Qualitative analysis, by definition, is the process of identification of the components present in a given sample.
Chemical analysis has two parts
a) qualitative aspect
b) quantitative aspect.
The qualitative aspect is the primary property determined, and the quantitative is the next one in most determinations.
Qualitative analysis is quality control for many products emerging out of the industry.
This quality is checked by standard institutes like “International Organization for Standardization -ISO.”
Qualitative analysis
For instance, if a Cop asks a person to breathe into an alcohol breath analyzer or meter.
The machine first detects if the breathing air contains ethanol in it.
If ethanol is present, then it goes for estimating the percentage amount of ethanol in the breath air.
So in the above instance, the identification of ethanol (alcohol) is a qualitative aspect.
If there is ethanol, then how much ethanol is present is the quantitative aspect.
If there is no ethanol (qualitative aspect), then the concept of quantitative analysis doesn’t come into the picture.
Thus qualitative analysis says “what the substance is,” while quantitative analysis reveals “how much the substance is.”
Qualitative analysis methods
Quantitative analysis is the direct estimation of the component. Hence the techniques focus on estimating the presence of the components.
For example, if you take toothpaste, the components in it can be
- Particles (to polish the teeth),
- Foaming agents (to produce foam and dissolve fatty matter),
- Flavoring agent (to impart good odor),
- Sweetening agent
- Antiseptic (to kill microbes).
So to detect all the components, we may need
- physical methods
- Separation methods
- Chemical methods
- Biological methods
- Spectro-Photometric methods.
Physical methods
Here, physical features like weight, temperature, and color are used for evaluation.
In the above case, toothpaste can be evaluated by particle size analysis, their shape and size by using a microscope or magnifying glass.
Very rough and large-sized particles are not an indicator of quality toothpaste.
Other physical methods include measuring the refractive index for oils, friability (for tablets), etc.
Separation technique
Chromatography is a separation technique that helps to identify a substance.
This is another type of qualitative analytical technique using physical and chemical methods combined.
Chemical methods
This is wide and mostly applicable to most substances we use or chemicals of some sort. In the above example, the sweetening agent can be determined by chemical reactions.
Chemical reactions used for qualitative analysis come mostly under either of the categories like
- Neutralization reactions
- Oxido-reductive reactions
- Complexation reaction.
- Precipitation reaction.
In the above example of toothpaste, the sweetening agent can be determined by the oxidoreductase reaction, as most sugars have oxygen molecules in their chemistry.
Biological methods
These are used when the biological agent is involved. In toothpaste, an antiseptic’s ability to kill microbes can be tested by microbiological assay.
In general biological methods used are
a) Immunoassays (ELISA) Bio-assays using small animals (for ex: histamine shows contractions in the ileum of a guinea pig as a specific reaction not shown by other bio-molecules,
b) Western blot, gel electrophoresis, radioimmunoassay, RT-PCR for determination of DNA, RNA, or protein molecules in a sample.
c) A behavioral method using experimental animals: For example, morphine administration shows Straub tail (i.e., the elevation of tail specifically) in mice.
Similarly, the administration of D-tubocurarine to the neck muscles of the rabbit makes rabbits’ heads hang without voluntary muscle control. These reactions are so specific and stand as qualitative markers for morphine & D-TC.
Spectro-Photometric methods
These are the most advanced methods for qualitative analysis and control. They are more precise, super-fast, and easy to use than other methods.
This uses spectroscopy techniques like UV, Visible, IR, Florescent, atomic absorption spectroscopy, optical dichroism, x-ray, and other electromagnetic radiation methods for the detection of compounds.
In toothpaste, most substances, like flavoring agents, sweetening agents, coloring agents, foaming agents, particle size, number, etc., can be identified precisely by photometric methods.
These photometric methods not only identify the sample but the exact internal chemistry can be known as the presence of nitrogen, oxygen, sulfur or double, triple bonds, etc.
Qualitative analysis is a necessary procedure to check the quality of any product being prepared for the market for large-scale use.
Examples of qualitative analysis
The above-mentioned methods have some examples. However, some specific examples include
Thin-layer chromatography (TLC)
This is regularly used to identify compounds in a mixture spread on the TLC plate.
Here, the compounds of a mixture get separated and run with relative differences on the TLC plate. On spraying detecting reagents, the samples will have different colors, which helps to identify them with ease.
Bioassay
When a sample is administered to rabbits’ eyes, based on constriction or dilation of the pupil, the compound can be said to be a myotic or mydriatic agent, respectively.
Qualitative Analysis Uses and applications
- To test the purity of water, milk, and solvents.
- To identify the components within an unknown substance.
- To check the self-life and half-life of drugs.
- In the diagnosis of the diseases.
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