Question

Discuss the principle of estimation of halogens, sulphur and phosphorus present in an organic compound.

Short answer

Halogens are estimated using Lassaigne's test, sulphur by Carius method, and phosphorus by converting it to phosphomolybdic acid for analysis.

Step-by-step solution

Introduction to Estimation Principle

The estimation of halogens, sulphur, and phosphorus in an organic compound is quantitatively determined using specific methods. These elements are identified and measured to understand the compound's composition.

Principle of Estimation of Halogens (Lassaigne's Test)

Halogens in an organic compound are estimated using Lassaigne's test. The test involves heating the organic compound with metallic sodium to form sodium halides. These halides are then treated with silver nitrate after acidification, forming a precipitate that confirms the presence of halogens. The quantity of halogens is calculated based on the weight of the precipitate formed.

Principle of Estimation of Sulphur

Sulphur is estimated using the Carius method. The organic compound is oxidized using fuming nitric acid in the presence of barium chloride to form barium sulphate. After filtration and weighing, the amount of sulphur is calculated from the mass of the barium sulphate precipitate produced.

Principle of Estimation of Phosphorus

Phosphorus is estimated using its conversion to phosphomolybdic acid. The organic compound is heated with fuming nitric acid. Phosphorus present is converted into orthophosphate ions, which are then reacted with molybdic acid to form phosphomolybdic acid. This complex can be quantitatively analysed to determine the amount of phosphorus present.

Key concepts

Lassaigne's Test

Lassaigne's Test is a classical method used to identify the presence of halogens, such as chlorine, bromine, and iodine in an organic compound. The process begins by heating the organic compound with metallic sodium. This is because sodium can bond with halogens present in the compound to form sodium halides.
Once the sodium halides are formed, the mixture is then treated with silver nitrate after acidifying the solution. The acidification step is crucial as it prevents the formation of alkali metal hydroxides that could interfere with the test. The presence of halogens is indicated by the formation of a distinct precipitate.

• Chlorine forms a white precipitate of silver chloride.
• Bromine results in a pale yellow precipitate of silver bromide.
• Iodine gives a yellow precipitate of silver iodide.

By weighing the precipitate and using stoichiometric calculations, the amount of halogen present in the compound can be determined. Lassaigne's test is valuable for its simplicity and effectiveness in detecting halogens.

Carius Method

The Carius Method is a time-tested technique for estimating the amount of sulphur in organic compounds. This process involves the oxidation of the organic compound by heating it with fuming nitric acid, a strong oxidizing agent.
In the presence of barium chloride, all sulphur in the compound is converted to barium sulphate, which is insoluble. This conversion is vital because barium sulphate is a well-defined compound, allowing precise gravimetric analysis.
After the reaction completes, the barium sulphate precipitate is filtered, dried, and weighed. The weight of barium sulphate is directly proportional to the amount of sulphur in the original compound. This process not only confirms the presence of sulphur but also provides a quantitative measure:

• The weight of the barium sulphate is used to calculate the sulphur content.
• It ensures accuracy and reproducibility.

The Carius Method remains a reliable means of quantifying sulphur content due to its robust procedural framework.

Phosphorus Estimation

Phosphorus estimation in organic compounds is achieved through its conversion into phosphomolybdic acid. This conversion starts by treating the organic compound with fuming nitric acid. Amid this reaction, phosphorus is transformed into orthophosphate ions.
These ions further react with molybdic acid to form phosphomolybdic acid, a complex that can be quantitatively analyzed. This method hinges on the formation of a colored complex, allowing for measurement through spectrophotometric methods.
The principle of phosphorus estimation includes several critical steps:

• Treat the sample with fuming nitric acid to ensure complete oxidation.
• Convert phosphorus into orthophosphate ions for further reaction.
• Form a measurable phosphomolybdic acid complex.

Once the complex is formed, the concentration of phosphorus is determined by measuring the intensity of the color of the complex. This method provides accuracy and precision in the estimation of phosphorus content in organic compounds.