Electrolysis - Part III

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4. Definition of standard solutions
In the process of electrolysis, we have seen that the electrolyte has to be in a molten state or in the form of a solution. By solution, we mean that the solute is completely dissolved in the solvent. A solution thus is a homogeneous mixture. Example of solution is brine, sugar dissolved in water, water in alcohol, etc. Oil in water is a heterogeneous mixture and hence is not called as a solution.

In a solution, one would be interested in knowing the amount of solute dissolved. Therein comes the concept of strength of a solution. Normally you can say that x grams of solute is dissolved in y grams of the solvent. The x and y are variables and can take any values. But this may cause confusion for electrolysis, as the entire process parameter for electrolysis namely, the current, the type of electrodes used, etc. all will depend on the strength of the solution. To simplify matters, standard solutions are used. Standard solutions are a fixed weight of solutes, mixed or dissolved in a fixed weight of the solvent. 

There are two types of standard solutions

  •  Molar solution

  •  Normal solutions

Molar Solution (M)
A Molar solution (M) is a solution that contains 1 mole of solute in each litre of solution. A mole is the molecular weight (MW) expressed as grams (sometimes referred to as the ‘gram molecular weight’ (gMW)).

Thus  1 M = 1 gMW of solute per litre of solution.

For example, how much sodium chloride is needed to make 1 litre of a 1 M solution?

First, we find out the molecular weight (MW) of sodium chloride. This is calculated as follows :

The atomic weight of sodium (Na) is 23.
The atomic weight of chlorine (Cl) is 35.5

So the molecular weight of sodium chloride (NaCl) is:
Na (23) + Cl (35.5) = NaCl (58.5)

Therefore, a 1 M solution of sodium chloride contains 58.5 grams of sodium chloride in 1 litre of solution.

Similarly, a 2M solution contains 117 grams of sodium chloride per litre.

And a 0.1M solution contains 5.85 grams/litre of sodium.

Normal Solution (N)
A Normal solution (N) is a solution that contains 1 ‘gram equivalent weight’ (gEW) of solute per litre of solution. The gram equivalent weight is equal to the molecular weight expressed as grams divided by the ‘valency’ of the solute.

To understand valency, consider the following acids:

Hydrochloric acid (HCl) has one replaceable hydrogen ion (H), sulphuric acid (H2SO4) has two replaceable hydrogen ions (H2) and phosphoric acid (H3PO4) has three replaceable hydrogen ions (H3). The valencies of these acids are determined by their respective replaceable hydrogen ions:

HCl,       Valency = 1
H2SO4,  Valency = 2
H3PO4,  Valency = 3

So, for 1N HCl the MW is 36.5, the EW is 36.5 and therefore 1N would correspond to 36.5 grams/litre.  In case of HCL 1M solution is same as 1N solution.

For 1N H2SO4 the MW is 98, the EW is 98/2 = 49 (that is, valency = 2) and so a 1 N solution would be 49 grams/litre.

Similarly, for 1N H3PO4 the MW is 98, the EW is 98/3 = 32.7 and 1N would be 32.7 grams/litre.

In case of alkalis, to understand valency, consider the following alkalis:

Sodium Hydroxide (NaOH) has one replaceable hydrogen ion (OH), calcium hydroxide  (Ca(OH)2) has two replaceable hydrogen ions ((OH) 2) The valencies of these acids are determined by their respective replaceable hydrogen ions:

NaOH,    Valency = 1
Ca(OH)2, Valency = 2

So, for 1N NaOH the MW is 40, the EW is 40 and therefore 1N would correspond to 40 grams/litre.  In case of NaOH 1M solution is same as 1N solution.

For 1N Ca(OH)2 the MW is 74, the EW is 74/2 = 37 (that is, valency = 2) and so a 1 N solution would be 37 grams/litre.

Since normal solutions are a bit confusing, these days the practice is to quote strength of solutions in molar terms.

5. Faraday’s Law of Electrolysis
Michael Faraday is a renowned English scientist who had worked in the field of electricity and has made some pioneering contributions. He established the relationship between the electric current that is passed through an electrolyte and the mass of the substance released at the anode or the cathode.  The relationship is known as the Faraday’s law of electrolysis. The law states that the mass of substance released at any electrode is directly proportional to the electric charge that is passed through the electrolyte.

Thus if m = mass of the substance released at the electrode

And      Q = amount of electric charge that is passed through the electrode

Then according to Faraday’s Law of Electrolysis

m     Q

If I is the current in amperes then Q = I x t (t = time duration in seconds)

Thus     m      It

M = Z x It

Z is the constant of proportionality and is known as the electromechanical equivalent.

Thus the electromechanical equivalent Z of a substance is defined as the amount of substance in grams liberated at any electrode when one coulomb charge is passed through an electrolyte.  

 

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