Electroplating can be defined as the deposition of a metallic coating through the subjection of an object with a negative charge to a solution containing a salt containing positive metal ions. Under the influence of an electric charge, the positive ions are attracted to the negatively charged object where they are reduced hence forming a coat on the surface of the negatively charged object. Electroplating employs the principle of electrolysis to apply a thin coat of metal onto another metal (Bird 2003). If two electrodes of zinc and copper are connected to a voltage supply and placed into a beaker containing copper nitrate on the copper electrode tank and zinc nitrate on the zinc electrode tank, electrons move from the positive to the negative electrode terminal in accordance with the electrochemical series. Usually the electrochemical series classifies electrodes in terms of their electrode potential (the potential difference measured with respect to a standard hydrogen electrode). Therefore, by knowing the emf of each electrode with respect to the standard electrode the electrochemical series is established and it details the sequence of metals according to their electric potentials.
When copper electrode is immersed in an electrolyte solution of copper and electric potential develops between the metal electrode and the solution. Some of the copper ions released may deposit on the metal itself in the case of a single cell. This leaves the metal electrode with a positive charge while the copper nitrate solution, a negative charge. The same reaction applies to zinc metal. In the event that the two reacting phases are brought together, an interphase electric potential difference develops. It is this development that is exploited in electrochemical plating.
Even though there exists a potential difference between each single metal and its electrolyte, experimental disregard these liquid junction potentials and instead focus on the metal solution interphases. Consider an electrochemical cell consisting of a zinc electrode immersed in a ZnNO3 solution on one side of the electrochemical cell and on the other a copper electrode immersed on CuNO3 solution. Contact between these two solutions is made possible by a fritted glass disk or salt bridge of KCl solution. The salt bridge aids in the passage of electrons without enabling the mixing of the two dissimilar solutions. When the two metal electrodes are connected through a voltmeter an electric potential is developed. This potential is the sum of the potential from the four sources: the copper-zinc junction where the voltmeter is connected to the zinc interphase; the zinc solution interphase; the junction between CuNO3 and ZnNO3 solution and lastly the solution copper interphase. In a galvanic cell, electrons move to the negative terminal (copper electrode) from the positive terminal (zinc electrode) through the salt bridge (Rieger 1993).
Industrial Uses of Electroplating
Generally this method is used in the deposition of layers of material like chromium to a combustion ampere of charges of up to 563V to produce the desired chemical and physical properties. This technique is also used in increasing thickness of undersized components of industrial products. Another application of electroplating is in the refinery of metals such as zinc and copper in the manufactures of currency coins. Besides, the technique is also utilized in the preparation of halogens such as chlorine on an industrial scale.
Factors Affecting the Rate of Electroplating
Catalytic reactions such as electroplating require energy to proceed. In most cases this energy is the voltage supplied to the reaction. Increase in voltage supply increases dissociation hence increasing the rate of the electroplating process. In some cases energy is supplied in form of heat usually as a result of the reaction mechanism. Temperature measurements are used as measures of energy supplied to the plating solution. However, energy can sometimes be independent of temperature. Usually like all variables, the electrolyte bath temperature is variable. The amount of energy determines the kinetics and the rates of deposition. Increase in temperature increases the rate of reaction and vise versa until an equilibrium is reached.
These rates can be established by taking measurements of the rate of the decrease in reactant concentration or alternatively the rate of increase in the concentration of reactants. In reactions where the concentrations of the reactants are kept constant the changes in temperature act as the only determinants of the reaction rate. Catalysts increase the reaction rate to the desired level by lowering the activation energy of the reaction.
The Effect of Surface Area on the Rate of Electroplating
Generally the rate of deposition increases with increase in the surface area of the plating surface. Deposition is often considered to occur at a single point and based on this assumption the rates of deposition on all surfaces is deemed to be equivalent. Even though the kinetics of deposition and the behavior of mass transfer may be equivalent in all the surfaces, the actual interfacial voltage on a plated surface drives the position rate. Interfacial voltage varies and is different from the center and the edges of the electrode.
Dependent and Independent variables
Variables in the experiment included; temperature, pH, reactant concentration, the surface area of the zinc electrode and the voltage input. Other variables include; impurities, build up products of the electrolytic reaction, loss of exaltants if any, the shape of the object being plated and finally the electrolyte and the position of the electrode in the medium.
The Reduction-Oxidation Reactions
Electroplating is an electrolytic process and electrolysis is a redox reaction. Oxidation occurs at the zinc electrode through dissociation of the elemental zinc to ionic form while reduction occurs at the copper electrode. On atomic and sub atomic scales, the zinc electrode releases electrons which flow through the external conductor constituting an electric current. These electrons are consumed by the copper electrode through the reduction of cupric ions. The voltmeter through which the electrons pass through measures the electric potential also called the electromotive force (emf). This potential can be a consequence of either;
The connection of the external electrode leads hence creating a closed circuit, making the cell generate an electromotive force. This case implies the operation of the galvanic/voltaic cell. The thermodynamic phenomenon is simply the transformation of chemical energy to electrical energy.
Alternatively, in the presence of a greater external potential which reverses the reactions, the reverse conversion of energy occurs. This is the case of an electrolytic cell.
In a spontaneous operation (the galvanic cell) or in an non spontaneous operation (electrolytic cell), oxidation occurs at the anode while reduction occurs at the cathode. In spontaneous operation, the zinc electrode is the anode and the copper electrode; the cathode. However, in a non spontaneous operation the zinc electrode becomes the cathode and the copper electrode becomes the anode.
According to the experimental set up, electroplating was done according to the non spontaneous operation owing to the inclusion of the 12V. For this reason the redox reactions that take place in the cell are;
Zn2+ + 2e- → Zn (Oxidation; electrons are lost)
Cu → Cu 2+ + 2e- (Reduction; electrons are gained)
Zn2+ + Cu→ Zn + Cu 2+ (Redox Reaction)
Zn|ZnNO3(0.2M)||CuNO3(0.2M)|Cu Designation of the electrochemical cell
Diagrammatic representation of the experimental set up
The oxidation-reduction reaction occurs due to the electron transfers across the electrode solution interfaces. With the presence of an additional voltage of 12V, the cell can be said to operate reversibly. The electrical energy produced in the reaction is free energy which constitutes the maximum work done; the work of expansion not included. With reference to the standard electrode potentials in a Daniels cell, the more positive the electrode is the more its oxidizing power. For this reason it is predictable that since Cu has greater oxidizing power, it will oxidize Zn. However, this argument is only systematic with a spontaneous reaction. In a non spontaneous reaction the reverse is true as the calculations of cell potentials will yield a negative value.
In an electrolytic reaction the zinc electrode is the cathode, it thus supplies electrons to the electrolyte which takes up the electrons and transfers them to the anode which accepts these electrons hence electrodeposition.
Due to the inclusion of an electrical charge, the reaction transforms into a non spontaneous operation with zinc as the cathode; the increase in the surface area of the zinc electrode results in a proportionate increase in the rate of electrodeposition.
Table: Surface Area, Mass Loss and Rates of Reaction
Rate of Reaction
Calculations of Rates of Reactions
Given that the change in concentration is directly proportional to the difference between the initial mass(which is constant for all the electrodes) and the final mass after the elapse of five minutes, the rate of the equation can be calculated as;
Rate = [Change in Reactant Concentration]
[Change in Time]
The rate at Surface Area:
2 cm2 = 0.057 = 0.0114
3 cm2 = 0.058 = 0.0116
4 cm2 = 0.060 = 0.012
5 cm2 = 0.064 =0.0128
6 cm2 = 0.067 =0.0134
Graphical Representation of the Surface Area against the Rates of Reaction
Results Evaluation and Conclusion
From the calculations of the effect of surface area of an electrode on the rate of electroplating and the subsequent presentation of these results in form of a graph, it is easy to discern that an increase in the surface area of the zinc electrode necessitates a proportional increase in the rate of reaction. An increase in surface are increases the interfacial voltage across the plating surfaces hence an increase in the rate of ionization of the zinc metal (dissociation of the solid zinc to ions in the ZnNO3 solution). An increase in the rate of dissociation leads to an increase in the concentration of Zn2+ in the electrolyte. These excess ions are taken up and transferred to the CuNO3 electrolyte where they drive the reduction process and consequently the zinc plating of copper.
The results therefore confirm the hypothesis which posited that due to the inclusion of an electrical charge, the reaction transforms into a non spontaneous operation with Zinc as the cathode; the increase in the surface area of the zinc electrode results in a proportionate increase in the rate of electrodeposition.
To improve the efficiency of the electroplating plating, cleanliness of the plates of metals is crucial since molecular layers of substances such as oil can prevent the adhesiveness of the coat. Standard guides for electroplating require that metals should be taken through solvent cleaning, electrocleaning, hot alkaline detergent cleaning or acid cleaning. Cleaning yields hydrophilic surfaces that are capable of retaining an unbroken water sheet that does not drain off or bead up. If cleaning is done using surfactants then through rinsing is necessary as these substances reduce the sensitivity of the electroplating process.
The current density; which results from the amperage of the current of electroplating divided by the surface area of the electroplating surface, greatly influence the plating adherence, the deposition rate and the plating quality. This density is variable and is higher on the surfaces than the inside surfaces of the metals. Higher densities increase the rate of deposition. However, there exists a practical limit beyond which adherence and plating quality are negatively impacted on.
For more accurate results, the experiment could employ the high rate electroplating cell method where an electroplating cell that comprises of a rotating anode or jet assembly are immersed in the respective electrolytes and the high pressure electrolyte jets directed towards the cathode (substrate). The presence of high pressure facilitates efficiency in the turbulence agitation on the surface of the substrate. When this is coupled to high current densities, plating rate and efficiency is increased. However, in a laboratory experiment, higher current densities are detrimental to the deposition process, hence lower densities are preferable. Moreover, for the improvement of the composition micro-uniformity, both the current density and the rate of deposition must be reduced.
Bird, John. (2003). Electrical and Electronic Principles and Technology. Newnes Press
Rieger, H. Phillip. (1993). Electrochemistry. Springer Press