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UNIT Il, , , , , , Chapter ... 4, PRECIPITATION TITRATIONS, , , , , , ¢ LEARNING OBJECTIVES «, © ‘To understand the principle of precipitation titration., © To calculate and plot a precipitation titration curve in which a single precipitate is formed., © To determine the graphic end point of a precipitation titration curve., © To explain how formation of a coloured precipitate can be used to determine end point., , , , OUTCOMES, , On satisfying the requirements of this course, students will have the knowledge and skills to:, , * Anunderstanding to the basic principles involved in precipitation titration., , * Estimation of analytes by using various end point detection methods of precipitation, titration., , 4.1 PRECIPITATION REACTION |, Precipitation is the formation of a solid in a solution, and solid formed is called the, , precipitate. Precipitation reaction occur when water solutions of two different ionic, , compounds are mixed and an insoluble solid separates out of solution. Silver nitrate is, , commonly used as a precipitating reagent in argentometric titrations., , Example:, KCl e AgNO3 — AgCll + KNO;, , (CI” solution) (Precipitating agent) (Precipitate), , The precipitation is itself an ionic process, the cations comes from one solution and, precipitation of ionic products forces the reaction to completion. The, titrimetric analysis are:, , , , , , , , , , , , anions from another;, requirements for a reaction to be useful in, * The precipitate must be practically insoluble., , * The precipitation reaction should be rapid and quantitative., , © The titration result should not be hampered by adsorption (co-precipitation) effects., , * It must be possible to detect the equivalence point during titration., , (4.1)

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dy, , Pharmaceutical Analysis 4.2 Precipitation Titration,, , Other example, , NaCl + AgNO; <> AgCl 1 + NaNO;, , AgNO,, , , , pe, , NaCl (aqueous solution), /——— AgCI (Precipitate), , , , , , , , , , , , , , , , , , that the solvent environment must be simi, , milar to that provided by the crystal structures. This is, , solute forces replace the solute-solvent forces., , soluble salt “BA” in equilibrium with excess of soli, , BAiy <> B* + AM, Where;, , BA,s) represents the solid phase,, , quilibrium constant for equation (4.1) may be’, written as,, , g, 7°, W, , (B*) [47], = Solubility product (which is constant), Solubility product is important because it Permits the calc, concentration if the concentration of the other is known. A s, the product of the ionic concentration exceeds the Ksp value., , ~, a, 3, , f, , ulation of one of the iof, ubstance precipitates out when, , In the equation (4.1) solid BA will precipitates out, when the product of [A*, , ] exceeds Kspie. Ksp < [B*}IA7), , J

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Pharmaceutical Analysis 43 Precipitation Titrations, [43 EFFEC, Effects of Acids:, , The solubility of a salt will be increased by decrease in pH (or increasing acidity); if the, anion of the salt is the conjugate of weak acid., , Example: Consider the slightly soluble salt BA, the anion of which [A] is the conjugate, base of a weak acid HA., , , , ace, , SLUBILITY:, , , , , , , , , , , , Here,, Two equilibrium in operation, BA (s) <> B* +A +++ 4.2), HA <—>3H* +A .-» (4.3), , The [A’] from equation (4.2) will shift the equilibrium (4.3) towards left., While, equation (4.2) will itself be shifted to right., , Hence, solubility of BA is increased with increase in H* (or decrease in pH)., The equilibrium expression is,, , Kp = (B*] [A7] w- (4.4), H*] [A7, Ka = aa ws (4.5), , Molar solubility of BA is equal to [B*] which is equal to the total concentration of [A™], ie. the [A7] dissolved from BA and that which is present in HA., , s = [B*] = [A] + [HA] «+» (4.6), From equations (4.4) and (4.5),, , K., -) _ =e, (Al = Br], , And ;, HY) TAT, Put the values of [A"] and [HA] in equation (4.6), : K. H*) [A", s=B1= Ct” Bt (47), , Again put the value of [A7] in equation (4.7),, Ko (H*LIAJK., , s= [B*] = [B*) + Ka. (B’] eae (4.8), K. H*, s = [B*] = moe, Kip (2 + (H*, , s = [B*? = = w- (4.9), , s = VK (1 + [H*V/Ka) .-. (4.10), , Equation (4.10) relates molar solubility (s) to Ksp, Ka and [H*) ion concentration.

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Pharmaceutical Analysis 44 Precipitation Titration,, , Effect of Temperature:, , Solubility of the most inorganic salts is increased by elevating the temperature of the, solution. Solubility of some substances influenced by temperature is small, but with Others it, is more. It gives advantage if the precipitation process is carried out in hot solution because, impurities are dissolved more readily and filtration is faster. However, in the case of fairly, soluble compounds, like magnesium ammonium phosphate, the solution must be Cooled in, ice water before filtration to prevent the loss of precipitate in filtrate., , Examples:, , Solubility of AgCI at 10°C is 1.72 mg/L and 100°C 21.1 mg/L., , Solubility of BaSO, at 10°C is 2.2 mg/L and 100°C 3.9 mg/L., Effect of Solvent:, , Most of the inorganic salts are more soluble in water as compared to organic solvents,, Water shows large dipole movement and attracts both cations and anions to form hydrated, , ions. The solubility of most inorganic compounds is reduced by the addition of organic, solvents such as methyl, ethyl and n-propyl alcohols., , Example:, Addition of about 20% by volume of ethanol shows the solubility of lead sulphate, (PbSO,) practically Negligible and it is used for quantitative separation,, , Dried nitrate mixture of calcium and strontium can be separated by treatment of alcohol, , or ether. Calcium nitrate dissolves in the Presence of alcohol or ether, but strontium nitrate, precipitates out,, , Potassium can be separated from sodium by, platinate (K2PtCle) from alcohol water mixed solvent., , 4.4 ARGENTOMETRIC TITRATION:, , Y precipitating Potassium hexa-chloro, , , , , Titration methods based upon utilizing AgNOs (silver nitrate) as a Precipitating agent is, , eful in Precipitation reactions, , known as argentometric titrations. Silver ions are extremely us, for halides (CI", Br” , T) and pseudohalides (S?”, HS", CN7, SCN7), Requirements for Argentometric Titration:, , © — The precipitate formation should be stoichiometric., , ¢ — Equilibrium of the reaction should be attained rapidly., , e — The precipitate must be of low solubility in the solution., , e Method to detect the stoichiometric point of the titration must be available,