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I'm not an expert teacher or lecturer of chemistry. I was only a student from SMA NEGERI 15 SURABAYA who had been one of the Bronze Medalist Participants of Olimpiade Sains Nasional X (2011) of Chemistry In Manado, North Sulawesi, 11 - 16 September 2011 and graduated in 2012. Now, I'm studying at Universitas Airlangga in Surabaya, Indonesia. I do love chemistry and I would like to help them who had difficulties in studying chemistry. That's why, please understand me if you found some misconcepts in my entries. Suggestions are always necessary in order to develop this blog. And I'm sorry because my English isn't so well.

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Sunday, December 18, 2011

Argentometric Titration - Fajans Method

I already posted about argentometric titration in the previous month. Now let me continue about this titration. As I told you before that there are 3 well-known methods in argentometric titration: Mohr Method, Vollhard Method, and Fajans Method. Still we have to keep this in our memory tightly: Titration is a method to measure quantitatively an unknown concentration of solution using a standard solution and indicator.

In Mohr Method, we use silver nitrate as the titer reagent and potassium dichromate as the indicator. Because the solubility of silver halides (Ex: Chloride and Bromide ion) are less than silver dichromate so that if there are precipitates of silver dichromate formed, that means that the titration reach the end-titration point.

In Vollhard Methode, we use silver nitrate (in nitric acid solution to make a lower pH) first as the titer reagent. Like I said before, Because the solubility of silver halides (Ex: Chloride and Bromide ion) are small, we can easily make precipitates of silver halides. We don't need to worry whether the titration already reached the end point or not because the excess of silver nitrate will be titrated again with Potassium thiocyanate as the titer reagent and ferri ion as the indicator. The purpose of the second titration is to calculate the excess of silver nitrate titrated to halides solution.


Fajans Method


A Halide salt solution is titrated with a standard solution of silver nitrate in a neutral environment (pH 7 - 10) with a fluorescein indicator (adsorption indicator). In equivalent point, white precipitates of silver chloride become rose because of the adsorption of silver chloride precipitates to the indicator.

See the picture above:


This method must be done with low intensity of the sunlight and the erlenmeyer glass must be coated with carbon paper in the process of titration.

Source:
Petunjuk Praktikum Kimia Analitik I Universitas Airlangga Surabaya 2005


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