-fold amounts. These errors seem likely to be due to formation of an extractable complex anion and the adsorption of antimony by irbn(iii) hydroxide, respectively. Tin(II) gives rise to a positive error for the determination of antimony(v) because of the reduction of antimony(v) to antimony(iii), whereas the interference of tin(iv) is not as severe as that of tin(ii). Differential determination of antimony(iii) and antimony(t~) As reported in the previous papers the most characteristic difference between antimony(iii) and antimony(v) is the rate of reaction with a-hydroxy acid. Antimony(V) reacts very slowly at room temperature (20 C), but rapidly on heating, whereas antimony(iii) reacts quickly irrespective of the temperature. Accordingly, similarly to the case of mandelic acid described previously, differential determination of antimony(iii) and (V) can be done by means of this difference in reaction rate. In order to determine either antimony(iii) or antimony(v), auxiliary complexing agents were investigated as masking agents. Agents tested were succinic acid, malonic acid, tartaric acid, citric acid, maleic acid and phthalic acid; they were used as 0.1 M-solution adjusted to ph 3.0 with sodium hydroxide solution. When they were used as buffer solution, neither antimony(iii) nor antimony(v) could be extracted. However, when they were added after heating the sample solution, only in the cases of citrate or tartrate antimony(v) could be extracted and determined, whereas antimony(iii) less than 100 µg could not be done. As shown in Fig. 3, net absorbance for antimony(v) depends on the shaking time on extraction in the presence of citrate, and the tendency to slightly decrease was observed. The significant difference between citrate and tartrate was not observed, except for the absorbance of the reagent

5 ANALYTICAL SCIENCES FEBRUARY 1986, VOL blank (0.164 for citrate and for tartrate). Accordingly, 1.0 ml of the citrate solution(0.1 M ph 3.0) and 5 min were selected as auxiliary complexing agent and shaking time, respectively. The apparent molar absorptivity for antimony(v) obtained by procedure B was 4.27±0.10X104 dm3 mol-' cm'. Consequently, as the amounts of antimony(v) alone can be determined by procedure B, that of antimony(iii) can be calculated from the total absorbance obtained by the procedure A. Recovery tests showed(table 4) that the combination of the recommended procedures gave satisfactory results. References E. Eegriwe, Z. Anal. Chem., 70, 400 (1927). E. B. Sandell, "The Colorimetric Determination of Traces of Metals", 3rd Ed., Interscience, New York (1959). R. W. Ramette, Anal. Chem., 30,1185 (1958). H. M. Neumann, J. Am. Chem. Soc., 76, 2611(1954). M. Tanaka and M. Kawahara, Bunseki Kagaku, 10, 185 (1961). H. Imai, Bunseki Kagaku., 11, 806 (1962). S. Sato, S. Uchikawa, E. Iwamoto and Y. Yamamoto, Anal. Lett.,16, 827 (1983). S. Sato, Talanta, 32, 341 (1985). R. W. Burke, Anal. Chem., 38,1719 (1966). (Received October 4, 1985) (Accepted November 28, 1985)

Spectrophotometric Determination of Anionic Surfactants in River Water with Cationic Azo Dye by Solvent Extraction- Flow Injection Analysis

ANALYTICAL SCIENCES JUNE 1987, VOL. 3 265 Spectrophotometric Determination of Anionic Surfactants in River Water with Cationic Azo Dye by Solvent Extraction- Flow Injection Analysis Shoji MOTOMIZU, Yoshito