In this article we will discuss about:- 1. Basic Principle of Electrolysis 2. Faraday’s Laws of Electrodeposition 3. Calculation of Current Required for Depositing Given Amount of Metal 4. Current and Energy Efficiency Required 5. Power Supply.
Basic Principle of Electrolysis:
In nature atoms of electrolyte (chemical compound) are closely bound together but bond becomes weaker, when dissolved and the molecules of the electrolyte split up into two types of ions carrying electric charges, called the cations and anions, and moving freely in the solution. Now if two electrodes are dipped into the electrolyte and connected to the dc supply, ions associated with positive charge (cations) and moving freely in the solution are attracted by the cathode (electrode connected to the negative terminal of the supply) and the ions associated with negative charge (anions) and moving freely in the solution are attracted by the anode (electrode connected to the positive terminal of the supply).
For example, when copper sulphate (CuSO4) is dissolved in water, immediately it gets dissociated into +vely charged copper ions (Cu++) and negatively charged sulphions (SO–4–) moving freely in the solution and if a potential difference is applied between the two electrodes immersed in the solution the +vely charged copper ions will move towards the cathode and the -vely charged sulphions will move towards the anode.
Each of the positively charged copper ions reaching the cathode will take two electrons from it and become a metallic atom of copper, and similarly each of the negatively charged sulphions reaching the anode will give up two electrons to it and cease to be anion. Thus the electrons will move from anode to cathode in the external circuit and constitute flow of current from anode to cathode in the electrolyte. Thus the function of source of supply seems only to serve as an electron pump pumping electrons from +ve side of the supply and supply to the -ve side of the supply.
ADVERTISEMENTS:
As mentioned above each +vely charged copper ion on reaching the cathode takes two electrons from it, becomes atom of metallic copper and deposite there. Similarly each -vely charged sulphion on reaching the anode gives up two electrons to it and becomes SO4 radical but since SO4 radical cannot exist in the electrical neutral state so it will attack the anode and will form the corresponding sulphate of the material of the anode; for example if the anode is of copper then copper sulphate (CuSO4) will be formed, but if the anode is of such a material which cannot be attacked by SO4, for example if the anode is of carbon the SO4 will react with water and form sulphuric acid and liberate oxygen according to chemical reaction-
2SO4 + 2H2O –> 2H2SO4 + O2
The whole process described above is called the electrolysis and the effect is that the copper gets dissolved from the anode and deposited on the cathode. During the process there is no accumulation of charge at any point in the circuit and the mass of copper deposited at the cathode is exactly equal to that removed from the anode.
Faraday’s Laws of Electrodeposition:
The laws governing the electrolytic processes were formulated by Michael Faraday, an English Scientist, and are known after his name.
ADVERTISEMENTS:
These may be stated as below:
Faraday’s First Law:
According to this law the chemical deposition due to flow of current through an electrolyte is directly proportional to the quantity of electricity (coulombs) passed through it.
i.e., mass of chemical deposition,
ADVERTISEMENTS:
m ∝ Quantity of electricity, Q
or m ∝ It ... Q = It
or m = ZIt …(8.1)
where I is the steady current in amperes flowing through the electrolyte for t seconds and Z is a constant of proportionality and is known as the electrochemical equivalent of the substance.
ADVERTISEMENTS:
If I = 1 A and t = 1 second, then Z = m
Thus electrochemical equivalent, Z, of a substance is defined as the amount of the substance deposited on passing a steady electric current of 1A for one second through its solution. It is usually expressed in terms of milligrams per coulomb. The SI unit of electrochemical equivalent Z is the kilogram per coulomb (kg C-1).
Faraday’s Second Law:
This law states that when the same quantity of electricity is passed through several electrolytes, the mass of the substances deposited are proportional to their respective chemical equivalents or equivalent weights.
ADVERTISEMENTS:
From this law it follows that the constant of proportionality Z in Eq, (8.1) is proportional to the chemical equivalent.
Table 8.1 gives the values of atomic weight, valency, chemical equivalent and electrochemical equivalent of various elements.
Calculation of Current Required for Depositing Given Amount of Metal:
The theoretical value of current required for depositing a given quantity of metal and the time for which this current should be passed through the electrolyte, can be calculated from the Faraday’s laws, if the electrochemical equivalent of the metal is known. The following example will make the method of calculation clear.
Example:
Calculate the ampere-hours required to deposit a coating of silver 0.05 mm thick on a sphere of 5 cm radius. Assume electrochemical equivalent of silver = 0.001118 and density of silver to be 10.5.
Solution:
Surface area of the sphere, S = 4πr2 = 4π x (5)2
= 314 cm2
Thickness of coating, t = 0.05 mm = 0.005 cm
Mass of silver to be deposited, m = S x t x density of metal
= 314 x 0.005 x 10.5
= 16. 493gm
= 0.016493 kg
ECE of sliver, Z = 0.001118 gm/coulomb
= 0.001118 x (3,600/1,000)
= 0.0040248 kg/A-h
Ampere- hours required = m/Z = 0.016493/0.0040248
= 4.12 Ans.
Current and Energy Efficiency Required for Electrolytic Processes:
Current Efficiency:
Owing to impurities, which cause secondary reactions, the quantity of substance or substances liberated is slightly less than that calculated from Faraday’s laws. This is taken into account by employing a factor, called the current efficiency.
The current efficiency is defined as the ratio of the actual quantity of substance liberated or deposited to the theoretical quantity, as calculated from Faraday’s laws. i.e., Current efficiency
= Actual quantity of substance liberated or deposited/Theoretical quantity of substance liberated or deposited.
Its value usually lies between 90 and 98%.
In certain cases this efficiency is very low. For example in chromium plating it is roughly 12 to 15 per cent. It is because only 15 per cent of the total current passed through the electrolyte consisting of some chromium acid solution, is used in depositing chromium and the rest is wasted in producing oxygen and hydrogen gases, which for the purpose in hand, are useless.
Voltage:
The voltage necessitated to pass the current through any electrolytic cell is equal to the sum of voltage drop in the resistance of the electrolyte and the voltage drops at electrodes (anode and cathode). From the resistivity of the electrolyte and the x-sectional area and the length of the electric current path, the resistance of electrolyte can be determined. For economy the electrolyte resistance should be reduced to minimum and to achieve it in many cases special conducting agents are added to the electrolyte.
The addition of sulphuric acid to copper sulphate solution in copper plating is an example of this. There is some potential difference between the cathode and the electrolyte and between the electrolyte and anode. This potential difference is a measure of the tendency of the metal to go into the solution and is known as electrode potential.
The electrode potential depends upon the exact conditions (i.e., temperature and concentration), and also upon the nature of the metal and the electrolyte. Under ideal conditions the value of electrode potential for most of the substances lie between 0.5 to 1.0 volt. The total potential difference required to pass the necessary current through electrolytic cell is about 1 or 2 volts.
Energy Efficiency:
On account of various secondary effects and reactions the substance deposited by a given quantity of electricity is less than that determined theoretically from Faraday’s laws. Voltage required is also higher than that determined theoretically. Hence actual energy consumption will be higher than that determined theoretically for depositing a given quantity of the substance.
The ratio of theoretical energy required to the actual energy required for depositing a given quantity of metal is known as energy efficiency.
i.e., Energy efficiency = Theoretical energy required/ Actual energy required.
Power Supply for Electrolytic Processes:
Power supply required for electrolytic processes is direct current and at very low voltage. The power required for electrodeposition is usually very small (between 100 and 200 amperes at 10 or
12 volts) and can be obtained either by employing a motor- generator set consisting of a standard induction motor driving a heavy-current low-voltage dc generator (preferably separately excited) or by employing the copper oxide rectifier.
The latter is preferred because of low maintenance cost, occupying less space and higher operating efficiency. Mercury-arc rectifier cannot be used because it has low efficiency at low output dc voltage on account of constant voltage drops at cathode and anodes. The plate rectifier unit is usually placed along with its transformer in the oil so that it may be protected from the corrosive fumes of the electrolyte.
Recently the solid state rectifying devices employing germanium and silicon diodes have been developed for use. These solid state devices occupy very small space even as compared to metal rectifiers. Output dc voltage can be controlled by controlling the excitation of the generator in case of motor-generator set and by means of continuously variable auto-transformer in case of rectifier supply. This method of control is suitable where only one bath is being supplied.
In case, more than one bath are supplied, a variable resistance is connected in series with each bath so that the supply to each bath can be controlled independently. With the development of SCRs or thyristors, which can control output voltages of power supplies, their use in power supplies for electrochemical processes has increased. They are also compact and light in weight, even cooling attachments are included. Voltage control is by using output transformers.
Power supply required for extraction and refining of metals and large scale manufacture of chemicals is in very large amounts. Since most of the processes are continuous, therefore, have a load factor of 100 per cent. Because of power requirements in huge amount and at 100 per cent load factor, such plants are located near the hydroelectric power stations or atomic power stations even if extra transportation of raw material is necessitated.
The advantage of a high load factor is greater with such stations than with steam stations and also transmission costs are eliminated. Nangal fertilizer factory producing calcium ammonium nitrate and heavy water and utilizing power of 180 MW from left bank Bhakra power house and Sri Ram Fertilizer factory located at Kota (Rajasthan) are instances in the support of the above statement.
The voltage of each cell is about 10 volts, but if many cells are connected in series, current of the order of several thousand amperes will be required at the voltage of the order of 500 to 800 volts. Thus by employing heavy current motor-generators, rotary convertors or even mercury-arc rectifiers, the required supply may be obtained from the modern grid network.