Understanding Faraday’s First Law of Electrolytic Quantitation

Faraday’s First Law: The Foundation of Electrolytic Quantitation

Faraday’s First Law is the cornerstone of understanding the quantitative relationship between electricity and chemical change in electrolysis. It elegantly states:  

• The mass of a substance deposited or liberated at an electrode is directly proportional to the electricity quantity. The more electricity passed through the electrolyte, the more substance is deposited or liberated.

Think of it this way: the more “electricity” you put in, the more “stuff” you get out (at the electrodes).

Breaking Down the Law

• “Mass of a substance deposited or liberated”: This refers to the amount (in grams) of a pure element or compound. It is formed or released at one of the electrodes during electrolysis. For example, if you’re electroplating copper, this would be the mass of copper deposited onto the object. If you’re electrolyzing water, this could be the mass of hydrogen or oxygen gas liberated.
• “Quantity of electricity passed”: This refers to the total electric charge. It is measured in coulombs (C). This charge flows through the electrolytic cell. A coulomb is a unit of electric charge. A coulomb relates to current (amperes, A) and time (seconds, s). The equation is: Q = It (Charge = Current x Time).  
• “Directly proportional”: This indicates that doubling the amount of electricity (charge) will double the amount of substance deposited (mass). If you triple the charge, you’ll triple the mass, and so on.  

Mathematical Expression

The proportionality can be expressed mathematically as:

• m ∝ Q

To turn this proportionality into an equation, we introduce a constant of proportionality, called the electrochemical equivalent (Z):  

• m = ZQ

Where:

• m: mass of the substance deposited or liberated (in grams)  
• Z: electrochemical equivalent (in grams per coulomb, g/C)  
• Q: quantity of electricity passed (in coulombs, C)  

Understanding the Electrochemical Equivalent (Z)

The electrochemical equivalent (Z) is a crucial factor. It represents the mass of a substance deposited or liberated per unit of electric charge (1 coulomb). Each substance has its own unique electrochemical equivalent. It’s essentially a measure of how much of a substance is produced by a given amount of electricity.  

Example

Let’s say the electrochemical equivalent of silver (Ag) is 0.001118 g/C. If you pass 100 coulombs of electricity through a solution of silver nitrate, the mass of silver deposited would be:

m = ZQ = (0.001118 g/C) * (100 C) = 0.1118 g

In Summary

Faraday’s First Law provides a precise way to calculate the amount of a substance produced during electrolysis. It’s a fundamental principle used in various applications, from electroplating to the extraction and purification of metals. The key is understanding the relationship between mass, charge, and the electrochemical equivalent.