H8
Monitoring Water Quality
Water bodies, such as rivers and lakes, have also been polluted by the activities of
humans. The nature and source of these pollutants are different from those in the
atmosphere and different techniques are employed by chemists to monitor their
presence and concentrations. Important analytic techniques are studied.
Characteristics of
good quality drinking
water
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Completely colourless and clear
Odourless
Pleasant taste
Relatively low salt content
Contains no pathogens or poisonous chemicals
Criteria for assessing
water quality
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Turbidity
Total dissolved solids (TDS)
pH
Temperature
Dissolved oxygen
Amount of biochemical oxygen demand (BOD)
Concentrations of nitrates and phosphate
Hardness
Presence of pathogens
Concentrations of heavy metals
Presence of toxic organic compounds
Turbidity
Means cloudiness or lack of transparency.
Turbidity can be assessed by measuring the depth of water
needed to render invisible a mark on the flat bottom of a
specially made tube.
Turbidity is measured in nephelometric turbidity units (NTU).
Total dissolved solids
(TDS)
Mass of solids dissolved in unit volume of water (mg L-1 or
ppm). TDS can be measured by evaporation but more
accurately by electrical conductivity.
TDS (in ppm) = 0.65 x (conductivity in mS cm-1)
Dissolves oxygen
Oxygen has a very low solubility (9ppm at 20oC) but if it falls
below 5ppm many aquatic species of animals will die or fail to
reproduce.
Dissolved oxygen may be measured by
 Titration method
 Electrolysis method (oxygen sensor)
Biochemical oxygen
demand (BOD)
The BOD of a water body is a measure of the concentration of
dissolved oxygen that is needed for the complete breakdown
of the organic matter in the water by aerobic bacteria.
Typical BOD levels are
 Unpolluted water
 Well treated sewage
 Raw sewage
 Storm water run off from urban areas
<5ppm
20 to 30ppm
150 to 300ppm
100 to 500ppm
The low BOD of municipal water supplies is measured by
adding nutrients to the sample, incubating in a sealed air-free
container in the dark for 5 days and then measuring the
residual dissolved oxygen.
Samples with high BOD are analysed by adding oxygen to
keep the sample saturated and measuring the amount of
oxygen used.
Hard water
Water that does not form a good lather with soap. The stearate
ions of the soap react with dissolved calcium and magnesium
ions in the water to form a grey scum.
Ca2+(aq) + 2C17H35COO-(aq)
(C17H35COO)2Ca(s)
Mg2+(aq) + 2C17H35COO-(aq)
(C17H35COO)2Mg(s)
Water hardness is measured by determining the total
concentration of calcium ions in the water and expressing the
result as milligrams of CaCO3 per litre.
A volumetric titration is used to determine the concentration
of magnesium and calcium ions. This titration involves the
formation of a complex between calcium and magnesium ions
in an ammonia/ ammonium ion buffer solution with the
quaternary charged anion of ethylenediaminetetra-acetic acid
(EDTA).
Tests used to identify
cations
(Insert Conquering Chemistry HSC page 270 Table 8.3)
Concentration of
cations
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Gravimetric analysis or titration are of limited used
because the concentrations are so low
Atomic absorption spectroscopy or atomic emission
spectroscopy are used.
Factors affecting the
concentration of ions
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The pathway from rain to water body
The pH of the rain
The nature and amount of human activity in the
catchment
Effluents discharged into the water body
Leaching from rubbish dumps
Algal bloom
An excessive growth of algae and other aquatic organisms in a
river, lake or ocean bay.
The detrimental affects of algal blooms are
 Water becomes unsuitable for normal use
 Cyanobacteria produce poisons that can kill livestock and
cause serious illness in humans
 The consumption of dissolved oxygen during the night
causes fish to suffocate and die.
Eutrophication
A natural process that involves deep lakes with limited life
forms changing to shallow ones with thousands of life forms,
to marshes and swamps and eventually to firm land.
The process can be hastened by human activity that adds
nutrients (nitrates and phosphates) to the waterways. Sources
of nutrients are
 Sewage
 Fertilisers
Water treatment
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Membrane filters
Lime or sodium hydroxide is added to raise the pH of the
water above 7
Iron (III) hydroxide is added to clear the water by
flocculation
The precipitate formed is filtered through sand
Chlorine is dissolved in the water to kill bacteria and
some viruses.
Fluorine is added in some places to strengthen the tooth
enamel of children
A thin film of a synthetic polymer through which there are
pores of fairly uniform size.
The advantages of membrane filters include
 They can filter out much smaller particles than paper or
sand filters.
 They filter out virtually all particles larger than their
specified pore size.
 They are quite thin and so liquids flow through them
fairly rapidly.
 They are reasonable strong and can withstand 2 to 5
atmosphere pressure differences.
 They can be cleaned (back-flushed) and reused.