Determine the Amount of Sulfate by Turbidimetric Method

Introduction: Sulfates occur naturally in the form of minerals, like barite (BaSO4) and gypsum (CaSO4·2H2O). Sulfate also occurs naturally in drinking water. Sulfates are used in the production of fertilizers, chemicals, dyes, glass, paper, soaps, textiles, fungicides, insecticides, astringents and emetics. Sulfates are also used in the mining, wood pulp, metal and plating industries, in sewage treatment and in leather processing (Greenwood & Earnshaw, 1984). Aluminium sulfate (alum) is used as a sedimentation agent in the treatment of drinking-water. Copper sulfate has been used for the control of algae in raw and public water supplies (WHO, 1996).

Sources of Sulfate in Environment: Sulfates are discharged into water from mines and smelters, Kraft pulp and paper mills, textile mills and tanneries. Atmospheric sulfur dioxide, formed by the combustion of fossil fuels and in metallurgical roasting processes, may contribute to the sulfate content of surface waters. Sulfur trioxide, produced by the photolytic or catalytic oxidation of sulfur dioxide, combines with water vapor to form dilute sulfuric acid, which falls as “acid rain” (WHO, 1996)

Sampling: The samples to determine sulfates were provided by the instructor.

Principle: Under controlled conditions sulfate ion is converted into BaSO₄ suspension. The resultant turbidity is measured by Nephelometer or Spectrophotometer and compared to a curve prepared from standard sulfate solution.

Preparation of Standard Solutions of Sulfate:

Stock Solution: Prepare 1000ppm stock solution of sulfate from Na₂SO_4:  take 1g of SO₄^– and dissolve it in 1000ml of water

1g of SO₄^–   = Molecular mass of Na₂SO_{4  /}   Molecular mass of SO₄

1g of SO₄^–   = 22.990 x 2 + 32.066 + 15.99 x 4 / 32.066+15.99 x 4 = 142.006 / 96.026   = 1.47g of Na₂SO₄

Dissolve 1.47g of Na₂SO₄ in 1000ml of water to prepare 1000ppm standard solution of sulfate.

Prepare 5ppm, 10ppm, 15ppm, 20ppm and 25ppm Solutions from the stock solution:

For 5ppm solution:

                        C₁V₁                =          C₂V₂

                                1000 x V₁          =          5 x 100

V₁          =          5 x 100 / 1000 = 0.5ml

Take 0.5ml of stock solutions in the flask of 100ml and add distilled water upto the mark to prepare 5ppm solution.

Similarly take 1ml, 1.5ml, 2ml and 2.5ml of stock solutions in 5 different flasks of 100ml and add distilled water upto the mark to make 10ppm, 15ppm, 20ppm and 25ppm Solutions respectively.

Procedure:

Temperature is a prominent factor it must be maintained between15-30 ⁰C.

Take 100ml of sample, 5ml conditioning agent and 0.3g BaCl₂.H₂O. Stir it exactly for 1min. Now pour it in the cell and measure the turbidity 8 times after an interval of 30sec. Record the maximum reading. Adopt the same procedure for every standard.

Calculations:

Conc. (ppm)	Absorbance									Max. Absorbance
5	0.584	0.588	0.592	0.595	0.597		0.598	0.598	0.599	0.599
10	0.635	0.642	0.646	0.65	0.653		0.655	0.658	0.658	0.658
15	0.688	0.692	0.695	0.697	0.698		0.699	0.699	0.721	0.721
20	0.797	0.8	0.802	0.804	0.804		0.804	0.805	0.854	0.805
25	0.856	0.858	0.858	0.859	0.857		0.856	0.855	0.854	0.859
Sample
6.5	0.531	0.598	0.599	0.59	0.63	0.613		0.613	0.613	0.613

RESULT: The concentration of sulfate in the given sample is 6.5ppm

Also see related Links:

Determine the Amount of Sulfate by Gravimetric Method