Gravimetric - determination of sulfate in fertiliser
Aim - To find the proportion of sulfate in a commercial fertiliser.  


- Safety goggles an lab coat
- Solutions of barium compounds are toxic
- Silver nitrate solutions stain skin and clothing



- Supply of fertiliser
- Solutions of 2 M HCl
- Solution of 0.1 M AgNO3
- Solution of 0.5 M BaCl2
- Distilled water
- 2 X 100 mL beakers and one 600 mL beaker
- 10 mL and 20 mL measuring cylinders
- Filter paper and funnel
- Mortar and pestle.
- Electronic balance
- Bunsen burner, tripod, gauze mat and heat mat.
- Vacuum filtration unit.




Step 1 Using a mortar and pestle, grind to a fine powder a small sample of the fertiliser. Accurately weigh a 100mL beaker. Accurately weigh about 1.0 g of the fine powder and place it in a 100 mL beaker. Record the mass of the fertiliser and beaker.
Step 2 Add about 50 mL of distilled water to dissolve all the sulfate in the fertiliser, then filter the mixture into a 600 mL beaker. Wash the residue thoroughly.
Step 3 Add 3.00 mL of 2.0 M HCl to the filtrate and make it up to 200 mL with distilled water. Heat the solution over a Bunsen burner and bring it to boil. Remove the Bunsen burner and add 15.0 mL of 0.5 M BaCl2. A white precipitate of BaSO4 will form.
Step 4 Return the beaker to the heat and continue to boil for another minute. Remove the beaker from the heat and allow to stand until the precipitate settles. Add a drop or two of 0.5 M BaCl2. If a precipitate forms add 3.0 mL of 0.5 M BaCl2 if no precipitate forms proceed to the next step.
Step 5 Weigh and record the mass of a round filter paper.
Step 6 Place the filter paper in the funnel of a vacuum filtration unit and wet with distilled water.
Step 7 Filter the solution in the 600 mL beaker using the vacuum filtration unit making sure to collect al the precipitate in the filter paper. Wash the precipitate with distilled water.
Step 8 After the final washing collect a sample of the filtrate and test it by adding one drop of 0.1 M AgNO3. If a white precipitate forms continue to wash the precipitate in the filter paper with distilled water.
Step 9 Allow the filter paper to dry overnight and weigh on an electronic balance.
Complete the table below  
Mass of 100mL beaker  
Mass of fertiliser and beaker  
Mass of fertiliser  
Mass of filter paper  
Mass of filter paper and barium sulfate  
Mass of barium sulfate  
1) Determine the percentage, by mass, of sulfate (SO4 2- ) in barium sulfate (BaSO4 ). Use percentage composition
2) Using the percentage composition of sulfate in barium sulfate to determine the mass of sulfate in the precipitate.
3) Find the percentage of sulfate in the fertiliser.
4) Now determine the percentage, by mass, of sulfur in the fertiliser.

5) How will the percentage of sulfur in the fertiliser that you calculated change:
- if in step 2 you did not wash thoroughly the residue
- if in step 8 you did not test the filtrate with silver nitrate.

Offer an explanation to your answers above.

6) Carbonates are found in the fertiliser and in the water when carbon dioxide dissolves.
a) Why is HCl added at step 3) above?

b) Write a balanced equation for the reaction between H+ and CO32-

c) Why is the solution heated?

d) What is the effect on the final percentage, calculated, of sulfur in the fertiliser if step 3) is not done?
7) Why is it important for the precipitate to have a high molecular formula?