Vocational implications: Methods used By Severn Trent Water to identify
ions
Read the information in the box, 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 effect 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.
Why do Seven Trent need to analyse the ions present in water?
Why is it important these tests are quantitative?
What qualitative tests are carried out on water, and why?
Read the following information on ions and their effect on water and
answer the questions. Delete the red notes after you are finished.
Calcium and magnesium ions cause water hardness and result from
limestone-type 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 eczma. Max calcium = 250 mg/l
Table 3. Hardness expressed as Water hardness
mg/l of CaCO3.
0-100 Soft
100-200 Moderately soft
200-300 Moderately hard
300-400 Hard
400-500 Very hard
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 = 150mg/l
Potassium is an essential nutritional element, but its concentration in most
drinking water is trivial and quantities seldom reach 10 mg/l. Max = 12mg/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 low-salt 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 = 400mg/l
Sulphate ions may give water a bitter taste and have a laxative effect on
people not adapted to the water. Max =250mg/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 =50mg/l
Iron ions are nuisance chemicals that cause troublesome stains and
deposits on light-colored clothes and plumbing fixtures. Iron causes yellow,
red or reddish-brown 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 =3mg/l
Comparing my method to the Method used by Severn Trent Water
Use the information in red above and the background from your flame
and chemical tests write up to complete the table.
Ion Effect on How we How Severn Trent Max
water detected it in detect the ions in amount
the lab (both their lab (mg/l) in
flame and tap
chemical tests water
and results)
Iron Qualitative Heat in nitric acid to
chemical test - dissolve metals then
Added sodium use spectrometry.
hydroxide and a To quantify results
brown or green they compare
precipitate results to standard
forms solutions
Calcium
Copper
Potassium
Sodium
Chloride Chemical added to
make iron III
thiocyanate.
Colourimetry sat at
wavelength 480nm.
Concentration
determined by
comparing to
standard solutions.
Nitrate Chemical added to
make pink azo dye.
Colourimetry used
at wavelength 510
nm. Concentration
determined by
comparing to
standard solutions.
Calcium Calculation- -
Carbonate Calcium result from
(hardness) above Result
multiplied by 2.5
Sulphate 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….
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 drug-testing 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 innacurate. 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?
Look back to the table at the start. How do Thames water carry out colourimetry for
different ions?
3 – 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?