To determine the Sulphates in given sample of water

To determine the Sulphates in a given sample of water

Method DISCUSSION:

Sulphate Occur naturally in water as result of leaching from gypsum and other common minerals. In addition, Sulphate may be added to water system in several treatment processes.

The Sulphate content of municipal water supplies is usually increased during clarification by alum. Sulphate contributes to the total solids content and the determination of Sulphate is sometimes used to control the washing of turbine blades to free them from deposits.

PRINCIPLE:

Sulphate ions are precipitated as barium Sulphate crystals of uniform size in an acid medium.

Light absorbed by the precipitate is measured using a spectrophotometer or an absorptiometer.

REAGENTS:

Conditioning reagent: Dissolve 75g sodium chloride in 300 ml distilled water and add 300 ml conc: HCl is 100 ml 95% ethyl alcohol (or isopropyl alcohol). Add 50ml glycerol and mix well.

Barium Chloride: crystals 20 to 30 mesh.

Standard Sulphate solution: Dissolve 147.9 mg anhydrous sodium Sulphate in distilled water and make up to 1000ml.

PROCEDURE:

Standards:

1. Measure into 250ml conical flasks 5, 10, 15, 20, 25, 30, 35, 40 ml of standard Sulphate solution and dilute to 100ml.
   
2. Add 5.0ml conditioning reagent and mix well using a magnetic stirrer. The speed of the stirring should be the same for standard and samples. stirring should be the same for both standards and samples.
   
3. While stirring acid about 0.5g barium chloride crystals and continue to stir exactly one minute.
   
4. Immediately after one minute, pour some of the solution into the absorption cell of the photometer and measure the optical density at 30-second intervals for 4 minutes taking the maximum reaction which will normally be after a period of 2 minutes after completion of the stirring. Measure the optical density using a spectrophotometer at a wavelength of 420nm or a filler photometer with a violet filter having a maximum transmittance near 420 mm with cells of 40 to 50 mm.
   
5. Carry out a blank determination on the reagents used
   
6. Prepare a calibration graph relating net optical density to ug of Sulphate (as SO4–) using a semi-logarithmic graph paper.
   

Samples:

1. Measure in a 250ml conical flask a suitable quantity of the sample containing not more than 4 mg Sulphate and dilute to 100ml, and proceed from steps No. 02-04 as for standards.
   
2. From the calibration graph, read the mg of Sulphate equivalent to the optical density
   
3. If the sample is coloured or turbid, conduct a sample blank using the same amount of sample and conditioning reagent, without the addition of barium chloride. Measure the optical density and deduct the value from the optical density of the barium chloride added sample and find out the mg equivalent Sulphate for the deducted value from the calibration graph.
   

CALCULATIOS:

OBSERVATION: