Analytical chemistry is the branch of chemistry in which samples are analysed in order to determine their components qualitatively and quantitatively.
Analytical chemistry consists of two main types of analysis
Qualitative analysis: This branch focuses on identifying the chemical components present in a sample.
Quantitative analysis: This branch aims to determine the amount or concentration of a specific component in a sample. One of the techniques used in quantitative analysis is volumetric analysis.
Importance of qualitative analysis
Food safety and quality control: Qualitative analysis is used to detect the presence of harmful substances or contaminants in food products. This ensures that the food we consume is safe and meets quality standards.
Forensic investigations: Qualitative analysis assists in identifying substances present at crime scenes and providing evidence for legal proceedings. It can help identify and link evidence to crime scenes such as the presence of specific drugs, blood stains, toxins, or chemicals present in samples.
Medical diagnosis: Qualitative analysis of body fluids such as urine and blood, can help detect the presence of specific substances or compounds that may indicate certain medical conditions or diseases.
Quality control in industries: qualitative analysis is employed in various industries, such as pharmaceuticals, textiles and electronics, to verify the composition and purity of raw materials and products. This ensures consistent quality and compliance with industry standards.
Consumer products safety: Many consumer products such as cosmetics, household cleaners and personal care items undergo qualitative analysis to detect the presence of potentially harmful chemicals or to verify compliance with safety regulations.
Research and education for discovery and development of new products.
Laboratory qualitative analysis
Apparatuses used:
Pyrex test tubes
Test tube holders
Test tube rack
Heat source.
Watch glass/ Petri dish
Nichrome /platinum wire or glass rod.
Dropper
Spatula
Qualitative guide sheet – a booklet containing procedures to be followed when one needs to identify a certain ion and their expected result in each procedure.
Procedures for carrying out qualitative analysis
These procedures can be grouped into three categories.
1. Preliminary test
Preliminary test gives important information which simplifies further process of analysis.
Preliminary tests provide a starting point for further analysis but are not sufficient for conclusive identification of ions present in the sample.
This test involves:
Appearance of solid sample: this involves observation of its colour, texture and deliquescence.
Flame tests: This test is used to identify the presence of certain metal ions (cations), based on the characteristic colour they produce when heated in a non-luminous flame.
Procedure:
Dip a glass rod/wire in a concentrated hydrochloric acid then heat in a non-luminous flame until no colour imparted. This procedure is used to clean the glass rod/ wire.
Dip the cleaned wire or glass rod in concentrated hydrochloric acid again then to the sample. This makes a small amount of the sample stick to the glass rod.
Introduce the sample stuck on the glass rod to the non-luminous flame.
Note the colour of the flame evolved.
Action of heat on a solid sample: tests the gases evolved or the residue.
Action of dilute hydrochloric acid on a solid sample.
Action of concentrated sulphuric acid on a solid sample
Solubility of a solid sample in water.
2. Tests in solution
Here a salt sample reacts with a number of different reagents while it is in solution state.
In this step a solid sample is dissolved in water or solvent to make its solution.
Most of the following tests will require the sample to be in solution state.
In order to avoid making the solution many times, a test tube containing a solution is half filled with the water or a solvent used to make this solution.
The resulting solution is known as stock solution or original solution.
Preparation of stock solution
solid sample + cold water (shake)
If it is insoluble in cold water, warm the contents till the salt completely dissolves.
insoluble in hot water?
Take the fresh solid sample in a test tube and add dilute hydrochloric acid into it.
insoluble in cold hydrochloric acid?
Heat the mixture
insoluble in cold/hot hydrochloric acid?
Solid sample + concentrated hydrochloric acid
insoluble in concentrated hydrochloric acid?
Solid sample + nitric acid
insoluble in nitric acid?
Solid sample + mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3:1(aqua regia)
Precipitation of hydroxides
This is the specific tests for cations.
Cations can be identified by adding aqueous sodium hydroxide or ammonia solution.
Most cations give precipitates (insoluble hydroxides) with ammonia solution or sodium hydroxide (except H+, Na+, K+, NH4+).
In this test, ammonia solution or sodium hydroxide solution is added dropwise then in excess.
Teacher
Demonstrate the behavior of some cations upon addition of ammonia solution and sodium hydroxide solution dropwise and in excess eg. Pb2+, Cu2+.
3. Confirmatory tests
Confirmatory test is a controlled chemical reaction used to determine the presence of a specific ion or compound in a sample. The ions confirmed are cations and anions.
Teacher
Discuss the chemical tests of various anions and cations with students. Then lead the class on how to identify the anions and cations present in a sample using the reagents discussed and identifying the unknown salt sample.
Note
For examination purpose, you will have to carry out the qualitative analysis by being given the instructions that aimed at identifying the ions present and the unknown sample (guided analysis) or you will have to use the steps analyzed on the qualitative sheet to systematically identify the sample (unguided or open analysis)