In order to extract the crystalline iron oxide bound organic carbon in the soil. We prepared the 57.4 mM dithionite solution. However, we found that all dithionite reagents we have tested exhibit high reagent blank C concentrations. Why?
Okay Dan, the explanation would be that dithionite is a reducing agent - 0.057 M is not very concentrated but it depends on the conditions... this is the only way I could explain. do you see the same level of DOC when you use tap or city water?
What method are you using to measure DOC? Can't give an answer without that knowledge. (Johan is almost certainly correct if your method relies on oxidation of the carbon.)
Sodium dithionite is stable when anhydrous, but the water has to be removed without heating. For this reason, the commercial product is usually crystallized from aqueous methanol and/or washed with methanol, and all of the purification methods I've found use methanol. To make things worse, commercial material may contain various carboxylate stabilizers. Solubility in water is very high, making recrystallization difficult, and the resulting dihydrate is air-sensitive.
An anion exchange resin could retain and then elute dithionite on an as-needed basis. Or you can make your own zinc dithionite solution on demand, by passing aqueous SO2 through a column of amalgamated zinc. A batch method for zinc dithionite is given at http://www.prepchem.com/synthesis-of-sodium-dithionite
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