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CONCENTRATION

Definition

The quality of a specified substance in a given amount of the medium throughout which it is distributed. Several units are currently used to describe the concentrations of substances in water. Frequently, the weight of the substance contained within 1 litre is used in a weight/volume ratio (e.g. mg/l). Such units are often used interchangeably with ratios such as parts per million (ppm; e.g. 1mg of a substance in 1 litre, i.e. 1000g, of pure water = 1 ppm). Care should be taken, however, as these latter units are volume/volume or weight/weight. Parts per billion (ppb) may also be encountered but, because of the different interpretations of billion in various countries, this unit may be somewhat confusing. Weight/volume or volume/volume ratios may be converted into percentages (e.g. a 1% w/v solution = 1g of a substance in 100 ml). In addition concentration is often expressed as a number of atoms or molecules of a substance in a given volume. The standard unit is the mole that is equivalent to Avogadro¬´s number (6.02x1023 ) of atoms or molecules. In practical terms, the weight (in g) of 1 mole of atoms or molecules is given by the atomic or molecular weight of a substance and concentration expressed as mol/l.

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