eng-chem

MAKING-UP SOLUTIONS

Analytical solutions are made up in volumetric flasks with volumes ranging from one cubic centimetre to as much as five litres.

Although they are all accurately calibrated, the largest flask will always give a solution with the most precisely measured concentration. This is mainly because any balance has a limit to its accuracy and this will produce a greater percentage error when weighing out smaller quantities of solute. Thus the smaller flasks should only be used when the greatest precision is not required. The very large flasks are used for making up standard solutions of reagents that will be used many times.

Solutions may deteriorate, so standard solutions should never be stored for long periods of time. Furthermore, some solutions may need to be transferred to dark bottles, to limit photochemical decomposition.

A common reason for making up a solution of a solid is to determine the solid's purity or composition - say to determine the percentage copper in this copper salt. The most commonly used flask for this purpose has a volume of 250 cubic centimetres. Like all volumetric flasks, it has a stopper, and a single calibration line which accurately marks the stated volume at room temperature - 20 degrees Celsius.

To determine the purity or composition of a solid, first weigh out a suitable quantity on an analytical balance to an accuraccy of four decimal places. Record the weighing in a notebook. Solids which absorb atmospheric moisture must first be dried in an oven and then allowed to cool in a desiccator, before weighing. Tip the solid into a 100 cubic centimetre beaker, then reweigh the weighing bottle to accurately determine the amount of solid added. Add any other necessary reagent, such as dilute acid then add about 50 cubic centimetres of de-ionised water, and stir the mixture until the soolid has completely dissolved It's bad practice to tip the solid directly into a volumetric flask - some solids are very slow to dissolve at room temperature. In such cases, gently warm the beaker carefully stirring the contents from time to time until the solid dissolves. Do not allow the mixture to boil, in case some drops of liquid splash out. Once a clear solution has been formed, remove it from the heat and allow it to cool. When the solution has cooled, first carefully wash the clock glass and the stirrer rod with de-ionised water. The remaining procedure is the same for solutions of solids, like the copper salt, which dissolve easily at room temperature.

Transfer the solution to a clean volumetric flask using a funnel to make sure that all the solution gets into the flask. Then rinse out the beaker with more water. Pour in de-ionised water until the level is about one centimetre below the calibration mark. Remove the funnel then add water dropwise using a wash-bottle, until the bottom of the meniscus is at the calibration level. Place the stopper in the flask, then thoroughly mix the solution by inverting the flask several times, keeping a finger firmly over the stopper. Notice that the flask has at all times remained at room temperature. Heating the flask will alter its calibrated volume so never heat a volumetric flask - it is a precision piece of equipment.

Solutions of standard reagents such as potassium dichromate or EDTA can be made up in a similar fashion. Alternatively, standard ampoules containing concentrated solutions of standard reagents can be purchased. There are several different types of ampoule in production, some made of glass and some made of plastic.

It is sometimes necessary to make up solutions of different known concentrations, starting from a standard stock solution. The easiest way to accurately dilute by a factor of 10 is to take 25 cubic centimetres of the stock solution in a pipette, then transfer it to a 250 cubic centimetre volumetric flask. Dilute the solution to the mark with deionised water. Then stopper the flask and thoroughly mix the contents. As the volume containing the solute has been increased 10 fold its concentration has been reduced 10 fold. If several different volumes need to be measured out. to give a range of concentrations, it may be easier to deliver the stock solution from a burette. Note that because 10 cubic centimetres can be measured out to higher accuracy than 1 cubic centimetre, normally a dilution of 100 fold is better done by diluting 10 cubic centimetres of stock solution to 1 litre, rather than one cubic centimetre to 100 cubic centimetres.