Page 374 - The Veterinary Laboratory and Field Manual 3rd Edition

Clinical chemistry 343

latter, chemicals must be weighed and added to
the appropriate volume of solvent but the need
for precision is less than when preparing solu-
tions such as standards for quantitative tests.
Some guidelines for the preparation of accurate
solutions are provided below.

Guidelines for preparing accurate
solutions
• Use a sensitive balance.
• Select analytical grade chemicals.
• Hygroscopic (deliquescent) chemicals need
to be weighed rapidly to avoid errors.
• Use accurately calibrated clean glassware.
• Use a funnel to transfer the chemical(s) from
the weighing container into a volumetric
flask.
• Wash any chemical(s) remaining in the con-
tainer into the flask with a little of the solvent. Figure 7.11 Reading the level of a fluid column
• Make the solution up to its final volume only (meniscus). When using a calibrated flask, beaker or
when it has cooled to the temperature used to measuring cylinder to measure a specified volume
graduate the flask (this temperature is writ- of a liquid read the level at the base of the ‘menis-
ten on the flask). cus’. This is the bottom of the concave surface of
• To avoid over-shooting the graduation mark the fluid seen when ‘eye level’ is the same as that
use a Pasteur pipette or wash bottle to add of the liquid. See also Figure 2.58.
the final volume of solvent to the flask.
• Make sure the bottom of the meniscus of the
fluid is on the graduation mark when viewed with 1 mole of another substance. Owing to
at eye level (Figure 7.11). the fact that chemicals interact in relation
to their molecular masses it is recommended
that the concentration of solutions be expressed
Expressing the concentration of
solutions in terms of the number of moles of solute per
litre of solution. However, if the relative molec-
As mentioned above, in a well-prepared solu- ular mass of a substance is not known, the
tion there should be an even distribution of concentration of such a substance in solution be
the molecules or ions of the solute throughout expressed in terms of mass (weight) concentra-
the solvent. Concentrations of solutions can be tion, that is, grams or milligrams per litre (per
expressed as the percentage solution (weight 100 ml is less commonly used).
for volume [w/v] or volume for volume [v/v]),
or as a molar (M) solution. In considering the MoLE PEr LItrE SoLutIonS
ways of expressing the concentration of solu- A mole per litre (mol/1) solution contains one
tions, it should be remembered that in chemical mole of solute dissolved in and made up to 1 l
reactions it is 1 mole of a substance that reacts with solvent.

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