TITRASI ARGENTOMETRI (PART 3) : METODE VOLHARD || KIMIA ANALITIK

Summary

The Volhard method is an indirect titration technique used to determine the concentration of halides like NaCl. The process involves adding an excess of silver nitrate to the sample, followed by titration of the remaining silver nitrate with potassium thiocyanate in an acidic solution. The endpoint is identified using a ferric ammonium sulfate indicator, forming a red complex. The method is efficient for halides, although precautions are necessary to avoid interference from precipitates. The procedure is demonstrated with a practical example, determining the NaCl content in table salt, which is calculated to be 96.5%.

Takeaways

• 😀 The Volhard method is an indirect titration technique used for determining halide ions like chloride (Cl⁻) in a solution.
• 😀 In the Volhard method, excess silver nitrate (AgNO₃) is added to the test solution, and unreacted AgNO₃ is titrated with a potassium thiocyanate (KSCN) solution.
• 😀 Ferric ammonium sulfate is used as an indicator in the Volhard method, and the endpoint is marked by the formation of a red color due to the ferric-thiocyanate complex.
• 😀 The Volhard method is conducted in an acidic environment to prevent the formation of ferric hydroxide, which would interfere with the titration.
• 😀 A common challenge in the Volhard method is the interference of silver chloride (AgCl) precipitate formed during the reaction, which may react with the titrant.
• 😀 To avoid interference, the silver precipitate can be filtered out, dissolved with nitric acid, or wrapped in nitrobenzene to prevent it from reacting with the titrant.
• 😀 The Volhard method works well for halide ions like chloride, bromide, and iodide, which have lower solubility compared to silver sulfide (Ag₂S).
• 😀 The concentration of thiocyanate used in titration is typically 0.1 M to ensure the endpoint is observable despite the very low concentration needed in some cases.
• 😀 The general procedure includes adding nitric acid, silver nitrate, an indicator, and nitrobenzene to the sample, followed by titration with potassium thiocyanate.
• 😀 An example of the Volhard method involves dissolving 0.2 grams of table salt in 50 mL of water, adding silver nitrate and an indicator, and titrating with potassium thiocyanate, resulting in a NaCl content calculation of 96.5%.

Q & A

What is the Volhard method in titration?

The Volhard method is an indirect titration or back titration where a known concentration of AgNO3 solution is added in excess to a test solution. The excess AgNO3 is then titrated using a thiocyanate solution with a red color formation as the endpoint, indicating the formation of a ferric-ammonium thiocyanate complex.

Why does the Volhard method work in an acidic environment?

The Volhard method requires an acidic environment because, in a neutral or basic environment, the ferric indicator would change into ferric hydroxide, which is dark orange in color, making it difficult to detect the endpoint.

What is the role of the ferric-ammonium thiocyanate complex in the Volhard method?

The ferric-ammonium thiocyanate complex is responsible for the red color change observed at the endpoint of the titration. This color change signifies the completion of the titration.

How is the amount of analyte determined in the Volhard method?

The amount of analyte is determined by subtracting the moles of excess AgNO3 from the total moles of AgNO3 added to the solution, using the stoichiometric relationship between AgNO3 and the analyte.

What type of anions can be determined using the Volhard method?

The Volhard method is effective for determining anions such as cyanide (CN-), bromide (Br-), iodide (I-), and chloride (Cl-), among others, due to their solubility properties with silver ions.

What problems might occur during the Volhard method titration?

A potential problem is the precipitation of the analyte as an AgX compound, which can react with the excess silver nitrate or interfere with the titration. To avoid this, methods such as filtering the precipitate or using a modifying agent like nitrobenzene are employed.

What is the purpose of adding nitric acid in the Volhard method?

Nitric acid is added to the test solution to lower the pH and create the acidic environment necessary for the proper reaction during the titration process.

How can the reaction of the AgX precipitate with the titrant be prevented?

To prevent the AgX precipitate from reacting with the titrant, the precipitate can be filtered out, dissolved with nitric acid, or treated with nitrobenzene to encapsulate it.

Why is potassium thiocyanate typically used as the titrant in the Volhard method?

Potassium thiocyanate is used because it forms a well-defined red complex with ferric ions, allowing for a clear endpoint detection in the titration.

How is the NaCl content in a sample determined using the Volhard method?

The NaCl content is determined by calculating the moles of silver nitrate added, subtracting the moles of potassium thiocyanate used in the titration, and using stoichiometric relationships to find the percentage of NaCl in the sample. In the example provided, the NaCl content was found to be 96.5%.