Supplementary Materials for Article Solvent and Emulsion Extractions of Gallic Acid By

SUPPLEMENTARY MATERIALS FOR ARTICLE “SOLVENT AND EMULSION EXTRACTIONS OF GALLIC ACID BY TRIBUTYLPHOSPHATE: MECHANISMS AND PARAMETRIC STUDY” K.H. YIM; M. STAMBOULI; D. PAREAU

1. Analysis of gallic acid

1.1. Method

Gallic acid was analyzed bythe classical Folin-Ciocalteu method. 1 mL of sample was introduced into a test tube. 2 mL of Folin-Ciocalteu reagent (200mmol.L-1) and 2 mL of Na2CO3(1 mol.L-1) were added. The tubes have been heated in a water bath at 50°C for five minutes.

The absorbance of the resulting solution was measured with a Varian Scan 50 visible spectrophotometer at 760 nm. Standard solutions were prepared with solid gallic acid dissolved in water or in phosphate solutions (H3PO4, NaH2PO4 and Na2HPO4), at different pH. The blank was either pure water or the same phosphate solution. Beer-Lambert Law was used for the estimation of the concentrations:

A = ε*L*C (1)

where A is the absorbance, ε the molar extinction coefficient, L the optical path (here 1 cm) and C the concentration of gallic acid.

1.2. Results

After addition of Folin-Ciocalteu reagent, gallic acid was oxidized and blue oxides of tungsten-molybdenum were obtained, whose absorbance A was read at 760 nm (± 0.03). When gallic acid was dissolved in distilled water, the resulting pH was about 4. The absorbance was proportional to the concentration of total gallic acid in water until high values (A = 2), according to the Beer Lambert law (Figure 1).

Figure 1: Beer Lambert law of gallic acid in water at 760 nm (Folin-Ciocalteu method)

The slope of the obtained straight line is 3478 L.mol-1 (r²=0.9995), giving a molar extinction coefficient of 3478 L.mol-1.cm-1 as the optical path is 1 cm.

Figure 2: Spectra of gallic acid solutions (294 µmol.L-1) in water and in Na2HPO4 0.125 mol.L-1 (Folin-Ciocalteu method)

Figure 2 shows the spectra obtained for the same concentration (294 µmol.L-1) of gallic acid in water and in a phosphate solution. An increased absorbance is observed for the phosphate solution, suggesting the formation of new oxides of tungsten-molybdenum.

Two parameters were studied for phosphate solutions: total phosphate concentration and pH. The figures 3 and 4 respectively show influences of pH at a total constant phosphate concentration (125 mmol.L-1) and of Na2HPO4 concentration on the absorbance.

Figure 3: Molar extinction coefficient of oxides of tungsten-molybdenum obtained from gallic acid in 0.125 mol.L-1 phosphate solutions at different pH (Folin-Ciocalteu method)

Figure 4: Molar extinction coefficient of tungsten-molybdenum obtained from gallic acid for different Na2HPO4concentrations (Folin-Ciocalteu method)

The molar extinction coefficient ε at 760 nm varies with pH, as shown on figure 3. For pH lower than 4.5, ε is constant at about 3500 L.mol-1.cm-1 as for pure water: there is no influence of phosphate on the absorbance as the major phosphate species is H2PO4-, unable to react with gallic acid; the reference can then be gallic acid in distilled water. For pH greater than 6, i.e. pH from which HPO42- significantly appears in the solution and reacts with gallic acid, the molar extinction coefficient at 760 nm increases.

Moreover, the molar extinction coefficient depends on the total phosphate concentration at constant pH equal to 9.5 (figure 4). It increases until a level of about 4000 L.mol-1.cm-1 and remains constant for Na2HPO4 concentrations greater than 200mmol.L-1. So, the analysis of gallic acid in Na2HPO4 solutions has to be calibrated with standard Na2HPO4 solutions.

2. Liquid-liquid extraction: study of stripping parameters

2.1. Method

Two parameters were studied: the concentration of the stripping agent, [Na2HPO4] and the volume ratio of the organic phase and the stripping phase, Vorg/VS.

The efficiency was measured by the stripping yield:

(2)

with morg,0, the gallic acid mass in the initial organic phase and mSthe gallic acid mass in the stripping solution.

The operating procedure was the following:

Preliminary extraction step: 200 mL of gallic acid aqueous solution (294 µmol.L-1) and 200 mL of organic phase (TBP 10% v/v in dodecane) were agitated and settled in a separatory funnel.

Stripping step: 20 mL of the obtained organic phase (light phase) were put in a new separatory funnel with different volumes of Na2HPO4250mmol.L-1 and with 20 mL of solutions containing different concentrations of Na2HPO4.

2.2. Results

The effect of Na2HPO4 concentration on stripping was studied. The results are given on figure 5, in terms of stripping efficiency. There is not significant influence of Na2HPO4 concentration in the range studied: stripping was in all cases quantitative due to the large excess of the stripping reagent compared to gallic acid, pH remaining at high values. The smallest concentration 0.05 mol.L-1 can then be used for a quantitative stripping.

Figure 5: Influence of [Na2HPO4] on stripping of gallic acid

(10% v/v TBP and Vorg/VS = 1/1)

The influence of the volume ratio Vorg/VS was studied too (Figure 6). In the range studied (1 to 10), there is only a slight influence of this parameter on the stripping efficiency for the same reason as before. A significant concentration factor can then be obtained in the stripping step (till 10).

Figure 6: Influence of Vorg/VS on stripping of gallic acid

(10% v/v TBP and [Na2HPO4] = 0.25 mol.L-1)