Peroxide Value determination by titration method.

 

Peroxide Value determination by titration method.

Peroxide Value: The determination of peroxide value involves a redox reaction. Peroxides in the oil react with iodide ions to produce iodine, which is then measured. This is an indicator of the degree of oxidation in fats and oils.

Peroxide Value is a measure of the amount of peroxide compounds in oils and fats. Peroxides are formed when fats and oils are exposed to oxygen. This process is called oxidation. The peroxide value tells us how much oxidation has happened in the oil or fat, indicating how fresh or rancid it might be.

The peroxide value increases when oils and fats start to oxidize.

Exposure to Oxygen: When oils and fats come into contact with air, the oxygen reacts with the fat molecules. This reaction creates peroxides. Light and Heat: Exposure to light (especially UV light) and heat speeds up the oxidation process. Presence of Metal Ions: Metals like iron and copper can act as catalysts in the oxidation process. Even small amounts of these metals in oils and fats can increase the rate of oxidation, leading to a higher peroxide value. Oils and fats made up of polyunsaturated fatty acids (like fish oil or sunflower oil) oxidize faster than those with monounsaturated or saturated fats. This is because polyunsaturated fats have more double bonds, which are more reactive to oxygen.

Fats and oils are primarily made up of triglycerides, which are molecules consisting of glycerol bonded to three fatty acid chains. When these fatty acids are exposed to oxygen, they undergo oxidation.The initial step in the oxidation process involves the formation of free radicals. The double bonds in unsaturated fatty acids are particularly susceptible to oxidation. When these double bonds react with oxygen (O2), they form free radicals (R•). 

RH+O2→R•+HOO•

• RH represents a fatty acid molecule.
• R• is a free radical formed after the removal of a hydrogen atom.
• HOO• is a hydroperoxide radical.

These free radicals then react with molecular oxygen (O2) to form peroxide radicals (ROO•): 

R•+O2→ROO•

• ROO• is a peroxide radical.

The peroxide radicals (ROO•) can further react with another fatty acid molecule (RH), creating a hydroperoxide (ROOH) and another free radical (R•), propagating the chain reaction:

ROO•+RH→ROOH+R•

• ROOH is a hydroperoxide, the primary product measured by the peroxide value.
• The new R• free radical can react further, continuing the oxidation process.

The peroxide value increases as more hydroperoxides (ROOH) are formed through these oxidation reactions. Factors that contribute to the increase, as explained before, include exposure to air (oxygen), light, heat, metal ions, and the type of fatty acids present. 

Procedure

Apparatus and Reagents Required

Apparatus:

1. Conical Flasks (e.g., 250 mL)
2. Pipettes and Burettes (for accurate measurement and titration)
3. Glass Stirring Rod
4. Graduated Cylinder
5. Beakers
6. Titration Stand and Clamp
7. Pipette Fillers
8. White Tile (for better visibility of color change)
9. Safety Equipment (gloves, goggles, lab coat)

Reagents:

• Solvent Mixture:

1. Acetic Acid (Glacial, pure)
2. Chloroform

• Potassium Iodide (KI) Solution:

1. Typically prepared as a 10% solution.

• Sodium Thiosulfate (Na2S2O3) Solution:

1. Standardized, usually 0.1 N.

• Starch Solution.

1. Used as an indicator during titration.

• Distilled Water

1. For dilution and washing purposes.

• Sample Preparation: Weigh approximately 5 grams of the oil or fat sample and transfer it into a 250 mL conical flask.

• Addition of Solvent Mixture: Add 30 mL of the solvent mixture (acetic acid 18ml, and chloroform12mlin a 3:2 ratio) to the flask. Swirl gently to dissolve the sample completely.

• Addition of Potassium Iodide (KI): Add 1-2ml saturated potassium iodide (KI) solution to the flask. This reacts with the peroxides to produce iodine (I2).

• Reaction with Peroxides: The chemical reaction is,

ROOH+2KI→I2+ROH+KOH

• ROOH is the peroxide present, I2 is iodine liberated in the reaction.

• Titration: Add 50 mL of distilled water to the flask to separate the layers.

Titrate the iodine (I2) with a 0.1 N sodium thiosulfate (Na2S2O3) solution. Add sodium thiosulfate until the yellow iodine color fades.

• Addition of Starch Indicator:

Near the endpoint of the titration, add a few drops of starch solution. The solution will turn blue. Continue titration until the blue color disappears, indicating the endpoint.

Calculation:

Calculate the peroxide value (PV) using the formula:

PV = (𝐕𝐨𝐥𝐮𝐦𝐞 𝐨𝐟 𝐍𝐚𝟐𝐒𝟐𝐎𝟑× 𝐍𝐨𝐫𝐦𝐚𝐥𝐢𝐭𝐲 𝐨𝐟 𝐍𝐚𝟐𝐒𝟐𝐎𝟑) / W𝐞𝐢𝐠𝐡𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐒𝐚𝐦𝐩𝐥𝐞 (𝐠𝐫𝐚𝐦𝐬) ×1000

• Volume of Na2S2O3Volume of sodium thiosulfate solution used.
• Normality of Na2S2O3: Concentration of the sodium thiosulfate solution.
• Weight of the Sample: Weight of the oil or fat sample in grams.
• Multiplying by 1000 converts the peroxide value from milliequivalents per gram to milliequivalents per kilogram. 

 1. Acetic Acid (Glacial)

• Purpose: Acetic acid is used as a solvent to dissolve the oil or fat sample. It provides an acidic environment which helps in stabilizing the iodine released during the reaction with peroxides.
• Function: It reacts with chloroform to create a medium in which the oil or fat is effectively dissolved and the subsequent reactions can occur.

2. Chloroform

• Purpose: Chloroform is used in combination with acetic acid as a solvent to dissolve the oil or fat sample.
• Function: It helps in dissolving both non-polar and moderately polar components of fats and oils, facilitating the reaction with potassium iodide.

3. Potassium Iodide (KI)

• Purpose: Potassium iodide reacts with the peroxides in the sample to liberate iodine (I2).
• Function: The chemical reaction is:

ROOH+2KI→I2+ROH+KOH

• ROOH represents the peroxide present in the sample.
• I2 is iodine, which is released and then measured.

4. Sodium Thiosulfate (Na2S2O3)

• Purpose: Sodium thiosulfate is used to titrate the liberated iodine.
• Function: The reaction with iodine is:

2Na2S2O3+I2→Na2S4O6+2NaI

• This reaction quantifies the amount of iodine and, by extension, the amount of peroxide in the sample.

5. Starch Solution

• Purpose: Starch is used as an indicator during the titration process.
• Function: It forms a blue complex with iodine, which helps to clearly identify the endpoint of the titration when the blue color disappears.

I2+Starch→Blue Complex 

Peroxide Value and Conversion to Percentage

1.00meq/kg means that there is 1 milliequivalent of peroxide per kilogram of the sample.

Milliequivalent (meq): This is a unit of measurement that represents the amount of substance based on its ability to react or its equivalence in chemical reactions. In this context, it measures the concentration of peroxide.

convert the peroxide value (meq/kg) to a percentage, you need to consider the specific chemical nature and density of the peroxides, which is a bit more complex. Typically, direct conversion from meq/kg to percentage isn't straightforward because:

• For simplicity, assume hydrogen peroxide (H2O2) as the peroxide in question.
• Molar mass of H2O2 is approximately 34.01 g/mol.

1 meq is equivalent to 1 mmol (millimole) because 1 equivalent is 1 mole of reactive species (for H2O2).

• 1 mmol of H2O2 is 34.01 mg.

Convert to Percentage: 

Mass of peroxide = Peroxide Value × Mass per mmol

Mass of peroxide = 1meq/kg × 34.01mg = 34.01mg

Convert mg to g:

Mass of peroxide = 34.01mg / 1000 = 0.03401g

 Percentage of peroxide:

Percentage = Mass of peroxide / Total mass × 100

Percentage = 0.03401 / 1000g × 100 ≈ 𝟎.𝟎𝟎𝟑𝟒%