Quantitative Analysis

We might have learnt about the qualitative analysis of various organic compounds. However, without knowing the exact quantity of each of the compounds, we won’t be able to use them in the best possible way. So, how do we do that? Quantitative analysis will help us! This form of analysis also involves certain tests! Yeah, you can never escape tests in chemistry! Let us start with the basics of what quantitative analysis is.

Suggested Videos

 

What is Quantitative Analysis?

Quantitative analysis is an analysis method that we can use to determine the quantity of the elements or molecules produced during the reaction. Organic compounds consist of carbon and hydrogen. It also comprises of elements like oxygen, nitrogen, phosphorus, sulphur and halogens. We will now explain the various methods for the measurement of percentage composition of elements in an organic compound.

Quantitative Analysis of Halogens

Carius Method

Here, we heat a mixture of organic compound and fuming nitric acid in the presence of silver nitrate contained in a tube (hard glass). This tube is known as the Carius tube in a furnace. Carbon and hydrogen present in the organic compound oxidise to carbon dioxide and water respectively.

The halogens present in the organic compound reacts with the silver nitrate to form the corresponding silver halide (AgX). After this, we filter, wash, dry and weigh them.

Calculations

Let us decide the mass of the given organic compound as mg. Suppose the total mass of the compound, AgX formed = m₁ g. We know that 1 mol of AgX has 1 mol of X. So, in m₁ g of AgX,

Mass of halogen = (atomic mass of X × m₁ g)(molecular mass of AgX)

Percentage of halogen = (atomic mass of X × m₁ × 100)(molecular mass of AgX × m)

Quantitative Analysis of Sulphur

In this segment, we continue to a process of heating a fixed mass of an organic compound containing sulphur in a Carius tube. This tube has sodium peroxide or fuming nitric acid. Sulphur present in the organic compound oxidises into sulphuric acid. It, then, precipitates as barium sulphate by the addition of barium chloride solution in water.

We then wash, filter, dry and weigh the precipitate. The mass of barium sulphate that we use in calculating the percentage of sulphur in the given organic compound.

Calculations

Let us consider the mass of organic compound = mg. Let the mass of the compound, barium sulphate formed = m₁ g. We know that 32 g sulphur is present in 1 mol of BaSO₄. Therefore, 233 g BaSO₄ contains 32 g sulphur

⇒   M₁ g of BaSO₄ contains 32 ×  m₁/233 g  of sulphur

∴ Percentage of sulphur = 32 ×  m₁ × 100/233 × m

Solved Example for You

Q: What is Aluise’s method?

Ans: It is a method that we use to calculate the weight of oxygen in an organic compound. In this method, we heat the organic compound containing oxygen with graphite. CO gets formed. We convert this CO quantitatively into CO₂ on reaction with I₂O₅. Thus, we get the weight of the oxygen in the compound.