Qualitative analysis primarily focuses on the visual characteristics of a chemical reaction, for example, a change in colour or the production of bubbles. Unlike quantitative analysis, qualitative analysis does not involve measurements or numerical values and calculations.
Qualitative analysis is relatively easy to perform because it generally doesn’t require expensive, high-tech equipment or precise measurements.
In this post:
What is Qualitative Analysis?
Qualitative analysis is a form of preliminary analysis that helps determine the types of chemicals involved in a reaction. A trained chemist can also identify a wide range of elements, compounds and mixtures by using simple qualitative analytical techniques.
The five senses
Qualitative analysis may simply involve the five senses – sight, touch, hearing, smell, and taste. For example, you can easily identify the aromatic compound vanillin from its vanilla smell.
Similarly, you can identify hydrogen from the gas bubbles generated in the chemical reaction between hydrochloric acid and a metal. This is an example of a simple displacement reaction, wherein the hydrogen in the acid is displaced by the metal.
The classical approach
The classical approach to qualitative analysis involves both a ‘dry’ test and a ‘wet’ test.
A dry sample of a solid substance is heated in a flame. The colour of the flame typically indicates the presence of certain elements. For example, zinc gives off green light when subjected to a flame. The appearance of residue is also an identifier, such as in the case of char, which indicates the presence of carbon.
After the dry tests are complete, a substance may be subjected to a wet test, where the sample substance or residue is dissolved in water. You can then use several qualitative analytical techniques to identify the positively charged and negatively charged ions (cations and anions).
Reagents such as hydrochloric acid, hydrogen sulphide, ammonium hydroxide, and ammonium chloride can be added to a solution of an unknown substance to determine its constituents based on the precipitates and other products.
For example, if a solution treated with hydrochloric acid produces a white precipitate, it may contain silver chloride, lead or mercury. See the tabulated summary below of the wet test, otherwise known as the cation-anion test.
As you can see, the products of reactions using the wet test can be categorised into seven groups. This narrows down the identification of the constituents of the analyte. Further qualitative tests can be done to pinpoint the exact composition.
The following ions produce specific colours in solutions:
- Cu2+ (blue)
- Cr3+ (green)
- CrO4- (yellow)
- Cr2O7- (orange)
- MnO4- (violet)
- Ni2+ (green)
- Co2+ (pink or blue)
- Mn2+ (pink)
Qualitative analytical techniques
Qualitative analytical techniques in chemistry may involve the following:
- Colour change test – a colour change may indicate the presence of certain elements in the form of ions. It may also signal the acidity or basicity of a solution.
- Flame test – when a solid substance is subjected to a flame, it produces colours that are specific to certain elements. The flame test may also produce residue that can be tested further.
- Distillation – you can identify liquid substances in mixtures or solutions such as alcohol through fractional distillation. Hydrocarbon mixtures, for instance, can be separated into their constituent compounds by using this technique.
- Extraction – techniques that are used to extract substances from a mixture or solution may involve multi-filtrations. These are usually applied in water analysis. In some cases, ultrasound can also be used to separate substances. This is called sonication.
- Precipitation – this involves the use of reagents that react with solutions. Precipitates of the reactions can be identified based on specific groupings.
What is the Purpose of Qualitative Analysis?
Qualitative analysis is mainly used to determine the composition of a substance or the absence of certain substances. It may also be performed to rule out certain substances or to determine the mechanism of reactions.
In many cases, qualitative analytical techniques are carried out as a form of preliminary analysis. They can often be completed on the spot, such as at a suspected crime scene. For example, spraying luminol on surfaces and then shining an ultraviolet light can reveal minute traces of blood, even if the surfaces have been thoroughly cleaned.
Qualitative analysis is usually followed by quantitative analysis if more precise results are needed.
Advanced Qualitative Analytical Methods
The main objective is to determine the functional groups, structure of organic molecules, and ligands present in the sample. Some examples of advanced qualitative analytical methods are listed below.
- X-ray crystallography – this method is used to analyse the composition and structure of inorganic, organic, and biochemical compounds like medicines. It involves examining how x-rays diffract as they pass through the crystalline structure of crystallised substances. This is how the double-helix structure was discovered by Watson and Crick (with the help of Rosalind Franklin).
- Chromatography – this is a technique used to physically separate the constituents of a mixture by allowing a substance to flow through a capillary tube, plate or sheet. You can also perform DNA analysis using this technique.
- Spectroscopy – various elements produce specific bands or lines on the visible spectrum when vaporised. You can then analyse these bands or lines to determine the composition of substances, including distant objects like stars.
- Nuclear magnetic resonance spectroscopy – this is a spectroscopic technique that involves systematically studying metabolites as they’re formed in the body of an organism. It relies on the magnetic resonance of atomic nuclei that are placed under very strong magnetic fields. Nuclear magnetic resonance spectroscopy can be used to construct a three-dimensional image of a molecule.
What is Thematic Analysis in Qualitative Research?
Thematic analysis focuses on the interpretation of meanings, patterns or themes in a collection of qualitative data. It actually refers to a broader concept as opposed to one particular method. What’s more, thematic analysis doesn’t just apply to chemical analysis; it’s also employed in various scientific fields that use qualitative analysis to interpret data.
Thematic analysis is concerned with three main methodological issues: the reflexibility of journals; the coding process; and sample size. It involves examining multiple data sets to find general patterns. It also follows the six phases of Braun and Clarke’s thematic analysis, as shown in the diagram below. Source
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