Qualitative analysis: Methods, Types, Examples

Qualitative analysis

Qualitative analysis is an important chemical analysis component in organic and inorganic chemistry. The qualitative analysis provides information about the chemical compound’s quality. The qualitative analysis examines the chemical makeup of a material. It denotes the presence of various elements or sets of elements, such as functional groupings, in the sample. As a result, a qualitative analysis of a sample can be used to establish whether or not a specific component is present in a sample. However, this study provides no information on the amount of that chemical component. Color, aroma, melting point, boiling point, reactivity, precipitation, and other properties of the sample are frequently addressed in the analysis. It refers to a group of analytical chemistry techniques that provide nonnumerical data about a specimen.

The chemical characteristics of an unknown substance are determined in the qualitative analysis technique by systematically reacting the unknown with a variety of different reagents. The ions in the solution can be identified by predicting what the specific reaction will create if a specific ion is present.

Methods of qualitative analysis

There is no clear way to categorize qualitative analytical procedures because they vary on a large scale and one analytical procedure involves many separate test methods to determine a certain chemical.

The qualitative analytical method’s complexity varies depending on the nature of the sample to be analyzed. A qualitative analytical method has two distinct properties. It needs to be specific and sensitive. The ability to recognize a certain component or element in the presence of additional components is referred to as specificity. Sensitivity refers to the ability to detect the testing ingredient even when it is present in trace amounts.

The following are the seven methods for performing qualitative analysis on a chemical compound:

  • Change in Colour: A shift in color could indicate the presence of specific elements in the form of ions. It can also indicate if a solution is acidic or basic.
  • Flame test: When a solid substance is exposed to a flame, it emits colors that are unique to each element. The flame test may also generate residue, which can then be evaluated further.
  • Distillation: Fractional distillation can be used to identify liquid components in mixes or solutions, such as alcohol. This approach, for example, can be used to separate hydrocarbon mixtures into their constituent constituents.
  • Extraction: Multi-filtration procedures may be used to remove compounds from a mixture or solution. These are commonly used in water analysis. Ultrasound can also be used to separate compounds in specific circumstances. This is known as sonication.
  • Precipitation: This entails the use of reagents that react with solutions. Specific categories can be used to identify the precipitates of the processes.

Types of Qualitative Analysis

Organic qualitative analysis (such as the iodine test) and inorganic qualitative analysis (such as the flame test) are the two primary groups of qualitative analysis.

The inorganic analysis examines a sample’s elemental and ionic composition, typically by examining ions in an aqueous solution. The organic analysis focuses on molecular types, functional groups, and chemical bonding.

1. Organic Qualitative Analysis

Organic qualitative analysis is the identification of the components and compounds contained in an organic sample. A qualitative study of an organic sample is normally performed by evaluating its physical and chemical properties. The color, odor, and melting point of an organic sample can be used to determine its physical properties. The reactivity and solubility of an organic sample can be used to determine its chemical properties. Physical tests provide specific information for the qualitative analysis of organic samples.

Color

Organic substances can be solid or liquid. The chemical makeup and impurities of organic substances affect their color.

Carbohydrates, amides, and anilides are colorless solids.

Color compounds include nitro compounds, amines, and phenols. Iodoform, benzoin benzil, and quinones are yellow colors. Compounds with a brown color include diamines, aromatic higher amines, and amino phenols.

Odor

Organic substances have a distinct odor. Examples include:

Nitrobenzene, Benzaldehyde = colour of bitter almonds

Fruity odor = ester

Chloral = Odour of cucumber

Fishy odor = aliphatic and aromatic amine

Pleasant odor = methyl ethyl ketone, acetone, acetophenone

Solubility

The nature of organic compounds is reflected in their solubility in various solvents such as water, alcohol, dil NaOH, and dil. HCl, conc H2SO4, ether, and others.

Carbohydrates, Methanol, Ethanol, Glycerol, Formalin, Acetaldehyde, Acetone, Citric acid =  Soluble in Coldwater

Starch, Quinones, Benzoic acid, Salicylic acid, and Phthalic acid = soluble in hot water

Oxalic acid, Benzoic acid, Salicylic acid, Phenol, Rsorcinol, Amino acid = Soluble in dil.NaOH

2. Inorganic Qualitative Analysis

The identification of an unknown inorganic chemical is accomplished through qualitative inorganic analysis. This can be achieved by a variety of methods, including testing for reactivity, solubility, and the presence of specific elements. The physical test involves the identification of compounds depending on their color state and solubility.

Colour:

In organic salts salts of alkali and alkaline earth metals (except chromate and permanganate are colourless. Salt of Al, Zn, Cd, Bi Sn, and Hg are white. While the salts of transition metals have a specific color. For  example:

Fe 3+ = Brown

Fe 2+ = light green

Cu +  or Cu 2+ = bluish-green or blue

Ni 2+ = greenish

Co 2+ = pink

States

Salts with the water of crystallization are crystalline in the state. Most of the carbonates and sulfides are amorphous.

Solubility

All nitrates and nitrites are soluble in water. Water dissolves almost all alkali metals and ammonium compounds. The majority of halides and sulfides are also water-soluble. The majority of carbonates and sulfides are water-insoluble.

 Some examples of qualitative analysis

Flame test

A flame test can be used to determine the presence of a specific metal or its ions. Depending on the metal atoms present, the flame takes on different colors. The flame color produced when a portion of the sample is burned in the Bunsen burner can be used to assess the presence of some metal ions. For example:

 Zn = Green Flame

 Sr = Crimson

Ca= Brick red

K = Violet

Borax bead test

The borax bead test is a type of test used to assess the presence of specific metals, most notably in salts or minerals. A type of bead is generated when the borax is exposed to the flame. A coating of the mineral or salt under consideration is placed on the surface of this bead, which is subsequently exposed to the burning flame. The flame then takes on a distinct color that corresponds to certain minerals; in other words, the color of the flame determines the metal in the mineral.

Iodine test

The presence or absence of starch is determined using the iodine test. Sugar is an organic substance. In this test, liquid iodine is the indicator. Use a spot test (take a white tile and drop some drops of the samples to be analyzed on it) is used to perform this experiment. Add a drop of iodine solution to each drop of the sample. The presence of starch is indicated by a change in the sample’s color to brown.

Lassaigne’s test

Lassaigne’s test is used to detect nitrogen, sulfur, and halogens in organic molecules. In this process, a small amount of Na metal is heated with the organic compound in a fusion tube. The basic idea is that Na changes all of the elements present into ionic form.

Na + C + N → NaCN

2Na + S  → Na2S

Na + X →  NaX (where X = Cl, Br, or I)

Boiling the fused mixture with distilled water extracts the produced ionic salts. This is known as sodium fusion extract.

Unsaturation test

Unsaturation tests can be performed to identify the presence of double and triple bonds in an organic molecule. The bromine test and Baeyer’s test can be used to determine the presence of unsaturation (carbon-to-carbon double or triple bonds),  in organic unsaturated Hydrocarbons.

Bromine test

The solution of bromine is brown. When a bromine solution is introduced to an unsaturated hydrocarbon in this test, the brown color disappears if the hydrocarbon is unsaturated. Bromine reacts with the unsaturated hydrocarbon to create an addition product.

Baeyer’s test 

When alkaline potassium permanganate is introduced to an unsaturated hydrocarbon, the pink color of potassium permanganate is discharged. The disappearance of pink coloration may occur with or without the formation of brown manganese oxide precipitate.

Advanced qualitative analysis methods

In organic chemistry and biochemistry, advanced qualitative chemical analytical procedures are commonly utilized. The primary goal is to determine the functional groups, structure of organic molecules, and ligands present in the sample. The following are some instances of advanced qualitative analytical methodologies.

  • X-ray crystallography: X-ray crystallography is used to determine the nature and arrangement of inorganic, organic, and biological molecules such as pharmaceuticals. It involves investigating how X-rays diffract when they pass through the crystalline structure of crystallized substances.
  • Spectroscopy: When certain elements are vaporized, they form specific bands or lines on the visible spectrum. These bands or lines can then be analyzed to establish the composition of objects, even distant objects such as stars.
  • Nuclear magnetic resonance spectroscopy: It is a spectroscopic technique that involves analyzing metabolites as they form in an organism’s body. It is based on the magnetic resonance of atomic nuclei placed in extremely strong magnetic fields. It is possible to create a three-dimensional image of a molecule using nuclear magnetic resonance spectroscopy.

References

  1. https://byjus.com/jee/qualitative-analysis-of-organic-compounds/
  2. https://ncert.nic.in/pdf/publication/sciencelaboratorymanuals/classXII/chemistry/lelm107.pdf.
  3. https://unacademy.com/content/neet-ug/study-material/chemistry/qualitative-and-quantitative-analysis/.
  4. https://www.britannica.com/science/qualitative-chemical-analysis.
  5. https://infinitylearn.com/surge/chemistry/qualitative-chemical-analysis/

About Author

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Kabita Sharma

Kabita Sharma, a Central Department of Chemistry graduate, is a young enthusiast interested in exploring nature's intricate chemistry. Her focus areas include organic chemistry, drug design, chemical biology, computational chemistry, and natural products. Her goal is to improve the comprehension of chemistry among a diverse audience through writing.

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