First-Hand Investigation – Determination of the Sulfate Content (Gravimetric Determination) of a Lawn Fertiliser
The sulfate content of ammonia fertiliser can be determined gravimetrically by converting the sulfate in a weighed sample of fertiliser to barium sulfate.
The highly insoluble barium sulfate is collected, dried and weighed of Gravimetric Determination.
- Dissolve 1.00 g of fertiliser in 500 mL of water in a volumetric flask.
- Transfer 100 mL of the fertiliser solution from the volumetric flask to a large beaker using a burette. Acidify with hydrochloric acid and bring to the boil.
- Add a 5% w/w solution of barium chloride until precipitation of barium sulfate is complete.
- Filter through a weighed filter paper. Wash the precipitate with several aliquots of hot water and dry the precipitate.
- Reweigh the dry precipitate and filter paper. Calculate the mass of barium sulfate collected.
Analysis
0.329 g of dry barium sulfate was collected after treating 100 mL of the fertiliser solution with barium chloride.
Validity
Validity of the analysis depends on the assumptions:
- precipitation is quantitative i.e. the filter has collected all BaSO4 particles
- no other species present in the fertiliser are precipitated by barium ions
Accuracy
Can be improved by using:
- an electronic balance rather than a beam balance
- a fine analytical grade quantitative filter paper rather than a qualitative filter paper
- precipitation from hot solution also helps in preventing occlusion of other species in the barium sulfate precipitate
Reliability
Can be improved by:
- repeating the analysis several times and averaging the results
- precipitate must be thoroughly washed with warm water to remove any soluble species
- then thoroughly dried in a desiccators
- filtration must be complete otherwise the percentage of sulfate will be too low.
- in order to achieve high accuracy, a pre-weighted filtering crucible is used so that no filter paper is needed