A titrimetric method based on the formation of a slightly soluble precipitate is called a precipitation titration. PRECIPITATION TITRATION

Involves formation of a precipitate. Must determine the volume of a standardized titrant needed to justprecipitate all of the ion.Need an indicator or electrode to determine when the precipitation is complete.PRINCIPLE OF PRECIPITATION TITRATIONS

SOLUBILITY OF PRECIPITATES It refers to the amount of solute capable to dissolve in a volume of solvent. Therefore , the chemical substances can be classified depending upon its solubility to : soluble , or partially soluble , or insoluble .

Concentration of ion solubilitysaturated

SOLUBILITY PRODUCT CONSTANT (K.S.P) : K.S.P is an important constant which is applied to saturated solution of slightly soluble but completely ionized such as salts or metal hydroxide. It is calculated from multiplying of anion and cation concentrations : AB A+ + B-

K.S.P = [ A+] [ B- ]

SOLUBILITY PRODUCTPrecipitation titrations produce sparingly soluble salts. Consider a precipitate, AgCl AgCl (AgCl) Ag+ + Cl- Keq = Ksp = [Ag+][Cl-] = solubility product. Precipitation will not occur unless the product of [Ag+] and [Cl -] exceeds Ksp.

FACTORS AFFECTING SOLUBILITY OF A PRECIPITATE1. Temperature 2. Solvent3. Common Ion Effect4. Effect of pH 5.Effect of Hydrolysis6. Effect of Complexation

When anion or cation of precipitate is capable to react with [H3O+] or [OH-] of solvent . CaF2 Ca+2 + 2F- 2H3O+

2HF + 2H2O

EFFECT OF HYDRONIUM ION CONCENTRATION (

CaF2 is a salt of a weak acid (HF). According to Le Chatelier principle,increasing in [H3O+] concentration causes an increasing in HF concentration and decreasing in F- concentration. Then solubility of precipitate will be increase.

EFFECT OF TEMPERATURE Increasing of temperature will causes an increasing in solubility of precipitate. It leads to an increase in KSP value for precipitates which are slightly soluble.

EFFECT OF SOLVENTInorganic salts are more soluble in water. A decrease in solubility in organic solvents can be used to separate two compounds. Example : mixture of Ca(NO3)2 and Sr(NO3)2 can be separated in solvent mixture containing alcohol and ether, which yields a soluble Ca(NO3)2 and an insoluble Sr(NO3)2

Argentometric methodMercurimetric methodKolthoff titrationThe different types of precipitation titrations are:

Argentometry is the useful method of precipitation titration, as it is caused of very low solubility product of halide salts. There are 3 techniques of end point determination:

Mohrs method (indicator : chromic potassium)Volhard method (indicator : ferric salt)Fajans method (indicator : fluorosceine)

ARGENTOMETRIC TITRATION

MOHRS METHODDeveloped for the determination of Cl- and Br -.Uses AgNO3 as titrant and Na2CrO4 as indicatorTitration reaction:Ag+ + Cl- AgCl(s) Ksp= 1 x 10-10

Indicator reaction:2Ag+ + CrO42- --> Ag2CrO4

(excess) (yellow) (red)Ag2CrO4 is precipitated at the equivalence point

VOLHARDS METHODVolhard titration is an indirect (back titration) technique which is used if reaction is too slow or if there is no appropriate indicator selected for determining the equivalent point.

Titration principle : Silver solution is added excessively to halide: Br- + Ag+ AgBr (precipitate) excess

After reaction has completed, the precipitate is filtered, then the filtrate is titrated with a standardized thiocyanate solution. Ag+ + SCN- AgSCN (solution) Fe(III) indicator reacts with thiocyanate ion to form a red colour solution : Fe3+ + SCN- [Fe(SCN)]2+

The reaction requires acid condition, as in basic solution the ferric ions form Fe(OH)3 precipitate.

FAJANS METHODFajans titration use adsorption indicators, i.e. organic compounds which is adsorbed into colloidal precipitate surface during the titration processes. Example : Fluoreseine in form of its fluorescenate (yellowish green)

Anion react with Ag+ to form an intensive red precipitate which is adsorbed to AgCl precipitate surface caused by ionic pair interaction Adsorption of colour organic compound on a precipitate surface could induce intermolecular electronic shift then change solution colour. This phenomenon is usually used for end titration detection of silver salt precipitation.

Fluorescein is a weak organic acid, forms a fluoresceinate ionwhich is not adsorbed by colloidal AgCl precipitate during excess of Cl-. But in the solution with excess of Ag+ the fluoresceinate anion will be adsorbed to form a Ag+ shielding layer, followed by change the solution colour to pink.

INDICATOR ANALYTE TITRANT REACTIONCONDITION

Dichlorofluorescein Cl- Ag+ pH = 4Fluorescein Cl- Ag+ pH = 7 8Eosin Br-, I-, SCN- Ag+ pH = 2Thorin SO42- Ba2+ pH = 1,5 3,5 Methyl violet Ag+ Cl- acid solutionBromphenol blue Hg22+ Cl- solution of 0,1 MOrthochrome T Pb2+ CrO42- neutral 0,02 M soln

MERCURIMETRIC TITRATIONWhen halide ions is titrated with mercuric nitrate solution,[Hg2+] is not found at the equivalent point.It is found in HgCl2 precipitate. After equivalent point, [Hg2+] increases, react with indicator to form a Hg-indicator complex.Hg2+ +2 Cl- HgCl2 e.g. Nitropruside form white precipitate, acid solution of diphenylcarbazide in forms intensive violet colour solution.

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