37
Chlorine
Note that a too high chlorine level (>8 mg/l) can cause bleaching of the
pink coloration formed in the DPD test and give a false negative or low
result (this doesn’t occur with DPD-XF and XT reagents). If a colourless or
weakly coloured test solution is obtained when chlorine is known to be
present, check for the possibility of bleaching by repeating the test on a
sample diluted with chlorine-free water.
Very high levels of calcium hardness (>1000 mg/l as CaCO
3
) may lead to
turbidity when performing the test. If this occurs, you need to add one
EDTA (AT 090) tablet to your sample prior to adding your DPD tablet.
Non-chlorine shock dosing treatments for swimming pools are widely used
to support chlorine treated pools. Chemicals containing monopotassium
persulphate (MPS) are used to oxidise organic contaminants in the water
and regenerate free chlorine from chloramines.
Typical products are Oxybrite, Oxysure and Purolyte Plus.
Excess MPS reacts in the DPD test. It does not affect the free chlorine
result with DPD No 1, but raises the DPD No 3 reading, thus yielding a false
high result for combined chlorine. Unwary pool operators may add further
shock dose at this point, and this further raises the false total chlorine
result. Palintest’s DPD Oxystop has been developed to prevent this
problem.
The DPD Oxystop tablet should be added after the DPD No 1 Free Chlorine
test result has been read and before the DPD No 3 tablet is added. The
tablet must be thoroughly crushed and mixed well into the solution, and
allowed a one-minute stand before proceeding with the DPD No 3 tablet.
Thus, Oxystop suppresses the response from MPS and allow accurate
measurements of free and total chlorine on non chlorine shock dosed pool
waters.
The DPD Oxystop tablet may also be used in exactly the same way with the
DPD-XF and DPD-XT tablets.
To assess the level of non-chlorine shock dose chemicals in the pool, request
supplementary instructions Phot.7.2.AUTO.
Summary of Contents for Pooltest 25
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