The Use of Latimer Diagrams to followed the Reduction of Dissolved Oxygen to Water
By Nicholas M. Martyak, Ph.D.
Oxygen is readily soluble in water and can cause a variety of service troubles in water treatment processes. Dissolved oxygen (DO) in water can be detrimental to ferrous-based metals, particularly in low- to high-pressure boilers. DO promotes the dissolution of iron-forming soluble ferrous, and ferric hydroxide, or oxide species (1-3). This can be seen in Equation 1. (Editorﾒs note: All article equations appear together in a separate Equations table that is placed within the article.) In the presence of excess oxygen, the ferrous hydroxyl species will oxidize to ferric hydroxyl (Equation 2), which upon precipitation leads to the red rust that is commonly seen on ferrous-based metals, hematite. This is shown in Equation 3. At sea level total, the atmospheric pressure (atm) is 760 millimeters (mm) mercury (Hg) (1 atm; 1.013 bar). This is gravity-induced weight of the atmosphere on the surface of the earth, and it generates enough force to move a sufficient volume of Hg 760 mm up a tube. The atmosphere is composed of several gases, the two most abundant being nitrogen, about 78%, and oxygen about 20.8%. Therefore, the partial pressure (percent of the 760 mm Hg) of the atmosphere due solely to oxygen is 158 mm Hg (0.208 x 760 mm Hg). The solubility of a gas in a particular medium such as water is governed by Henryﾒs Law(4) and for oxygen in water, the Henryﾒs Law constant is 1.7 x 10-6 molal/mm Hg at 25ﾰC. The concentration of dissolved oxygen in molality (m) (moles solute per mass solvent in kilograms) is shown in Equation 4. Equation 5 illustrates the number of milligrams per liter (mg/L) of oxygen that will dissolve in water at 25ﾰC.
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