CiteWeb id: 19600000022

CiteWeb score: 2960

DOI: 10.1016/B978-0-08-009235-5.50026-7

The oxidation potential of dithionite (Na 2 S 2 O 4 ) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na 2 S 2 O 4 oxidized. Tests show that NaHCO 3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays.