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Idea Transcript
DETERMINATION OF SULFATE IN LAWN FOOD Measuring sulfate
Fertiliser composition
2(SO4 )
The presence of sulfate ions in solution cannot be directly measured using instrumentation or titration methods. A solution of excess barium (Ba2+ ) ions is added and all sulfate ions are precipitated out of solution as barium sulfate (BaSO4), leaving excess barium ions still dissolved in solution.
2- +
2+ Ba
SO4 1 : 1
→ BaSO4 (s) + excess : 1
2+ Ba
(NH4)2SO4 (s) → 2NH4 + SO4 1 : 2 : 1 +
There are two methods used to quantitatively measure this reaction. 1. Gravimetric Methods: Filter off the formed barium sulfate precipitate and weigh the dried material. 2. Chemical Methods: Colourmetric titration to measure the remaining barium ions left over after reaction (back titration ).
Measuring excess Barium
The fertiliser used in this experiment contains sulfate in the form of ammonium sulfate (NH4)2SO4. When dissolved, this ionic compound forms 2 molecules of ammonium (NH4+) ions, and 1 molecule of sulfate (SO42-) ions. This ratio of ions is shown below:
2-
Every molecule of fertiliser releases 1 molecule of sulfate when dissolved (1:1 ratio)
2+ (Ba )
A method first is needed to visualise the presence of excess barium in solution. In this experiment, a weak complexometric indicator named Eriochrome black T (EBT) is added to achieve this. This compound weakly binds to any metal cations (Ba2+ ) present in solution, forming a bright red coloured solution. When eriochrome black T is not bound to metal ions, it produces a bright blue solution. Now the presence of barium can be seen, the amount remaining in solution can be measured. This is achieved with a titration, which adds a known volume of solution with a known concentration (titrant) to the barium solution (analyte).
Ba[EBT] 1
+ :
[EDTA] 1
→ :
Ba[EDTA] + [EBT] 1 : 1
Eriochrome black T bound to a barium ion (left) and not bound (right)
The titrant used in this experiment is ethylenediaminetetraacetic acid (EDTA). It is very strong complexing agent and strongly binds with metal ions in a 1:1 ratio but all EDTA complexes are colourless. Any metal ions (Ba2+ ) that are bound in the eriochrome indicator complex are stolen by the EDTA molecules being added. This leaves the eriochrome complex in its unbound form, resulting in a colour change to blue indicating all barium has been reacted and the reaction is complete.
Reaction Summary Equal amounts of Ba2+ is added to both the test and blank solutions.
Test
EBT is added & binds to remaining Ba2+ (red)
EDTA is added until all Ba2+ is stolen (blue)
Test titration with EDTA used 31.70 mL. Blank titration with EDTA used 57.20 mL. Difference = 57.20 – 31.70 = 25.50 mL
Ba2+
contains SO42-
Blank
All SO42- ions precipitated as BaSO4
Example Results
filter Barium sulfate is a fine white solid.
This difference in EDTA addition is due to the amount of barium ions lost from solution when reacted with all the sulfate ions.
EDTA
The overall reaction ratio is 1:1:1 SO42- : Ba2+: EDTA. Only excess barium remaining Ba[EBT]
Ba2+
All barium as Ba[EDTA]
Using the equation: n = CV n = number of moles C = concentration (M) V = volume (L)
EDTA
no SO42All barium remaining Ba[EBT]
The number of moles of EDTA in 25.50 mL is equal to the moles of barium that reacted with sulfate, and equal to the moles of sulfate present.
All barium as Ba[EDTA]
where:
The number of moles of EDTA can be calculated if the concentration of the solution is known (on the bottle)