Download Eriochrome black T bound to a barium ion

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)