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5
TROUBLESHOOTING
Chemistry Troubleshooting
It is very important to specify Palintest Photometer
Grade tablets when ordering replacement reagents
for this instrument. Using an alternative grade of
tablet may lead to a turbid sample which in turn
leads to inaccurate results.
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 does not 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.
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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.
pH
:
Ionic strength, temperature and other water factors
will have an effect on pH readings. This test has been
calibrated for conditions most likely to be encountered
in a typical swimming pool.