A stock solution of AuCl4- was prepared by dissolving weighed amounts of the solid in water

23 CHAPTER 2

Material and methods

2.1 Materials

All the chemicals not expressly cited are of analytical grade. All the reagents were employed without further purification. Water, purified by pumping demineralized water through a Milli-Q apparatus, was used to prepare the solutions and also as the reaction medium.

2.1.1 Metals

Gold was used in the form of hydrated chloroauric acid, HAuCl4•3H2O, purchased from Sigma-Aldrich. A stock solution of AuCl4- was prepared by dissolving weighed amounts of the solid in water.

Copper was used in the form of hydrated copper perchlorate, Cu(ClO4)2•6H2O, purchased from Fluka. Stock solutions were prepared by dissolving weighed amounts of solid in water; the concentration of copper stock solution was evaluated by titrating with EDTA.

2.1.2 Ligand

The ligand pyridine-2-azo-p-dimethylaniline (PADA), which structure is shown in Figure 2.1, was obtained from Sigma-Aldrich. Stock solution was prepared by dissolving weighed amounts of solid in water. During the preparation of the stock solution 1% of ethanol was added to increase the ligand solubility.

Figure 2.1 Molecular structure of pyridine-2-azo-p-dimethylaniline (PADA).

N

N N N

CH₃ CH₃

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2.1.3 Surfactans

Sodium dodeclysulphate (SDS) was from Sigma-Aldrich and dodecyltrimethylammonium chloride (DTAC) was from Fluka. The structures of surfactant monomers are shown in Figure 2.2. In both cases stock solutions were prepared by dissolving weighed amounts of solid in water.

CH3 O

S

O O

O

(a)

(b)

Figure 2.2 Molecular structures of sodium dodecyl sulphate (a) and dodecyltrimethyl ammonium (b).

2.1.4. Ultrafiltration membranes

Ultrafiltration membranes made of regenerated cellulose (YM 3, Millipore) of 44.5 mm diameter and with a molecular weight cut-off (MWCO) of 3000 daltons were used. Ultrafiltration membranes having higher MWCO could also be used taking into account the aggregation number of SDS (Yoshikiyo et al., 1987) and DTAC (Bales and Zana, 2002), and this will result in a increase in the permeate flux. The change of MWCO has, however, only a minor effect on the metal rejection. The membranes were pre-treated and stored according to the method recommended by Millipore.

2.2 Methods

2.2.1 Spectrophotometry

A Perkin-Elmer Lambda 35 spectrophotometer was used to record absorption spectra and to perform spectrophotometric titrations. The apparatus is

CH3 N CH3

CH3

CH3

25 equipped with jacketed cell holders with temperature control and all the measurements were made within 25 ± 0.1 °C.

The spectrophotometric titrations, aimed to evaluate the dissociation constants of pyridine-2-azo-p-dimethylaniline in the presence of surfactants (SDS and DTAC) were performed as follows: increasing volumes of HCl solution were added with a microsyringe (Mitutoyo) to 2.0 ml of the ligand solution directly in the spectrophotometric cell, kipping the surfactant concentrations constant at the desired value during titration. After each addition of acid e spectra of the solution was recorded.

Complex formation reactions between gold(III) and pyridine-2-azo-p- dimethylaniline, were investigated in the absence and in the presence of surfactants (SDS or DTAC) by adding with a microsyringe (Mitutoyo) increasing volumes of metal solution to 2.0 ml of the ligand solution directly in the spectrophotometric cell where the acidity, ionic strength and concentration of surfactant (when present) were kept constant. Also in this case spectra were recorded after each addition of metal.

2.2.2 Kinetics

The kinetics of the reaction of complex formation between gold(III) and PADA in water, SDS and DTAC are rather complex. They exhibit kinetic effects sufficiently slow to be studied by the spectrophotometric method. The reaction was started by adding a known amount of HAuCl4 to a known volume of PADA already contained in the spectrophotometric cell at prefixed value of acidity and ionic strength. The course of the reaction was monitored at 633 nm.

The reaction investigated also displays fast kinetic effects; hence we had to resort to the stopped-flow method. The apparatus, assembled in our laboratory, uses a Hi-Tech SHU-61SX2 stopped-flow sample handling unit capable of rapidly mixing two reagents (Figure 2.3). The two reactant solutions were rapidly mixed by being forced into a mixing chamber, and then through an observation cell (Figure 2.4). The flow of the reacted mixture was suddenly stopped, and the reaction course monitored using a suitable spectroscopic

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probe, such as absorbance. The change in the spectroscopic signal as a function of time is recorded. This technique enables reactions taking place between milliseconds and many minutes time range to be studied.

Figure 2.3 HI-TECH scientific mixing unit; front and rear views.

Figure 2.4 Internal scheme of a stopped flow apparatus. A) Light input from monochromator and mirror box, B) light output to photomultiplier tube, 1) plunger, 2) syringes, 3) reservoir syringes, 4) mixing chamber, 5) cell, 6) thermostating block, 7) drain tube, 8) drain valve block, 9) stop syringe, 10) adjustable stop nuts, 11) mechanical stop, 12) leaf-type trigger switch.

The reaction transient is stored in a Tektronix 2210 digital oscilloscope. The kinetic curves were analyzed using a non-linear least square procedure provided by Jandel (Oregon).

27 2.2.3 Ultrafiltration

The MEUF and LM-MEUF experiments were carried out in batch stirred cells (Amicon, model 8050), as that shown in Figure 2.5, with a capacity of 50 ml and an effective membrane area of 13.4 cm². The extraction procedure is the following: the cell was initially loaded with 20 ml of solution under an applied nitrogen pressure of 1 bar; the permeate solution was collected until 2 ml of the retentate solution remained.

Figure 2.5 Schematic view of ultrafiltration cell.

In the stripping procedure the retentate was then mixed with 10 ml of stripping solution and then a second ultrafiltration step was performed which allows separation of the metal-containing phase (the permeate in this case) from the surfactant-containing phase (the retentate); this procedure enabled us to collect 10 ml of permeate solution. Shortly after the conclusion of the procedure the ultrafiltration membranes were flushed with deionized water and, if necessary, they were regenerated according to the method recommended by Millipore (Amicon). The amounts of metal ions extracted or recovered by ultrafiltration were assessed through atomic absorption spectroscopy. In any case, the only permeate was analyzed, while the metal ion in the retentate was obtained by difference between the metal ion in permeate and in initial solution. The reproducibility of concentration measurements in the extraction and stripping experiments was within ± 2 % and 4% respectively.

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2.2.4 Atomic Absorption Spectroscopy

A Perkin Elmer HGA-800 atomic absorption spectrophotometer was used to measure the gold ions concentration in permeate. Samples were atomized in an air/acetylene flame; in the case of low concentration of gold a graphite furnace was employed too. The light source was a hollow-cathode lamp of gold.

2.2.5 pH measurements

A Metrohm 713 pH-meter equipped with a combined glass microelectrode was used in order to measure the pH of the solution. The electrode was calibrated automatically employing buffers in the 4-9 pH range and solution of known HCl concentration (below pH 4).