Geoscience Reference
In-Depth Information
There are a range of spectrophotometers available
to perform rapid analysis of water quality in either
a laboratory or field situation. Many ions of interest
in water-quality analysis can be assessed using
colorimetric analysis. These include nitrate, nitrite,
ammonia and phosphate.
Colorimetry
Colorimetric analysis depends on a reagent causing
a colour to be formed when reacting with the par-
ticular ion you are interested in measuring. The
strength of colour produced is assumed to be pro-
portional to the concentration of the ion being
measured (Beer's law). The strength of colour can
then be assessed using one of four techniques:
comparison tubes, colour discs, colorimeter or
spectrophotometer.
Comparison tubes are prepared by using standard
solutions of the ion under investigation which the
reagent is added to. By having a range of standard
solutions the strength of colour can be compared (by
eye) to find the concentration of the water sample.
The standard solutions will fade with time and need
remaking, hence this is a time-consuming method.
Colour discs use the same principle as comparison
tubes, except in this case the standards are in the
form of coloured glass or plastic filters. The coloured
sample is visually compared to the coloured disc to
find the corresponding concentration. It is possible
to buy colour disc kits that come with small packets
of reagent powder for assessment of a particular
ion. This method is extremely convenient for rapid
field assessment, but is subjective and prone to
inaccuracy.
A colorimeter (sometimes called an absorptio-
meter) takes the subjective element out of the
assessment. It is similar to a turbidity meter in that
a beam of light is shone through the reagent in a
test tube. The amount of light emerging from the
other side is detected by a photo-electric cell. The
darker the solution (caused by a high concentration
of reactive ion) the less light emerges. This reading
can then be compared against calibrations done for
standard solutions.
A spectrophotometer is the most sophisticated
form of colorimetric assessment. In this case instead
of a beam of white light being shone through the
sample (as for the colorimeter) a specific wavelength
of light is chosen. The wavelength chosen will
depend on the colour generated by the reagent and
is specified by the reagent's manufacturer.
Ion-selective electrodes
In a similar vein to pH meters ion-selective elec-
trodes detect particular ions in solution and measure
the electrical potential produced between two
reactive substances. The tip of the electrode in the
instrument has to be coated with a substance that
reacts with the selected ion. With time the reactive
ability of the electrode will decrease and need to
be replaced. Although convenient for field usage
and accurate, the constant need for replacing elec-
trodes makes these an expensive item to maintain.
There are ion-selective electrodes available to
measure dissolved oxygen, ammonium, nitrate,
calcium, chloride and others.
Spectral techniques
When ions are energised by passing electricity
through them, or in a flame, they produce distinc-
tive colours. For instance, sodium produces a
distinctive yellow colour, as evidenced by sodium
lamps used in some cars and street lamps. Using
spectral analysis techniques the light intensity
of particular ions in a flame are measured and
compared to the light intensity from known stand-
ard solutions. The most common form of this
analysis is atomic absorption spectrophotometry,
a laboratory technique which is mostly used for
metallic ions.
PROXY MEASURES OF WATER
QUALITY
Any measurement of water quality using individual
parameters is vulnerable to the accusation that it
represents one particular point of time but not the
