Can anyone explain the process by which Cyanuric Acid protects Free
chlorine from UV degradation in swimming pools.  And why the ORP would
increase between sunset and sunrise (only in stabilized pools) holding
constant all variables of water balance including Free Availble
Chlorine?

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Clarification of Question by ctate-ga on 02 Jun 2004 20:15 PDT

ORP means Oxidation Reduction Potential, often refered to outside the
U.S as Redox potential.  ORP is expressed as a Mv reading, and
measures the qualitative work value of a sanitizer such as Sodium
Hypochlorite, ect..

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Clarification of Question by ctate-ga on 04 Jun 2004 21:38 PDT

I do not have a membership to ACS publications, they seem to require a
$2500 memebership, I have a call into them to see if they will allow
access to the single document referenced for a more resonable price.

My primary interest is why ORP increases after the sunset holding
constant all other water chemistry paramaters.  I have confered with
several other expeirenced persons who have noticed the same phenom,
but no who can explain why it happens.  I would be willing to raise
the price / tip if anyone can provide a documented explaination

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Clarification of Question by ctate-ga on 07 Jun 2004 12:54 PDT

Comment for Amal:
I have studied and worked with ORP for sometime now and have never
heard anyone say that ORP is a function of Dissolved Oxegen.  Are you
sure about this, if so please document.

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Clarification of Question by ctate-ga on 08 Jun 2004 11:52 PDT

Clarification for Amal:
I think the DO influence on ORP reference mentioned applies to
microbiology and cell research not swimming pool water.

It is possible such claims are advertising technabable. If I recall
correctly a popular sanitiser with a very long name = tri clor......
of the late 1980s decays into cyanuric acid after it releaces one 
atom of oxygen which does the sanitizing. The State of Florida
reccomends no more than 50 parts per million of cyanuric acid in pool
water. As a decay product the level rarely exceeded 5 parts per
million, so some pool stores sold cyanuric acid so you could increase
the level to perhaps 40 parts per million, for which they claimed
vauge benefits. I used the sanitizer in my vynal pool when the ph of
the water was high. Otherwise I used common bleach = Sodium
Hyprochlorite as the sanitizer with lower cost and good results. I did
not use buffer = sodium bicarbonate, nor adjust the total alkalinity,
which is desirable for grouted pools. I rarely used algacide, nor
clarifier or any other chemicals.
 UV is short for ultra violet photons (a component of ordinary
sunlight) which do decay all types of sanitizers.  If you can explain
the meaning of DRP it may be helpful to the reasearchers.   Neil

It's not technobabble.  Isocyanuric acid (C3N3O3H3) reacts with the
hypochlorous acid (HClO), present in a swimming pool to form a variety
of chlorinated isocyanuranates (e.g., chloroisocyanurate: C3N3O3H3 +
HClO <-> C3N3O3ClH2 + H2O; dichloroisocyanurate: C3N3O3H3 + 2HClO <->
C3N3O3Cl2H + 2H2O, trichloroisocyanurate: C3N3O3H3 + 3HClO <->
C3N3O3Cl3 + 3H2O, plus the associated deprotonated acid anions). 
HClO, which is the "active" disinfectant in chlorinated pools, is
UV-sensitive, and it breaks down rapidly when exposed to sunlight. 
The isocyanurate species are several times more stable with respect to
UV exposure than is HClO, so they form a relatively "unreactive" pool
of compounds that react to "replenish" the concentration of HClO as it
is progressively destroyed by sunlight (i.e., the reactions written
above "run in reverse" to generate HClO).

The equilibrium constants governing the chemistry of this system were
determined back in the 1960's.  If you have access to the Journal of
the American Chemical Society, the relevant reference is at
http://pubs.acs.org/cgi-bin/archive.cgi/jacsat/1963/85/i20/pdf/ja00903a011.pdf
(Brady et al., 1963, Equilibria in Solutions of Cyanuric Acid and its
Chlorinated Derivatives, J. Am. Chem. Soc., v85, #20, pp3101-4.)

In case you are interested, the chlorinated isocyanurates are
considered fairly "safe" compounds, and are approved for use as
disinfectants in swimming pools.  See
http://ehp.niehs.nih.gov/members/1986/069/69036.PDF.

Hello ctate, 
ORP is a function of dissolved oxygen(DO) content. Since DO content
varies day/night cycle, the ORP changes.
I can find documents regarding DO varition and the correlation between
DO content and ORP. Will it prove anything?

Amal

Hello ctate, 
Please refer to the second paragraph in the introduction part of this
pdf document that refers to a relation between DO and ORP.
http://web.umr.edu/~amal/ORP_DO.pdf
Three other references are given there for further details. (these are
relatively older references and are not available online)

This http://web.umr.edu/~amal/orp1.doc also hints at this. 

Amal

Hello ctate-ga
Oxidation Reduction Potential or Redox is the activity or strength of
oxidizers and reducers in relation to their concentration.  Oxidizers
accept electrons.  Reducers lose electrons.  Examples of oxidizers
are: chlorine, hydrogen peroxide, bromine, ozone, and chlorine
dioxide.  Examples of reducers are sodium sulfite, sodium bisulfate
and hydrogen sulfide.
Dissolved Oxygen will affect ORP but not by much and it will cause it
to reduce not increase.  My guess is that the ORP increase is due to
temperature or pH decrease (or both) of the pool water. Lower pH will
cause more HOCl- to be available. Lower temperature will cause the ORP
reading to be higher.  Tony