Terrace: title

Year 12 Chemistry - Unit Four

Titrations - Volumetric Analysis

Term	Explanation
Volumetric analysis	A quantitative technique used to analyse solutions using volumes of solutions.
Burette	Calibrated apparatus with a delivery tap at the base, used to deliver up to 50 mL of solution
Pipette	The analytical pipette is designed for accurate transfer of fixed volumes of solution. Common sizes include 5.0 mL, 10.0 mL, 20 mL, and 50 mL. A pipette bulb is used to draw up solution safely into the pipette.
titrate	To add the solution from the burette into the receiving flask while watching for a colour change.
titre	The volume of solution added from the burette to exactly reach the end-point.
aliquot	The accurate volume of solution transferred by pipette to the receiving flask.
end point	The point at which a colour change occurs. The solution added from the burette is usually just in excess.
equivalence point	The point at which the ratio of moles of reactants is equal to the ratio in the reaction equation; neither reactant is in excess.
indicator	The suitable acid-base indicator that changes colour at the end point. This indicator should be chosen carefully so that the end-point is as close as possible to the equivalence point.
primary standard	The substance used to make a solution of accurately known concentration.
standardisation	The process of determining the concentration of a solution by titrating it with a primary standard.
secondary standard	a solution that has been standardised using a primary standard. This solution now has a known concentration and can be used to titrate against a further solution.
volumetric flask	This flask is accurately calibrated and contains an exact volume at 20^oC. Used to make up primary standard solutions or to dilute solutions accurately.

Steps for Solving Titration Problems

Identify your known and unknown in the problem.
Find the amount of the known . i.e. find n (either from n = m/M or n = cV)
Write a balanced (non-ionic) overall equation for the reaction.
From the equation, using proportion (ratios) find n for the unknown.
From n and V of your unknown solution find concentration c (c = n/V)

Titration Problems

Titration Techniques

1. To prepare 250 mL of a standard solution of accurately known concentration.

Calculate the mass of solid needed to make 250 mL of a solution of the required concentration.
Accurately weigh out close to this amount in beaker.
Record the mass of the solid.
Dissolve the solid in some distilled water.
Transfer to a clean 250 mL volumetric flask rinsed with distilled water. (Note: It does not matter if it is not dry as the presence of distilled water will not affect the mass and thus the number of moles of the solid solute.)
Rinse the beaker with some more distilled water and transfer this to the volumetric flask. Repeat if necessary. Ensure that all the weighed solid is transferred to the volumetric flask.
Fill the volumetric flask with distilled water until the bottom of the meniscus just sits on the calibration mark. Stopper and gently mix the solution.
Calculate the concentration of the solution and record the concentration on a label on the flask. (Remember you can only be as accurate as the accuracy of you mass balance.)

2. The properties of a primary standard.

It is easily obtainable pure.
It does not react with oxygen or carbon dioxide in the air.
Its reaction with the substance to be analysed is complete and the equation for the reaction is known.
It is soluble under the conditions in which it is to be used.Other useful features:
High molar mass so that weighing errors are minimised.
Cheap and readily available.Examples of primary standards -
anhydrous A.R. sodium oxalate Na₂C₂O₄,
anhydrous A.R. sodium carbonate Na₂CO₃,

3. Techniques for using a burette

Clean the burette with detergent and water and rinse it with distilled water.
Rinse the burette twice with 5-10 mL portions of the solution to be transferred.
Using a small funnel, nearly fill the burette with the solution to be transferred. Remove the funnel.
Open the tap to run out air bubbles in the burette tip and allow the meniscus to fall to a level between the 0 and 5 mL calibrations.
touch a beaker against the tip to remove any drop that has not fallen off.
With the burette vertical and the meniscus at eye level, estimate to the nearest 0.02 mL the level of the bottom of the meniscus. Record this initial reading.
Lower the burette so that the tip is lower than the top of the receiving flask.
Transfer solution from the burette into the receiving flask until the end-point is reached. with practice you will find this is best done by turning the tap with your left hand. Use your right hand to swirl the receiving flask
Record your final reading. Th difference between your initial and final reading is the titre. (i.e. the volume of liquid delivered from the burette.)
Do a rough titration followed by 2 or 3 accurate titrations. The average titre is found by averaging the volumes of your accurate titrations.

4. Techniques for using a pipette

Clean the pipette with detergent and water.
Rinse the pipette thoroughly, first with tap water and then with distilled water.
Rinse the pipette twice with about 10% of its volume of the solution to be transferred.
Using a safety filler, fill the pipette above its calibrations mark.
Use a tissue to remove drops on the outside of the pipette near the tip.
Hold the pipette vertically with the tip against a beaker. With the calibration mark at eye level, allow the solution to drain slowly until the bottom of the meniscus is level with the calibration. If the bottom of the meniscus falls below the calibration, start again to refill using the safety filler.
Allow the contents to drain into the conical flask. Hold the pipette tip against the flask wall to avoid splashing.
After the pipette appears to have drained, allow five seconds further drainage. Do not force the contents out of the pipette.
Wash down the inside walls of the flask with distilled water from a wash bottle.

5. Rinsing of apparatus used in titrations

Receiving apparatus - volumetric flasks, conical titration flasks (Rinse with water)
Transfer apparatus- burettes, pipettes.(Rinse with the solutions they will be transferring.)