# Peroxide value

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Measure of peroxide content of a fat or oil

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Titration setup for finding the peroxide value

Detection of [peroxide](/source/Peroxide) gives the initial evidence of [rancidity](/source/Rancidity) in unsaturated fats and oils. Other methods are available, but **peroxide value** is the most widely used. It gives a measure of the extent to which an oil sample has undergone primary oxidation; extent of secondary oxidation may be determined from [p-anisidine](/source/P-anisidine) test.[1]

The [double bonds](/source/Double_bond) found in fats and oils play a role in [autoxidation](/source/Autoxidation). Oils with a high degree of [unsaturation](/source/Unsaturated_fat) are most susceptible to autoxidation. The best test for [autoxidation](/source/Autoxidation) (oxidative rancidity) is determination of the peroxide value. [Peroxides](/source/Peroxide) are intermediates in the autoxidation reaction.

Autoxidation is a [free radical reaction](/source/Free_radical_addition) involving [oxygen](/source/Oxygen) that leads to deterioration of fats and oils which form off-flavours and off-odours. Peroxide value, concentration of peroxide in an oil or fat, is useful for assessing the extent to which spoilage has advanced.

## Definition

The peroxide value is defined as the amount of peroxide oxygen per 1 [kilogram](/source/Kilogram) of fat or oil. Traditionally this was expressed in units of [milliequivalents](/source/Milliequivalent), although in [SI units](/source/SI_unit) the appropriate option would be in [millimoles](/source/Millimole) per [kilogram](/source/Kilogram) (N.B. 1 milliequivalents = 0.5 millimole; because 1 mEq of O2 =1 mmol/2 of O2 =0.5 mmol of O2, where 2 is valence). The unit of milliequivalent has been commonly abbreviated as mequiv or even as meq.

## Method

The peroxide value is determined by measuring the amount of [iodine](/source/Iodine) which is formed by the reaction of peroxides (formed in fat or oil) with [iodide](/source/Iodide) ion.

- 2 I− + H2O + R−OOH → R−OH + 2 HO− + I2

The [base](/source/Base_(chemistry)) produced in this reaction is taken up by the excess of [acetic acid](/source/Acetic_acid) present. To measure the iodine, a [redox titration](/source/Redox_titration) is performed using [sodium thiosulfate](/source/Sodium_thiosulfate).

- 2 S2O2−3 + I2 → S4O2−6 + 2 I−

The acidic conditions (excess acetic acid) prevents formation of [hypoiodite](/source/Hypoiodite) (analogous to [hypochlorite](/source/Hypochlorite)), which would interfere with the reaction.

A [starch indicator](/source/Starch_indicator) solution is used, for which [amylose](/source/Amylose) forms a blue to black solution with iodine and is colorless when the iodine is converted to iodide [titrated](/source/Redox_titration).

## Taste

Peroxide values of fresh oils are less than 10 milliequivalents/kg; when the peroxide value is between 30* and 40 milliequivalents/kg, a rancid taste is noticeable.

## See also

- [Acid value](/source/Acid_value)

- [Amine value](/source/Amine_value)

- [Bromine number](/source/Bromine_number)

- [Epoxy value](/source/Epoxy_value)

- [Hydroxyl value](/source/Hydroxyl_value)

- [Iodine value](/source/Iodine_value)

- [Saponification value](/source/Saponification_value)

## References

1. **[^](#cite_ref-1)** Chemistry And Technology Of Oils And Fats [Chemistry And Technology Of Oils And Fats By Dr. M.M. Chakrabarty](http://www.alliedpublishers.com/BookDetails.aspx?BookId=228) [Archived](https://web.archive.org/web/20180925180548/http://www.alliedpublishers.com/BookDetails.aspx?BookId=228) 25 September 2018 at the [Wayback Machine](/source/Wayback_Machine)

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