Personal Study: Preparing a Primary Standard Solution

There are two methods of preparing primary standard solutions.

1. By direct weighing of a pure reagent and adding the solvent to make up a known volume of solution.
   
   
2. By the dilution of a prepacked ampoule containing an accurately known volume of a highly concentrated solution with an accurately known concentration.

Solute purity
The concentration of a primary standard needs to be known accurately. Therefore, the solute must be pure. To ensure this, the following guidelines should be followed.

• When using powdered or crystallized reagents use a clean spatula to transfer the reagent from its container to the weigh boat. If you weigh out too much, do not place the excess back into the container; discard the extra. If absolutely necessary, use a clean spatula to break-up clumps of chemicals.
• Solutes should be analytical reagent grade (AR).
• Solutes should be dried before being weighed and made up, to the volume required, with solvent.

Water quality

• Water quality is just as important as the purity of the solute used to make solutions.
• Tap water contains chemicals and other impurities and is not suitable for making up solutions.
• Distilled water can be used for making up solutions.
• Distilled water is prepared by boiling water and collecting the vapour given off in a condenser. The water is purified because only the vapour is collected, and the contaminants, such as dissolved minerals and microbial contaminants, are left behind in the heating chamber.
• Distilled water must be protected in storage. Capped winchesters or large polythene drums may be used for storage.

Analytical balance

Analytical balances are used in many laboratories to weigh out small masses (up to about 100 g, depending on the model). They usually measure to 4 decimal places (ie 0.0000 g).

Analytical balances are highly accurate and as such have specific siting and maintenance requirements.

They must be:

• on a vibration-free surface (usually very heavy and stable)
• away from air draughts
• away from areas of high pedestrian traffic, to reduce vibration and air currents
• level, to ensure accuracy
• cleaned routinely to remove the build up of spilled chemicals that will affect the accuracy of the measurement
• cleaned routinely to remove spilled chemicals that may etch the surface of the balance pan and affect accuracy
• used with the glass doors closed to reduce the effects of drafts
• used only to weigh chemicals that are at room temperature, as hot materials will set up air currents that will affect the accuracy of the measurement.

Top loading balance

Top loading balances have a larger capacity than analytical balances. These balances are available in a range of capacities, but typically a top loading balance will have a capacity of about 5000 g, and can measure to two decimal places (ie 0.00 g).

Top loading balances are more durable than analytical balances, but it is still important to use them correctly. The following general rules apply to the use of top loading balances.

• Do not place chemicals directly on the balance pan. Weigh material in or on something - a weighing dish, a weighing bottle (if the substance then has to go in the oven), a beaker or a piece of folded paper. However it is important to weigh the container first so that you can adjust for the weight of the container.
• Any chemicals spilled on or around the balance should be cleaned up immediately.
• Position the balance in a location that is horizontal, stable and vibration free.
• Avoid placing the balance in areas exposed to direct sunlight.
• A top loading balance should be left on through the working day. If the components of an electronic balance are cold, they will drift while warming up causing changes to the measurements. Leaving it on will ensure the components are kept warm.
• The top loading balance should be covered, to protect it, when it is not being used.

Volumetric flask

Volumetric flasks are used when diluting a sample or solution to a certain volume. They are designed 'to contain' (hence marked TC) an accurate volume at the specified temperature (20°C).

When using volumetric glassware, such as volumetric flasks or burettes, the bottom of the meniscus should coincide with the calibration mark when held at eye level, to reduce parallax error.

Primary standard preparation procedure
The concentration of a primary standard needs to be known accurately. To achieve this the pure reagents used to make up the solution must be precisely measured.

When preparing a primary standard by the direct weighing of a pure reagent and then addition of solvent to make up a known volume of solution, the following procedure is followed.

1. In a weighing bottle, measure out the amount of solute required to make the required volume of solution at the required concentration, to an accuracy of 3 significant figures.
   
   
2. Dry the solute in an oven for 2 hours.
   
   
3. Cool the dried solute to room temperature in a desiccator, with the lid off the weighing bottle.

4. Reweigh the dried solute to an accuracy of 4 significant figures.
   
   
5. Check that the volumetric flask is free from oil or grease residue.
   
   
6. Rinse the clean volumetric flask with distilled water three times.
   
   
7. Fill the volumetric flask approximately 1/10 full with distilled water.
   
   
8. Transfer the dried and weighed solute into the volumetric flask using a funnel.
   
   

9. Use a distilled water wash bottle to rinse all traces of the weighed material, from the weighing boat and the funnel, into the volumetric flask.

10. Fill the volumetric flask approximately 1/3 full with distilled water.
    
    

11. Stopper the volumetric flask and shake it thoroughly to dissolve all traces of the solute.

12. Fill the volumetric flask with distilled water until the bottom of the meniscus corresponds to the calibration mark when the flask is held vertically and the mark is at eye level.

13. Mix the solution thoroughly.