1. Take a 100.0mL aliquot of the boiled beverage in a 250mL beaker.
2. Place the beaker on the magnetic stirrer, add the stirrer “bug” (more correctly a magnetic
follower) and carefully clamp the burette into position above the beaker (so that you can
comfortably add base) and the pH electrode into position. Turn the stirrer on and make sure that
it does not come into contact with the pH electrode. Stirring should be a steady, constant rate
– not too rapid. This will help achieve effective mixing with each addition of the base.
3. With the pH electrode in place, titrate the solution with standard (approximately 0.10M NaOH
– record the exact concentration) sodium hydroxide solution, recording the pH after each
addition. The first two additions of titrant volume (Vb) can large (about 1 mL), as the pH change
at this point in the titration is only small (see diagram on page 15). The volume added should
however be decreased beyond this point the pH will begin to rise rapidly. You will need
to make adjustments to Vb based on the change in ΔpH / ΔV. Make sure that you calculate
ΔpH/ΔV for each new Vb addition as this will show when you are nearing the titration end-point
(by becoming large). When the pH is changing rapidly record the volume of addition should be
reduced to no more than 0.1mL - 0.2mL.
Drop-wise addition of NaOH in the vicinity of the equivalence point is essential to
create a satisfactory plot. Make sure that you calculate ΔpH/ΔmL as you go. This
permits precise determination of the end points.
4. Once you have passed through the first end point ΔpH / ΔV will fall off in magnitude. Have the
supervisor check your results at this point, then proceed to ascertain the second endpoint using
the same approach in step 3. The volume of NaOH may again be increased in the initial stages
(~5 mL) until you approach the second end point. The volume of NaOH added between the 1st and
2nd endpoints (V2) will be about the same as that observed for the first equivalence point (V1).
5. When the second inflection point has been reached, turn the magnetic stirrer off, remove the
pH electrode from the solution, wipe carefully with a clean tissue and place in the original flask
of distilled water. Leaving pH electrodes in solutions of high pH can damage them.
6. The titrated solution can be discarded down the sink.
DO NOT THROW THE BUG OUT WITH THE SOLUTION!
Replace it on the magnetic stirrer.
1. Take a 100.0mL aliquot of the boiled beverage in a 250mL beaker.2. Place the beaker on the magnetic stirrer, add the stirrer “bug” (more correctly a magneticfollower) and carefully clamp the burette into position above the beaker (so that you cancomfortably add base) and the pH electrode into position. Turn the stirrer on and make sure thatit does not come into contact with the pH electrode. Stirring should be a steady, constant rate– not too rapid. This will help achieve effective mixing with each addition of the base.3. With the pH electrode in place, titrate the solution with standard (approximately 0.10M NaOH– record the exact concentration) sodium hydroxide solution, recording the pH after eachaddition. The first two additions of titrant volume (Vb) can large (about 1 mL), as the pH changeat this point in the titration is only small (see diagram on page 15). The volume added shouldhowever be decreased beyond this point the pH will begin to rise rapidly. You will needto make adjustments to Vb based on the change in ΔpH / ΔV. Make sure that you calculateΔpH/ΔV for each new Vb addition as this will show when you are nearing the titration end-point(by becoming large). When the pH is changing rapidly record the volume of addition should bereduced to no more than 0.1mL - 0.2mL.Drop-wise addition of NaOH in the vicinity of the equivalence point is essential tocreate a satisfactory plot. Make sure that you calculate ΔpH/ΔmL as you go. Thispermits precise determination of the end points.4. Once you have passed through the first end point ΔpH / ΔV will fall off in magnitude. Have thesupervisor check your results at this point, then proceed to ascertain the second endpoint usingthe same approach in step 3. The volume of NaOH may again be increased in the initial stages(~5 mL) until you approach the second end point. The volume of NaOH added between the 1st and2nd endpoints (V2) will be about the same as that observed for the first equivalence point (V1).5. When the second inflection point has been reached, turn the magnetic stirrer off, remove thepH electrode from the solution, wipe carefully with a clean tissue and place in the original flaskof distilled water. Leaving pH electrodes in solutions of high pH can damage them.6. The titrated solution can be discarded down the sink.DO NOT THROW THE BUG OUT WITH THE SOLUTION!Replace it on the magnetic stirrer.
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