Test Method for Water-Soluble Sulphate Analysis in Soil.

 1) Principle:  

For the determination of water-soluble sulphate, barium chloride solution is added to the  
water extract and the precipitate of barium sulphate is collected, dried, and weighed. The  
sulphate content is then calculated from the mass of the material used in the analysis and  the mass of barium sulphate precipitated.   

2) Reagents and Apparatus:  

I. Drying oven  
II. Weighing balance  
III. Desiccator  
IV. Test sieves  
V. Pestle and mortar 
VI. Riffle boxes  
VII. Glass weighing-bottle  
VIII. Beaker, 500 mL  
IX. Measuring cylinder  
X. Hot plate  
XI. Filter paper  
XII. Filter funnel  
XIII. Crucible  
XIV. Muffle furnace  
XV. 10 % Hydrochloric Acid solution  
XVI. 5 % Barium chloride solution  

3) Procedure:  

3.1) Preparation of test specimen  

Prepare a sample of material passing the 2 mm test sieve. 
Divide the material passing the 2  mm sieve by successive riffling through the 15 mm divider to produce a sample weighing  approximately 60 g. 
Pulverize this sample so that it passes the 425 µm test sieve.  Throughout this and any subsequent operation, mix the material thoroughly before any  division and take care to avoid segregation during riffling. 
Place the specimen in a glass  weighing bottle and dry at a temperature of 75 °C to 80 °C. The specimen is deemed to be  dry when the differences in successive weighing at intervals of 4 h do not exceed 0.1 % of  the specimen mass. 
Allow the specimen to cool to room temperature in the desiccator.  

3.2) Preparation of water extract.  

Obtain the 2 : 1 water-soil extract for the determination of the water-soluble sulphate content  from each of the prepared specimens. 
Weigh a sample of 50.00 g on a watch glass and  transfer it to a clean and dry extraction bottle. 
Add exactly 100 mL of distilled water to the  extraction bottle, stopper it tightly and place it in the shaker and agitate for 16 h.    
Filter the soil suspension into a clean and dry flask through a suitable filter paper, e.g., Whatman No. 50, on  the Buchner funnel. transfer 50 mL of filtered extract to a 500 mL conical beaker. 
Add distilled  water to about 300 ml.  

3.3) Gravimetric method for analysis of water extract.  

Analyze each filtrate sample obtained from the water extract procedure. 
Test the solution with  litmus paper and if necessarily make slightly acid by the addition of 20 drops of hydrochloric  acid. 
Dilute to 300 mL (if necessary), bring the solution to the boil, and add 10 mL barium  chloride solution drop by drop with constant stirring. Continue boiling gently until the  precipitate is properly formed. 
Allow the solution to stand at just below boiling point for at  least 30 min, then leave to cool to room temperature. 
Transfer the liquid and precipitate of  barium sulphate with extreme care to a previously ignited and weighed sintered silica filter  crucible using suction. 
Alternatively transfer the precipitate with extreme care to a suitable filter paper, e.g., Whatman No. 42, in the glass filter funnel and filter. 
In either case wash the  precipitate several times with hot distilled water until the washings are free from chloride as  indicated by absence of turbidity when a drop is tested with the solution of silver nitrate. 
If the  precipitate is filtered through a filter paper, transfer the filter paper and precipitate to a  previously ignited and weighed crucible. 
If an electric muffle furnace is used, place the  crucible and contents in it at room temperature and gradually raise the temperature to red  heat. 
If a Bunsen or other burner is used first dry the filter paper and precipitate slowly over a  small flame, taking care to char the filter paper slowly rather than to allow it to inflame,  otherwise some of the precipitate may be lost. 
Ignition for about 15 min at red heat is usually  sufficient in either case. cool the crucible to room temperature in the desiccator and weigh it  to the nearest 0.001 g. 
Calculate the mass of the precipitate (m4) from the increase in the  recorded mass of the crucible.  

4) Calculation:  

  SO3,  (%) = 1.372 m4  
Where: m4= mass of ignited precipitate, g  

5) Reference:   

BS 1377 Part 3:1990 Amd 9028 :1996