4.1 Turbidity measurements
Water samples are frequently cloudy (or turbid), a property that arises from suspended solid particles of varying sizes being dispersed through the liquid rather than dissolved to form a solution. Turbidity provides an instant visual guide to water quality.
The suspended particles, which include, clay, silt and algae, can be problematic. In natural waters particles of all sizes tend to reduce light penetration; this inhibits the rate of photosynthesis and therefore causes a reduction in the growth of plant life. Sudden changes in turbidity may tell you a new pollution source has emerged or, in the case of drinking water, there may be a problem in the treatment process.
One piece of kit designed to measure the clarity of water is named after Fr Pietro Angelo Secchi, an astronomer and scientific advisor to the Pope, who investigated the clarity of water in the Mediterranean Sea in 1865. The Secchi disc is a flat circular plate, 20-30 cm in diameter, either all white or with two quadrants painted black and two painted white (Figure 6). It is lowered into the water until such a depth that it can't be seen, which indicates its turbidity.
However, a more accurate approach is to shine light though a sample and measure scattering using a nephelometer. A beam illuminates a pre-determined volume of water which scatters light in all directions, and a detector measures the amount of scattered light at an angle 90° to the light source (Figure 7).
The amount of scattered light is reported in Nephelometric Turbidity Units (NTU), although other scales are also used.
A range of portable nephelometers are available on the market offering all the advantages of the photometric apparatus we've looked at so far. Infrared LED light sources emitting at 850 nm are common (using this wavelength avoids interferences due to colour in the sample). Calibration, as ever, is essential.
Nephelometers are calibrated using suspensions of the polymeric molecule formazin (Structure 2), where n indicates variable chain length. The formazin chains fold in different ways to form particles with a distribution of sizes, reproducing those found in real world samples.
To give you a feel for the NTU scale, the turbidity of deionised water is 0.02 NTU, drinking water is typically between 0.02 and 0.5 NTU and untreated waste water is anywhere between 70 and 2000 NTU.