MONTO DEL CAPITAL PENDIENTE Y RENDIMIENTOS POR COBRAR Se debe reportar el balance de capital y rendimientos por cobrar en las cuentas contables

When connected in a circuit with a power supply, a potential difference is achieved between the electrodes and electrons accumulate on the negative electrode. Within the electrolyte, the positively charged species, mainly metal cations2, are attracted to the negative electrode while negatively charged species in solution will be attracted to the positive electrode. The applied potential which ultimately results in polarization of the solution is sometimes referred to as the polarization potential. The reduction of metal cations to metal occurs by the acquisition of electrons by the cations on the surface of the electrode supplying the electrons. The electrode upon which reduction occurs is named the cathode. At the other end of the cell, electrons are removed from the electrode by the driving electromotive force, EMF, of the battery or power supply and sent to the cathode resulting in oxidation of the positive electrode. The electrode at which oxidation occurs is known as the anode. It should be noted that it is a common misconception that the anode is always positive and the cathode is always negative such as in electroplating cells; for fuel cells the anode and cathode have opposing polarity as the term anode and cathode are based on the location of oxidation and reduction within the system and not electrical connections. Moreover, in the case of rechargeable batteries, the anode and cathode switch depending on whether the battery is being charged or used. This is because the recharging of a battery resets the system by oxidizing what is the cathode during operation and rebuilding through reduction the operating anode. While both the anode and cathode are electron conductors, the anode within the electroplating cell may be either inert or consumable. Inert anodes, often made of platinum or carbon, require the replenishment of metal ions in solution as they are consumed. Conversely, consumable anodes match the identity metal ion species in solution and replenish the ions in solution as the anode is oxidized and the electrons removed from the metal result in the liberation of metal cations into the electrolyte.

In addition to the anode and cathode, electroplating cells often make use of a third, inert, electrode known as a standardized reference electrode, or standard electrode. The purpose of the standard electrode is to provide a stable reference with respect to which the potential between the anode and cathode is measured. The need for the standard electrode is due to the significant difficulty in maintaining a constant potential at an electrode while a current is passed through the electrodes for the purpose of redox reactions. The difficulty originates, in part, from the electrical double layer which is a parallel structure of charges produced when a surface is exposed to a fluid. The electrical double layer, which may be constructed of solid particles or gas bubbles, is cause by the accumulation of charges at the surface of an electrode. The charges at the electrode polarize the electrolyte resulting in the formation of a layer of oppositely charged ions, or polarized molecules, at the interface and another layer of charges, or molecules, attracted by the first layer. The double layer, which behaves like a capacitor storing charge, causes a variation of electrical potential at the surface and is described by several models including the Helmholtz model, the Gouy-Chapman model, and the Gouy-Chapman- Stern model.

The standard electrode provides a known reduction potential, while the other electrode, anode, passes all the current needed to balance the current provided by the cathode. The standard electrode is connected to the cathode with a large resistance placed in the connection between the standard and working electrode to ensure the circuit is not disturbed. The standard, or reference, electrode is itself isolated from the solution by means of a salt bridge or a glass frit so that any minimal electron flow will not result in reduction on the electrode. The salt bridge is constructed by filling a glass tube with a conductive electrolyte such as sodium chloride {NaCl} or potassium chloride {KCl}. The electrolyte is often turned into a conductive gel by mixing it with agar, or may be kept within the tube by sealing an end with glass frit, small glass beads which provide a porous barrier allowing the flow of ions but not the bulk liquid. The standard electrode, isolated by the bridge or frit, is placed as close as possible to the cathode in order to maintain no potential difference between the cathode and standard potential. A summary of the reactions for a consumable anode in a deposition cell containing a standard

electrode for the deposition of nickel {Ni} from aqueous dissociated nickel chloride {NiCl2} electrolyte is depicted in Figure 2.1.

Figure 2.2: Depiction of an electroplating cell

using a standard, or reference, electrode, housed within a KCl solution filled salt bridge, for the deposition of Ni from an aqueous NiCl2 solution onto the cathode. Equations displayed show the dissociation (black), oxidation (blue) and reduction (red) reactions of Ni within the cell.

Standard electrodes are constructed to have a stable equilibrium potential for reversible half-reactions, meaning no current flow is present between electrode and internal electrolyte of the standard electrode. A number of standard electrodes exist, the most common of include the standard hydrogen electrode (SHE), the standard calomel electrode (SCE), and silver-silver chloride electrode (SSCE). The scale of standard electrode potentials is based on the half reaction of hydrogen, 2H+(aq) + 2e– ↔ H2(g), which by convention is defined as having a standard potential of 0.00 V at an effective concentration of 1 M and pressure of 1 atm at 25 °C. Given that half-cell potentials cannot be measured, a relative electrode potential for the reaction is measured against the 0 V potential of the SHE.

Standard hydrogen electrodes are constructed using platinised3 platinum {Pt}, electrode in an acidic solution having a 1.00 M concentration of hydrogen ions {H+} [2]. Pure hydrogen gas {H2} at a pressure of 1 atm is bubbled around the Pt electrode, and equilibrium of the hydrogen in the two phases, aqueous and gaseous, within the system

3

Platinized Pt, also known as black Pt for its black color, is composed of black platinum powder deposit on a shiny platinum surface which results in a highly catalytic surface due to the increased surface area of the

establishes the half-reaction. Due to the difficulty in setting up the SHE, other standard electrodes, such as the silver/silver chloride {Ag/AgCl}, are more commonly used. The SSCE is composed of solid AgCl, usually as a coating on Ag metal, immersed in an aqueous Cl salt solution, often 4 M potassium chloride {KCl}, saturated with AgCl and has a relative electrode, or reduction, potential, E°, of 0.197 V compared to the SHE [2].

The SSCE, along with the SCE, represent electrodes of the second kind in which the equilibrium potential is a function of the concentration of an anion in the solution as it controls the cation concentrations by means of the solubility product of the slightly soluble metal salt [3]. Like electrodes of the second kind, which often are used as standard electrodes, electrodes of the first and third kind both operate on the equilibrium potential determined by the cation in the solution. Electrodes of the first kind consist of a metal salt in solution with the cation in solution matching the metal electrode [3]. Electrodes of the third kind consist of a metal in contact with two slightly soluble salts of differing cations, only one of which matches the electrode, and identical anions immersed in a solution containing a salt of the differing cation [3]. The series of equilibrium within electrodes of the third kind result in instability and hence limited use.