Chris both of these chemicals gives off SO2, I really like hydrogen peroxide for dechlor.

Chris:

A correction to Barrie66's post.....neither sodium thiosulfate or sodium bisulfite give off SO2, and both offer substantially lower dechlorination treating costs, without the possible handling issues, of H2O2.

Depending on your delivered material costs, sodium bisulfite will probably give the best economics of treatment, although either of your choices will dechlorinate suitably.

Orenda

Orenda

Both of these sulfur based compounds are used for dechlorination. The decision on which chemical to use is based on availability and cost. On a weight basis, you need less sodium sulfite than sodium thiosulfate, but if the sulfite costs twice as much per ounce or pound, the thiosulfate is less expensive for the application.

Sodium thiosulfate is a colorless, transparent crystal widely used by municipalities for dechlorination. It undergoes multiple reactions with free and combined chlorine, depending on solution pH.  Reaction with chlorine yields the following:

Na2S2O3 +  4HOCl +   H2O  > 2NaHSO4  +    4HCl

Na2S2O3    +   HOCl     > Na2SO4   + S  +  HCl

2Na2S2O3   +   HOCl   >  Na2S4O6    +  NaCl +    NaOH

On a weight-to-weight basis, approximately 2.25 parts of sodium thiosulfate are required to remove one part of chlorine.  It should be noted that the thiosulfate reduction depends quite strongly on pH. Sodium thiosulfate is a reducing agent.  However, it scavenges less oxygen than sodium sulfite.

Sodium thiosulfate is a skin, eye, nose and throat irritant.  It has a NFPA Rating of 1,0, 0 for health, fire and reactivity, respectively. An EPA toxicity study indicated that sodium thiosulfate is not very toxic to aquatic species.  Sodium thiosulfate may react slowly with chlorine under some conditions, and requires more time for dechlorination than most dechlorination chemicals.  

Sodium sulfite is another dechlorinating agent that is widely used. Sodium sulfite is a skin, eye, nose and throat irritant.  It has a NFPA Rating of 2,0, 0 for health, fire and reactivity, respectively.

Sodium sulfite is available in powder/crystalline and tablet form. It undergoes the following reaction with free chlorine:

Na2SO3  +    HOCl    > Na2SO4  +  HCl

On a weight-to-weight basis, approximately 1.775 parts of sodium sulfite are required to remove one part of chlorine.  Sodium sulfite is a reducing agent and is reported to scavenge more oxygen than sodium thiosulfate.  
 

While sulfates may or may not harm fish they will both deplete oxygen levels if overdosed.  Although it costs more, vitamin C will also remove chlorine but will only slightly lower oxygen levels if over dosed.  Vitamin C has gained wide acceptance in locations were certain fish are on the endangered species list, and accidental fish kills give bad press.
Hydrae

What exactly is the water used for ? If it is for a cooling water system, one must take care that the solubility limit for calciumsulfate is not exceeded. If high cycles of concentration are used this might be the case.
This means that depending on the application and the use of other sulfate containg compounds, it could be interesting to choose the treatment which produces the least sulfate.

Edwin Muller
KW2 Burau Veritas
www.kw2.nl