Quantitative Analysis: Definition, Methods, Formula and Sample Questions

Quantitative Analysis method is used to determine the quantity of a given substance. In this process, the known substance is dissolved in an unknown composition of water to form a precipitate. The formed precipitate is then isolated, dried and weighed. 

Key Terms: Quantitative Analysis, Carbon, Hydrogen, Ion, wate, Chemical analysis, compound mass, percentage mass, atmosphere

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Quantitative Analysis 

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Quantitative Analysis is a method that is used to determine the percentage of a particular element or ion in a sample. 

Several methods are used to determine Quantitative Analysis. These methods use both physical and chemical properties of the substance. Gravimetric Quantitative Analysis is a chemical analysis method that has great significance. This analysis can be used to determine the percentage of a particular element or ion in a given sample. 

With the help of the chemical formula along with the mass of the precipitate, the mass of the ion of interest can be determined from the percentage mass composition of the precipitate. The ion of the present interest within the main compound can further be calculated from the mass of the ion of interest and the original compound mass.

Also Read: Electrochemistry

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Quantitative Analysis of Organic Compounds

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The estimation of the percentage composition of the various elements present in a compound helps in determining the formula of the compound. The different methods that are used for the estimation of various elements are described below:

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Estimation of Carbon and Hydrogen

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Estimation of Carbon and Hydrogen is done together by Liebig’s Combustion Method. According to the method, the already weighted amount of the given compound is strongly heated with excess copper oxide in an atmosphere involving air and oxygen. This way both carbon and hydrogen get oxidised to water or H₂O and CO₂ or Carbon dioxide. Both of them are separately collected and weighted. The percentage of Carbon and Hydrogen within the compound is as follows:

% C= 12/44 x Mass of formed CO₂/ Mass of the Substance x 100

% H= 1/18 x Mass of formed H₂/ Mass of Substance x 100

Also Read: Mole fraction

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Estimation of Nitrogen

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Estimation of Nitrogen can be done by two methods- the Dumas method and Kjeldahl’s method.

Dumas Method 

Dumas method applies to all the Nitrogen-containing compounds. The basic principle includes an organic compound containing nitrogen gets heated with Copper Oxide and it releases Nitrogen while the oxides are produced in addition to other elements like water vapour, carbon dioxide, etc. On passing over the heated copper, the nitrogen oxides are decreased into free nitrogen and the entire nitrogen is collected over the Potassium Hydroxide or KOH solution. The collected nitrogen volume is measured and from this, the percentage of the present nitrogen within the compound can be calculated.

% N= 28/22400 x Volume of the Nitrogen at NTP/ Mass of Compound x 100

Kjeldahl’s Method

Kjeldahl digestion or Kjeldahl’s method is used to determine the nitrogen amount in organic fertilizers and other food materials. Even though this method is far easier than the Dumas method, it cannot be applied to all forms of nitrogenous compounds such as nitrate. This method is developed by Johan Kjeldahl in the year 1883 and is considered important for analysing protein elements.

Estimation of Halogen

Carius Method is used to estimate the amount of Halogen present. In this process, the organic substance which contains the halogen group is heated with the fuming Nitric acid along with some Silver Nitrate crystals in a sealed tube. When the silver halide is extracted, it gets separated, washed, cleaned, dried and weighed. From that accumulated mass of silver halide, the Halogen percentage can be calculated.

% Halogen= Obtained Mass of Silver Halide x Atomic Mass of Halogen X 100/ Molar Mass of Silver Halide x Mass of the compound taken

This method cannot give accurate results when it deals with Iodine. This is because Silver Iodine is quite soluble in Nitric acid and some iodine is also formed even in the presence of excess silver nitrate.

Also Read: Batteries

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Estimation of Sulphur

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Sulphur can also be estimated through the Carius method. Here, instead of the Silver Nitrate, only Nitric acid is used with the compound. This way, the Sulphur from the compound is transformed into Sulphuric Acid after the oxidation process. Then Barium Chloride is used to precipitate the Barium Sulphate. After that, the Barium Sulphate or BaSO₄ gets cleaned, dried and weighed. The Sulphur percentage can be calculated using the following formula:

% S= 32/133 x Mass of BaSO₄/ Mass of the Compound x 100

Estimation of Phosphorus

The compound containing phosphorous is heated in presence of the fuming Nitric acid. The Phosphorus within the compound is transformed into Phosphoric acid after the oxidization. After adding the Magnesia mixture, the Phosphoric acid is precipitated. To obtain Magnesium Pyrophosphate, the precipitated Magnesium Ammonium Phosphate is ignited.

2MgNH₄PO₄ → Mg₂P₂O₇ + 2NH₃ + H₂O

The Phosphorus percentage is calculated using the following method:

% P= 62/222 x Mg₂P₂O₇ /Mass of Compound x 100

Also Read: Anode and Cathode

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Determination of the Molecular Formula of the Compound

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The molecular formula of the compound indicates the actual number of atoms of various elements present in the molecule of the compound. The molecular formula can be the same as the empirical formula or can be a simple multiple of it. 

Molecular formula= N x Empirical Formula

Where, 

N is the whole number. The value of N can be calculated as: 

N= Molecular Mass/ Empirical Formula Mass

Also Read: Noble gases

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Things to Remember

• Quantitative Analysis is used to determine the percentage of a particular element or ion in an organic compound. 
• The formula of a compound can be estimated with the percentage composition of the various elements present in the compound. 
• Carbon and Hydrogen are estimated by Liebig’s Combustion Method. 
• The mass percentage of Nitrogen is determined with the help of the Dumas method and Kjeldahl’s method.
• The Carius method is used to determine the percentage of Halogen and Surphur. 

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