High current–density anodic electrodissolution in flow–injection systems for the determination of aluminium, copper and zinc in non–ferroalloys by flame atomic absorption spectrometry

An automatic procedure with a high current-density anodic electrodissolution unit (HDAE) is proposed for the determination of aluminium, copper and zinc in non-ferroalloys by flame atomic absorption spectrometry, based on the direct solid analysis. It consists of solenoid valve-based commutation in a flow-injection system for on-line sample electro-dissolution and calibration with one multi-element standard, an electrolytic cell equipped with two electrodes (a silver needle acts as cathode, and sample as anode), and an intelligent unit. The latter is assembled in a PC-compatible microcomputer for instrument control, and for data acquisition and processing. General management of the process is achieved by use of software written in Pascal. Electrolyte compositions, flow rates, commutation times, applied current and electrolysis time were investigated. A 0.5 mol l-1 HN03 solution was elected as electrolyte and 300 A/cm2 as the continuous current pulse. The performance of the proposed system was evaluated by analysing aluminium in Al-alloy samples, and copper/zinc in brass and bronze samples, respectively. The system handles about 50 samples per hour. Results are precise (R.S.D. < 2%) and in agreement with those obtained by ICP-AES and spectrophotometry at a 95% confidence level.


Introduction
Aluminium and brass alloys are widely employed in automotive, aeronautic and railway industries, in the electric packaging section, civil and mechanical engineering, etc. due to the favourable physical characteristics of this metal, e.g. low-density, high resistance/weight ratio, high resistance to corrosion, high electric and thermal conductivity, suitability for superficial treatments, etc. [1][2][3].
According to a recent review made by Dulski [4], the analytical techniques usually employed in industrial quality control are optic emission spectroscopy (AOES), X-ray fluorescence spectroscopy (XRF), inductively-coupled plasma mass spectroscopy (CP-MS), inductively-coupled plasma atomic emission spectrometry (ICP-AES), atomic absorption spectrometry (AAS) and spectrophotometry. The first two tools are characterized by relatively specific and fast techniques, since the analysis is performed directly on the metal surface. The others, in spite of their good sensitivity, simplicity and low operational cost, determine elements only in liquid samples [5,6]. Among the available techniques for direct metallic alloy analysis are spark and AC/DC arc [4,7,8], glow discharge (GDL) [4,9] and laser ablation [4,10,11].
Although efficient, these accessories are relatively expensive and hardly portable. The anodic electro-dissolution approach (AE) [12][13][14][15] has been suggested as an alternative procedure for solid sample dissolution due to its simplicity, low operational cost, portability and feasibility of coupling with flow-injection analysis (FIA) [16]. On-line metallic sample dissolution, based on FIA-ED coupling, is very attractive in analytical routine work involving spectrometric or spectrophotometric determinations [17,18]. In spite of the wide applicability of ED technique, the required solid standards for building analytical curves are the Achilles' tendon of this approach [13][14][15]. This cumbersome procedure can be circumvented by using an electro-dissolution cell which operates at close to 100% current efficiency. Although existing electro-dissolution cells with different configurations have been used for analytical routine work [12-15, 17, 19], an electrochemical cell operating at high current efficiency has not yet appeared in scientific literature. This study reports on a new procedure for determining A1, Cu and Zn in non-ferroalloy samples by exploiting the approach of high current-density anodic electrodissolution (HDAE) for on-line metallic sample dissolution in a flow-injection system.

Experimental
Reagents, standards and samples All solutions were prepared with pro-analysis chemicals and distilled-deionized water (Milli-Q, Millipore). Zinc stock solution (100rag l -) was prepared by dissolving 100mg Zn powder (Aldrich, 99.99%) in 10ml concentrated nitric acid. The resulting solution was diluted to 1000 ml with water.
Before the analysis, the metallic samples were treated by polishing their surfaces with sandpaper #400. After, they were washed thoroughly with water.
The flow-injection system A Hitachi Model Z-8200 atomic absorption spectrometer, a home-made multi-channel peristaltic pump circuit IC1 was used in the configuration of a power driver in order to provide an interface between the current source and the solenoid valves (Vl-V4) and to the microcomputer. In the circuit of the current source, LM350T (IC2) was used, a positive voltage regulator of three terminals capable of supplying up to 3.0 A of maximum current. This device was used as a current precision regulator by connecting resistors RI-R4 between pins and 2 of this component. Each resistor defines the level current supplied by the circuit. The binary combination of 24 of bits 2-5 of the parallel port of the microcomputer (C) sets at a low level the exit pins of IC1 (1-4), saturating transistors T-T4, which, switching the resistors with IC2, supplies the 16 current levels demanded by the process.
The electro-dissolution cell (EC, figure 2) used in this work was based on that proposed by Souza et al. [18]. It was machined in a perspex block acrylic resin. The cell compartment consisted of a ml well, machined in a resin block.
A 2 mm hole in the bottom of the well was made in order to fix a silver needle (cathode). The electrolyte solution was injected into the cell through this electrode. After the electrolysis step, the inner solution was removed from the EC to the chamber H by a hole located on the lower right-side. A mm hole in the top of the cell promoted the gas purge originated during the electrolysis. The circular site engraved on the upper surface of the chamber with a latex ring avoided the leakage of solution when a sample was positioned in the chamber.
The adjustable sample holder was used for entrapping samples of variable thickness on the electro-dissolution chamber. This procedure allowed accommodation of samples with a height of up to 5cm. Electrical contact was made by means of this metallic holder.

Procedure
The flow diagram of the system used for the determination of aluminium, copper and zinc in non-ferroalloys is depicted in figure 2. It consists of a solenoid valve-based commutation (V]-V4) in a flow-injection system for online sample electro-dissolution and calibration with one multi-element standard. After pre-treatment, the solid sample is positioned in the EC chamber and clamped by the adjustable holder. The start of an analytical cycle begins through a microcomputer command which activates the peristaltic pump (P) for approximately 30s. This period is necessary for cleaning the transmission lines, removing the air bubbles and stabilizing the flow rates. Thereafter, the system calibration is carried out by opening the solenoid valve V4 for various periods of time.
In this situation, each multi-element standard volume undergoes a time-based injection into a carrier stream (water), and the established plug is directed towards the detector (FAAS). The passage of the processed aliquot through the spray chamber results in a large dispersion of the analyte. Transient atomic signals are measured as a peak with height proportional to the analyte content in the injected solutions. After maximum peak measurement, another cycle starts. With this procedure, only one order to obtain different aliquots of the standard solution to be injected into the system. The signal-time diagram of an analytical cycle involving electrolysis is depicted in figure 3(b). The peristaltic pump is switched on again for 30s by a P pulse at high level. Two seconds after P, a pulse P2 opens the solenoid valve V] for 7 s and electrolyte solution (E) is injected into the EC. Three seconds after P, another pulse P3 turns the electro-dissolution source on for 5 s. The P3 interval was longer than the electrolysis time in order to avoid the absence of electrolyte solution during electrolysis. After 7 s, P2 and P are at a low level again.
The pulse P4 is set at a high level and this opens the solenoid valve V2 for 6 s. Air is directed towards EC and helps the electro-dissolved material removal from this chamber to the H chamber. This is achieved by aspirating the electro-dissolved material through the bottom of

Results and discussion
The flow diagram depicted in figure 2, and used for A1, Cu and Zn determinations in brass and Al-alloy samples, was designed as a compromise between system hydrodynamic pressure, electrolyte residence time and flow rate. Thermal oscillations markedly affected the conditions of direct current established at the beginning of the procedure because of the enormous dependence of electrolyte ionic conductivity on temperature [21]. The electrolyte, E, flowing through the cathode, is introduced into EC and reaches the sample (anode) in jet form.
Besides dissipating the heat generated in the inter-electrodic region [21,22], the higher rotation speed of the peristaltic pump promotes the continuous removal of the product of reactions from the anodic surface. The occurrence of secondary reactions is then drastically reduced [23]. During electrolysis, the concentration of the metallic ion produced in adjacencies of the anodic surface is very different from that produced in the bulk of the solution. The establishment of a concentration gradient implies the reduction of ionic mobility and conductivity of solution because the mass transport is governed by a pure diffusion mechanism [18]. Under conditions of high flow rate and low residence time of the electrolyte, the diffusion layer is minimized and the conductance is kept almost constant.
The influences of the current intensity and the electrolysis time on the amount of electrochemically removed material are presented in figures 4   In both cases, the efficiency of the process increased with either current increase or electrolysis time. This effect was based on the Faraday law [24] which indicates that the quantity of the electro-dissolved mass depends directly on the Coulomb quantity. Aiming to maintain a compromise between analytical rate and low current, the subsequent studies were carried out with 250 mA current and 3 s electrolysis time.
With regard to selected Al-alloy samples, iron, copper, magnesium, lead, tin and silicon are the majority constituents. Alloys having very different electrode potentials present preferential dissolution of the metal with smaller reduction potential. This is very critical for low-current density conditions. On the other hand, high-density current conditions minimize this effect, since the ddp between anode and cathode is sufficiently high to guarantee adequate potentials, making the dissolution of the constituents easily attainable. As the current density and 20 inter-electrodic distance are inversely proportional, higher efficiency in the electro-dissolution process was observed when the inter-electrodic distance was maintained at 500 gm. This permitted the performing of the electrolysis under high-density current conditions. The electrochemical cell configuration adopted in this work has been efficiently applied to the routine analysis of nonferrous alloys since current densities up to 600 A/cm 2 are easily attained. In this way, the on-line anodic electrodissolution proposed by Bergamin et al. [15] was improved in electro-dissolution systems involving highefficiency currents, as suggested by Souza et al. [18].
In the passive region of potential, aluminium is one metal which is characterized by the formation of self-protecting films with extremely low conductivity [22]. At high anodic potentials, the thickness of the film can increase significantly. Indeed, aluminium anodization depends on the electrolyte composition. This can originate film of several gm thickness which can stand a potential difference higher than 100 V [22]. The aluminium dissolution which uses high-density current (transpassive region) involves localized rupture of the passive film when concentrated acids are involved [22]. In this sense, the electrolyte composition is a relevatt parameter in this procedure. Strong inorganic acid sblutions were elected as electrolytes because they avoid the formation of insoluble products. The presence of this anodic mud in the flow-system may result in line-clogging. The influence of the nature of the electrolyte on the analytical signal is depicted in figure 6.
Sulphuric acid solution was the worst electrolyte. This is probably due to its characteristic passivity. Although the signals obtained with hydrochloric acid-based electrolytes were higher than those obtained with sulphuric acid, these solutions also presented passivity properties. Even at high concentration and current density, chloride ions generated a thin layer on the electrolysed sample surface. When the electrolyte was a nitric acid solution,  The system calibration in direct solid analysis based on the electro-dissolution technique can be carried out by using aqueous standards instead of solid ones. The main requisite is an electrochemical cell operating at high anodic-current density in order to permit sample dissolution at high removal rate. In this way, the calibration of the system with aqueous standards was elected. However, the conventional manual preparation of standard solution sets is laborious and time-consuming, so an autodiluter system was adopted as an alternative analytical tool. This calibration procedure combined the use of FIA ing to 250 and 6000 ms g 4 opening time. As the opening time for the introduction of a processed sample volume is variable, the following protocol is used for the analyte quantification: for a given V3 valve opening time (ts; sample injection time) that produces a signal S1 should be fitted in the linear calibration range), select the V4 valve opening time on the calibration curve (tstd, standard injection time) that coincides with the Sl value; thereafter, by applying the law of proportions, the analyte concentration (Ca) can be computed by Ca--" (tstd/ts) Cstd.
The spectrometer readings were transferred to the microcomputer through a serial port. During data acquisition, the signals from both the analytical solution aliquots and the samples were stored in an ASCII file. After acquisition, the software computed the corresponding peak area, plotted the analytical curves and estimated the analyte concentration.
With the proposed system, aluminium was determined in several Al-alloy samples (table 1), and copper and zinc were determined in brass samples (table 2).
Results were precise (R.S.D. < 2%) and in agreement with those obtained by ICP-AES and spectrophotometry at a 95% confidence level. With this system, 50 samples could be run per hour. The on-line electrochemical dissolution with an auto-diluter system improved the sampling rate and quality of the analytical results by reducing the potential source of contamination which is present in conventional procedures.

Conclusions
With the HDAE approach, the aqueous standard could be used for calibration. HDAE-FIA-FAAS coupling can be efficiently applied to aluminium, copper and zinc determination in Al-alloy and brass samples in largescale analysis.