This picture is from Asahi's website. It shows the Asahi Chemical's ion exchange process for salt electrolysis.
The following discussion provides an introductory guide regarding electrolysis, which is also used to produce and refine metals for industries, making jewelries, protecting metal from corrosions and more. We deal with the fundamental science of electrolysis here.
Electrolysis can be carried out in solutions or molten salts (liquid). Because the atoms and ions have to move physically, the medium has to be a fluid. The products, like the reactants in a galvanic cell, can be in a solid, liquid, or gas state.
Reduction always takes place at the cathode, by definition. In the electrolysis of molten salt, NaCl, the cathode and anode reactions are:
|Anode oxidation||DE||Cathod reduction|
|2 Cl- = Cl2 + 2 e-||e- pump||2 Na+ + 2 e- = 2 Na|
|2 Cl- + 2 Na+ = Cl2 + 2 Na|
In the above setup, if the current is 1.0 A, the rate at which Na deposits at the cathode will be 1.0E-5 (= 1/96485) mol (or 0.24 mg) per second. Of course, it takes 96485 seconds (or 26.8 hrs) to deposit one mole (23 g) of sodium metal.
A potential of -2.06 V is the standard cell potential for,
|Anode oxidation||DE||Cathod reduction|
|H2O = 4 H+ + 4 e + O2||>2.06 V||4 H2O + 4 e = 2 H2 + 4 OH-|
|2 H2O = 2 H2 + O2|
Aluminum (Al) is the third the most abundant elements on Earth crust, in the form of bauxite or alumina Al2O3. Because it is very reactive, this metal remained unknown to mankind until 1827. By then, Wohler obtained some Al metal by reducing Al2O3 with potassium vapore.
In 1886, two young men working in two continent apart electrolyzed molten cryolite Na3AlF6 (melting point 1000° C). Alminium was not produced when pure cryolite was used. Electrolysis is successful only if the ions move to the electrodes, and the reactions take place. That was not the case for molten cryolite.
However, both Hall and Heroult tried to mix about 5% alumina in their molten cryolite, and their discovery is now known as the Hall-Heroult process, which is a commercial process. In a modern process, the reactions are:
Electrolysis causes chemical reactions. Amounts of reactants, products, energy, and charge are inter-related. The following examples illustrate the stoichiometry of electrolysis.
Study the following conversion method to get from 1 Kg of Al to number of Faradays and coulombs. Note that values in the numerators are equivalent to those in the denominators in the factors.
1000 g 1 mol 3 F(araday) 1 kg Al ------- ------- -------- 1 Kg 26.98 g 1 mol Al 96485 C = 111 M -------- 1 M = 1.1e7 C.Producing Al is an expensive process.
Using the same method as indicated above, you have
1 mol 2 F 3.175 g Cu ------- -------- (At.wt. Cu = 63.5) 63.5 g 1 mol Cu 96485 C = 0.100 M --------- 1 M = 9650 CTo calculate the current, you divide the charge (C) by the time period (sec).
I = 9650 C/(30*60 sec.) = 5.36 A.
The charge passed the cell is
0.10 (C/sec)*2*3600 sec = 720 C. 1 M 1 mol Fe 55.8 g Fe 720 C ------- -------- --------- 0.90 96485 C 3 M 1 mol Fe = 0.12 g Fe.Note that the last factor corresponds to 90% efficiency.
Taking the current efficiency of 95% into consideration, the effective charge passed through the cell is
0.200 (C/sec) * 3600 sec * 0.95 = 684 CThe atomic weight of M is thus,
1 M 1 mol M 684 C ------- -------- 96485 C 2 M = 3.54E-3 mol M
0.399 g -------- = 112.7 g/mol 3.54E-3Checking the results against a table indicates that the element is cadmium Cd.
Electroplating different from electrolysis in that the metal deposited from electrolysis plates out on the surface of another metal. The electrolyte contains the plating metal in the form of dissolved ions and the annode usually is made of the plating metal. The object to be plated is the cathode.
An Industrial Chemistry on electroplating considers a more complete syllabus.
As I search for recent information on electroplating, I came accross a websit for Jobs - Plating Chemist / Technician.
Electroplating technology is frequently use in metal finishing, metallic coating and finishing, and salt water pool chlorination.
If you are interested in electroplating companies, here are some websites:
The Samson Technology Corporation sells portable electroplating system. The following paragraph is from Samson Samson's portable electroplating systems are built for continuous heavy duty use, light weight and easy to operate. Auto emblems can be plated without removing them from the car. Faucets can be plated on the sink. 24K gold, sterling silver, nickel and copper can be brush plated with ease using Samson's electroplating systems. Samson offers a complete line of chemicals and supplies for electroplating, buffing and polishing.
Answer . . . anode
Convention in cell notation: Oxidation anode cell is on the left, reduction cathode cell is on the Right.
Just another question to help you to remember that the cathode provide electrons for the reduction reaction,
2 H+ + 2 e = H2.
Facts: Two Faradays is required to produce 22.4 L H2 at STP. Two mili-Faradays (0.002 mole) is required to produce 22.4 mL, 1 mili-mole or 0.001 mole, H2 at STP.
1 mol 2*96485 C 1 s 22.4 mL ------- --------- ----- = 193 s 22.4 L 1 mol 1 CNote: 1 A = 1 C/s; 1 s / 1 C = 1 / (1 A)