TITRASI ARGENTOMETRI (PART 4) : METODE FAJANS || KIMIA ANALITIK

Summary

The Fajans method is an argentometric titration technique that uses adsorption indicators to detect the endpoint. By adding silver nitrate (AgNO3) to a solution containing chloride ions, a precipitate forms. The indicator, adsorbed onto the precipitate, causes a color change once the equivalence point is reached. This method is fast, accurate, and can be used in acidic conditions. However, it is limited to anions that form colloidal precipitates with Ag+, such as chloride, bromide, iodide, and selenide. The endpoint is indicated by changes in the solution's color and clarity.

Takeaways

• 😀 The Fajans method uses an adsorption indicator to detect the endpoint of a titration with AgNO3 solution.
• 😀 The adsorption indicator absorbs onto the precipitate and causes a color change when it interacts with the silver ions (Ag+).
• 😀 These indicators are typically weak acids or bases that form precipitates with Ag+ and can be used across different pH ranges.
• 😀 Some common adsorption indicators used in the Fajans method include Fluorescein (pH 7-8), Dichlorofluorescein (pH 4-8), Eosin (pH 2-8), and Bromphenol Blue (pH 3-8).
• 😀 The mechanism of indicator absorption involves the indicator binding to the surface of the AgCl precipitate during titration.
• 😀 Before reaching the equivalence point, the AgCl precipitate is negatively charged due to unreacted chloride ions (Cl-).
• 😀 At the equivalence point, chloride ions have reacted with Ag+, and the precipitate becomes neutral, leading to changes in its charge.
• 😀 After the equivalence point, adding more AgNO3 causes the AgCl precipitate to absorb Ag+ ions and become positively charged.
• 😀 The endpoint of the titration is indicated by a color change in the indicator, such as the formation of a red color at the endpoint.
• 😀 The advantages of the Fajans method include its speed, accuracy, and versatility in both acidic and basic conditions, though its application is limited to specific anions (e.g., chloride, bromide, iodide).

Q & A

What is the Fajans method used for?

The Fajans method is used for argentometric titrations, which involves using a silver nitrate (AgNO3) solution to determine the concentration of halide ions in a sample.

How is the Fajans method similar to the Mohr method?

Both methods use AgNO3 as a titrant, but the Fajans method differs by employing adsorption indicators instead of chromates or dichromates as in the Mohr method.

What is the role of adsorption indicators in the Fajans method?

Adsorption indicators are weak acids or bases that are adsorbed onto the surface of silver halide precipitates and cause a color change, which helps to identify the endpoint of the titration.

What happens to the silver halide precipitate during the titration in the Fajans method?

During the titration, the silver halide precipitate initially adsorbs chloride ions (Cl-) and becomes negatively charged. After the equivalence point, the precipitate adsorbs silver ions (Ag+) and becomes positively charged.

Can you name some commonly used adsorption indicators in the Fajans method?

Some commonly used adsorption indicators in the Fajans method include fluorescein, diklorofluorescein, eosin, and bromophenol blue, each effective at different pH ranges.

What is the mechanism behind the adsorption of the fluorescein indicator on silver chloride (AgCl) in the Fajans method?

Before the equivalence point, fluorescein ions are adsorbed onto the surface of AgCl, resulting in a negatively charged precipitate. After the equivalence point, the AgCl adsorbs Ag+ ions, becoming positively charged, and fluorescein reacts, indicating the titration's endpoint.

What color change indicates the endpoint of the titration in the Fajans method?

The endpoint is indicated by a color change from a pale white precipitate to a pink-red color, signaling the formation of a colored complex between the silver halide and the adsorption indicator.

What observations can be made during the titration to confirm the endpoint?

At the endpoint, three observations can be made: (1) the precipitate color changes from white to red-pink, (2) the solution becomes clearer, and (3) a yellow-green solution turns nearly colorless.

What are the advantages of using the Fajans method?

The Fajans method is fast, accurate, and can be used in acidic pH conditions, making it versatile for various titrations involving silver halides.

What are the limitations of the Fajans method?

The method is limited to halide ions that can form colloidal precipitates with silver ions, such as chloride, bromide, iodide, and selenide.