dpd tablet

DPD Tablets used for testing the chlorine or bromine content of swimming pool and spa bath water. The DPD tablets shown are foil wrapped for convenient dosing and optimum storage

DPD Tablet general information

DPD is an abbreviation of diethyl-p-phenylene diamine. DPD tablets react with pool water containing chlorine or bromine, colouring the water sample various shades of red. The deeper the colour, the higher the concentration of sanitiser. DPD tablet tests have largely replaced orthotolidine testing as this is considered carcinogenic. The DPD tablet No.1 is used to measure free or "available" sanitiser, the useful hypochlorous or hypobromous acid required for sanitising. DPD tablet No.3 is used after DPD1 to measure total chlorine or bromine. The difference between the two measurements is the combined chlorine (chloramine) or bromine (bromamine) value. Chloramines are an undesirable form of chlorine which do not sanitise and cause irritation. Bromamines have some sanitising value but do not generally irritate like chloramines. Either though indicates bather pollution.

DPD tablet use in domestic pools

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 chlorine and bromine, these are often used, with the correct tablet, to measure pH and alkalinity.

 pool testing kit
A typical pooltester kit using tablets

The complete test method, which should be carried out daily when the pool is in use, is as follows:

Do not sample the water with anything breakable especially glass

  • Remove the cap and rinse out the testing cell with the water to be tested
  • Fill the cells with the sample water directly from the pool
  • Place a rapid dissolving DPD1 tablet in the Chlorine or Bromine test cell. (Note: Each has a different scale) Place other tablets such as Phenol Red for pH, in the other cell(s) as appropriate
  • Replace the cap and shake to dissolve the tablets
  • Hold the cell up to the light, ideally a north natural light. Avoid greenish fluorescent lights, and evening light which has a red cast to it
  • Within 15 seconds read the values
  • Weekly, or more frequently if the pool has had heavy use, after testing the free chlorine add a DPD3 tablet to the cell and test again for total chlorine
  • Note this value and subtract the free chlorine (DPD1 test) value. The difference is the combined chlorine value
  • Make the necessary chemical adjustments to the water and retest

Note: If the colour "flashes" pink and then disappears, it is probably a sign that the water is heavily chlorinated, rather than has no chlorine. This is because the chlorine is bleaching the colour. This happens typically when shock dosing above 10 parts per million. If you suspect this to be the case, dilute the sample exactly in half using unchlorinated water. If you then get a reading, multiply it by 2 to get the true value.

More information about shock dosing

DPD tablet use in commercial pools

 disc comparator with dpd tablets
A disc comparator using dpd tablets for bromine and chlorine testing. A full range of tablets and tests are available for all pool water tests

 
Hand held type of photometer which measures up to 5 parameters
  
Portable photometer using DPD tablets and other types to measure up to 9 parameters

In commercial pools where a "duty of care" exists, a far stricter regime of water testing needs to be employed. Chlorine and pH testing should take place when the pool opens, when it closes and intermittently throughout the day depending upon bather load. Frequently 2 hourly testing can be justified. Above this manual testing becomes a full time job and some sort of automatic system will be used.

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 dpd 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 free and total chlorine using dpd tablets would be:

Do not sample the water with anything breakable especially glass

  • Collect a sample by filling a clean plastic bottle below the water surface excluding any air
  • Test as soon as possible to avoid deterioration of the sample
  • Switch on photometer and select the Free Chlorine key
  • Rinse the test tube with the sample water, leaving about 1ml of water in the tube
  • Add a photometer grade dpd1 tablet to the tube and crush it. Note a colour change from colourless to red. See note above about colour bleaching
  • Fill the tube up to the 10ml mark with the sample water. Mix to dissolve the tablet
  • Set the wavelength to 520nm
  • Optionally cover the tube with the light shield if working in bright light
  • Note the free chlorine reading immediately
  • Add a photometer grade dpd3 tablet, crush and mix
  • Stand for 2 minutes
  • Select the Total Chlorine key and the total chlorine reading
  • Calculate the combined chlorine reading by subtracting the free chlorine reading from the total reading

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