NUMBAT OER - Open Educational Resources

5. Making a range of standard solutions

Having made a solution of known concentration, it is possible to make various mixtures of the standard ('stock' solution) and the pure solvent in order to prepare a range of standard solutions. This set of standards can be used to calibrate an analytical technique.

The usual way of making standard solutions is to prepare individual mixtures of the stock standard solution and the pure solvent. Adjustable pipettes are used to provide an exact amount of each. The table below indicates a standard series based on the sodium nitrate solution described in the last section, where a series of 20 ml standard samples have been prepared.

Amount of stock solution (8.5 g l-1) Amount of distilled water Final concentration (g l-1) Final concentration (mol l-1)
0 ml 20 ml 0 g l-1 0 mol l-1
5 ml 15 ml 2.125 g l-1 0.25 mol l-1
10 ml 10 ml 4.25 g l-1 0.5 mol l-1
15 ml 5 ml 6.375 g l-1 0.75 mol l-1
20 ml 0 ml 8.5 g l-1 0.1 mol l-1

If you are unfamiliar with the use of adjustable piston pipettes, you may find it useful to look at a University of Michigan practical schedule at:

http://www.umd.umich.edu/casl/natsci/slc/slconline/MICRPIP/index.html

In this example, the same volume of solution (20 ml) is being produced in each case. Sometimes, analytical tests may involve 'spiking' a constant amount of solvent with different amounts of the stock solution. In this case, the final volume of each standard is different, so that the concentration of each of the standard series has to be calculated based on the added solute and the final volume. For instance, if you use a pipette that dispenses 250 μl you can make a standard series like this, by adding different numbers of 'shots' to 10 ml batches of solvent:

Number of 250 μl additions Total solute added (mg) Final volume (ml) Final concentration (mg ml-1) Final concentration (mmol l-1)
0 0 10 0 0
1 2.125 10.25 0.207 2.44
2 4.25 10.5 0.405 4.76
3 6.375 10.75 0.593 6.98
4 8.5 11 0.773 9.09
5 10.625 11.25 0.944 11.11

Note that as more shots of standard are added, the final volume increases so that the fractional increase in concentration declines.