Estimation of Carbon, Hydrogen, Nitrogen, Halogens, Sulphur, Phosphorus

Halogens are estimated by the process known as the Carius method. In this method, a known quantity of the organic compound is reacted with fuming nitric acid in the presence of silver nitrate. We use IR iodate titration to determine sulphur. In phosphorus,a known quantity of organic compound is heated with fuming nitric acid. Phosphorus is oxidised to phosphoric acid. Ammonia and ammonium molybdate is added to the solution, which precipitates phosphorus as ammonium phosphomolybdate. 

INTRODUCTION:

Estimation (or estimating) is the process of finding an estimate, or approximation, which is a value that is usable for some purpose even if input data may be incomplete, uncertain, or unstable. The value is nonetheless usable because it is derived from the best information available. Organic compounds are composed of different atoms (ingredients) and in order to make the compound, those atoms have to be present in defined amounts. We will be discussing how the relative amounts of different atoms in an organic compound can be estimated and deduced. Our focus will centre around carbon, hydrogen, nitrogen, halogens, sulphur, and phosphorus since those atoms tend to be the most common inorganic compounds. 

Liebig’s method is basically used for the estimation of carbon and hydrogen in which we heat a known mass of organic compounds in the presence of pure oxygen after that the carbon dioxide and water formed are collected and weighed. The specific piece of laboratory equipment typically used for this type of experiment is called a bomb calorimeter.

The Carius method is an excellent technique that allows for the determination of the amount of chlorine, bromine, fluorine, iodine, or sulphur contained in a compound. The compound is first heated in the presence of nitric acid and silver nitrate, which converts any halogens or sulphur present into their corresponding silver salt. This salt can then be collected, weighed, and the amount of halogen or sulphur calculated based on the amount of silver salt recovered.

If we suspect that our compound may contain phosphorus, we can actually utilise a modified Carius method to determine how much it may contain. The organic compound is first heated with concentrated nitric acid and magnesium, which converts all of the phosphorus present to a salt called magnesium ammonium phosphate. 

Estimation of carbon and hydrogen

Principle. A known mass of the organic compound is heated strongly with an excess of dry copper oxide in a current of dry air or oxygen (free from carbon dioxide). Under these conditions, the carbon present in the organic compound is oxidised to carbon dioxide and hydrogen is oxidised to water.

C (from organic compound) + 2CuO—>CO₂ + 2 Cu

2H (from organic compound) + CuO—>H₂O+Cu

The water thus produced is absorbed in a U-tube containing anhydrous calcium chloride or anhydrous magnesium perchlorate while CO₂ produced is absorbed in another U-tube containing a strong solution of KOH or ascarite (NaOH+ CaO). The tubes are weighed before and after the combustion. The increase in the mass of CaCl₂ or Mg(CIO4)2 U-tube gives the mass of water produced while the increase in the mass of KOH or ascarite U-tube gives the mass of CO₂ produced.

Estimation of  Nitrogen

In 1893, a scientist by the name of Johan Kjeldahl successfully pioneered and developed a method known as Kjeldahl’s method in which the amount of nitrogen in an organic compound can be determined. In this analytical technique, the organic compound is heated in the presence of sulfuric acid, which leads to decomposition of the compound followed by the capture of the nitrogen atoms to form ammonium sulphate.

The ammonium sulphate is then reacted with sodium hydroxide which produces ammonia as the reaction product. Once the ammonia is collected, it is neutralised with a known amount of an acid in an acid/base reaction. Since we know how much acid it takes to react with the ammonia, the amount of nitrogen in the original sample can be back-calculated easily.

Percentage of nitrogen in sample= (1.4V*N)/W

V = acid used in titration (ml)

N = normality of standard acid

W = weight of sample (g)

Estimation of  halogens

A known mass of the compound is heated with Conc. HNO3 in the presence of AgNO3 in a hard glass tube called the Carius tube. C and H are oxidised to CO2 and H2O. The halogen forms the corresponding AgX. It is filtered, dried and weighed.

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

Estimation of  sulphur

A known quantity of organic compound is heated with fuming nitric acid or sodium peroxide in a hard glass tube (Carius tube). Sulphur is oxidised to sulphuric acid. Excess barium chloride is added which precipitates barium sulphate. The precipitate is filtered, washed, dried and weighed.

A known mass of the compound is heated with conc. HNO3 in the presence of BaCl2 solution in the Carius tube. Sulphur is oxidised to H2SO4 and precipitated as BaSO4. It is then dried and weighed.

Percentage of S= ((Atomic mass of S)/(Molecular mass of BaSO4)) x (( Mass of BaSO4)/(Mass of the compound)) x 100

 Estimation of  phosphorus

If we suspect that our compound may contain phosphorus, we can actually utilise a modified Carius method to determine how much it may contain. The organic compound is first heated with concentrated nitric acid and magnesium, which converts all of the phosphorus present to a salt called magnesium ammonium phosphate. This solid is then burned to produce Mg2 P2 O7 (magnesium pyrophosphate).

The mass of this compound recovered is then used to calculate the relative amount of phosphorus that was present in the original starting compound. Just like in the previous Carius method, this technique relies on selectively converting the element of interest to a solid, which can easily be isolated and its mass determined.

Percentage of P= ((Atomic mass of P)/(Molecular mass of (NH4)3PO4.12MoO3)) x ((Mass of (NH4)3PO4.12MoO3)/(Mass of compound)) x 100

CONCLUSION

Detection of elements present in an organic compound constitutes an important step in its analysis. All the organic compounds contain carbon. Hydrogen is also present in most of the organic compounds (the few exceptions are the compounds such as CCl4, CS2, etc.). In addition to carbon and hydrogen other elements which are generally present in organic compounds are oxygen, nitrogen, sulphur and halogens. AS,The Kjeldahl method or Kjeldahl digestion in analytical chemistry is a method for the quantitative determination of nitrogen contained in organic substances and Halogens are estimated by the process known as Carius method. 

Since nearly all the organic compounds contain carbon as well as hydrogen it is usually not necessary to carry out tests to detect them and their presence can be assumed without testing for them.