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I am not allowed to use glacial acetic acid at my school, which all procedures online for finding the peroxide value of oils use through an iodometric titration. Is there a replacement?

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  • $\begingroup$ It vastly depends on where you determine the peroxide value. Sometimes you can use mineral acids, sometimes not. $\endgroup$
    – andselisk
    Mar 2 '19 at 20:42
  • $\begingroup$ I am analyzing the effects of gamma-Tocopherol on olive oil using the IFRA procedure to determine peroxide value. I cannot use chloroform. So I wanted to use a 3:2 ratio of glacial acetic acid to cyclohexane, however, I cannot use glacial acetic acid. Would mineral acids work in this scenario or do I have to change my method? $\endgroup$
    – hi101
    Mar 3 '19 at 0:09
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Your school is not allowing use of glacial acetic acid, but you insist of using glacial acetic acid in determining peroxide value, you may use a solvent system, which consists of glacial acetic acid. For example, you can use 60% ($v/v$) glacial acetic acid in chloroform. I give that example because it is commercially available online at Reagents. If you are determining peroxide value of fats, this is an ideal solvent to dissolve fat and resultant solution is soluble in water.

I'm suggesting this because there is a method recommended by International Fragrance Association (IFRA) in Belgium. Breifly, one of the IFRA analytical methods for determination of the peroxide value is described as follows:

Transfer approx. $\pu{3.0 g}$ of the sample, accurately weighed, into a 250-mL Erlenmeyer flask with glass stopper. Add $\pu{50 mL}$ of the appropriate solvent mixture (glacial acetic acid : chloroform 3:2; glacial acetic acid : cyclohexane 3:2; glacial acetic acid : isooctane 3:2) and saturated potassium iodide solution, $\pu{1 mL}$, freshly prepared and allow to react for $\pu{60 ± 1 sec}$ and shaking thoroughly during this period. Then add water, $\pu{100 mL}$ and shake. Titrate with $\pu{0.01 mol/L}$ sodium thiosulfate solution, or $\pu{0.1 mol/L}$ sodium thiosulfate solution at higher usages, using $\pu{1 mL}$ starch solution or $\pu{0.1 g}$ of Thyodene indicator. The indicator should be added towards the end of the titration but while the pale straw color is still present. During titration shake until the blue color disappears. Carry out a blank titration under the same conditions. No more than $\pu{0.5 mL}$ of $\pu{0.01 mol/L}$ sodium thiosulfate solution should be consumed for this purpose.

Note: Use $\pu{0.1 mol/L}$ sodium thiosulfate solution when the expected peroxide values are $\gt \pu{20 mmol/L}$. Use $\pu{0.01 mol/L}$ sodium thiosulfate solution when the expected peroxide values are $\lt \pu{20 mmol/L}$.

Thyodene indicator is also commercially available in Fisher.

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  • $\begingroup$ Thanks, I was using this procedure to give me an idea of how to measure the peroxide value of olive oil. I am using cyclohexane instead of chloroform, however, I cannot purchase a solution containing glacial acetic acid. $\endgroup$
    – hi101
    Mar 3 '19 at 0:14
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As an alternative you can use Ferric Thiocyanate Method, instead.

The ferric thiocyanate method: Chapman and McFarlane (1943) have developed colorimetric method to determine peroxides in food such as milk powder (Ref.1). The method is based on the oxidation of ferrous ($\ce{Fe^{II}}$) to ferric ($\ce{Fe^{III}}$) iron by the peroxides present and the colorimetric determination of the $\ce{Fe^{III}}$ iron as ferric thiocyanate (red color). They also applied the method to determine peroxides in fats and oils (Ref.2).

In this method, a reagent containing 0.1% ferrous ammonium sulfate and 0.4% ammonium thiocyanate in 96% acetone ($\pu{9 mL}$) is added to a $\pu{1 mL}$ aliquot of an acetone solution of the fat, and the mixture is heated for 10 minutes in a water bath at $\pu{50 ^{\circ}C}$. The red color developed (ferric thiocyanate) is measured in a Spectrophotometer at $\pu{485 nm}$. A calibration curve should be prepared from readings obtained with standard solutions of ferric chloride ($0.2$ to $\pu{14.0 \mu g/ mL}$) in acetone.

Authors’ Note:

This procedure gives more readily reproducible results than the usual peroxide titration methods and also measures more completely the extent of oxidation, particularly with highly unsaturated fats.

References:

  1. R. A. Chapman, W. D. McFarlane, “A Colorimetric Method for the Determination of Fat-Peroxides and Its Application in the Study of the Keeping Quality of Milk Powders,” Canadian Journal of Research 1943, 21b(7), 133-139 (https://doi.org/10.1139/cjr43b-017).
  2. A. Lips, R. A. Chapman, W. D. McFarlane, “The application of the ferric thiocyanate method to the determination of incipient rancidity in fats and oils,” Oil & Soap 1943, 20(11), 240-243 (https://doi.org/10.1007/BF02630882).
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  • $\begingroup$ Thanks so much, all the materials fit the approved chemicals list! $\endgroup$
    – hi101
    Mar 3 '19 at 2:57
  • $\begingroup$ My pleasure. Please also remember, to make calibration curve, need to use known concentrations of $\ce{FeCl3}$ solutions in place of the unknown sample. $\endgroup$ Mar 3 '19 at 14:12

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