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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.

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Figure 1 The water cycle.
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Figure 1 A cross section through the surface of the Earth. On the left hand side there are mountains, which slope down to the right to lower land, and then about two-thirds of the way across the diagram, the coast. To the right of this is the ocean, which gradually deepens up to the right hand edge.

In the top right of the diagram is a picture of the sun. Below this, two arrows labelled 'evaporation' point vertically upwards from the surface of the ocean. Above these arrows it says 'water vapour', and above that is a picture of a cloud.

An arrow labelled 'wind' points to the left from the cloud to another cloud. Another arrow, this time labelled 'cloud rising' points to the left and a little up to a third cloud. Rain is falling from this cloud into a lake at the edge of the mountainous area. From the lake, a river meanders its way past a town to the ocean.

From the third cloud, another arrow points up and to the left to a final cloud. From this cloud snow is falling onto the tops of the mountains.

Small blue arrows point vertically down from the surface of the land into the ground. The bottom part of the cross section through the surface is shaded a darker colour than the rest. On the left hand side the boundary between the light and dark areas is about half way between the bottom of the diagram and the peak of the mountain. The boundary slopes gently down to the right, while the light coloured area on top gradually becomes thinner, so that at the coast the lighter coloured area vanishes, and the entire area beneath the ocean is the darker colour.

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.

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Figure 2 Typical analysis for a bottle of mineral water values shown in mg l-1.
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Figure 2 A photograph of the label on a bottle of water. At the top it says 'Typical analysis'.

Calcium 55, Magnesium 19, Potassium 1, Sodium 24, Bicarbonate 248, Chloride 37, Sulphate 13, Nitrate

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 Cl−13.5-374.573.9
nitrate NO3−6.30.63.520.6
sulfate SO42−8.11051310120
bicarbonate HCO3−71.0258248357n.a.

Footnotes  

* 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?

Answer

Cations such as calcium and magnesium.

Question 2

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

Answer

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?

Answer

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.)

Answer

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).