Science, Maths & Technology

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Test kits for water analysis

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# 6.3 Electrical conductivity measurements

The electrical conductivity of water arises from the movement of dissolved ions in solution.

In general, the greater the concentration of dissolved solids, the greater the conductivity of water.

For low concentrations of dissolved ions, the relationship between concentration of cations (expressed as millimoles of charge, mmolc, per litre) and conductivity is approximately linear (Figure 11).

Figure 11  Relationship between the electrical conductivity of dilute salt solutions and cation concentration, expressed as millimoles of cationic charge per litre. 'S' stands for siemens, the unit of electrical conductivity, where 1 siemens stands for 1/(1 ohm).

As a measure of dissolved ion concentration, we tend to use total dissolved solids (TDS), which is the amount of solids dissolved in a water sample. This can be determined by gravimetric analysis but is more usually determined by measurement of electrical conductivity, taking the raw conductivity value and multiplying by a conversion factor.

There are two types of conductivity sensor: contacting and electrodeless. The former have their electrodes in direct contact with the test solution. Electrodes were originally made from gold or platinum because these metals could tolerate solutions that were highly acidic or caustic. Today, however, graphite, titanium or stainless steel are used and have been shown to have good resistance to such harsh test solutions.

One of the problems with contacting sensors is that some solutions tend to damage or coat the electrodes. Electrodeless sensors, however, contain two coils within a waterproof housing surrounding a tube which is open to the test solution. They induce an alternating current in the solution and measure the magnitude of this current to determine the solution conductivity.

The conductivity of a solution increases with temperature as ionic mobility increases, and so test results are reported at a specified temperature, usually 25 °﻿C. Instruments usually have built-in temperature compensation, either manual or automatic, to allow the readout to be corrected to 25 °﻿C.

## Activity 4  MCERTS

Timing: Time allowed: 1 hour

The UK Environment Agency established its monitoring certification scheme called MCERTS to deliver quality environmental measurements. MCERTS covers the monitoring of emissions to air, land and water.

Visit the website of the Environment Agency [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] and find out what the requirements are for portable test kits for water analysis.

## Activity 5  Test kits in action: health and safety considerations

Timing: Time allowed: 2 hours

Watch the Videos 7-9 showing water quality test kits being used in three locations.