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RE: thiosulphate

> The neutralizing reaction of the thiosulphate ion with 
> chlorine is given by:
> 2S2O3-- + Cl2  -----> S4O6-- + 2Cl-
> Tap water typically will not have more than 1ppm chlorine at 
> the tap, some
> treatment plants aim for 1ppm Cl at the exit of the treatment plant so
> that by the time it reaches your tap, there should still be a 
> residual.

Free chlorine gas is not present in chlorinated water.   When chlorine gas
is added to water, it reacts to form hypochlorite and hydrochloric acid:

CL2 + H2O --->HOCl + H + Cl

the hypochlorite can further ionize to form:

HOCL <--->H + OCl

"Free residual chlorine" is the usual measure of chlorination effectiveness,
which is the sum of [HOCL] and [OCl].  Many treatment plants have a target
value of 1 ppm free residual chlorine, not  simply Cl.  I think your
calculations are a bit off.

Furthermore, many municipalities now use chloramine instead of chlorine.
They add ammonia to chlorinated water to form:
HOCl + NH3 <--> NH2Cl (monochloramine) + H2O
HOCl + NH3 <--> NHCL2 (dichloramine) + H2O
HOCl + NH3 <--> NCl3 (nitrogen dichloride) +H2O

These reactions are dependent upon pH, temperature, contact time, and the
initial ratio of chlorine to ammonia, but in most cases the predominant
forms are monochloramine and dichloramine.  The chlorine present in these
compounds is called the "combined available chlorine".  The addition of
thiosulphate will neutralize the chlorine but will also liberate the
ammonia, which can cause an ammonia spike in our aquaria.  Thiosulphate is
not recommended in chloramine-treated water.

Source:  Tchobanoglous, G and E. Schroeder.  1985.  Water Quality:
Characteristics, Modeling, and Modification.  Addison-Wesley.