1. Factors effecting the solubility of precipitation
1.1. Temperature
1.2. Solvent
1.3. Acid
1.4. The formation of complex ions
2. Methods
2.1. Mohr's method
2.1.1. This method utilized choromate as indicator.
2.1.2. 2Ag+ + CrO42- -->Ag2Cr04
2.1.3. The method is applicable to the determination of Cl- and Br- , but not for I- or SCN- . Titration: Ag+ (aq) + Cl- (aq) AgCl (s) End point: Ag+ (aq) + CrO4 2- (aq) Ag2CrO4 (s) The first drop of excess Ag+ will react with the chromate indicator (reddish precipitate) after all Cl- has been consumed to formed AgCl.
2.2. Volhard's method
2.2.1. This method is to determine Cl- by titrating with Ag+ using back titration. First, Cl- is precipitated by excess standard AgNO3 (known quantity). Ag+ (aq) (excess) + Cl- (aq) AgCl (s) The precipitate is isolated and the excess Ag+ is titrated with standard thiocyanate (KSCN) solution in the presence of Fe3+. Ag+ (aq) + SCN- (aq) = AgSCN (s) After all of the Ag+ has been consumed, the SCN- reacts with Fe3+ to form a red complex. Fe3+ + SCN- = Fe(SCN)2+ The indicator system is very sensitive and usually good results are obtained.
2.3. Fajan's method
2.3.1. Fluorescein and its derivatives are adsorbed in the surface of AgCl. After the Cl is completed, Ag+ will react with FI-
2.3.2. Ag+ + FI- --> AgF
3. Limitations
3.1. It is a titrimetric method which involves the formation of precipitates during the experiment of titration. The titrant reacts with the analyte and forms an insoluble substance. The titration is continued till the last drop of the analyte is consumed. When the titrant is excess it reacts with the indicator and signals to terminate the titration process.
3.2. A few of halide ions will titrated with precipitation titration as Cl-, Br-, I-
3.3. Co-precipitation maybe occured
3.4. It is difficult to detect the end-point
4. Indicators
4.1. K2CrO4
4.2. Ag2Cr04
5. Denifition
6. Characteristics
6.1. fast, stoichiometry is known, secondary reaction is interference
6.2. depend on the solubility of product. The precipitation complete when Kps < 10^-8
7. Applications
7.1. Analyze drugs like :
7.1.1. Carbromal
7.1.2. NaCl Infusion
7.1.3. KCl infusion
7.1.4. Thyamine Hydrochloride