Synthetic dyes are widely used in the food industry to impart, enhance, or restore the color of various products. The widespread use and negative impact of synthetic dyes on the human body necessitates the control of their concentrations [1–3]. The main difficulty encountered in the determination of dyes is nonquantitative extraction from complex matrices and the need for preconcentration. Liquid–liquid extraction, cloud point extraction, ultrasonic extraction [4–6], and solid-phase extraction [4–8] are commonly used to extract dyes from foodstuffs. Most methods for extracting dyes require the use of organic solvents [9–11], which have disadvantages such as volatility, flammability, toxicity, and carcinogenicity. In this regard, environmentally friendly extraction systems that do not contain organic solvents based on ionic liquids [12–17], deep eutectic solvents [18–21], or aqueous biphasic system (ABSs) [22] are in demand.

Aqueous two-phase liquid–liquid systems with high water content in the extracting phase are especially promising for the extraction of hydrophilic and ionizable compounds, including dyes. The formation of ABSs based on water-soluble polymers, surfactants, hydrophilic solvents, ionic liquids, and organic salts usually requires the introduction of a salting-out agent and a change in temperature or pH [12, 23–27]. In addition to low volatility, an obvious advantage of ABSs based on organic salts is the high polarity of the extracting phase and the ability to ion exchange.

Synthetic sulfo-containing dyes are present as multiply charged anions in aqueous solution; therefore, they are usually extracted into organic solvents (chloroform, toluene, n-butanol, and 1-hexanol/isooctane) in the form of ionic associates with hydrophobic counterions using quaternary ammonium salts: tetrabutylammonium bromide, tetrabutylammonium hydrogen sulfate, octadecyltrimethylammonium bromide, trioctylmethylammonium chloride, and cetyltrimethylammonium chloride [11, 28–31]. The interstate standard GOST 33457-2015 for the analysis of fruit and vegetable products includes a method for the qualitative detection of synthetic dyes (Amaranth, Azorubine, Sunset Yellow, Tartrazine, Allura Red, Red 2G, etc.) based on ion-pair extraction into chloroform in the presence of tetrabutylammonium bromide [32]. However, a two-phase liquid–liquid system based on a quaternary ammonium salt can also be obtained in the absence of an organic solvent by mixing the salt with water. Thus, two immiscible liquid phases are formed on contact between tetrahexylammonium bromide and water [33], and the formation of a two-phase system does not require the introduction of a salting-out agent or any other influences. Quantitative extraction of dyes (Allura Red, Red 2G, Azorubine, and Fast Green) was achieved in ABSs based on tetrahexylammonium bromide in 1 min [34].

Some quaternary ammonium salts form ABSs in the presence of salting-out agents, in particular, tetrabutylammonium bromide (TBABr). This organic salt, which is highly soluble in water, with a melting point of 99–100°C occupies a boundary position between ordinary salts and ionic liquids [33]. When a sufficient amount of a salting-out agent is added to an aqueous solution of TBABr, the solubility of TBABr sharply decreases to cause the separation of a liquid phase enriched in TBABr. The following compounds can be used as salting-out agents: (NH4)2SO4, Na2SO4, NaNO3, Na2CO3, K3C6H5O7 [35–37], NaH2PO4, Na2HPO4, Na3PO4 [36], KH2PO4, K2HPO4, and K3PO4 [38]. Two-phase aqueous systems based on TBABr with ammonium sulfate were used for the quantitative extraction of metal ions [25, 35, 37]. Song et al. [39] demonstrated the possibility of using this system in a microfluid device for the extraction of methyl orange (the extraction efficiency was no greater than 55%). The use of similar ABSs based on tetrabutylammonium chloride with potassium citrate [40] or sodium malate [41] as salting-out agents for the extraction of caffeine, carbamazepine, and nonsteroidal anti-inflammatory drugs was described.

Smirnova et al. [25] found that an ABS based on TBABr can be obtained in a wide range of concentrations of phase-forming components. The optimal choice of a concentration ratio between the components and the volumes of the resulting ABS phases can make it possible to preconcentrate dyes from a large volume of the analyzed sample into a small volume of the extract with high preconcentration factors. The high water content of the TBABr-rich phase and the ability of the system to ion exchange create conditions for the efficient extraction of hydrophilic ionizable compounds, such as anionic dyes.

The purpose of this work was to study the applicability of the TBABr–H2O–(NH4)2SO4 ABS to the microextraction preconcentration of the synthetic food dyes Tartrazine (E102), Azorubine (E122), Allura Red (E129), Sunset Yellow (E110), and Fast Green ( E143) and their determinations in drinks, liquid media, and other materials.

EXPERIMENTAL

Reagents, solutions, and equipment. We used Tartrazine (85%, Sigma, the United States), Azorubine (50%, Aldrich, China), Allura Red (80%, Sigma-Aldrich, India), Sunset Yellow (90%, Aldrich, India), Fast Green FCF (≥85%, Sigma-Aldrich, India), tetrabutylammonium bromide (TBABr) (99+%, Acros Organics, India), ammonium sulfate (97%, RusKhim, Russia), 37% hydrochloric acid (Reag. USP; Panreac, Germany), and sodium hydroxide (98%, Panreac, Germany). Scheme 1 shows the structural formulas of synthetic food dyes used in this work.

Scheme 1 . Structural formulas of synthetic food dyes: (I) Tartrazine, (II) Azorubine, (III) Allura Red, (IV) Sunset Yellow, and (V) Fast Green.

Stock solutions of dyes (5.0 × 10–2 M) were prepared by dissolving accurately weighed portions in distilled water. All working solutions were prepared daily by diluting the stock solutions with distilled water. Solutions of reagents and dyes were stored at +4°C in a place inaccessible to sunlight. Nonwoven polypropylene (NWPP) (wiping material, Kimberly-Clark Kimtech Pure W4, the United States) was used to collect and separate the extracts.

Absorption spectra were recorded on a U-2900 UV-VIS double-beam spectrophotometer (Hitachi, Japan) using cells with the optical path length l = 1 cm. The values of pH were measured on a model 410 pH meter (Akvilon, Russia) with an ELSK-13.7 combined glass microelectrode. A Hettich EBA-20 centrifuge (Tuttlingen, Germany) was used. Substances were weighed using a ViBRA HT analytical balance (Shinko Denshi, Japan). Mechanical mixing of the solutions was carried out on an ELMI S-3 orbital shaker (ELMI, Latvia). The water content of the extracts was determined by Karl Fischer titration using an 870 KF Titrino plus titrator (Metrohm, Switzerland).

In order to determine the boundary of a phase separation region in the TBABr–H2O–(NH4)2SO4 (21 ± 2°C) system by isothermal titration, we prepared initial solutions of the phase-forming components with the concentrations cTBABr, init = 1.00 M and \({{c}_{{{{{\left( {{\text{N}}{{{\text{H}}}_{{\text{4}}}}} \right)}}_{{\text{2}}}}{\text{S}}{{{\text{O}}}_{{\text{4}}}}{\text{,}}\,{\text{init}}}}}\) = 3.93 M (44 wt %) by dissolving accurately weighed portions (±10–4 g) in distilled water. Two-phase systems obtained by mixing the initial solutions of TBABr and (NH4)2SO4 in the volume ratios of VTBABr : \({{V}_{{{{{\left( {{\text{N}}{{{\text{H}}}_{{\text{4}}}}} \right)}}_{{\text{2}}}}{\text{S}}{{{\text{O}}}_{{\text{4}}}}}}}\) from 3 : 1 to 1 : 99 with a total volume 10.0 mL were titrated with distilled water with constant stirring with a magnetic stirrer until the opalescence disappeared. The single-phase point of the system (transparent solution) was detected visually [42]. The volume of water (\({{V}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}\)) used for the titration was noted, and the concentrations \({{c}_{{{{{\left( {{\text{N}}{{{\text{H}}}_{{\text{4}}}}} \right)}}_{{\text{2}}}}{\text{S}}{{{\text{O}}}_{{\text{4}}}}{\text{,}}\,{\text{tep}}}}}\) and cTBABr, tep at the end point of titration were calculated using the following formulas:

$$\begin{gathered} {{c}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}},\,{\text{tep}}~}}} = \frac{{{{c}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}},\,{\text{init}}~}}}{{V}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}~}}}}}{{{{V}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}~}}} + {{V}_{{{\text{TBABr}}}}} + {{V}_{{{{{\text{H}}}_{2}}{\text{O}}~}}}}}; \\ {{c}_{{{\text{TBABr}},\,{\text{tep}}~}}} = \frac{{{{c}_{{{\text{TBABr}},\,{\text{init}}~}}}{{V}_{{{\text{TBABr}}}}}}}{{{{V}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}~}}} + {{V}_{{{\text{TBABr}}}}} + {{V}_{{{{{\text{H}}}_{2}}{\text{O}}~}}}}}. \\ \end{gathered} $$

The concentrations of the components at the calculated points correspond to the related points in the binodal curve.

Preparation of two-phase aqueous systems and extraction of dyes. To obtain an ABS and study the extraction of dyes, 3.0 mL of an ammonium sulfate solution with a concentration of 30 to 47 wt % was introduced into a plastic test tube, and 0.4 mL of a 5.0 × 10–4 M dye solution (1.0 × 10–4 M for Fast Green), 0.4 mL of distilled water or a solution of HCl or NaOH (to adjust pH), and 0.2 mL of a 1.5 M solution of tetrabutylammonium bromide were added. The tubes were shaken for 1 min. To separate the phases, the contents of the test tube were filtered through a glass funnel, the neck of which was filled with nonwoven polypropylene. In this case, the aqueous phase freely passed through the NWPP, and the phase enriched in tetrabutylammonium bromide with the extracted dye was retained on the filter. The equilibrium value of pH was measured in the aqueous phase, and this phase was used for the spectrophotometric determination of residual dye concentrations based on intrinsic absorption at the maximum absorption wavelengths of 503, 516, 426, 481, and 624 nm for Allura Red, Tartrazine, Azorubine, Sunset Yellow, and Fast Green, respectively. In the determination of the dyes in the test materials, the extract was washed off from NWPP with 2.0 mL of distilled water and the spectrophotometric determination of the dyes in the eluate was carried out.

The extraction efficiency of dyes (R, %) was calculated using the formula

$$R = \frac{{c_{{{\text{aq}}}}^{0}V_{{{\text{aq}}}}^{0} - {{c}_{{{\text{aq}}}}}{{V}_{{{\text{aq}}}}}}}{{c_{{{\text{aq}}}}^{0}V_{{{\text{aq}}}}^{0}}},$$

where c0aq and caq are the initial and final (equilibrium) dye concentrations in the aqueous phase, respectively, M; and V0aq and Vaq are the initial and final volumes of the aqueous phase, respectively, mL.

Extraction–photometric determination of dyes in drinks, liquid media, and other test materials. The dyes Allura Red, Sunset Yellow, and Fast Green were determined in “Lifeline” and “Mirinda” drinks and “Listerine” rinse without preliminary sample preparation; aliquot portion volumes were 1.0, 1.0, and 0.5 mL, respectively. To determine Azorubine and Tartrazine in an Easter kit, 100 mg of a sample containing Azorubine (or Tartrazine) was dissolved in 100.0 mL of distilled water; the volume of an aliquot portion was 10.0 mL.

To determine a dye, an aliquot portion of the analyzed sample (1.0–10.0 mL) was introduced into a 15‑mL plastic test tube and distilled water was added to a total volume of 10.0 mL. Ammonium sulfate (4.5 g) and 0.5 mL of a 1.5 M TBABr solution were added. The tubes were shaken for 1 min, and the phases were separated by filtration through a funnel filled with NWPP. The extract retained on the polypropylene filter was eluted with 2.0 mL of distilled water. The absorption spectra of the eluate were recorded and the absorbance of the solution was measured. The dye content was calculated from calibration dependences.

In the determination of Azorubine in the shell of the drug Ibuprofen, the volume of an aqueous phase was reduced to 4.0 mL due to low dye content. The concentrations of the phase-forming components corresponded to the optimal values for the preparation of ABS and quantitative extraction (cTBABr = 0.075 M and \({{c}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}}}}\) = 33 wt %). A 4.0-mL portion of distilled water was added to a drug tablet, and the aqueous solution was separated by decantation after the dissolution of the shell. After centrifugation for 5 min at 4000 rpm and separation of the insoluble residue, a 2.0-mL aliquot portion was taken and a weighed amount of 1.2 g of ammonium sulfate and 0.5 mL of a 1.5 M TBABr solution were added; the volume was adjusted to 4.0 mL with distilled water. Dye extraction was carried out in the same manner as described above.

Preparation of nonwoven polypropylene strips. Nonwoven polypropylene was washed with distilled water and then dried at room temperature. The NWPP sheet was cut into strips of the same size (20 × 30 mm) and weight (50 ± 1 mg). Then, a strip of NWPP was twisted and placed in the neck of a glass funnel.

RESULTS AND DISCUSSION

Conditions for the formation of a two-phase aqueous system. An aqueous biphasic system based on tetrabutylammonium bromide and an inorganic salt is formed due to the salting-out effect when the phase-forming components are mixed in certain ratios. Salts with multiply charged anions are effective for salting out. The ability of sodium salts to salt-out tetrabutylammonium bromide corresponds to the Hofmeister series \({\text{PO}}_{4}^{{3 - }} \gg {\text{ HPO}}_{4}^{{2 - }}\) > \({\text{CO}}_{3}^{{2 - }}\) > \({\text{SO}}_{4}^{{2 - }}\) > H2PO\(_{4}^{ - }\) [36]; ions with a high charge density, such as the phosphate ion, are the most effective. Note that, as a result of the use of different salting-out agents, the values of pH in the coexisting phases of a two-phase system are different. With the use of phosphates and carbonates, separation occurs in the alkaline region. With a decrease in pH, the ability of the system to separate decreases due to the protonation of anions, a decrease in the charge, and, as a consequence, a decrease in the salting-out effect [43]. In an acidic medium, the precipitation of salts (salting-out agents) is possible due to a decrease in solubility.

Ammonium sulfate was used as a phase-separating agent, which made it possible to salt-out TBABr in a wide range of pH. The solubility of ammonium sulfate is almost three times higher than the solubility of sodium sulfate [44], and it can contribute to the salting out of tetrabutylammonium bromide from its aqueous solutions with lower concentrations.

The formation of the TBABr–H2O–(NH4)2SO4 ABS occurs in a wide range of concentrations of the phase-forming components. The volume ratio of the coexisting phases depends on the composition of the system. Varying the concentrations of TBABr and (NH4)2SO4 makes it possible to change the volume ratio of the ABS phases. With a decrease in the concentration of TBABr and an increase in the concentration of the salting-out agent in the system, the volume of the extracting phase decreases.

To study the possibility of using the ABS for microextraction preconcentration, we determined the boundary of the phase separation region at high concentrations of ammonium sulfate (to 3.5 M or 38 wt %). Figure 1 shows the boundaries of the phase separation region at (21 ± 2)°C. The binodal curve separates the concentration region of the components that form two immiscible aqueous phases L1 + L2 (above the curve) from the single-phase region L (in the curve and below). Phase separation in the system occurs spontaneously and quickly after mixing the components. The resulting phases are transparent and highly mobile liquids.

Fig. 1.
figure 1

Boundaries of the phase separation region in the tetrabutylammonium bromide–H2O–(NH4)2SO4 system (21°C).

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From a comparison of the obtained binodal with published data [36], it follows that the amount of sodium sulfate required to salt-out tetrabutylammonium bromide is somewhat smaller than the amount of ammonium sulfate. For example, the salting-out of TBABr from a 0.55 M aqueous solution requires a 0.89 M solution of Na2SO4 or a 1.15 M solution of (NH4)2SO4. Nevertheless, the boundary of the phase separation region is extended by the use of ammonium sulfate in the part of the diagram corresponding to a high inorganic salt concentration. The use of ammonium sulfate as a component causing phase separation makes it possible to salt-out tetrabutylammonium bromide from its aqueous solutions with concentrations to 0.009 M (at an ammonium sulfate concentration of 3.5 M or 38 wt %, Fig. 1). Hooshyar and Sadeghi [36] salted out tetrabutylammonium bromide from its aqueous solutions with concentrations of no lower than 0.119 M (or 0.1226 mol/kg) at sodium sulfate concentrations to 20 wt % (1.56 M or 1.7762 mol/kg), which is close to the solubility of sodium sulfate in water [44].

To select conditions for the extraction preconcentration, we studied the effect of the concentrations of ammonium sulfate and tetrabutylammonium bromide on the volume ratio of the separated phases and on the efficiency of dye extraction.

To extract dyes from small volumes of aqueous solutions (to 10 mL), the ABS preparation conditions were chosen by varying the concentrations of the phase-forming components. It was established that, at a TBABr concentration of 0.188 M, the formation of the TBABr–H2O–(NH4)2SO4 ABS occurred at an ammonium sulfate concentration of >22 wt % (Fig. 2). Quantitative extraction of all of the test dyes was achieved at an (NH4)2SO4 content of ≥29 wt %. Under these conditions, the recovery was higher than 97% for all of the dyes (Fig. 2). When the concentration of ammonium sulfate was higher than 29 wt %, the volume of the TBABr-enriched phase remained constant and amounted to 0.28 ± 0.02 mL. With a decrease in the ammonium sulfate content of the system to 25 wt % or lower, the volume of the extract decreased, and the system did not separate into two phases at a salting-out agent concentration lower than 20 wt %. In the subsequent experiments, the ABS was obtained at an ammonium sulfate content of 33 wt % (2.97 mol/L).

Fig. 2.
figure 2

Dependence of the extraction efficiency of dyes and the volume of a separated phase enriched in tetrabutylammonium bromide on the ammonium sulfate content of the aqueous biphasic system; cTBABr = 0.188 M, pH 5.2–5.9: (1) Allura Red, (2) Azorubine, (3) Tartrazine, (4) Sunset Yellow, (5) Fast Green, and (6) volume of the separated phase enriched in tetrabutylammonium bromide.

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The optimal concentration of TBABr for the formation of ABS, which ensured the quantitative extraction of dyes, was varied in a range of 0.019–0.188 M by adding 0.05–0.5 mL of a 1.5 M solution of TBABr. The total ABS volume was 4.0 mL, and the (NH4)2SO4 content was 33 wt %. It was found that the system remained single-phase at a TBABr concentration of 0.019 M; however, already at a concentration of 0.038 M TBABr, microdroplets of a new phase were formed. As the concentration of TBABr was increased to 0.056 M, the extraction of dyes was quantitative; however, the extract volume was too small for accurate measurements and reproducible results (Fig. 3).

Fig. 3.
figure 3

Dependence of the extraction efficiency of dyes and the volume of a separated phase enriched in tetrabutylammonium bromide on the concentration of tetrabutylammonium bromide in the aqueous biphasic system; \({{c}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}}}}\) = 33 wt %, pH 5.2–5.9: (1) Allura Red, (2) Azorubine, (3) Tartrazine, (4) Sunset Yellow, (5) Fast Green, and (6) volume of the separated phase enriched in tetrabutylammonium bromide.

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For operations in the microextraction mode, the lowest possible concentration of TBABr in the system, 0.075 M, was used to ensure the quantitative extraction of dyes and satisfactory reproducibility of the results. In this case, the volume of the upper separated TBABr-enriched phase was 70 ± 5 μL, and the volume of the phase enriched in ammonium sulfate was 3.93 ± 0.01 mL. Thus, the volume ratio between the aqueous and organic phases was 56 : 1, which is much higher than those reported in a number of publications on extraction in ABSs based on tetrabutylammonium bromide [35, 37, 39].

Selection of conditions for the extraction of dyes in a two-phase aqueous system. Azorubine, Allura Red, Tartrazine, Sunset Yellow, and Fast Green are water-soluble sulfo compounds whose ionic state depends on pH. It was found that the dyes were quantitatively extracted in the TBABr–H2O–(NH4)2SO4 ABS in a wide range of pH from 2.3 to 9.5 (Fig. 4a), where they occurred as differently and multiply charged anions; that is, the ionic state did not affect the degree of extraction of dyes: three- and four-charged anions were extracted just as efficiently as singly charged ones. Extraction of multiply charged ions is not characteristic of traditional extraction systems with molecular solvents. A decrease in the degree of extraction with the charge of dye ions was also noted in systems based on ethoxylated nonionic surfactants [26].

Fig. 4.
figure 4

Dependences of the extraction efficiency of dyes on (a) pH and (b) phase contact time. cTBABr = 7.5 × 10–2 M, \({{c}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}}}}\) = 33 wt %, and Vaq/Vо = 56: (1) Allura Red, (2) Azorubine, (3) Tartrazine, (4) Sunset Yellow, and (5) Fast Green.

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Because dyes in the studied range of pH occur in the form of multiply charged anions, it is reasonable to assume an anion-exchange extraction mechanism in the TBABr–H2O–(NH4)2SO4 system: the exchange of a dye anion for bromide ions as a result of competition for the tetrabutylammonium cation TBA+. The extracting phase of the ABS contains a hydrophobic tetrabutylammonium cation as the main phase-forming component, which can act as a counterion in the extraction of anionic forms of the dye. In addition, it is likely that the efficiency of dye extraction in the TBABr–H2O–(NH4)2SO4 system is positively affected by the high water content of the extracting TBABr-enriched phase: 36.7 wt % or 91 mol % (at a TBABr concentration of 0.075 M in the initial solution and 33 wt % (NH4)2SO4, as determined by the Karl Fischer titration). The water content of this phase remained almost unchanged in a range of pH from 3 to 8. It is likely that, in this case, the transfer of highly hydrophilic/hydrated compounds from the aqueous phase to the extractive phase with high water content does not require significant dehydration of the extracted compounds.

The independence of the extraction efficiency from the ionization state of the solute in the aqueous phase was observed earlier in the two-phase tetrahexylammonium bromide–water system [34] and in systems based on hydrophilic–hydrophobic ionic liquids [45]. All these systems are characterized by a high concentration of water (at least 68 mol %) in the extracting phase [34, 45] and the quantitative extraction of hydrophilic compounds.

We found that the dyes Allura Red, Azorubine, Tartrazine, Sunset Yellow, and Fast Green were quantitatively extracted in a concentration range from 1 × 10–5 to 1 × 10–2 M; the degree of extraction was not lower than 98% at pH 5.2–5.9. As can be seen in Fig. 4b, a extraction efficiency of at least 99% was achieved after 5 min. High recovery rates of 92–98% were observed for all dyes at a phase contact time of 1 min. A high rate of extraction favorably distinguishes the TBABr–H2O–(NH4)2SO4 system from extraction systems in which quantitative extraction is achieved upon a longer phase contact or at a higher temperature [26, 28, 46].

A high volume ratio between the aqueous and organic phases makes it possible to obtain a high preconcentration factor. However, the separation of a small volume of a low-viscosity extract having a density lower than that of water from the aqueous phase for the subsequent analysis is a significant problem. To separate and collect a small-volume extract, the sample was filtered through a glass funnel the neck of which was densely filled with nonwoven polypropylene [25], which let water through but retained the “organic” phase. The filtration procedure took no more than 30 s and made it possible to avoid the stage of centrifugation. High porosity due to the presence of voids between the fibers of the material and the oleophilic and hydrophobic properties of the material ensured the retention of an extract, whereas the aqueous phase freely (no pressure or vacuum required) passed through the NWPP filter.

Because the extract is highly soluble in water that does not contain a salting-out agent, it was washed off the polypropylene filter with 2.0 mL of distilled water after the separation of phases. This eluant volume ensured the quantitative extraction of dyes from the polypropylene filter, and it was optimal for the subsequent spectrophotometric determination.

Determination of dyes in drinks, liquid media, and the shell of a drug. The chosen conditions for the extraction of dyes in the TBABr–H2O–(NH4)2SO4 ABS (cTBABr = 7.5 × 10–2 M, \({{c}_{{{{{({\text{N}}{{{\text{H}}}_{4}})}}_{2}}{\text{S}}{{{\text{O}}}_{4}}}}}\) = 33 wt %) were used for their extraction–photometric determination. Because a significant volume of a concentrated solution of the salting-out agent (NH4)2SO4 should be added to produce the ABS, solid ammonium sulfate (an accurately weighed portion) was added in an amount corresponding to the condition of ABS formation to provide quantitative extraction for the determination of dyes in the samples.

For the construction of calibration graphs and the determination of dyes, a 1.0-mL aliquot portion of a standard dye solution (or 1.0–10.0 mL of the analyzed sample) was placed in a 15-mL plastic test tube, and distilled water was added. The total volume was 10.0 mL. Ammonium sulfate (4.5 g) and 0.5 mL of a 1.5 M solution of TBABr were added. The test tubes were shaken for 1 min; then, the phases were separated by filtration through a funnel filled with NWPP. The extract held on the polypropylene filter was eluted with 2.0 mL of distilled water. The absorption spectra of the eluate were recorded and the absorption was measured. A similar procedure was performed for blank solutions.

Table 1 characterizes the procedure used for the extraction–photometric determination of dyes (the calibration equations, the linearity ranges, and the limits of detection cmin). The calibration functions for each dye were plotted as the dependences of the absorption of the organic phase (extract diluted with water or eluate) on the concentration of a dye (mg/L) at 503, 519, 426, 481, and 609 nm for Allura Red, Azorubine, Tartrazine, Sunset Yellow, and Fast Green, respectively. The limits of detection were calculated using the 3s test. The relative standard deviations obtained in three repeated experiments (n = 3, P = 0.95) at a dye concentration of 1.0 mg/L were 0.03, 0.04, 0.05, 0.04, and 0.05 for Allura Red, Azorubine, Tartrazine, Sunset Yellow, and Fast Green, respectively.

Table 1. Analytical characteristics of the spectrophotometric determination of dyes after extraction in the aqueous biphasic system of tetrabutylammonium bromide and (NH4)2SO4 (n = 3, P = 0.95)
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The dyes Allura Red, Sunset Yellow, Fast Green, Azorubine, and Tartrazine were determined in “Lifeline” and “Mirinda” drinks, a “Listerine” mouthwash, an Easter kit, and the drug “Ibuprofen” (Table 2 summarizes the compositions of the test materials declared by the manufacturers). Conditions for the sample preparation of all test materials are described in the Experimental section and in Table 2.

Table 2. Composition of the test materials and preliminary sample preparation
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The interfering effects of sucrose, ascorbic acid, and citric acid, which are usually contained in various foods and drinks in large quantities, on the extraction of dyes were studied at molar concentration ratios between dyes and interfering components from 1 : 1 to 1 : 1000. It was found that, even at a 1000-fold excess of an interfering component with respect to the dye, the degree of extraction of dyes in the TBABr–H2O–(NH4)2SO4 system was no lower than 92%.

The accuracy of the extraction–photometric determination procedure was checked using the standard addition method (Table 3). The dyes were identified by comparing the molecular absorption spectra of extracts obtained after the extraction of a dye from a standard solution and from the analyzed material. As can be seen in Fig. 5, the shapes of the spectra and the absorption maximums of the extracts coincided.

Table 3. Results* of the extraction–photometric determination of dyes in various test materials (n = 3, P = 0.95)
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Fig. 5.
figure 5

Molecular absorption spectra of the extracts of (a) Allura Red, (b) Sunset Yellow, (c) Fast Green, (d) Tartrazine, and (e) Azorubine: (1) after extraction from a standard dye solution and (2, 3) after dye extraction from the test material; (e) (2) Easter kit and (3) “Ibuprofen”.

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Table 3 summarizes the results of the determination of dyes in the samples by the standard addition method; these data indicate the correctness and good reproducibility of the procedure. The concentration of Azorubine in the shell of an Ibuprofen tablet found using the proposed extraction–photometric procedure was 5.3 ± 0.4 μg (n = 3, P = 0.95), which is consistent with the data declared by the manufacturer within the confidence interval (Table 2).

CONCLUSION

Thus, the TBABr–H2O–(NH4)2SO4 aqueous biphasic system provides quantitative extraction of the dyes Allura Red, Azorubine, Tartrazine, Sunset Yellow, and Fast Green from aqueous solutions; it can be easily prepared based on available reagents without the use of toxic organic solvents. The procedure proposed for the extraction–photometric determination of dyes is characterized by simplicity, rapidity, availability of the equipment, and ease of implementation in the laboratory.