H₂O Na₂SO₃ 2NaOH SO₂ H₂O SO₂ H₂SO₃ (acidic) N₂H₄ O₂ H₂O N₂ (oxygen  consumed) 2C₄H₉NO CO₂ 2H₂O C₄H₉NO   C₄H₉COOH HNO₃ H₂ (carbon  dioxide  consumed) 2N₂H₄ 2 NH₃ N₂ H₂ NH₃ H₂O NH₄OH DOE-HDBK-1015/2-93 Principles of Water Treatment DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL Rev. 0 CH-04 Page 19 Sodium sulfite reacts rapidly with oxygen and is a very efficient scavenger.  However, being a solid and the source of another solid (Na  SO  ) that is produced during the reaction, sodium 2 4 sulfite has the potential of fouling heat transfer surfaces.  An additional problem associated with the  use  of  sodium  sulfite  is  corrosion  of  secondary  system  components  resulting  from  its decomposition products.  At the temperatures present in the steam generators, sodium sulfite can decompose as follows. (4-13) Sulfur dioxide (SO  ) is a gas and is carried over to the remainder of the steam facility.  With 2 water (in the steam or in the feed/condensate system), the SO   reacts in the following manner. 2 (4-14) This acidic condition is corrosive to all components in the secondary system. Because of the problems associated with sodium sulfites, many facilities use volatile chemistry control  of  the  secondary  steam  system  to  control  dissolved  gases  in  conjunction  with  air removal systems.  This control utilizes hydrazine (Reaction 4-11) and morpholine (Reaction 4- 15) to eliminate oxygen and carbon dioxide, respectively. (4-11) (4-15) As can be seen by Reaction (4-11), no solids are formed; thus, the tendency of fouling heat transfer surfaces is reduced.  An additional benefit results from the decomposition of hydrazine by the following reactions. (4-16) (4-17) These reactions result in an alkaline pH condition that decreases corrosion in the steam facility. As can be seen in Reaction (4-15), the consumption of CO   takes place.  Two benefits result 2 from this reaction; 1) the inventory of dissolved gases in the steam facility is reduced, and 2) is the reaction contributes to maintaining a higher pH by eliminating carbonic acid (H  CO  ), 2 3 thus reducing corrosion.