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What chemical compounds might be present in drinking water?
What chemical compounds might be present in drinking water?

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1.1 The water cycle and drinking water

Water moves around the Earth in the water cycle (Figure 1). Natural water on Earth is not completely pure as compounds may dissolve at several points in the cycle and this may lead to pollution if above accepted guidelines.

Described image
Figure 1 The water cycle.
  • Predict where inorganic pollutants might enter the water cycle?

  • At several steps in the water cycle inorganic compounds can become dissolved and, depending on their level, pollution may occur. For example:

    • Anions such as nitrate and phosphate may become dissolved from agricultural fertilisers, sewage and the natural breakdown of organic matter.
    • Air pollution from burning fuels can produce nitrogen oxides, NOx compounds and sulfur oxides, SOx compounds, to form dissolved nitrates and sulfates respectively.
    • Cations such as calcium or magnesium may dissolve naturally due to the weathering of minerals in rocks or via anthropogenic, or human-derived, contamination of ground water by pollutants from industry, roads, or mining.

For drinking purposes water has to meet government guidelines but will not be absolutely pure as it is also a natural source of ions needed by the human body. Some typical values are shown in Figure 2 and Table 1.

Described image
Figure 2 Typical analysis for a bottle of mineral water values shown in mg l-1.
  • What will the [H+(aq)] be in the water in Figure 1?

  • pH equals minus log left parenthesis left square bracket cap h super plus left parenthesis aq right parenthesis right square bracket divided by mol dm super negative three right parenthesis
    (Equation 1)

    Rearranging gives

    [H+(aq)] = 10−pH mol dm−3 = 10−7.4 = 4.0 × 10−8 mol dm−3

Table 1 Concentrations of ions in bottled and tap waters.
IonConcentration/mg l−1
Volvic®Vittel®Buxton®Evian®Tap water*
calcium Ca2+11.5915578102
magnesium Mg2+8.019.919248.81
sodium Na+11.67.324549.1
potassium K+6.2-11n.a.
choride Cl13.5-374.573.9
nitrate NO36.30.63.520.6
sulfate SO42−8.11051310120
bicarbonate HCO371.0258248357n.a.


* value for the area containing The Open University in Milton Keynes; - = too small to measure; n.a.= not available

Activity 1 Ions in drinking water

Timing: Allow at least 30 minutes

Question 1

This activity aims to access up-to-date information about ions in drinking water. You will access the web pages of the Drinking Water Inspectorate, DWI (2015) of England and Wales [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . (Hint: use the A-Z index.)

Search for information to answer the following questions:

Which ions in Table 1 are responsible for the hardness of water?


Cations such as calcium and magnesium.

Question 2

What is the World Health Organization (2015) guideline value for nitrate concentration in drinking water?


The World Health Organization (2015) guideline value for nitrate is 50 mg l−1 for drinking water and the EU also adopts this value.

Question 3

How does the concentration compare with the values for nitrate concentration in Table 1?


All the values for nitrate in Table 1 are well below 50 mg l−1.

Question 4

What is the allowed level of fluoride ions in drinking water? (Note this is often artificially added to water supplies.)


The maximum permitted value of fluoride in drinking water is 1.5 mg l−1.

You might wish to check the water quality report for your own water supplier. This is often available on the company website or see if the values for tap water in Milton Keynes in Table 1 have altered. Links to information on drinking water in other EU states can be found at European Commission (2015).