Question

When determining the peroixde value, explain why the volume of sodium thiosulfate needed to titrate the sample will be higher than the volume needed to titrate the blank

Answer 1

The peroxide value (PV) finds the conc. of hydroperoxide.

ROOH+KI→ROH+KOH+I2

The amount of ROOH is determined by measuring the amount of iodine formed, which is done by titration with sodium thiosulfate and using starch as indicator:

I2+starch+2Na2S2O3(blue)→2NaI+starch+Na2S4O6 (colorless)

The amount of peroxides is calculated back by the amount of sodium thiosulfate (Na₂S₄O₆) consumed. It is expressed as in units of milli-equivalents (meq) peroxide per 1 kg of fat extracted from the food.

For the blank solution with no double bond, for the ICl to react to. So, all excess ICl react with KI to produce I2. And because there are more I2, more Na2S2O3 is needed to titrate I2 at the endpoint.

So, the volume of sodium thiosulfate needed to titrate the sample will be higher than the volume needed to titrate the blank.