1

Turbidimetric

Determination of

Sulphate Ions in a

Water Sample

EXPT. 6 TURBIDIMETRIC DETERMINATION OF

SULPHATE IONS IN A WATER SAMPLE

Structure

6.1 Introduction Objectives

6.2 Turbidimetry

6.3 Principle

6.4 Requirements

6.5 Solutions Provided

6.6 Procedure

6.7 Observations and Calculations

6.8 Result

6.9 Precautions

6.1 INTRODUCTION

You have so far learnt about and performed a number of experiments based on

spectrophotometry. These experiments pertained to the determination of the

concentrations of inorganic or organic species and a physical constant for an organic

molecule. You would recall that in the previous experiments using spectrophotometer

you were required to measure the absorbance of the solutions and determine the

concentration of the analyte from it using Beer-Lambert’s law. In turbidimetric

determinations, the spectrophotometer is used to measure the percent transmittance which is related to turbidity. In this experiment you would learn about the turbidimetric

determination of sulphate ions in a sample of water.

You know that water samples from natural sources contain dissolved sulphates. For

example, the presence of sulphate in rain water may be traced down to the oxidation of

sulphur obtained as a result of the combustion of coal or petroleum oil. The sulphur

trioxide so obtained dissolves in rain water to form sulphuric acid. The presence of

sulphate can also be due to dissolved minerals or from some other source.

In the turbidimetric determination, the sulphate ions present in the sample are converted

into a suspension of barium sulphate which is then determined turbidimetrically. You

would be using a spectrophotometer to determine the turbidity of a solution. When

carefully performed, the turbidimetric method provides reproducible results for such an

important determination. This method is much faster and sensitive as compared to the

commonly employed gravimetric determination. In the next experiment you would

learn about the application of IR spectrometry in the determination of functional group

of organic compounds.

Objectives

After studying and performing this experiment you should be able to:

• explain the principle of turbidimetry,

• explain the principle underlying turbidimetric determination of sulphate ions in a

water sample,

• enumerate the general factors that may affect the results of a turbidimetric

determination,

• observe due precautions while performing the turbidimetric determination of

sulphate ions, and

• adapt the method for other turbidimetric determinations.

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SpectroscopicMethods

Lab. 6.2 TURBIDIMETRY

You have learnt about different spectroscopic methods of analysis in the MCH-003

course. However, you have not learnt about turbidimetry. Let us learn about the

meaning and the principle of turbidimetry. You know that some of the insoluble

compounds in small amounts may be obtained as stable suspensions. When light is

passed through such a suspension, the incident electromagnetic radiation interacts with

the suspended particles and a part of it gets dissipated due to absorption, reflection and

refraction. The unabsorbed part of the radiant energy gets transmitted. The intensity of

the transmitted radiation is a measure of the turbidity of the solution and is a function of

concentration of the dispersed particles in the suspension. The percent transmission can

be mathematically converted into turbidance (S) as per the following equation.

S = –logT = 2 – log%T … (6.1)

Where, T is the transmittance and %T refers to the percent transmittance. The

measurement of transmittance of a fine suspension as a measure of the turbidity of the

solution forms the basis of turbidimetry. The relationship between the turbidance of

the suspended particles and its concentration is semi-empirical in nature. Therefore, it is

necessary to construct a calibration curve for turbidimetry. The calibration curve is

obtained by using different dilutions of the standard solution and measuring their

transmittance under identical conditions. This curve is then used to determine the

sample concentration whose transmittance is also measured in the same way as the

standards.

The intensity of scattered light depends on the number and size of the suspended

particles. Therefore the precipitate must be very fine so that it does not settle rapidly.

The accuracy of the results depends a great deal on the reproducibility of the turbidity

formed. Therefore, while preparing the turbid solution; utmost care is necessary to

ensure reproducibility of turbidity. Further, for accurate and reproducible results with

turbidimetric techniques, it is critical that factors such as temperature, time and rate of

stirring, and time of standing of suspension before measurements, be as uniform as

possible for the standard solutions and the analyte samples. The following conditions

must be ensured for the reproducibility of turbidity;

• The ratio of concentration of the reactants must be uniform.

• The order of addition, rate of addition, rate of mixing, and time delay between

preparation of the suspension and the measurement of turbidity must be same for

the standards and the unknown samples.

• The presence of other salts (matrix) must be same for the standards as well as the

samples.

• Temperature must be maintained.

6.3 PRINCIPLE

In the turbidimetric determination of sulphate ions in a water sample, the sulphate ions

are converted to a suspension of BaSO4 under controlled conditions. The sulphate ions

present in the sample are precipitated by the addition of a highly acidified solution of

barium chloride to form the suspension as per the following equation.

−

+→+

−

Cl2BaSOBaClSO 4224

The percent transmittance of the suspension is then determined by a spectrophotometer.

This is then is mathematically converted to turbidance. The turbidance of the sample

solution is compared with a calibration curve drawn from standard solutions of sulphate

ions.

Barium is the only

common ion which forms

a precipitate with sulphate in highly acidic solutions.

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Turbidimetric

Determination of

Sulphate Ions in a

Water Sample

6.4 REQUIREMENTS

Apparatus Chemicals

Spectrophotometer/ Filter photometer

Matched cuvettes

Volumetric flasks (1 dm3)

Conical flasks (100 cm3)

Burettes

Graduated pipette (5 cm3)

Sieves (20 and 39 mesh)

1

2

2

10

3

1

1each

Potassium sulphate

Sodium Chloride

Barium Chloride

Hydrochloric acid

Glycerol

Absolute Ethanol

6.5 SOLUTIONS PROVIDED

Standard sulphate solution: It is prepared by dissolving 0.1810 g of analytical grade

potassium sulphate in about 200 cm3 of distilled water taken in a one litre volumetric

flask and diluting it to mark with distilled water. The standard solution so obtained

contains 0.1 mg of sulphate per cm3 (100 mg per dm

3).

Sodium chloride-HCl solution: It is prepared by dissolving 60 g of analytical grade

sodium chloride in about 150 cm3 of distilled water taken in a one litre volumetric flask

followed by addition of 5 cm3 of Analytical grade conc. HCl and diluting it to mark

with distilled water.

Glycerol-ethanol solution: It is prepared by mixing one volume of pure glycerol in

two volumes of absolute alcohol.

6.6 PROCEDURE

In the turbidimetric determination of sulphate ions, it is essential that,

• the sodium chloride -hydrochloric acid solution is added before the addition of

barium chloride. This is to inhibit the growth of micro crystals of barium

sulphate.

• the temperature and pH of the solution should be maintained.

• the order of addition, rate of addition, rate of addition and shaking of solution must be maintained uniform for all the standards and unknown samples.

• the turbidity is stabilized by the addition of glycerol-ethanol solution.

• the time interval between precipitation and transmittance measurement must be

maintained constant.

The following procedure takes care of the above mentioned requirements. Follow the

steps given below in sequential order to perform the experiment.

1. Take 6 conical flasks of 100 cm3 capacity labelled from 1 to 6 and transfer the

standard solution of sulphate ion and water in them as per the details given in

column 2 and 3 of the Observation Table 6.1. You may use burettes for the

purpose.

2. Take another 2 conical flasks of 100 cm3 capacity and label them as S1 and S2

and transfer 50 cm3 each of the unknown water sample in these. You may use

pipette / burette for the purpose.

3. To another 100 cm3 conical flask, labelled as blank, transfer 50 cm

3 of distilled

water to prepare the blank solution.

4. Transfer 5 cm3 of the sodium chloride-hydrochloric acid solution and 2 cm

3 of

glycerol-ethanol solution each to the solution in the flask labelled as S1 and the

The time at which the

turbidity of each solution

is measured is critical to

the determination.

Therefore, treat each

solution individually in as

similar a manner as possible and do not

attempt to treat all the

solutions

simultaneously.

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SpectroscopicMethods

Lab. flask labelled as blank. Keep the flask (NO-S1) on a magnetic stirrer and place a

stirrer bar in the solution. Turn on the stirrer and adjust the stirring rate so as to

obtain a shallow vortex in the solution. Note the position of the control knob on

the stirrer. The same position, i.e., the same stirring rate, is used for all of the

solutions.

5. Add 0.3 g of sieved barium chloride crystals (the crystals should pass through the

sieve of 20 mesh and be retained by 39 mesh sieve ) to the above solution and

start the timer. Stop the stirring after one minute but allow the timer to continue.

6. Immediately fill a cuvette with the solution and place in the spectrophotometer

holder.

7. Add 0.3 g of sieved barium chloride crystals to the ‘blank’ flask also and shake to

dissolve it.

8. Measure the percent transmittance of the solution at 420 nm after an interval of 5

± 1 minutes after the stirring was stopped (the total time elapsed after the addition

of barium chloride is 6 ± 1min.) The percent transmittance of the solution is

measured against the blank and is recorded in the column no 7 of the Observation

Table 6.1.

9. Repeat the same (steps 4 to 6) as above for the solution numbered 2 to 6 and

record the percent transmittance values in the column 7 of the Observation Table

6.1.

10. Treat the sample solutions, S1 and S2 in the same way (steps 4 to 6) and record

the percent transmittance appropriately in Observation Table 6.1.

11. Convert the percent transmittance values for the standard solutions as well as for

the sample solutions into turbidance using Eq. 6.1, and record the same in column

of Observation Table 6.1.

12. Plot a graph between the concentration at X-axis (column 4) and turbidance

Y-axis ( column 8) in the Fig. 6.1.

13. Determine the concentration of sulphate ions in the given sample solution with

the help of the calibration curve and report the result.

6.7 OBSERVATIONS AND CALCULATIONS

Observation Table 6.1: Transmittance/turbidance data for the standard and

sample sulphate solutions.

Column

1 2 3 4 5 6 7 8

S. No.

Vol. of

Std.

sulphate

solution

(cm3)

Volume of

Distilled

Water

(cm3)

Conc.

of standard

sulphate

solution

(ppm)

Vol. of

NaCl-

HCl

solution

(cm3)

Vol. of

Glycerol

-Ethanol

solution

(cm3)

Percent

transmit-

ance

Turbid-

ance

1 2 48 4 5 2

2 5 45 10 5 2

3 10 40 20 5 2

4 15 35 30 5 2

5 25 25 50 5 2

6 35 15 70 5 2

Blank - 50 0 5 2

Water Smaple

S1 - 50 ? 5 2

S2 - 50 ? 5 2

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Turbidimetric

Determination of

Sulphate Ions in a

Water Sample

0 20 40 60 80 100

Concentration (ppm) of sulphate ions

Fig. 6.1: Calibration plot between turbidance and concentration of standard sulphate

solution

The concentration of sulphate ions in solution S1 =.......ppm

The concentration of sulphate ions in solution S2 =.......ppm

Average =..........ppm

6.8 RESULT

The concentration of the sulphate ions in the given sample of water is found to be

= .....ppm

6.9 PRECAUTIONS

• Since suspended material in the sample can interfere with the analysis, it is

necessary to filter suspended matter from the solution prior to the determination.

• Any coloured substance that absorbs radiation at the wavelength which is chosen

for the analysis can also interfere.

• Take care to avoid any air bubbles adhering to the walls of the spectrophotometer

tube.

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