To answer ‘what is qualitative analysis in chemistry?’, you must first know that chemical analysis consists of two branches, namely qualitative analysis and quantitative analysis. We need both qualitative and quantitative measures to analyze an unknown sample. Qualitative analysis involves the identification of elements or identification of characteristic features in a sample whereas quantitative analysis measures the amounts of those elements precisely. This article focuses on the area of qualitative analysis in chemistry. There are various methods for qualitative analysis. The method varies depending on the nature of the sample to be analyzed.
What is qualitative analysis in Chemistry
Qualitative analysis is a main part of chemical analysis in both of Organic Chemistry and Inorganic Chemistry. It gives an idea about the quality of the chemical compound. The purpose of the qualitative analysis is to determine the composition or to identify the components or elements in an unknown sample. Qualitative analytical methods do not provide a precise answer to quantitative analytical questions (the amount or how much of each element present in the mixture, percentage of each cation present in a solution).
Qualitative analytical methods
There is no distinct way to classify the qualitative analytical methods as methods vary in a vast scale and one analytical procedure contains so many different test methods to determine a particular substance. The expected observation varies as the way you perform the analysis; some produce precipitates, some evolve gases with acids, some form coloured solutions, some give a colour to the flame, etc . In the analysis of a complex mixture, it needs to be systematic and carefully designed to identify all the constituents present.
Qualitative analytical methods and Selectivity and Sensitivity
The complexity of the qualitative analytical method varies depending on the nature of the sample to be analyzed. There are two special characteristics in a qualitative analytical method. It should be a specific one and sensitive one. Specificity involves the ability to detect a certain component or element in the presence of the other components. Sensitivity involves the ability to detect the testing element, even if it is present in trace quantities. In other words, sensitivity is defined as the smallest quantity of an element / compound that can be detected by a given method. Some methods are very sensitive and for some, it needs to have a fairly high concentration for the detection.
Example: Identification of SO42- ions
Method 1: Using mercury nitrate solution
When mercury nitrate is added to a solution containing sulphate ions, a yellow precipitate of basic mercury sulphate (HgSO4) is formed. This is a very sensitive test, which means it gives a precipitate even if the SO42- concentration is very low.
Method 2: Using silver nitrate solution
When silver chloride is added to a sulphate solution, a crystalline precipitate of silver sulphate is formed. This occurs only in concentrated solutions (solubility of Ag2SO4 = 5.8 gl-1 at 180C).
Note: In most of the qualitative analytical methods, interfering ions should be eliminated before carrying out the analysis. If not we will not get the expected observation. Sometimes we might have to perform another qualitative test to verify that interfering ions are absent in the analyte.
Qualitative Analysis – Summary
Qualitative analysis is used in both Organic and Inorganic Chemistry to analyze an unknown sample. It determines the chemical properties or chemical constituents present in a sample. The analytical procedure varies from simple to complex depending on the nature of the sample. It follows a systematic procedure, allowing reactions with different chemical reagents.