REG NO: 20BCL0062 NAME: Bishan Subedi
Estimation of sulphate in drinking water by conductivity method
Sulphate (SO42-) is found in almost all natural water. Origin of most sulphate compounds is the
oxidation of sulphite ores, presence of shales or the industrial wastes. Ground water moving
through soil and rocks containing sulphate minerals result in higher dissolved sulphate ions than
permissible limit.
Problems due to excess sulphate ion concentration in water:
Sulphates cause scale formation in boilers, pipes, etc.
High sulphate concentration will leads to corrosion on copper piping.
Sulphate has a laxative effect and creates diarrhoea leading to dehydration in humans and
animals.
High sulphate concentration leads to eutrophication of water bodies leads to reduction of
dissolved oxygen Sulphate will give bitter taste to water if the concentration exceeds
beyond 250 ppm.
Methods to estimate sulphate ion concentration in water:
1. Turbidimetry method: It involves the measurement of turbidity formed when an aliquot
of BaCl2-gelatin reagent is added to acidified sulphate solution.
2. Titrimetric method: By dissolving precipitated BaSO4 in excess of EDTA solution and
the excess EDTA is back titrated with standard Zinc solution.
3. Colorimetric Measurement: Based on the reaction of barium chloranilate with sulphate
ion at pH 4 in ethanol yield highly coloured acid-chloranilate ion and is measured at 530
nm.
4. Conductometric method: This method measures the conductivity of the solution as the
titration proceeds. Conductance tends to vary with the characteristics of the solvent,
number, size and charge of ions involved. When one ion is replaced by another ion
significantly during the titration, conductance will change in a linear manner until the
replacement is complete. After that, the line will change to different slope due to the
additional inclusion of another ion of difference conductance.
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Exp No:6
Experiment Estimation of sulphate in drinking water by conductivity method
Problem definition People using water with high levels of sulfate are vulnerable to
dehydration and diarrhea. Kids are more sensitive to sulfate than adults.
Methodology Conductivity of the soluble sulphate solution will change when it is
precipitated by BaCl2. Conductivity will reach minima when all sulphate
ions are precipitated, and from which, the total amount of sulphate ion
present in the water can be determined.
Solution Amount of BaCl2 required to remove the dissolved sulphate can be
estimated.
Student learning Students will learn to
outcomes a) perform conductometric method
b) remove sulphate ion from irrigate water
Principle:
Electrolyte solutions conduct electricity due to the presence of ions in solution. In case of
precipitation titration between BaCl2 and Na2SO4, the conductance decreases slowly due to the
replacement of Cl- ion by SO42- ion upto the equivalence point. After the equivalence point, the
conductance increases rapidly due to the excess addition of BaCl2 which remains in solution as
Ba2+ and Cl-. This makes detection of neutralization point easy from the conductance trend
plotted as a graph. This is the principle used in the estimation of SO42- from contaminated water
sample.
Requirements:
Reagents and solutions: BaCl2 (0.1 N), Na2SO4 (0.02 N), unknown sulphate solution and
distilled water.
Apparatus: Conductivity Bridge, Conductivity cell, Burette, Pipette, Volumetric flasks, Glass
rod, Beaker (100 mL).
Procedure:
Calibration of Conductivity meter: Place a freshly prepared 0.1 N KCl solution (given in
bottle) in a 100 mL beaker. Dip the conductivity cell in this solution and connect to the
Conductivity meter. Press “CAL” button and complete the internal calibration of the instrument.
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Standardization of BaCl2 (Titration – 1):
Pipette out 20 mL of 0.02 N Na2SO4 solution (from Bottle A) in a 100 mL beaker and add 10 mL
of distilled water to it to make the conductivity cell dip completely in the solution. Addition of
water will not affect the conductivity since the number of ions in the solution remains unaltered.
Dip the conductivity cell into the solution in the beaker and connect to the conductivity meter.
Fill the burette with ~0.1 N BaCl2 solution (from Bottle B). Record the conductivity of the
sulphate solution without adding any BaCl2 from the burette (0th reading). Add 1 mL BaCl2 of
known concentration into the beaker, stir with glass rod and note down the conductance.
Continue the addition of BaCl2 (1 mL each time) and note the conductance after each addition.
Continue the titration beyond the equivalence point for about 5 mL. The conductance will either
decrease slightly or remain constant until complete precipitation of BaSO4, and then starts
increasing on continuing the addition of BaCl2. A graph is now drawn by plotting conductance
vs volume of BaCl2 added. Intersection point from the plot gives the volume of BaCl2 required
for precipitating the sulphate present in the known sample.
Estimation of unknown sulphate in the given solution (Titration – 2):
Make up the unknown sulphate solution given in a 100 mL standard flask upto the mark using
distilled water resulting in a solution containing 0.96 mg/mL of sulphate ions (Eq. wt. of SO42- =
48.03). Pipette out 20 mL of this solution into a 100 mL beaker and add 10 mL distilled water to
it. Dip the conductivity cell and repeat the above procedure with the unknown sulphate solution
to determine the amount of BaCl2 required for precipitating the unknown sulphate in the sample.
From the two titrations carried out, calculate the amount of sulphate present in the effluent
sample.
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Table 1: Conductometric Titrations
Titration-1: Standardization of BaCl2 Titration-2: Estimation of sulphate content
Burette: BaCl2 solution (~0.1 N) Burette: std. BaCl2 solution
Beaker: 20 mL of Na2SO4 (0.02 N) + 10 Beaker: 20 mL of unknown sulphate solution +
mL of distilled water 10 mL of distilled water
Conductivity cell, Conductivity meter Conductivity cell, Conductivity meter
Volume of BaCl2 Conductance Volume of BaCl2 Conductance
added (mL) ( mhos) added (mL) ( mhos)
0.0 1 0.0 1.1
1.0 1 1.0 1.1
2.0 1 2.0 1.1
3.0 1 3.0 1.1
4.0 1 4.0 1.1
5.0 1 5.0 1.2
6.0 1.2 6.0 1.4
7.0 1.3 7.0 1.6
8.0 1.5 8.0 1.8
9.0 1.6 9.0 1.9
10.0 1.7 10.0 2.1
11.0 1.8 11.0 2.2
12.0 1.9 12.0 2.3
Fig 1: Model graphs – 1 and 2 for Conductometric estimation of known and unknown sulphate
sample solutions, respectively.
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Calculations:
A). Standardization of 0.1 N BaCl2:
(N x V) of BaCl2 solution = (N x V) of sodium sulphate
N of BaCl2 solution = 0.02 N x 20 mL =0.02N x 20 mL
Volume measured from Plot-1 (V1) 4.3
=____0.093____N of BaCl2 solution
B). Estimation of unknown sulphate:
(N x V) of irrigation water sample = (N x V) of BaCl2 solution
N of irrigation water sample = N of BaCl2 x Volume measured from Plot-2 (V2) =0.093 x 4.4
20 mL 20 mL
=_____0.020___N of irrigation water sample
Amount of sulphate present in 1L = Normality of irrigation water sample x Eq. wt. of SO42- (48.03)
Amount of sulphate present in given sample solution = Strength of irrigation water sample x 48.03 x100
1000
=0.02 x 48.03 x 100
1000
=______0.09606____ grams in 100 mL
Result: Amount of sulphate in given irrigation water sample =____0.09606_____ grams.
GRAPH
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