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DPD tablets (diethyl-p-phenylenediamine) are used to determine chlorine in water and can also be used to test bromine, iodine and other disinfectants. These tablets come in four different types and are used alone or in combination, depending on your measurement parameter. The convenience of using a rapid version is that it dissolves quickly but is used as a visual test only.
Image of DPD tablets by Lovibond
DPD types
There are four types of DPD tablets available. The choice of which to use is dependent on what parameter you are measuring. The different tablets and their analytes are listed below:
DPD 1 - these are used to measure free chlorine or bromine
DPD 2 - these are used in combination with DPD1 to give the combined chlorine value
DPD 3 - these are used to test for the total chlorine or bromine. DPD3 is used in combination with DPD1 tablets. A calculation is done by subtracting the total chlorine value obtained using DPD3 from the free chlorine value obtained from DPD1 to get the total chlorine measurement
DPD 4 - these tablets react to any oxidising agent, they test for the total available chlorine or ozone
These tablets are usually crushed and placed into a tube with a water sample. Then, following a set reaction time to allow for a colour change, the tube is placed in a comparator kit of photometer to give you a value. It is important to note that a visual assessment is made when using a comparator kit, which can be prone to human interpretation while utilising a photometer relies on a light source reading.
Image of Macherey-Nagel photometer
Rapid vs Non-rapid tablets
Rapid DPD tests dissolve quicker than standard DPD tablets. However, they produce cloudiness in the solution, resulting in the solution not being appropriate for use with a photometer, they can only be used in a comparator kit. Non-rapid DPD tablets produce a clear solution and can be measured in either a photometer or comparator kit
Rapid DPD tests are listed below:
DPD 1 rapid - Free chlorine
DPD 3 rapid - Total chlorine
DPD 4 rapid - Total available chlorine or ozone
Image of Lovibond comparator kit
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