PRACTICAL (3) Precipitation Titrations

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

2. Introduction: • In these interactions ions 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.

3. 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

4. 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) → AgCl(s)

5. 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) + CrO42-(aq) → Ag2CrO4(s) • Equation of (Mohr's method): • AgNO3 +NaCl→ AgCl+NaNO3

6. 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.

7. Equipments used: • burette and stand • 10 mL pipettes. • 250 mL conical flasks • Beakers • dropper

8. Reagents used: • Silver nitrate solution AgNO3(0.02 mol/L). • Sodium chloride NaCl solution of Unknown concentration. • Potassium chromate K2CrO4 solution as an indicator.

9. The Procedure: • Add 10ml of unknown standard solution of Sodium chloride in conical flask. • Add 4 drops of Potassium Chromate solution • Titrate using (0.02 M ) AgNO3silver 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.

10. The Procedure: • Use the following reaction equation to determine the moles of chloride ions reacting. • Ag +(aq) + Cl - (aq) → AgCl(s) • law's used: ( M x V) = (M\ x V \) n NaCln \ AgNO3 strength= concentration x Molecular weight C = M × M.wt

12. 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 red-brown colouration. • Right flask: If addition of Ag+ is continued past the endpoint, further silver chromate precipitate is formed.

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

14. Thank you