Qualitative and quantitative analysis of organic compounds


To understand the identification of chemical compounds, analysis is the most essential process. Quality and quantity are the 2 essential elements used as criteria to detect different elements involved in a compound. Quantitative analysis is used to determine the total number of elements or molecules produced while conducting an experiment. On the other hand, quantitative analysis is more about finding out how much of the elements are present. The common atoms found in most of the organic compounds are carbon, hydrogen, oxygen, nitrogen, phosphorus, Sulphur and halogens. To find out the proportion or percentage distribution of each element, a variety of tests being used. The following techniques are normally used to analyze the organic compounds qualitatively.



Method of detecting carbon and hydrogen atoms

Carbon and hydrogen atoms present in any organic compounds can be detected by heating the substance(s) with copper oxide in a hard glass tube. During this process, the coke/carbon present will reduce the copper (II) oxide to brown copper solid, and the brown copper gets oxidized to carbon (IV) oxide. CO2 gas in the laboratory can chemically be tested by use of lime water which forms a white precipitate. On the other hand, water formed can be tested using anhydrous copper (II) sulphate. 

CuO(s) + C(s)→ 2Cu(s) + CO2(g)

H2 (g) ++ CuO(s)  Cu(s) + H2O (l)



Test for Phosphorous

The sample is heated with an oxidizing agent, heating mixture results in the formation of phosphate by oxidizing the phosphorous. After a while, the solution is boiled with the concentrated HNO3 and then treated with ammonium molybdate. Formation of a yellow precipitate confirms the presence of phosphorous. The reaction can be written as;

Na3PO4 + 3HNO3 → H3PO4 + 3NaNO3

H3PO4 + 12(NH4)2MoO4 + 21HNO3 → (NH4)3PO4.12MoO3 + 21NH4NO3 + 12H2O



Test for halogens ( chlorine as an example)

Halogens are a group in the periodic table consisting of 5 chemically related elements including chlorine (Cl), fluorine (F), bromine (Br), iodine (I), and astatine (At). The halogens can be tested by using sodium fusion extract which is usually prepared by mixing nitric acid to the silver nitrate. When chlorine is added to a test tube containing the sodium fusion extract, a white precipitate soluble in ammonium hydroxide is formed.  On the other hand, bromine forms a yellow precipitate which is sparingly soluble in ammonium hydroxide solution. Lastly, iodine forms a yellowish precipitate with the sodium fusion extract and is insoluble in ammonium hydroxide solution. This can be represented as X- + Ag+ → AgX



Carius Method

An exact amount of the compound is heated with Concentrated  HNO3 alongside the silver nitrate in a tube called  Carius tube. C and H are oxidized to CO2 and H2O. The halogen forms the corresponding AgX. It is filtered, dried and weighed.





Estimation of Halogens by Carius method

Percentage of X =((Atomic mass of X)/(Molecular mass of AgX))x((Mass of AgX)/(Mass of the compound))x100



Preparation of sodium fusion extract

A small piece of sodium metal is heated with an organic compound in a fusion tube for about three minutes, and the red-hot tube is plunged into distilled water contained in a special dish. The products on the dish are in turn heated, cooled and filtered. This filtrate is what we refer to as sodium fusion extract.



Estimation of Nitrogen by Dumas Method

Damus method


Dumas method is among the quantitative methods that involve heating a known mass of the analyte with excess of copper (II) oxide in the presence of carbon (IV) oxide gas. Here, the nitrogen present in the organic compound is converted into nitrogen gas. The volume of nitrogen gas, N2 is converted into the standard temperature and pressure and finally compute the percent of nitrogen as follows:




Estimation of Nitrogen by Kjeldahl Method

In Kjeldahl's method, a known mass of an organic compound is heated with concentrated sulphuric acid in the presence of potassium sulphate and some copper sulphate solution in a special flask known as the Kjeldahl’s flask. Mercury can also be used instead of copper sulphate. Nitrogen present in the analyte is converted into ammonium sulphate which is boiled with an excess amount of sodium hydroxide solution. During the process, the ammonia gas produced is absorbed by either hydrochloric acid or sulphuric acid. Finally, the volume of the unreacted acid is obtained by titrating it with a standard alkali solution. The percent of nitrogen can be determined using the relationship below.







Estimation of Oxygen by using Aluise’s method

A calculated mass of the compound is decomposed with the help of heating in the presence of N2 gas. The mixture of gases so produced is passed on to the  red hot coke. This will help to convert the entire  O2 is converted into CO. This mixture is heated with I2O5 in which CO is oxidized to CO2 liberating I2. The reaction can be written as,

Organic compound → other gaseous products + O2

2C + O2 → 2CO

I2O5 + 5CO → 5CO2 + I2

Percentage of O = ((Molecular mass of O2/Molecular mass of CO2) x (Mass of CO2/Mass of the compound) x 100






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