Volumetric Analysis

1. Introduction

Quantitative analysis determines how much of a substance is present in a sample. It has two main branches:

• Gravimetric analysis: the constituent is separated as a pure solid of known composition, and its mass is measured (for example, precipitating chloride as $\ce{AgCl}$ and weighing it).
• Volumetric (titrimetric) analysis: the volume of a solution of known concentration that reacts exactly with a measured volume of the unknown is determined, and the amount is calculated from the reaction stoichiometry.

Volumetric analysis is quicker than gravimetric analysis and is the standard method used in the school laboratory.

1.1 Equivalent weight

The equivalent weight (gram equivalent) of a substance is the number of parts by mass that combine with or displace $1.008$ parts of hydrogen, $8$ parts of oxygen, or one equivalent of any other substance. It is calculated as:

$\text{Equivalent weight} = \frac{\text{molar mass}}{\text{valency factor}}$

• Acid: valency factor = basicity (number of replaceable $\ce{H+}$). For $\ce{H2SO4}$, equivalent weight $= 98/2 = 49$.
• Base: valency factor = acidity (number of replaceable $\ce{OH-}$). For $\ce{NaOH}$, equivalent weight $= 40/1 = 40$.
• Salt: valency factor = total positive charge. For $\ce{Na2CO3}$, equivalent weight $= 106/2 = 53$.
• Oxidising/reducing agent: valency factor = number of electrons gained or lost. For $\ce{KMnO4}$ in acid ($\ce{Mn}$ goes $+7 \to +2$, 5 electrons), equivalent weight $= 158/5 = 31.6$.