Voltammetric determination of Zn(II) in Zn-Fe alloy electroplating baths using square-wave voltammetry

Journal of the Brazilian Chemical Society

Favaron, Regiane¹  Universidade Estadual de Campinas, Campinas, Brazil¹  Aleixo, Luiz M.¹ 

关键词: zinc (II) determination;    square-wave voltammetry;    Zn-Fe alloy baths;   

DOI  :  10.1590/S0103-50532001000200008

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The requirements for good resistance to corrosion in the automotive and electronic industries have increased during recent years. Metallic electrodeposition techniques to protect the base metal from corrosion is now not sufficient. The protective layer must remain without alterations for a specific period. However, in the long-term, these materials do loose these characteristics and compromise the function of the metal piece1. Recently, the use of electrodeposited alloys has been developed, because this process increases the resistance of the metal to corrosion, relative to the actual processes of electrodeposition2.For example, we can have alloys based on Zn-Fe3-4, Ni-Fe5, Zn-Ni6 and Zn-Co7. The Zn-Fe alloy is the most used in electroplating procedures due to its low cost. The resistance of this alloy to the corrosion process, in relation to electrodeposited layers such as zinc (II), can triple, becoming significantly advantageous. Metal resistance to corrosion occurs when these metals are in a given concentration range, but at higher or lower concentrations than those required, they present undesired effects 8. Thus, to avoid further problems, it is necessary to carry out a periodic monitoring of the concentrations of these metals to keep them within the working limits.In general, zinc determination is carried out by a complex-formation titration employing EDTA9. It has been observed that this technique can lack precision due the presence of Fe3+, Pb2+, Cu2+ and others metals ions in the sample. Also, if the zinc (II) concentration in the alloy bath is lower than 4.0 g L-1, the results obtained are also inaccurate10. Other analytical techniques have been used for zinc determination in alloy galvanic baths, such as potentiometric titration in zinc baths made with zinc sulphate and boric acid (known commercialy as the acid zinc bath)11, the use of an ion selective eletrode based on tetradecylphosphonium12 for zinc determination in cyanidric and non cyanidric baths, and flow injection analysis using as the working electrode a chloride ion selective electrode in slightly acidic zinc baths13. Voltammetric analysis has recently been employed in metal determinations in alloy galvanic baths because this technique is very simple, fast and has low detection limits for several metals, mainly when more than one metal coexists in the alloy bath. This coexistence occurs in alloy baths when one or more metals are in the solution, acting as contaminants in the electrodeposited layer14-18.The aim of this paper is to report the development of a rapid analysis method, without previous treatment, for zinc (II) in alloy galvanic baths, using the Osteryoung square-wave voltammetric technique19, chosen due to the speed of the analysis. Different supporting electrolytes, as well different electrolyte concentrations, pH and sample volume were studied to evaluate the principal analytical parameters that could affect the voltammetric analysis. ExperimentalReagentsThe stock solution of Zn2+ (0.0940 mol L-1) was prepared from zinc (II) chloride (Merck), acidified with HCl and standardized with 0.0989 mol L-1 EDTA20 (disodium salt from Merck). The other zinc (II) standard solutions used in the experiments were prepared daily from stock solution as follows: 0.940 x 10-3 mol L-1 Zn2+ (dilution 1:100), 1.87 x 10-3 mol L-1 (dilution 1:50) and 3.74 x 10-3 mol L-1 (dilution 1:25). For the zinc (II) determination, 0.1 and 0.2 mol L-1 citric acid (Merck) solutions at pH values of 2.0, 3.0, 4.0 and 5.0, as well as a 1.0 mol L-1 NH3 / 0.20 mol L-1 NH4Cl (Merck) solution were used as supporting electrolytes. Standard solutions of some metallic ions for interference studies were suitably prepared using copper (II), manganese (II) and zinc (II) sulfates (Carlo Erba), lead (II) nitrate and chromium (III) chloride (Vetec).The test alloy galvanic bath was prepared with 3.0 mol L-1 NaOH solution, 1.0 x 10-1 mol L-1 zinc (II) and 9.0 x 10-4 mol L-1 iron (III) solutions, besides organic additives, according to the descriptions in a Technical Bulletin from Atotech of Brazil21. The iron added to the alloy bath was as an organic iron(III) complex, combined with different organic additives. Real bath samples were obtained from the electroplating industries. All reagents utilized were of analytical grade and all solutions used in the experiments were diluted adequately in deionized water.ApparatusThe voltammetric measurements were made with a PAR model 384B polarographic analyzer coupled to a PAR polarograph stand model 303-A, using a three electrode cell: static mercury drop electrode (SMDE) as working electrode (the large size mode of the mercury drop electrode adjusted by the polarographic stand was used), Ag|AgCl (saturated KCl) as reference electrode and a platinum wire as counter-electrode. The technique employed in the Zn2+ analysis was the square-wave voltammetry and the voltammograms were recorded on a DPM-40 digital plotter from Houston Instruments.The pH adjustments of the solutions were performed with a Radelkis OP-271 pH/ion analyzer (Hungary). For pH measurements, an OP-808P Radelkis glass electrode (Hungary) was used. The volume additions in the polarographic cell were carried out with pipettes (Finnpipette) from 20 mL to 5.00 mL.Analytical procedureThe voltammetric cells were prepared with the addition of 5.00 mL of supporting electrolyte, followed by deoxygenation of the solution by bubbling ultra-pure nitrogen for 10 min. Appropriate volumes of either Zn2+ standard solutions or bath samples were put into the cell followed by 2 more min of deoxygenation with nitrogen. The voltammograms were then registered over a potential range between -0.80 and –1.50 V vs. Ag|AgCl (satureted KCl). The optimized voltammetric conditions were: scan rate of 200 mV s-1, pulse height of 20 mV, pulse frequency of 100 Hz. Results and DiscussionZinc voltammetric determinationThe Zn-Fe alloy galvanic baths have about 15 mL per liter of organic additives, called purifiers, brightness and levellers. When the concentration of these additives are over this value they interfere in the complexometric determination of zinc using EDTA, giving rise to errors in the analysis. In the method described in this work there is no need for prior treatment to eliminate interferences, because the organic additives do not yield voltammetric peaks in the potential region near the potential of the Zn (II) voltammetric peak.Zinc (II) generates well-defined peak potentials in different supporting electrolytes and is reduced at a more cathodic region than other metals. Thus, its determination can be easily carried out in the presence of many potentially interfering metal ions. Acidic or alkaline supporting electrolytes can be used, presenting a half-wave potential for zinc (II) between –1.0 V and –1.3 V vs. Ag|AgCl. In the

【 授权许可】

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