Making a Volumetric Solution & Performing a Titration (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about AQA Required Practical 1: making a volumetric solution and performing a titration.

The two go together because they are the two halves of one measurement: volumetric analysis uses the volume and concentration of one solution to find the concentration of an unknown one.

Volumetric analysis is a process that uses the volume and concentration of one chemical reactant to determine the concentration of another, unknown solution, and the technique most commonly used is titration. Before the titration can be done, the standard solution has to be prepared, and specific apparatus must be used at both stages so that volumes are measured precisely. That is what makes these one practical rather than two.

We'll start with making the standard solution, then take the titration itself, and finish with the burette readings and the uncertainty on the titre.

Chemists routinely prepare solutions whose concentrations are known precisely. These are called volumetric solutions, or standard solutions, and they are made as accurately as possible using three-decimal-place balances and volumetric flasks, to reduce the impact of measurement uncertainties.

There are five steps. Weigh the solid, either by difference or by taring the beaker, on a balance reading to at least two decimal places. Dissolve it in distilled or deionised water, stirring with a glass rod until all the solid has dissolved. Transfer to a volumetric flask. Add the washings from the beaker and the glass rod. Then make up to the 250 centimetre cubed mark with water, stopper, and invert to mix.

Concentration is the amount of solute dissolved in a solvent to make one decimetre cubed of solution, usually expressed as moles per unit volume or as mass per unit volume. Everything the titration produces depends on this solution being exactly the concentration you think it is.

The main piece of apparatus in a titration is the burette, typically graduated in 0.10 centimetre cubed divisions.

A known volume, usually 20.0 or 25.0 centimetres cubed, is measured with a volumetric pipette and transferred to a conical flask. The other solution goes into the burette, filled close to zero, and the initial reading is recorded. A few drops of a suitable indicator are added to the conical flask, and the burette tap is opened carefully while the flask is swirled continuously. A white tile underneath makes the colour change easier to see. As the endpoint is approached the solution is added dropwise, and the tap is closed immediately after a single drop causes the indicator to change colour.

Two terms are worth separating. The equivalence point is reached when the amount of titrant added is chemically equivalent to the substance in the flask. The endpoint is the point at which the indicator changes colour, signalling that the reaction is complete, or very close to complete. The final reading is recorded, the titre is calculated, and the titration is repeated until concordant results are obtained.

Both burette readings are recorded to a precision of plus or minus 0.05 centimetres cubed, the same as the uncertainty. The burette is an analogue instrument, so its reading uncertainty is taken as half the smallest scale division.

The titre is the volume delivered from the burette, and it carries an uncertainty of plus or minus 0.10 centimetres cubed. That is doubled because two readings are used to calculate it: each reading has an uncertainty of ±0.05, and when you subtract the two values to find the titre, the uncertainties add. Concordant results are those differing by no more than 0.10 centimetres cubed. Those are averaged, and any result that is not concordant is excluded from the mean. The average titre is then used to find the unknown number of moles or concentration, which is the number the whole practical exists to produce.

Percentage uncertainty shows how significant an absolute uncertainty is compared with the size of the measurement: the absolute uncertainty divided by the measured value, multiplied by one hundred. It is not the same as percentage error, which compares an experimental result with a known or literature value.

Burettes and other laboratory glassware are often marked in millilitres. Centimetres cubed and millilitres are the same thing, but in exams this board only uses centimetres cubed as the unit for titration volumes.

Marks are most often lost on the standard solution. Name the volumetric flask, because saying beaker, conical flask, or vaguely a flask does not score. Include the washings. Say until all the solid has dissolved. Write distilled or deionised water rather than just water. And do not forget the final shake or invert to mix.

A standard solution is made by weighing accurately, dissolving completely, transferring with the washings, and making up to the mark. The titration then delivers that solution from a burette into a pipetted volume, dropwise at the end, repeated until the titres are concordant. Each burette reading carries plus or minus 0.05 centimetres cubed, so the titre carries plus or minus 0.10, and the concordant titres are averaged. Two halves, one measurement: the concentration you calculate is only as good as the solution you made and the titre you read.