Electrolysis - Part IV

6. Application of electrolysis
The process of electrolysis has many applications. But before we consider a few applications, let us first look at the factors that influence discharge of ions at the electrodes.

Suppose your electrolyte is a solution of NaCl in water. At the cathode, there will be a competition between release on Na of H ions as both of them are positively charged.  The question that might arise in your minds, is which of the ion will be released at the cathode?

The ion that will be released at the cathode or the anode will depend on the following factors :

• Relative position of the ion in the electromechanical series. The electromechanical series is a representation of how reactive the ion is. For example in a solution that contains Na⁺ and Hg⁺⁺ ions, Na⁺  ions will be preferentially released as Na accepts electrons easily (see its electronic configuration). But this happens if the concentration of Hg⁺⁺ ions is comparatively small. (If the electrolyte is made up of NaCL in H₂O, then H ions will be preferably released, as H is more reactive than Na). Table below gives some examples of the electrochemical series.

• Concentration of ions in the electrolyte. In the above example if the concentration of Hg⁺⁺ ions is very large, it will be preferentially released.

• The nature of the electrodes. Some electrode materials such as graphite or platinum are non-corrosive and are not affected by ions surrounding it. Some electrodes such as copper, may enhance the release of ions especially ions like Cu, Ag, Ni, etc.

The Electrochemical Series in order of increasing preference:

In the light of the above discussions, we will see how the process of electrolysis is applied  :

1. Electroplating of metals
2. Electro-refining of metals
3. Extraction of metals or Electro-metallurgy
4. Battery

Electroplating of metals
Electroplating is a process whereby a thin coating of desired material is applied on a required material. This is mostly done on stainless steel to prevent rusting, or on some decorative items, so that they look attractive. On stainless steel, generally nickel-chromium plating is done. On decorative items, such as spoons, plates, jewelry items, silver, gold or other plating is done. Electroplating is cheap and cost effective. It enhances the life of the object and makes it look better in appearance.

The following method is adapted :

• First the item to be electroplated is smoothened and cleaned thoroughly. It should not have any oily or dirt marks on it.

• An electrolyte is selected whose ions are required to be deposited on the item.

• Direct current is preferred to alternating current, as alternating current may result in non-smooth deposit.

• The item to be electroplated forms the anode or cathode of the electrolytic cell. This is the drawback of the electroplating process. The item has to be electrically conducting, or has to be made electrically conducting.

• For a smooth coating, the electrolytic process has to be optimized for time, temperature and current in the cell.

Figure below is a conventionally used electrolytic cell for depositing silver. Let the item to be silver-plated be a spoon.

Electrolyte used is sodium silver cyanide solution [Na{Ag(CN)2}]. Silver nitrate is not used as it is observed that the electrolyte does not give a uniform deposition. The cathode is made out of the item on which the electro deposition is to be done, in this case it is a spoon. Anode is made of a block of silver. 

The [Na{Ag(CN)₂}] dissociates as

Na{Ag(CN)₂}      Na⁺  + Ag ⁺   +2 CN 

Reaction at the cathode

Ag ⁺   +    1 e^-         Ag 

The positively charged Ag ⁺   ions  are attracted to the cathode (-) and accept one electron and get deposited as a thin film over the cathode material, in this case the spoon.

Reaction at the anode

Ag     -      1 e^-        Ag ⁺                           

The silver atoms at anode loose electrons and enter into the electrolyte as an ion.  This ensures that the concentration of the Ag in the solution remains constant.

The electroplating is complete when a desired thickness of the silver film is deposited.

The electroplating process parameters, such as the type of electrolyte, the type of anode or cathode, the temperature of the electrolyte, the strength of the current, the time for electroplating, etc. are very well documented in standard literature and these are usually followed.

Electro-refining of metals
Similar to the process of electro-deposition, electrolysis can be used to purifying metals that are obtained from the ores. The process is known as electro-refining of metals. The metals that are generally refined by this process are Zn, Ag, Ni, Cu, Pb, Al, etc.

Figure below is a conventionally used electrolytic cell for purifying metal ores, in this case the metal is impure copper.  

Let the cathode be made of pure copper sheets and the anode be made of impure copper block. The electrolytic solution is copper sulphate solution.

The electrolyte dissociated into

CuSO₄          Cu ⁺⁺    +   SO₄^{- -}

Reaction at the cathode

Cu ^{+ +}    +    2 e^-          Cu

The positively charged Cu ⁺⁺   ions  are attracted to the cathode (-) and accept two electrons and get deposited as a thin film over the cathode material, in this case the pure copper sheets.

Reaction at the anode

Cu  -  2 e^-            Cu ⁺⁺                          

The copper atoms at the anode loose electrons and enter into the electrolyte as an ion.  This ensures that the concentration of the Cu in the solution remains constant. Besides the copper atoms, other atomic impurities such as Ag and Au also get into the electrolytic solution.  Thus the impure block at the anode gets used up and pure copper is deposited at the cathode.

Extraction of metals or Electro-metallurgy
Extraction of metals by the process of electrolysis is known as electro-metallurgy. This process is used in  case highly reactive metals such as sodium. An ore containing sodium is used in a molten form. This forms the electrolyte. Anode and cathodes are generally carbon rods or steel. The Na atoms get attracted to the cathode of the cell and then the entire cathode with its coating is stored for further use.

Battery
All batteries that we come across in our day to day use, including car batteries, dry cells used in torches, calculators, hand-sets, etc. are all examples of an electrolytic cell. But in this case the reverse of an actual electrolytic process is being used. The chemicals inside the cells produce current (and voltage) which is utilized.

In a car battery, for example, two grids are used as anode (Pb) and cathode (PbO₂). The solution is H₂SO₄  of generally about 6 M in concentration.  

The overall reaction is:

PbO₂ + Pb + 2 SO₄^-2 + 4 H⁺¹    2 PbSO₄ + 2 H₂O

During the discharge process, the metallic lead atom (Pb) at the anode loses two electrons and becomes positively charged. This process is called "oxidation." The electrons flow from the anode through the bath to the PbO₂ cathode. At the cathode, the positively charged lead ion in the lead dioxide (Pb⁺⁴ O^-2₂) accepts two electrons through the external circuit. This electron acceptance process is called "reduction."

The discharge product on both electrodes is lead sulfate (PbSO₄). The overall voltage of these cells is about 2.0 V, so a 6 V battery has three of these sets of cells in series, while a 12 V battery has six of them. 

Next        Main        Previous