Volhard method: Reaction, Example, Procedure, Advantages & Precautions

It's a technique for determining chlorine, bromine, and iodine in halides by precipitating them with excess silver nitrate and titrating the excess with a solution of thiocyanate.

Precipitation titration is a type of titration in which a precipitate forms during the process of titration. Precipitation titration refers to a volumetric technique based on the formation of a slightly soluble precipitate, while the argentometric method refers to the titration using silver nitrate (AgNO3) as a precipitating agent. The most common application of this type of titration in analytical chemistry is to determine chloride using silver ions.

Table of Contents: What is Volhard's method? Reaction of Volhard method Example of Volhard method Procedure of Volhard titration Advantages of Volhard method

In a precipitation titration, the titrant reacts with a solute to produce an insoluble substance. It detects chloride levels using silver ions and continues until all solutes have been consumed. Precipitation titrations are classified as Mohr's method, Volhard’s method, and Fajan's method, depending on the type of application and method of detecting the endpoint.

What is Volhard's method of analysis?

In chemistry, Volhard’s method is a type of precipitation titration which is used for titrating silver with potassium thiocyanate standard. The titration is performed in an acidic solution. The first excess of titrant and the iron (III) indicator react to generate a soluble red-complex after the silver (I) is precipitated as white silver thiocyanate. Because the titration can be performed in an acidic solution, this technique is widely used for silver and chloride.

Volhard’s method was first reported in 1874 by German chemist Jacob Volhard. This technique determines halide ions (F, Cl, Br, I) and anions such as phosphates and chromates in acidic environments using silver ions. If the Volhard titration is not performed in an acidic medium, the iron ion will precipitate as hydrated oxide. Volhard's method uses iron ion as an indication. The first analyte (halide ion solution or any other anionic solution) is titrated with a measured excess of AgNO3.

The reaction involved in Volhard method:

If analyte contains chloride anions, the reaction will be as follows.

To titrate Ag+; determination of Cl- involves a back titration

First, Cl- is precipitated by excess AgNO3

Ag⁺ (aq) + Cl^- (aq)     AgCl(s)

In the presence of Fe3+, excess Ag+ is titrated with KSCN

Ag⁺ (aq) + SCN^- (aq)  AgSCN(s)

When Ag+ has been consumed, a red complex forms

Fe3⁺ (aq) + SCN^- (aq)   [FeSCN]²⁺(aq)

Example of Volhard titration method:

The titration of silver with ammonium thiocyanate using ferric alum as an indicator is an example of a Volhard titration in which a colored substance is formed in the solution.

Procedure of Volhard titration method:

• Procedure for determination of chloride ion concentration by titration (Volhard’s Method)
• Fill the burette with 0.1 M potassium thiocyanate solution and adjust the zero.
• Measure 100 ml of the sample and pour it into a conical flask.
• Add 01.00 ml of ferric ammonium sulphate solution as an indicator.
• Using the potassium thiocyanate (KSCN) solution, titrate the unreacted silver ions.
• Because of the ferric thiocyanate complex, the endpoint is the appearance of a dark red color.
• Repeat the titration three times to get an accurate result.
• Calculate the moles of potassium thiocyanate used.

Conditions and precautions of Volhard method:

• To avoid the precipitation of iron (III) as hydrated oxide, titration should be performed in an acidic medium.
• The concentration of the indicator should not exceed 0.2 M.
• The concentration of nitric acid (HNO3) in the resultant solution should be between 0.5 and 01.00 M during the titration.
• The temperature of the solution must be kept below 25°C as the red color of the ferric thiocyanate complex fades quickly at higher temperatures.

Advantages of Volhard method:

• It can analyze direct silver and indirect halide and has a very distinct color change.
• This method is useful where titration is to be carried out at low pH.
• The strong acidic conditions give an advantage for analysis halide since ions such as carbonate, oxalate, and arsenate do not interfere.
• It can provide accurate results due to back titration.

Frequently Asked Questions (FAQ):

What is the principle of precipitation titration:

The principle of precipitation titration is: Amount of added precipitating agent = the amount of a compound that is precipitated.

Why is Nitric acid used in the Volhard method?

It is used to provide the acidic medium and to prevent the precipitate of iron (III) as hydrated oxide.

Which indicator is used in the Volhard method?

In the Volhard method, Fe3+ (ferric ion) is used as an indicator, which produces dark red color at the endpoint of the reaction.

Why is the Volhard method performed in an acidic system?

To avoid Fe3+ precipitation as Fe (OH) 3, the titration must be performed in an acidic solution (pH 01-03) than the basic medium.

What is the use of the modified Volhard method?

The modified Volhard method is the advanced version of Volhard’s method used to determine the anions that precipitate with silver.

Why nitrobenzene is used in Volhard’s method?

To mask Ag+ ions of silver chloride; otherwise, these Ag+ ions will react with thiocyanate to form silver thiocyanate, resulting in a significantly higher titration reading and a high chloride content.