Now that you understand why the blank consumes more Na₂S₂O₃, what does this mean for your analysis?
In a back-titration (like determining Iodine Value or Peroxide Value), the blank titration always uses more sodium thiosulfate ( cap N a sub 2 cap S sub 2 cap O sub 3 Now that you understand why the blank consumes
contains everything except the lipid. Since there is no fat for the reagent to react with, the reagent remains fully intact. When you move to the titration step, you are neutralizing the entire initial amount of the reagent. 4. The Titration Step Sodium thiosulfate is used to neutralize only the remaining (unreacted) iodine In the Blank: You are titrating the amount of reagent added. This requires a high volume of cap N a sub 2 cap S sub 2 cap O sub 3 In the Sample: You are only titrating what the lipid When you move to the titration step, you
At first glance, this seems counterintuitive. One might assume that a "blank"—which contains everything except the analyte—should require less titrant. After all, the sample contains the hydroperoxides (the target molecules) that should react with iodide to liberate iodine, which in turn consumes thiosulfate. So why does the blank need more Na₂S₂O₃? This requires a high volume of cap N
Suppose a blank consumes of 0.01 N Na₂S₂O₃. A lipid sample theoretically contains enough hydroperoxides to liberate I₂ equivalent to 3.0 mL of thiosulfate. However:
The incubation step for peroxide value determination is often carried out in the dark (to prevent photochemical reactions). However, ambient room light or minor temperature fluctuations can still promote autoxidation in the blank. The lipid sample, being opaque or colored, can shield the reaction mixture from light, further reducing side reactions. The blank, typically clear and colorless, offers no such shielding.
The contains all reagents except the lipid sample . It measures any background I₂ formed from non-lipid sources (e.g., air oxidation of KI, impurities in solvents).