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Precipitation Titration

Analytical Chemistry Presentation

Shannon Gunn

on 22 April 2010

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Transcript of Precipitation Titration

Precipitation Titration The Determination of Chloride by Titration with an Adsorption Indicator The objective of this experiment was to determine the chloride content of an unknown salt by titration of silver nitrate using an adsorption indicator. The titration in this experiment is based upon a precipitation reaction between the silver in silver nitrate and the chloride in an unknown salt. Silver Nitrate (Just as an aside, getting silver nitrate on your skin discolors it for an extended period of time, so be careful!) The indicator used to signal the equivalence point of the reaction was dichlorofluorescein, an ADSORPTION INDICATOR. Adsorption indicators work by marking the point when excess chlorine ions on the surface of the precipitate are replaced by excess silver ions. The negatively charged dichlorofluorescein (In-) is attracted to the positively charged silver ions (Ag+), is adsorbed, and changes color. Procedure 10 mL unknown chloride solution
5 drops dichlorofluorescein
~0.1g dextrin * Contents of Flasks Contents of Burette burette rinsed with silver nitrate solution *
50 mL 0.02494M silver nitrate
check for bubbles
Titration Procedure flasks placed under burette
titrant added until light pink
performed on 3 samples
Results * * These readings are not correct. They should have two digits after the decimal place. The principle of the equation... ...and the actual calculations for a sample. The Math * Dextrin was used in order to stabilize the colloid and increase the surface area of the particles, as silver chloride in a uniformly distributed particle state is optimum for dichlorofluorescein. The formation of a precipitate decreases surface area and impedes the performance of the indicator. * This was done because the silver nitrate was a standard solution, and we have to protect against contamination or dilution. Conclusions It worked! As expected, the analyte turned yellow when the dichlorofluorescein was added, then milky white when the AgCl precipitate began to form, then light pink when the indicator adsorbed to the excess silver ion at the end point of the reaction. However... Mistakes were obviously made. Both the standard deviation and the relative standard deviation were high, indicating some sort of error. What happened? Possible sources of error include improper preparation of the standard silver nitrate or poor titration technique. The End The surface of AgCl precipitate after the endpoint, when positively charged silver ions are in excess.
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