ARTICLES
Micro-scale precipitation titration
By Chris Warren
In a school laboratory, students are familiar with carrying out
titrations using 25 mL pipettes for the aliquot and 50 mL burettes
for measuring the titre and, when repeated several times, this
can involve large volumes of reactants and waste products.
• 50 or 100 mL conical flask
• white tile
Hazard Analysis
A Mohr Titration is an established method to determine chloride
concentration in solution, for example, measuring the salinity
of seawater. In this titration, all of the chloride (and bromide/
iodide) ions in sea water will be precipitated. The titrant used is
silver nitrate which is problematic for two reasons: firstly, it is an
expensive reagent; secondly the wastes need to be collected.
Titrations can also be time consuming in terms of setting up,
rinsing and cleaning equipment. With this particular reaction a
blank titration or calibration is also required. For these reasons, a
microscale titration may be appropriate, especially if the level of
precision required is only 2 significant figures.
Wear eye protection when handling silver nitrate. Use skin
protection if there is any risk of spills. The chromate and silver
wastes are toxic to the environment, so should be retained with
your heavy metal wastes for reprocessing. It is the responsibility
of teachers supervising this experiment to carry out an
appropriate risk assessment.
Procedure
1. Rinse a 10 mL measuring cylinder with the 10% sea water and
discard. Refill the cylinder to 10.0 mL. Transfer to the conical
flask. Rinse the measuring cylinder with 10 mL of deionised
water and transfer to the flask.
In a Mohr Titration, halide ions are precipitated by adding silver
nitrate solution dropwise.
2. Add 2 drops of potassium chromate solution to the flask. It
should turn a transparent pale yellow colour.
Ag + (aq) + Cl - (aq) AgCl(s)
The solubility product for silver chloride (K sp = 1.77 × 10 −10 ) is
low, so a precipitate forms readily. This is a white colour that
darkens on exposure to light. K sp values for silver bromide and
iodide are even lower.
3. In another clean, dry measuring cylinder, add exactly 10.0 mL
of 0.100 M silver nitrate solution.
4. Using the disposable pipette, begin adding the silver nitrate
dropwise to the flask while swirling. Note a pale lemony white
colour.
The chromate ion is used as the indicator. Since silver chromate
is slightly more soluble than silver chloride, it will not begin to
precipitate until all the chloride ions have precipitated.
5. Keep adding silver nitrate dropwise while swirling. Note the
yellow gets darker and persists longer as you approach the
end-point.
Before reaching the end-point, the solution will appear a pale
lemon colour due to the yellow colour of the chromate indicator
and the white colour of the silver chloride precipitate. Just
before the end-point, it turns dark yellow. At the end-point, the
solution turns a reddish brown colour (or a pale pink) due to the
precipitation of silver chromate.
6. At the end-point, the indicator will turn a pale pink-red. You
may see black flecks of metallic silver in the solution as well.
7. Return any unused silver nitrate to the measuring cylinder and
record the volume used.
Equipment required
8. Repeat 3 times or until concordant values are reached.
• 10% sea water sample (this can be prepared in advance for
time saving)
• Deionised water
• 25-30 mL 0.10 M silver nitrate solution (sufficient for 4 titrations)
• 2 mL approx. 0.1 M potassium chromate solution
• Disposable plastic dropping (Berel) pipette
• 2 x 10 mL measuring cylinders
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SCIENCE EDUCATIONAL NEWS VOL 68 NO 3