Clinical Analytical Chemistry CLS 231 PRACTICAL 3 Precipitation

Clinical Analytical Chemistry CLS 231 PRACTICAL (3) Precipitation Titrations Lecturer: Amal Abu - Mostafa 1

Introduction: In these interactions combine in the solution forming salts less soluble in water For example : interactions of silver nitrate, which is the most important materials used in the interactions of precipitation. Objectives: Calculate the molarity mol/L , and The concentration in g/L of Sodium chloride. 2

Argentometric titration: • Titrations involving silver are termed argentometric, from the Latin name for silver, argentum. Argentometric methods involving precipitation titrimetry: Mohr’s Method Fajan’s Method Volhard’s Method 3

Determination of Chloride Ion Concentration by Titration (Mohr’s Method) Principle: This method determines the chloride ion concentration of a solution by titration with silver nitrate. As the silver nitrate solution is slowly added, a precipitate of silver chloride forms. Ag +(aq) + Cl - (aq) → Ag. Cl(s) 4

Continue the Principle: The end point of the titration occurs when all the chloride ions are precipitated. Then additional silver ions react with the chromate ions of the indicator, potassium chromate, to form a red-brown precipitate of silver chromate. 2 Ag +(aq) + Cr. O 4 2 -(aq) → Ag 2 Cr. O 4(s) Equation of (Mohr's method): Ag. NO 3 +Na. Cl → Ag. Cl+Na. NO 3 5

Mohr’s method: Note: This method can be used to determine the chloride ion concentration of water samples from many sources such as seawater, stream water, river water and estuary water. 6

Equipments used: burette and stand 10 m. L pipettes. 250 m. L conical flasks Beakers dropper 7

Reagents used: Silver nitrate solution Ag. NO 3(0. 02 mol/L). Sodium chloride Na. Cl solution of Unknown concentration. Potassium chromate K 2 Cr. O 4 solution as an indicator. 8

The Procedure: • Add 10 ml of unknown standard solution of Sodium chloride in conical flask. • Add 4 drops of Potassium Chromate solution • Titrate using (0. 02 M ) Ag. NO 3 silver nitrate solution gradually and slowly from the burette with constant shaking Until the appearance of white precipitate of silver chloride and a red of silver Chromate disappear upon shaking. • Continue to add silver nitrate until the appearance of brick red color • Repeat the experiment twice and then Calculate the concentration of Sodium chloride in mol/L and in g/L. 9

The Procedure: Use the following reaction equation to determine the moles of chloride ions reacting. Ag +(aq) + Cl - (aq) → Ag. Cl(s) law's used: ( M x V) = n Na. Cl (M x V ) n Ag. NO 3 strength= concentration x Molecular weight C = M × M. wt 10

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Left flask: before the titration endpoint, addition of Ag+ ions leads to formation of silver chloride precipitate, making the solution cloudy. The chromate indicator gives a faint lemon-yellow colour. Centre flask: at the endpoint, all the Cl− ions have precipitated. The slightest excess of Ag+ precipitates with the chromate indicator giving a slight redbrown colouration. Right flask: If addition of Ag+ is continued past the endpoint, further silver chromate precipitate is formed. 12

Notice: This method is used in neutral solutions because in acid solution, silver chromate gets dissolved. And in alkali solution, it makes a precipitate of silver hydroxide. 13

Thank you 14