What is Qualitative Analysis?

 A qualitative analysis is considered the division of experimental chemistry which is often concerned with identifying the range of components in the specified unknown mixture. The primary goal of qualitative analysis is to analyze and identify particular components including ions present throughout the molecular content of a sample. Qualitative research is employed for the identification and separation of cations and anions in a sample material. A qualitative study is a conceptual method of evaluation as opposed to quantitative analysis that attempt to evaluate the amount or quantity of the specimen.

The component and atom to be measured is transformed in quality assessment through chemical processes to such a specific product with certain typical characteristics based upon this information obtained the component could be identified. Qualitative analysis could also be done using systematic numerical techniques according to the quantitative measurement of compounds utilized for experimental processes.

Qualitative research is a method that is efficiently designed to recognize or identify specific components and ions that makeup the chemical information of a substance.

Dissociation

Qualitative research includes the detection of the -ve ion (acid) and +ve ion basic sections that is present as a single salt or mixtures of two or more chemical salts in an inorganic substance.

If a simple salt or even a combination of 2 or maybe more simple salts dissolves into the liquid, one bears a positive charge named as positive ion and often called as cation, or basic molecules, whereas the other bears a negative charge, called negative anion or acidic molecules. It breaks into two kinds of charged particles. This phenomenon of salt breaking into ions in a given solution is called dissociation.

The most common example of dissociation is the formation of Na⁺ and Cl^- ions from dissociation of NaCl.

Solubility Product

The qualitative research analyses the amount of oppositely charged ions in the provided inorganic salt as well as combination against which the amount of analysis is being conducted out also called as tests. The particular solution used for the experiment is named as a reagent and the chemical modification observed in the test is known as the analytical or chemical reaction solubility product.

If small portion of sparingly soluble salt is transferred to water and then stirred vigorously then smaller portions of the salt melt into water and undergo ionization giving saturated salt solution. Certain parts disintegrate yet remain unionized whereas a large portion continues to remain unresolved.

Common Ion Effect

The method, where an electrolyte ionization or dissociation is inhibited by adding some other solution possessing a common ion with the electrolyte, is referred to as common ion effect (CIE).

Relevance of Solubility Product and Common Ion Effect in Differentiating Cations into Groups

These methods can be used for the analysis of carbon and hydrogen and the components found in organic compounds. Oxygen, sulphur, nitrogen, halogens even phosphorus could also be found in the mixture through their qualitative property analysis.

Carbon and hydrogen analysis

This analysis includes heating of the copper (II) oxide complex that will measure carbon as well as hydrogen. The carbon contained in this mixture is converted to carbon dioxide through oxidation (tested for the development of turbidity by lime-water) or water by hydrogen (tested with blue anhydrous copper-sulfate).

For other elements

'Lassaigne's Test' is used for the analysis of nitrogen, sulphur, halogen and phosphorus present in an organic compound. This method works by merging of the molecules with sodium metal occurs where the components found in the mixture are transformed into the ionic state from their covalent state.

Nitrogen analysis

The extract fused with sodium is firstly boiled in presence of iron (II) sulphate followed by acidification with concentrated sulphuric acid. The qualitative presence of nitrogen is confirmed only on the development of Prussian blue color. Sodium hexacyanidoferrate (II) is formed when sodium cyanide bonds with iron (II) sulphate. This when heated along concentrated sulphuric acid few iron (II) gets oxidized into iron (III) ions this then bonds with sodium hexacyanidoferrate(II) to develop a Prussian blue in colour iron(III) hexacyanidoferrate(II) (ferriferrocyanide).

Phosphorous detection

For the analysis of phosphorous, the mixture is warmed by an oxidizer (sodium peroxide). The phosphorus in the mixture is phosphate oxidized. The liquid is boiled using nitric acid and later processed in presence of ammonium molybdate. A yellow pigmentation or precipitate suggests the existence of phosphorus through qualitative analysis.

Halogen detection

Halogen analysis includes the mixture of sodium fusion undergoing acidification with nitric acid which is again processed with silver nitrate. If bromine is present in the given sample, it will be observed in the form of a white precipitate that is sparingly dissolving in ammonium hydroxide whereas if chlorine is present it shows by a white precipitate that is completely dissolving in ammonium hydroxide and for iodine the precipitate is yellow color which is insoluble in ammonium hydroxide.

Analysis of Anions

Stage I : Preliminary analysis with dilute sulphuric acid:

In this stage the anions such as carbonate, sulphide, sulphite, nitrite and acetate will react with dilute sulphuric acid and emit different gases. The gas emitted will have specific characteristics which will give us information about the anions present in the mixture. This is mentioned in detail in the following table:

Stage II: Confirmatory tests

Confirmatory tests for anions can be performed in two ways depending upon the solubility of the compound. For the salt that are water soluble water extract is used whereas, for the salt that are water insoluble sodium carbonate extract is used.

Few common confirmatory tests for different anions are given below in the table:

Common Mistakes

The relation between the ionic concentrations of a mixture to its K_sp should be interpreted carefully to determine whether the given sample is saturated, unsaturated or supersaturated.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Chemistry.