Precipitation

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Precipitation titration

Titrations with precipitating agents are useful for determining certain analytes e.g. Cl- can be determined when titrated with AgNO3.

Detection of end point:
Chemical
Precipitation Type - Mohr’s method
Adsorption – Fajan’s method
For silver analyses –Volhard method
Sensors –Potentiometric or amperometric
The chemical types are also classified into:
1.Indicators reacting with titrant forming specific color.
2.Adsorption indicators.

Indicators reacting with the titrant:
Two methods will be discussed where this type of indicators are applied; namely: Mohr and Volhard.

I) Mohr method for determining chloride:

Chloride is titrated with AgNO3 solution. A soluble chromate salt is added as the indicator. This produces a yellow color solution. When the precipitation of the chloride is complete, the first excess of Ag+ reacts with the indicator to precipitate red silver chromate: 2 Ag+(aq) + CrO42–(aq) → Ag2CrO4(s)

Yellow red ppt The Mohr method must be performed at a pH about 8. This method is useful for determining Cl- in neutral or unbuffered solutions such as drinking water.

II) Volhard titration:

This is an indirect titration procedure for the determination of anions that precipitate with silver like CL-, Br-, I-, SCN-, and it is preferred in acid (HNO3) solution. A measured excess of AgNO3 is added to ppt the anion, and the excess of Ag+ is determined by back titration with standard potassium thiocyanate solution: Ag+(aq) + Cl–(aq) → AgCl(s) + excess Ag+

excess Ag+(aq) + SCN–(aq) → AgSCN(s)
The end point is detected by adding iron III (Fe3+) as ferric ammonium sulfate which forms a soluble red complex with the first excess of titrant. Fe3+(aq) + SCN–(aq) → [FeSCN]2+(aq)
These indicators must not form a compound with the titrant that is more stable than the precipitate or the color reaction would occur on addition of...
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