EXTRACTION-PHOTOMETRIC DETERMINATION OF TRACES OF PERCHLORATE IONS IN WATER WITH BRILLIANT GREEN

Yu. L. Lel’chuk, B. N. Besprozvannykh, G. N. Dembovskaya

(Presented by the scientific seminar of the Department of Analytical Chemistry)

For the determination of small amounts of perchlorate ions, extraction-photometric methods have been proposed that are based on photometry of extracts of perchlorate complexes with methylene blue [1], Brilliant Green [2], and Malachite Green [3]. Our experiments showed that more stable data are obtained with Brilliant Green and benzene as the extractant. We used this reagent for the determination of microamounts of perchlorate ions in water.

According to the literature data [2], Brilliant Green forms with \(\mathrm{ClO_4^-}\) ions a green compound that is well extracted by benzene, toluene, and metaxylene. It permits detection of \(0.5\ \mu\mathrm{g}/\mathrm{ml}\) \(\mathrm{ClO_4^-}\) in the presence of \(\mathrm{ClO_3^-}\) and \(\mathrm{Cl^-}\). The authors established that the composition of the compound formed is \(1:1\), and that the absorption maximum in the above-mentioned extractants is \(619\ \mathrm{m}\mu\). The optical density of the extracts remains constant if the compound is extracted at pH from 7 to 4.5. In extraction with benzene, a 15-fold excess of reagent is optimal; in extraction with toluene or metaxylene, a 20-fold excess is optimal. The influence of the anions \(\mathrm{Cl^-}\), \(\mathrm{ClO_3^-}\), \(\mathrm{Cr_2O_7^{2-}}\), \(\mathrm{CrO_4^{2-}}\), \(\mathrm{NO^-}\), \(\mathrm{NO_2^-}\), \(\mathrm{F^-}\), and of the cations \(\mathrm{Li^+}\), \(\mathrm{K^+}\), \(\mathrm{NH_4^+}\), and \(\mathrm{Ba^{2+}}\) was also studied.

Experimental Part

Twice-distilled water was used; all reagents were of analytical-reagent or chemically pure grade; benzene was for cryoscopy. The initial standard solution with a \(\mathrm{ClO_4^-}\) titer of \(1000\ \mu\mathrm{g}/\mathrm{ml}\) was prepared from sodium perchlorate dried to constant weight at \(50^\circ\mathrm{C}\). A working standard solution with a \(\mathrm{ClO_4^-}\) titer of \(10\ \mu\mathrm{g}/\mathrm{ml}\) was prepared on the day of use. A 0.4% aqueous solution of Brilliant Green was stored in dark glass and a fresh one was prepared every 10–15 days.

Preliminary experiments established that the colored complex is extracted most completely by threefold extraction with benzene. The first two times, \(10\ \mathrm{ml}\) of benzene are used each time; the last time, \(5\ \mathrm{ml}\). The optical density of the benzene extract changes practically not at all over 24 hours (Table 1). The excess of reagent must be regulated, since as it increases the optical density of the solution also increases (Table 2).

Table 1

Stability over time of benzene extracts of the perchlorate complex with Brilliant Green.
Content of $\mathrm{ClO_4^-}$: 10 $\mu$g; flask volume 25 ml; cuvette thickness 2 cm; reference solution—benzene

Figure 1 shows the spectrophotometric curves obtained by us for benzene extracts of the blank test (1), a solution with 2 $\mu$g $\mathrm{ClO_4^-}$ (2), and a solution with 5 $\mu$g $\mathrm{ClO_4^-}$ (3) in 25 ml, in cuvettes with a layer thickness of 5 cm, on an SF-4A spectrophotometer. According to our data, the absorption maximum is not 619 [2], but 638 m$\mu$. The correctness of our data was confirmed by measuring the optical density of solutions with the same $\mathrm{ClO_4^-}$ concentration on an FEKN-57 photoelectric colorimeter-nephelometer using light filter No. 7 with a transmission maximum at 610 m$\mu$ and light filter No. 8 with a transmission maximum at 656 m$\mu$ (Table 3). The latter light filter, being closer to the absorption maximum, proved to be more sensitive.

Table 2

Effect of an excess of reagent on the optical density of benzene extracts of the complex.
Flask volume 25 ml; cuvette with layer thickness 2 cm; light filter No. 8; reagent concentration 0.4%

Fig. 1. Spectrophotometric curves of benzene extracts of Brilliant Green and its complex with the perchlorate ion: 1—blank experiment; 2—3 $\mu$g $\mathrm{ClO_4^-}$; 3—5 $\mu$g $\mathrm{ClO_4^-}$.

We also studied the influence of chloride, bromide, bromate, and chlorate ions on the accuracy of the determination of the perchlorate ion with Brilliant Green. The results of these experiments are given in Table 4. These data show that the first three anions do not noticeably change the optical density of the solutions even when their concentration is two orders of magnitude higher than the concentration of the perchlorate ion. The chlorate ion interferes with the determination.

Determination of traces of perchlorate ions in water

To 10 ml of the water being analyzed, in a test tube of 20–25 ml capacity, add 1 ml of a 0.4% aqueous solution of Brilliant Green, mix, transfer to a separatory funnel, and extract ...

Table 3

Dependence of the optical density of benzene extracts of the complex on the light filter.
Flask volume 25 ml, cuvette with a layer thickness of 3 cm

the complex formed with 3 portions of benzene. The first 2 portions are 10 ml each, the last 5 ml. After addition of each portion of benzene, the contents of the funnel are shaken for 1 minute, allowed to stand for 2–3 minutes, and, after phase separation, the aqueous phase is drained through the stopcock into the same test tube, while the benzene extract is transferred through the neck of the funnel into a dry 25-ml volumetric flask.

Table 4

Effect of some anions on the optical density of the benzene extract of the perchlorate complex with Brilliant Green.
$\mathrm{ClO_4^-}$ content 10 μg, flask volume 25 ml, cuvette with a layer thickness of 2 cm

Then the aqueous phase is transferred from the test tube into a separatory funnel, the residues from the walls of the test tube are rinsed with a new portion of benzene, and the extraction is repeated.

After the 3rd extraction, the volume is brought to the mark with benzene, mixed, and the optical density of the extract is measured on an FEKN-57 photocolorimeter-nephelometer in cuvettes with a layer thickness of 2 cm, using red light filter No. 8 with a transmission maximum at 656 mμ. Benzene serves as the reference solution.

For construction of the calibration graph, experiments with specified volumes of the standard $\mathrm{ClO_4^-}$ solution are carried out under the same conditions. The volume of the aqueous solution before extraction is brought

Fig. 2. Calibration graph. Flask volume 25 ml, cuvette with a layer thickness of 2 cm, light filter No. 8, reagent concentration 0.4%.

Table 5

Verification of the accuracy of the photometric determination of the perchlorate ion with Brilliant Green by the method of additions in tap water

is brought in all experiments to 10–12 ml. The calibration graph constructed under the selected conditions obeys the Lambert–Beer law at a $\mathrm{ClO_4^-}$ content from 0 to 15 $\mu\mathrm{g}$ (Fig. 2).

The accuracy of the method was checked by the method of additions. Some of the data obtained are given in Table 5.

Conclusions

The optimum conditions have been studied for the extraction-photometric determination of trace perchlorate ions in water with Brilliant Green and benzene as the extractant. The sensitivity of the method is $10^{-5}\%$. The duration of the analysis is 1 hour. The relative error does not exceed $\pm 5\%$.

Literature

1. G. Charlot. Methods of Analytical Chemistry. 907. Khimiya Publishing House, Moscow–Leningrad, 1965.
2. V. A. Golosnitskaya, V. I. Petrashen. Proceedings of the Novocherkassk Polytechnic Institute, vol. 141, 65, 73, 1964.
3. V. A. Golosnitskaya, V. I. Petrashen. In: Advanced Methods of Chemical Technology and Production Control, 270. Rostov University Publishing House, 1964.