What is the Use of Nitrobenzene in the Assay of Halides, Ammonium Chloride, and Thiourea By Volhard’s Method

Everything in the physical world is a result of chemical reactions; the food we eat, the washing of our clothes, the effect of medicines, and adding polish to jewellery, everything involves different chemicals reacting with each other to produce phenomenal results. 

Often, an analyte, the substance to be analysed, is a mixture of various compounds, and it is necessary to know the concentration of each one of them.

In such cases, quantitative chemical analysis proves to be a crucial tool. Quantitative chemical analysis helps us determine the exact composition of a given analyte and determine how to recreate the analyte if the need arises. One important and most widely used method in quantitative chemical analysis is precipitation titration.

What is Precipitation Titration?

It is the process of determining the concentration of an unknown substance A with the help of a known substance B, whose concentration and volume are known to us. The known substance B is added in increments to the analyte, containing the unknown substance A.

The known substance B is known as the titrant, and the unknown substance A is the analyte. In precipitation titration, the goal is to attain an insoluble solid substance called the precipitate. The change in the weight of the precipitate serves as the basis of the titration.  

The titration is said to be complete when the addition of more titrant does not change the weight of the precipitate formed. A critical method of performing precipitation titration to analyse halides, chlorides, and thiourea is using the Volhard Method.

The Volhard Method 

Invented by the German scientist Jacob Volhard in 1874, the Volhard Method aids in identifying halide, chlorides and thiourea ions in a given compound. It relies on two significant chemicals to achieve this goal. The first chemical is a compound of silver and the second compound is potassium thiocyanate. The first chemical is often silver nitrate (AgNO₃).

The Volhard Method works because of the higher reactivity of halide, chloride and thiourea ions with respect to the silver ion. When a compound containing these halides or chlorides ions is added to AgNO₃, it forms silver chlorides or halides.

The indicator in the Volhard Method is the presence of the iron (III) ion. When the reactant is titrated with AgNO₃, the concentration of silver ions in the solution mixture drastically decreases. But the concentration of the cyanate ion SCN– does not change.

Therefore, as the titration proceeds, the concentration of silver ions decreases, giving the iron (III) ion a chance to form a complex with the cyanate ion. When all the silver ions are consumed, the iron (III) ion is free to form complexes, resulting in the formation of a red coloured complex called Ferric cyanate. The formation of the red coloured precipitate signals the completion of the titration.

Reactions and Advantages

The most common usage of the Volhard Method is to detect chloride in a compound. The process involves back titration. The reactions that are involved in the reaction are;

Hence, the first step is to reduce the silver concentration in the solution.

Ag⁺ (aq) + Cl^– (aq) → AgCl (s)

Since the Cl^– now gets precipitated with the Ag ion, the excess Ag⁺ ion must be titrated with KSCN or potassium cyanate. The reactions are

Ag⁺ (aq) + SCN^– (aq) → AgSCN (s)

After the point where all of the silver has been consumed, a red coloured iron (III) complex is formed. The reactions can be given as follows;

Fe³⁺ (aq) + SCN^–  (aq) → [FeSCN]²⁺ (aq)

The formation of the red coloured complex indicates the end of the titration.

A major advantage of the Volhard Method is the distinct colour change during precipitation. This allows fewer errors in determining the exact point when the titration should stop.

The Volhard Method effectively detects indirect halide ions and provides highly accurate results because it uses back titration. The acidic conditions in which the Volhard Method has to be carried out prevent the interference of other ions such as carbonate, oxalate, and arsenate.

Why is nitric acid used in the Volhard Method?

To detect halide and chloride ions, a compound of silver is used for back titration. This is the basis of the Volhard Method. The silver compound used for back titration is silver nitrate (AgNO₃).

Nitric acid is used along with a silver foil to form silver nitrate. This ensures that the chloride and the halide ions are precipitated with the help of nitric acid. The nitric acid also helps maintain an acidic medium that ensures that other ions like carbonates, oxalates, etc., do not interfere with the titration.

Conclusion

We need to know the composition of various complexes found in nature. But to find this composition, different quantitative chemical analysis methods have to be used.

One popular method of doing quantitative chemical analysis is precipitation titration. Precipitation titration involves adding a certain amount of a known substance to determine the concentration of an unknown substance. A popular method of doing precipitation titration is the Volhard Method. This method allows iron (III) ions to find the concentration of halides, chlorides, and thiourea in a compound.

The Volhard Method relies on back titration and uses it to form halide and chloride compounds of silver. After consuming the excess silver ions by the cyanate ion, the iron (III) ions react with the cyanate ion to form a red coloured complex that indicates the end of the titration. The main advantage of this method is the clear distinction it provides during titration and the acidic medium that prevents other ions from interfering.