Severn Trent Water Case Study – Chemical Testing
Key Skills References
How Science Works References
1a, 1b, 3b, 4a, 4b
Vocational Implications: Methods Used by Severn Trent Water to Identify Ions
Read the information. Make brief notes by answering the questions in full sentences.
Severn Trent Water provides millions of homes in the midlands with their water.
Water boards such as Severn Trent monitor ions in the water by carrying out
chemical tests to identify impurities and other dissolved substances that affect the
water used. Ions in water can affect the taste, and ‘hardness’ (hard water needs a lot
more soap to make a lather than soft water) and our health. It is important the water
board uses quantitative tests to determine the exact levels of these ions in the water.
If the levels get too high the water board needs to treat the water to lower the ion
levels.
The water boards and consumers also use qualitative tests on water include
 colour – should be clear and bright
 odour- smell – sometimes it will smell of chlorine. At low levels chlorine is not
harmful and disinfects water
 taste – if water has been standing in the pipes it may have a flat, stale, metallic
taste.
Questions
1. Why do Seven Trent need to analyse the ions present in water?
2. Why is it important that these tests are quantitative?
3. What qualitative tests are carried out on water, and why?
Ions in Water
Read the following information on ions and their effect on water and use it to
complete the table in the next section.
Calcium and magnesium ions cause water hardness and result from limestonetype materials in underground soil layers. Separate values are of minor concern but
they are combined for calculating hardness.
Hardness is the soap-consuming capacity of water; that is, the more soap required to
produce lather, the harder the water. Hard water also causes greasy rings on
bathtubs, film on dishes or hair after washing, limescale in kettles, and poor laundry
results. Hardness is reported as calcium carbonate in milligrams per litre. Hard
water may irritate eczema. Max calcium = 250 mg/l
Hardness expressed as mg/l of CaCO3.
0-100
100-200
200-300
300-400
400-500
Severn Trent Water Case Study – Chemical Testing
© Copyright of 3E’s Enterprises (Trading) Limited
Water hardness
Soft
Moderately soft
Moderately hard
Hard
Very hard
Severn Trent Water Case Study – Chemical Testing
Sodium ions in water may be of health significance to people on a low-sodium diet.
Sodium can be reduced or removed by expensive treatment systems, Sodium ions
occur in water if it has been ‘softened’. Max = 150 mg/l
Potassium is an essential nutritional element, but its concentration in most drinking
water is trivial and quantities seldom reach 10 mg/l. Max = 12 mg/l
Carbonates and bicarbonates are the major contributors to the "total alkalinity" that
may be determined in a routine water test. If the alkalinity is low problems could
occur due to corrosion of metal in plumbing systems.
Chloride ions concentrations in drinking water may be important to people on lowsalt diets. Most people will detect a salty taste in water containing more than 250 mg/l
of chloride. Expensive treatment methods are needed to remove chloride from water.
Max = 400 mg/l
Sulphate ions may give water a bitter taste and have a laxative effect on people not
adapted to the water. Max = 250 mg/l
Nitrate ions are of health significance to pregnant women and infants under 6
months. Do not use high-nitrate water in infant formulas or other infant foods. Most
adults tolerate considerably higher nitrate content. Max = 50 mg/l
Iron ions are nuisance chemicals that cause troublesome stains and deposits on
light-coloured clothes and plumbing fixtures. Iron causes yellow, red or reddishbrown stains and deposits. Excessive amounts also may cause dark discoloration in
some food and beverages and cause an unpleasant taste. Max = 0.2 mg/l
Copper ions will cause an undesirable metallic taste if concentrations are above the
recommended limits. Max = 3 mg/l
Comparing my Method to the Method Used by Severn Trent Water
Use the information above and the background from your flame and chemical tests
write up to complete the table.
Severn Trent Water Case Study – Chemical Testing
© Copyright of 3E’s Enterprises (Trading) Limited
Severn Trent Water Case Study – Chemical Testing
Ion
Effect on
water
Iron
How we
detected it in
the lab (both
flame and
chemical tests
and results)
Qualitative
chemical test Added sodium
hydroxide and a
brown or green
precipitate forms
Calcium
Copper
Potassium
Sodium
Chloride
Nitrate
Calcium
Carbonate
(hardness)
Sulphate
How Severn Trent
detect the ions in
their lab
Max
amount
(mg/l) in
tap
water
Heat in nitric acid to
dissolve metals then
use spectrometry. To
quantify results they
compare results to
standard solutions
Chemical added to
make iron III
thiocyanate.
Colourimetry sat at
wavelength 480 nm.
Concentration
determined by
comparing to
standard solutions.
Chemical added to
make pink azo dye.
Colourimetry used at
wavelength 510 nm.
Concentration
determined by
comparing to
standard solutions.
Calculation- Calcium
result from above
Result multiplied by
2.5
Chemical added to
make a barium
sulphate precipitate.
The amount of which
is determined
turbidimetrically.
-
Extension for top grades (distinction)
Read the 3 quantitative methods used by Severn Trent, make BRIEF notes on each
method and answer the questions comparing the Severn Trent Methods with our lab
methods….
Severn Trent Water Case Study – Chemical Testing
© Copyright of 3E’s Enterprises (Trading) Limited
Severn Trent Water Case Study – Chemical Testing
1) Atomic (Flame) Emission Spectrometry is Used to Analyse of Metals and
Metallic Ions.
Flame emission spectrometry techniques are often quantitatively used to measure the
concentration of chemicals in a sample. The sample to be tested is dissolved in water
and the resulting solution is then aspirated into a flame. The flame excites the atom
and releases light of a certain wavelength (colour). The intensity of the light given out
is measured so the amount of that ion can be worked out quantitatively. This method
is very accurate and is used in a wide variety of areas including the Olympic drugtesting program and in environmental studies such as water testing.
Comparing Atomic Spectrometry to Our Flame Test Method.
 This method is only good for ions, which are easily excited – similar to flame
tests.
 For a mass spectrometer to work the ions must be placed in a vacuum.
Otherwise interference from air stops the ions reaching the detector. Did your
flame tests and chemical tests need a vacuum?
 To reduce contamination from light produced by other chemicals in the
sample, a wavelength selector is used. In our flame tests why did we
sometimes see contaminatiuon in the flame?
 The detectors measure the exact wavelength of light. Is this more accurate
than subjectively ‘guessing’ the colour.
 For quick analysis the detector is linked to a computer which states the result.
In our flame tests we had to look at our background table to work out what
colours corresponded to what ion. Which method gives a quicker result?
 In order to make quantitative measurements, an instrument has to first be
calibrated. Instruments generally record some signal (e.g. the intensity of
light). If incorrectly calibrated the results will be inaccurate. Did our flame
tests need calibration?
2) Colourimetry
The ions are reacted with certain chemicals to produce a coloured solution. The more
ions present, the deeper the colour of the solution. Light is passed through a filter of
a certain wavelength (colour) and the amount of light absorbed (% absorbance) by
the solution corresponds to how much of the ion is present. The darker the solution,
the more light absorbed, the more ion present. This method also needs calibrating
and the solution must be coloured.




Did you have any coloured solutions in your chemical tests?
Which ions made coloured solutions?
Would more concentrated copper sulphate be darker or lighter than week
copper sulphate?
Was our chemical test methods qualitative?
Severn Trent Water Case Study – Chemical Testing
© Copyright of 3E’s Enterprises (Trading) Limited
Severn Trent Water Case Study – Chemical Testing

Look back to the table at the start. How do Thames water carry out
colourimetry for different ions?
3) Turbidity.
Turbidity is a measure of how cloudy a solution is by measuring the amount of
precipitate made.




Name the precipitates made in your chemical tests experiment and compare
this to the ones made using Severn Trent’s methods. Are any of them the
same?
What effect would increasing the ions in the chloride, sulphate, copper, iron II
and Iron III solutions have on the amount of precipitate?
Did you measure the amount of coloured precipitate?
How do Thames water measure the amount of precipitate?
Severn Trent Water Case Study – Chemical Testing
© Copyright of 3E’s Enterprises (Trading) Limited