Notice the . If ( B > S ), this yields a negative Net Titre—which is impossible for a fresh oil but can occur mathematically. However, for most oils with positive peroxide values (e.g., > 2 meq/kg), ( S ) is typically larger than ( B ), but ( B ) is still substantial.
$$\textI_2 + 2\textNa_2\textS_2\textO_3 \rightarrow 2\textNaI + \textNa_2\textS_4\textO_6$$ Notice the
Ideally, if all reagents were perfectly pure and chemically inert, the KI would not react with the acid or the solvent. In a perfect world, no iodine would be generated, and the blank titration would require zero milliliters of sodium thiosulfate. The reaction proceeds slowly but significantly over the
Under acidic conditions, iodide ions are thermodynamically unstable in the presence of oxygen. The reaction proceeds slowly but significantly over the typical 5–30 minute incubation period: no iodine would be generated
$$2\textKI + 2\textCH_3\textCOOH + \textROOH \rightarrow \textI_2 + 2\textCH_3\textCOOK + \textH_2\textO + \textROH$$
Finally, the liberated iodine is titrated with sodium thiosulfate using a Starch Indicator .
Consequently, the actual background I₂ produced in the sample flask is than in the blank, but this is a smaller effect compared to Reason 1.
