Determination of chloride in water samples using the Argentometric Method

Introduction

Chloride in the form of Cl- ion is one of the major inorganic anions in water and wastewater. In portable water, the salty taste produced by chloride concentrations is variable and dependent on the chemical composition of water. The chloride concentration is higher in wastewater than in raw water. There are five methods used for the determination of chloride ions in water. Selection of method is largely a matter of personal preference. The argentometric method is suitable for relatively clear water. The end point of the mercuric nitrate method is easier to detect. The potentiometric method is suitable for coloured or turbid samples in which colour-indicated end points might be difficult to observe. The ferricyanide method is an automated technique. Ion chromatography may also be used for chloride determination. In this practical, we will be using the Argentometric method. This procedure is based on the principle that 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.

Apparatus

General laboratory glassware
Burettes
Titration set up

Reagents

Standard silver nitrate titrant (0.0141N (0.0141M): Dissolve 2.395g AgNO3 in distilled water and dilute to 1000 ml. Standardize against NaCl 1.00ml = 500g Cl-

Potassium chromate indicator solution: Dissolve 50g K2CrO4 in a little distilled water. Add AgNO3 solution until a definite red precipitate is formed. Let stand 12 hours, filter and dilute to 1 l with distilled water.

Standard sodium chloride NaCl 0.0141N (0.0141M): Dissolve 824.0 mg NaCl (dried at 140OC) in distilled water and dilute to 1000 ml. 1.00ml = 500 g Cl-

Sodium hydroxide NaOH 1N: dissolve 4g of NaOH in 100 ml distilled water

Hydrogen peroxide H2O2, 30 %

Sulphuric acid H2SO4 1N

Standardization

Add 1 ml of K2CrO4 indicator to 100 ml of 0.0141 N NaCl in an Erlenmeyer flask
Titrate with standard AgNO3
Calculate actual normality of AgNO3 from end point with NaCl (M1V1 =M2V2)
Procedure

1. Take a 25 ml portion of well-mixed sample and make it up to 100ml.
2. If sample is highly coloured, add 3 ml (Al(OH)3 suspension, mix, let settle and filter
3. If sulphide, sulphite or thiosulphate is present, add 1 ml H2O2 and stir for 1 min
4. Directly titrate sample in the pH range 7 – 10. Adjust sample pH to 7 –10 with H2SO4 or NaOH if it is not in the range add 1.0 ml K2CrO4 indicator solution.
5. Titrate with standard silver nitrate titrant to a pinkish yellow end point. Be consistent in end point recognition
6. Establish reagent blank by titrating a blank with AgNO3.

Calculation



Where A = ml titration for sample
B = ml titration for blank
N = normality of AgNO3
d = dilution factor