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Volhard method is a technique to determine chlorine, bromine, and iodine in halides through precipitation with excess silver nitrate and titrating the excess with a solution of thiocyanate. A titration in which a precipitate is formed during the process of titration is known as precipitation titration. It is a volumetric technique based on the formation of a slightly soluble precipitate. Titration is performed by placing a burette that contains a known concentration of a titrant above a beaker containing a solute. The titrant is added in a slow manner until the reaction is complete.
Key Terms: Titration, Precipitation Titration, Hydrogen, Silver, Solution, Ions, pH, Acids, Oxalate
What is Volhard Method?
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A reaction in which an alpha hydrogen group of a carboxylic acid is replaced by a halogen is termed the Volhard method. It is used to titrate silver with potassium thiocyanate standard and is performed in an acidic solution. If it is not performed in an acidic medium, the iron ion will precipitate as hydrated oxide.
German chemist Jacob Volhard first reported the volhard method in 1874. This technique determines halide ions (F, Cl, Br, I) and anions like phosphates and chromates in acidic mediums using silver ions.
Titration
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Volhard Method Procedure
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The procedure for determination of chloride ion concentration by titration (the volhard method) is as follows:
- Fill a burette with 0.1 M potassium thiocyanate solution.
- 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.
- Titrate the unreacted silver ions using the potassium thiocyanate (KSCN) solution.
- The endpoint will have a dark red colour because of the ferric thiocyanate complex.
- Repeat the titration three times to get an accurate result.
- Calculate the moles of potassium thiocyanate used.
Reactions of Volhard Method
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To understand better, a few reactions involved in the volhard method are given below.
For an analyte that contains the chloride ions, the reaction would 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)
Advantages of Volhard Method
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There are several advantages of the volhard method which are illustrated below.
- It analyses direct silver and indirect halide and has a very distinct colour change.
- This method is useful when titration is to be carried out at a low pH.
- For the analysis of halide, the strongly acidic conditions give an advantage since ions such as carbonate, oxalate, and arsenate do not interfere.
- It provides accurate results due to back titration.
Things to Remember
- When in a reaction, an alpha hydrogen group of a carboxylic acid is replaced by a halogen, the reaction is termed the Volhard method.
- Volhard method is useful when titration is to be carried out at a low pH.
- When during the process of titration a precipitate is formed, the titration is known as precipitation titration.
- Volhard method is used to titrate silver with potassium thiocyanate standard and is performed in an acidic solution.
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Sample Questions
Ques: What are the methods of determination of chlorides? [2 marks]
Ans: The chloride ion concentration can be calculated by subtracting the titration findings of silver ion moles from the total moles of silver nitrate added to the solution. This approach is used when the solution has an acidic pH after the sample is prepared.
Ques: Which indicator is used in the volhard method? [1 mark]
Ans: In the volhard method, the Fe3+ (ferric ion) is used as an indicator.
Ques: Define the Argentometric method. [2 marks]
Ans: In analytical chemistry, argentometry is a form of titration which involves the silver(I) ion. This is usually used to determine the chloride content present in a sample. A solution of silver nitrate of known concentration is titrated against the sample solution.
Ques: Why is Nitric acid used in the volhard method? [1 mark]
Ans: Nitric acid is used to provide the acidic medium and to prevent the precipitate of iron (III) as hydrated oxide.
Ques: What is the principle of precipitation titration? [1 mark]
Ans: The principle of precipitation titration is, “amount of added precipitating agent = the amount of a compound that is precipitated”.
Ques: What is a precipitation reaction? [1 mark]
Ans: When two solutions containing soluble salts are mixed, a precipitation reaction occurs, resulting in the formation of an insoluble salt. The precipitate is the insoluble salt that emerges from the solution.
Ques: What are the various types of precipitation titration? [2 marks]
Ans: Precipitation titration is classified into three types which as follows
- The Volhard method
- The Fajan method
- The Mohr method
Ques: What are the uses of precipitation titration? [3 marks]
Ans: The precipitation titration is used as discussed below.
- It is used to determine the concentration of halide ions in a solution.
- It is used to determine the salt content of foods, beverages, and water.
- It is used to make sulphur, thiocyanate, dichromate, and other chemicals.
- It is used in many drugs, including carbromal, KCl infusion, NaCl infusion, and others.
Ques: Explain the difference between the mohr, fajan and volhard methods. [2 marks]
Ans: The main difference between the Mohr and Volhard methods is that the former refers to the reaction between silver ions and halide ions in the presence of a chromate indicator, whereas the latter refers to the reaction between excess silver ions and halide ions. Meanwhile, the Fajans method refers to the reaction of silver halide and fluorescein adsorption.
Ques: What are the limitations of the precipitation reaction? [2 marks]
Ans: There are several limitations to the precipitation titration which are as follows:
- The precipitation titration method can only titrate a small number of halide ions.
- Coprecipitation can also occur.
- The endpoint is extremely difficult to assess.
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