Qualitative Inorganic Analysis

Have you ever wondered what the world is made up of, or, thought about the things you use daily? From the chemicals in your shampoo to the compounds in your food, a small change in these can lead to a huge collapse that will work like slow poison. Such is the intimacy that we are sharing with the world of chemicals currently, much like technology and electronics. Hence, it is equally important to produce people who better understand these concepts as well. Thankfully, we have a division in chemistry that deals with this practical aspect called Qualitative Inorganic Analysis.

The subject of identification of molecules present in a compound goes a long way back.  Compounds can be briefly classified into covalent and ionic. Covalent compounds are bound by covalent bonds and can be identified much easily by observing physical properties like boiling point and melting point. It is because a relatively less amount of energy is required to break the covalent interactions between the molecules than in ionic bonds. So, how does one identify the ionic species present in a compound? The answer to this question will lead you straight to Qualitative Inorganic Analysis.

The analysis of these compounds are divided, intuitively, into a qualitative one – where we focus on finding out what is present in a compound – and also a quantitative one – where the focus is on how much is present in the compound. You might be wondering still, why should I pursue this? The first obvious answer to this question points to food, other important parameters being paints, computer chips and any manufactured product. They all are made up of various combinations of compounds and impurities in these, especially food, can have serious consequences. Hence it is very crucial to know how to identify impurities. Another important reason is the present environmental crisis. It is equally important to be able to identify pollutants in a given sample so that measures can be taken to prevent the accumulation of the same.

A further classification is made into organic and inorganic compounds, which have different techniques of identification. A formal definition of the inorganic analysis of compounds is:

“Qualitative chemical analysis is a branch of chemistry that deals with the systematic identification of elements or grouping of elements present in a sample.”

Now the techniques used in the above follow an order. They are based on the systematic elimination of compounds by observing reactions of the sample with various known compounds to form precipitates or substances with known properties. By observing the products of such reactions, the correct ion present in the sample can be identified. The classification divides this into two categories: identification of cations and identification of anions. Now cations and anions are further divided into groups of compounds that exhibit similar end products. After identifying the group, independent tests are conducted to zone in on the right ion.

The cation and anion groups have the following general scheme:

Cations

I

II

III

IV

V

VI

Pb+2, Ag+, Hg2+2 Cd2+, Bi3+, Cu2+, As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+, Hg2+ Fe2+, Fe3+, Al3+, Cr3+ Zn2+, Ni2+, Co2+, Mn2+ Ba2+, Ca2+, Sr2+ Mg2+, Li+, Na+, K+

The ammonium ion (NH4+) is also put into a group called the ZERO group.

Anions

I

II

III

CO32-, HCO3, CH3COO, S2−, SO32−, S2O32− , NO2

 

Cl, Br, I, NO3, C2O42−

 

SO42−, PO43−, BO33−

 

The above groups are divided such that each one forms a similar type of compound. The Group I cations from insoluble chlorides, Group II cations from acid-insoluble sulphides, etc. On the other hand, for the anions, a particular reagent is used that gives different effects with different anions. Group II anions use conc. H2SO4 etc.

A sequence should be followed to obtain meaningful results as some of the cations overlap with other groups too. But before directly going on to using the chemicals, a few basic tests can be used to pre-determine what kind of ions are present. These include:

  • Checking the Appearance: Some ions have characteristic colours like Cu+2 in blue.
  • Heating test: After heating a compound, the colour and odour of the gas produced can be used to identify the ions. Br2 is brown in colour.
  • Flame Test: Some ions give characteristic colours on heating. K+ ions burn to give a pale violet colour.
  • Solubility in Water: Solubility tests could also be used to identify the compounds. Carbonates, sulphites, and phosphates are insoluble in water.

The procedure for conducting this would not be outlined here, but the above methods are just some of the various methods used to identify the many different compounds present.

A few books that deal extensively with this topic are:

  • Vogel’s Qualitative Inorganic Analysis by G.Svehla
  • A Textbook of Inorganic Chemistry by O.P Tandon

Perfection in this field can be achieved, as in any field, by regular practice of theory matched by laboratory experiments. After a point of understanding the phenomenon, you would be able to identify the initial ions just by the basic tests and then crosscheck the accuracy by reacting a given sample with the right reagent to save resources and simultaneously confirm the identity of the compound.

This field of analysis is very practical. You could become a qualitative inorganic analyst and be the first one to successfully overturn the problem of impurities faced by today’s industries among many others – A true hero.

The qualitative inorganic analysis is an important part of inorganic chemistry. If prepared well, it’ll help you in the preparation of the entire inorganic chemistry. However, remember, revision is the key to this chapter. Learn to remember the things you read in this article of ours!

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