Sunday, 3 January 2016

Standardisation of Silver Nitrate (Indicator End-point Titration method)


1. Principle

AR silver nitrate has a purity of at least 99.9%, so that a standard solution can be prepared by direct weighing. AR sodium chloride has a purity of 99.9 - 100%; the substance is therefore an excellent primary standard. Sodium chloride is very slightly hygroscopic, and for accurate work it is best to dry the solid at 140oC prior to use. In a neutral or slightly alkaline solution, potassium chromate can indicate the end-point of the silver nitrate titration of chloride. Silver chloride is precipitated quantitatively before red silver chromate is formed.





2. Reagents

2.1 Sodium chloride (NaCl), AR – dried at 140 oC.

2.2 Standard sodium chloride, 0.0141 N: Dissolve 0.824 g NaCl in reagent grade water and dilute to 1L.

2.3 Calcium carbonate, AR

2.4 Potassium chromate (K2CrO4), AR

2.5 Silver nitrate (AgNO3), AR

2.6 Standard silver nitrate solution, ~0.141 N: Dissolve 23.95 g AgNO3 in reagent grade water and dilute to 1 L. Store in a dark bottle. Standardise as per Section 3.

2.7 Standard silver nitrate solution, ~0.0141 N: Dissolve 2.395 g AgNO3 in reagent grade water and dilute to 1 L. Store in a dark bottle. Standardise as per Section 3.

2.8 Potassium chromate indicator: Dissolve 50 g K2CrO4 in a small amount of reagent grade water. Add AgNO3 solution until a definite red precipitate is formed. Let stand 12 hours, filter and dilute to 1 L.



3. Procedure

Take a know aliquot of standard NaCl (e.g. 10 mL diluted to 100 mL for standardize ~0.0141 N AgNO3 or 50 mL diluted to 100 mL for ~0.141 N AgNO3). Add 1 spatula of calcium carbonate to solution, add 1.0 mL K2CrO4 indicator solution, and titrate with AgNO3 to a pinkish-yellow end-point. Be consistent in the end-point recognition. A blank must also be established by this titration. Record information on workbook. Carry out standardization in triplicate. Calculate the normality of AgNO3 as per section 5.



4. Frequency of Standardisation

Restandardise every 6 months.



5 Calculation




Where
B = Blank titre (mL)



6. Reference

APHA 20th Edition: 4500 - Cl – B; pp.4-67-68.

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1 comments:

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