If this is the case, the sample titration volume should be higher than the blank. If you are observing the reverse—that the blank is higher—it usually indicates a specific experimental condition or calculation error, , more commonly, it relates to the definition of "usage."
For students and novice technicians, the procedure often presents a puzzling observation: the "blank" titration consistently requires a higher volume of sodium thiosulfate to reach the endpoint than the titration containing the lipid sample. At first glance, this seems counterintuitive. If the sample contains chemical species that generate iodine, shouldn't the sample require more titrant to neutralize that iodine? If this is the case, the sample titration
However, in real-world chemistry, reagents are rarely perfect. The blank titration measures the "background noise" of the experiment. It accounts for any iodine that is liberated not by the peroxides in the oil, but by impurities in the reagents or environmental factors. The central reason the blank titration uses more $Na_2S_2O_3$ lies in the definition of the Peroxide Value calculation and the nature of the sample matrix. If the sample contains chemical species that generate
To understand why the reverse is true, we must dive deep into the stoichiometry of the reaction, the specific goals of a blank correction, and the unavoidable realities of laboratory reagents. This article explores the chemical mechanisms that dictate this phenomenon, explaining why the blank titration acts as the baseline "cap" for sodium thiosulfate usage. Before analyzing the volume discrepancies, we must establish the chemical framework. The determination of peroxide value is an indirect titration. We are not titrating the peroxides in the oil directly; rather, we are titrating the iodine liberated by the peroxides (or, in the case of blanks, the iodine liberated by the reagents). It accounts for any iodine that is liberated
As the titration proceeds, the amber color of the iodine fades. Just before the color disappears, a starch indicator is added, turning the solution a dark, bruised blue-black. The endpoint is reached when the blue color vanishes completely, leaving a colorless solution. In a blank titration, the chemist performs the exact same procedure as with the sample, but without the lipid . The flask contains the solvent (chloroform/isooctane), the acetic acid, and the potassium iodide.
Here, the hydroperoxides ($\text{ROOH}$) in the lipid oxidize the iodide ions ($\text{I}^-$) to molecular iodine ($\text{I}_2$). Once the iodine is generated, it is titrated with standardized sodium thiosulfate:
$$I_{total} = I_{blank} + I_{sample}$$