The assessed practical is based on the classical titration exercise. I hope the following notes will help give you some background to this procedure.
In a titration, a fixed volume of one solution in a conical flask is reacted with a second solution drained into the first from a burette. The volume added from the burette to the "end point" is called the titre. Either solution can be the "standard" i.e. the one whose concentration is known, and calculations ensue to determine the concentration of the other. Any further mathematics will depend on the ingenuity of the examiner!
The fixed volume is dipensed using a pipette, frequently, though not exclusively, 25.0 cm3. Special safety fillers are available. The pipette must be rinsed with solution first, before it is used to measure out the volume for the titration. If the pipette is wet, the water inside it will dilute the solution causing the titre to be too small. After draining the measured volume into the conical flask, the final drop must be left in the pipette and not blown out. Allow the pipette to drain for at least 30s before moving it away from the flask.
The burette must also be rinsed with the appropriate solution. If this piece of apparatus is wet then the solution will again be diluted, but this time the titre will be too large. The jet at the end of the burette must be seen to be filled with solution before starting the titration. The funnel must be moved from the top of the burette since it may drip and cause errors in readings. The burette scale must be read to 0.05 cm3, and two titrations to within 0.20 cm3 are required before any confidence in the readings can be held. The burette must be clamped vertically over the flask, with its jet just below the rim to prevent any loss by splashing.
During the titration a jet of distilled water must be used to rinse round the inside of the flask. Many candidates worry that this will somehow affect the titration. It will! It will make it more accurate since the rinsing will ensure that all of the reagents are in the body of the flask reacting with each other.
As the solution is drained into the flask, the contents of the latter must be swirled gently to ensure rapid mixing and reaction. As the end point is appoarched the rate of addition is slowed and time allowed for each drop to react before adding another. remember that as the end point is apporached the amount of reagent left in the flask has become very small so that the reaction with the solution entering from the burette will be very slow.
Posted by dcox at November 13, 2009 06:33 PM