[alberg30] chlorination of ships water

[Marianne King-Wilson]

 Hi, everyone
The recommendation about testing for residual chlorine at the outlet is the one used
for most municipal and industrial requirements.  The reason being, that if  "the
chlorine won", in other words, was not overcome by any bacteria in the water, nor in
the system, you have purified both.
While it seems more complicated, it does overcome variations in water quality and in
system cleanliness.  As you mentioned, sometimes it's rainwater, and sometimes it's
out of a questionable tap.  The outlet testing is the equalizer.


Here is a summary we found for the procedure:
Chlorine, hypochlorous acid and hypochlorite ion (OCl-) are
known as free residual chlorine while the
      chloroamines are called combined residual chlorine.

      The test for residual chlorine is important, widely
practiced and extremely simple to perform. A
      pocket-sized chloroscope is used in which 2 tubes,
each containing a measured quantity (normally
      10ml) of water sample under test, which can be
compared by eye for colour. One is coloured by the
      addition of a chlorine-sensitive reagent, the other by
the range of standard glass slides.

      The reagent commonly used now is DPD tablets
(NN-diethyl-p-phenylene diamine sulphate) which
      indicate the strength of the free chlorine in terms of
shades of red. The different proportions of free
      and combined chlorine can be ascertained by using
different types of DPD tablets as recommended
      by the supplier.


      Apparatus:

      Lovibond comparator, water samples, colour discs.



      Procedure:

      Pour 10 ml of the water sample into glass tube.

      Add one tablet of DPD no.1 into the tube and stir to
dissolve it in the water. Then place the tube in
      the left compartment of the comparator.

      Match the colour of the tube to which the tablet has
been added with that of the nearest standard
      colour shown in the right compartment by rotating the
colour disc and the concentration of free
      residual chlorine can be read off directly as
indicated on the disc.

      For every sample, repeat step 2 and 3 to get the
average concentration (ppm).

      To determine the combined residual chlorine
concentration use DPD tablet No.4 instead in step 2.
      Repeat steps 3 & 4.

      If the concentration of chlorine content in the
samples are too strong and beyond the range of the
      disc, dilute it with a known quantity of distilled
water and multiply the disc reading by the appropriate
      factor to get the actual concentration of the original
sample.

      Results

      Present the results according to the format below:

       Sample
                                   Average Combined
                                   Residual Chlorine (mg/l)

Average Free Residual

Chlorine (mg/l)

NB do not forget to include a dilution factor if  diluted.







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