(4) bimr (Civil/Environmental)
15 Dec 05 10:59
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.