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Phenol red tablets are based on a pH sensitive dye which varies in colour between yellow at pH 6.8 and red at pH 8.4. The optimum range for swimming pool water is pH 7.2 to 7.8.
Phenol Red is available as a liquid, powder but most frequently and conveniently in a tablet form.
A widely used type of test method for domestic pools is the dip cell or pooltester. This consists of a small plastic block with one or more cells into which the water sample is added. Coloured scales are provided next to each cell, showing the value of the measurement against the colour. In addition to pH, these are often used, with the correct tablet, to measure chlorine, bromine and alkalinity.
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A typical
pooltester kit using tablets |
Do not sample the water with anything breakable especially glass
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A disc comparator
using phenol red tablets for pH testing. A full range
of tablets and tests are available for all pool water tests |
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Hand held type of photometer which measures up to 5 parameters |
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Portable
photometer using phenol red tablets and other types to measure up to
9 parameters. A 25 parameter version is also available |
Many pools have used comparators which employ different colour discs with which to compare the colour generated by the tablet. Some use natural light and some artificial light sources, but nevertheless they still rely on the operator's eyesight and lighting conditions to obtain the reading. The latest instruments known as photometers use a fixed wavelength of light to examine the water sample as coloured by the phenol red tablet or other types of test. The operator merely has to select the test required by pressing a key pad, crush the right tablet in the sample and directly read off a digital numeric value. The typical test method for pH using phenol red tablets would be:
Do not sample the water with anything breakable especially glass
