Unit 4: Acid Value and Ascorbic Acid - Subjective Questions

Acid value is the number of milligrams of potassium hydroxide () required to neutralize the free fatty acids present in of fat or oil.

Where:

• = volume of standard used in mL
• = normality of
• = equivalent weight of in g equivalent
• = weight of the fat sample in g

Significance:

• It measures the amount of free fatty acids formed by hydrolysis of triglycerides.
• A high acid value indicates rancidity, decomposition, or poor storage conditions.
• It helps assess the freshness and quality of edible oils and fats.
• It is useful for monitoring the deterioration of fats during processing and storage.

The acid value determination is based on the neutralization of free fatty acids present in a fat sample with a standard alkali solution.

The fat is dissolved in a suitable neutralized organic solvent, usually an ethanol–ether mixture. Phenolphthalein is added as an indicator, and the solution is titrated against standard alcoholic until a persistent pale-pink endpoint is obtained.

The general neutralization reaction is:

The volume of consumed is proportional to the quantity of free fatty acids in the sample. The acid value is then calculated as the milligrams of required to neutralize the free fatty acids in of fat.

Reagents:

• Fat or oil sample.
• Standard alcoholic potassium hydroxide solution.
• Neutralized alcohol–ether solvent or another approved fat-solubilizing solvent.
• Phenolphthalein indicator.
• Distilled water, when required for cleaning and preparation.

Apparatus:

• Analytical balance for accurately weighing the fat sample.
• Conical flask for dissolving and titrating the sample.
• Burette for delivering standard .
• Pipette or measuring cylinder for transferring the solvent.
• Measuring flask for solution preparation.
• Glass rod or shaker for mixing.
• Water bath, if gentle warming is needed to dissolve the fat.

All glassware should be clean and dry. The solvent and alkali should be free from acidic or alkaline impurities because such impurities can cause an incorrect titre value.

Procedure:

1. Weigh accurately a known amount of the fat sample, usually between and , into a clean conical flask.
2. Add a measured volume of neutralized alcohol–ether solvent.
3. Warm the flask gently in a water bath if necessary and swirl until the sample dissolves completely.
4. Add a few drops of phenolphthalein indicator.
5. Fill the burette with standardized alcoholic solution and note the initial reading.
6. Titrate the sample with while continuously shaking the flask.
7. Stop the titration when a pale-pink color persists for approximately seconds.
8. Note the final burette reading and calculate the volume of used.
9. Perform a blank titration using the same solvent and indicator but without the fat sample.
10. Use the blank-corrected titre in the acid value calculation.

Here, is the sample titre, is the blank titre, is the normality of , and is the sample weight in grams.

Suppose mL of alkali of normality is required to neutralize the free fatty acids in g of fat.

The number of gram equivalents of used is:

The equivalent weight of is . Therefore, the mass of used is:

In milligrams:

This quantity corresponds to g of fat. Therefore, the quantity corresponding to g of fat is:

If a blank correction is required, the formula becomes:

where is the blank titre in mL.

Role of phenolphthalein:

• Phenolphthalein is an acid–base indicator.
• It is colorless in acidic medium and becomes pink in slightly alkaline medium.
• It indicates the endpoint when all free fatty acids have been neutralized and a slight excess of is present.
• A pale-pink color that persists for about seconds is generally taken as the endpoint.

Importance of blank titration:

• The solvent and indicator may contain traces of acidic substances.
• These substances can consume some of the standard .
• A blank titration measures this background alkali consumption.
• The blank titre is subtracted from the sample titre to obtain the true alkali volume used by the free fatty acids.

Blank correction improves the accuracy and reliability of the acid value.

Given:

• Weight of sample,
• Volume of ,
• Normality of ,
• Equivalent weight of

Use:

Substitution:

Therefore, the acid value of the fat is approximately:

Several factors can affect the accuracy of acid value determination:

• Incorrect sample weighing: An inaccurate sample mass directly affects the calculated value.
• Absorption of carbon dioxide: Alkali solutions absorb carbon dioxide from air and may form carbonate, changing their effective concentration.
• Improper standardization of : An incorrectly standardized alkali produces a wrong result.
• Unneutralized solvent: Acidic impurities in the solvent increase the titre value.
• Overshooting the endpoint: Adding excess alkali gives an artificially high acid value.
• Incomplete dissolution: Undissolved fat prevents complete reaction with the alkali.
• Incorrect indicator amount: Too much indicator may slightly influence the endpoint.
• Poor mixing: Inadequate shaking can delay neutralization and cause an incorrect endpoint.
• Moisture or contamination: Water and contaminants may interfere with dissolution or titration.

Careful standardization, blank correction, proper endpoint detection, and replicate analysis help minimize these errors.

The acid value mainly reflects hydrolytic spoilage, whereas the saponification value provides information about the overall ester composition of the fat.

Triglycerides can undergo hydrolysis during storage, especially in the presence of moisture, heat, enzymes, or microorganisms:

The fatty acids released are called free fatty acids. Since acid value measures the amount of alkali required to neutralize these acids, an increase in free fatty acids causes an increase in acid value.

Relationship with rancidity:

• Fresh fats generally have a low acid value.
• Hydrolytic breakdown releases free fatty acids and raises the acid value.
• Some free fatty acids have unpleasant odors and flavors.
• Oxidation of unsaturated fatty acids can also produce aldehydes, ketones, and other compounds responsible for rancid odors.
• Thus, a high acid value is commonly used as an indicator of deterioration, although it does not measure oxidative rancidity by itself.

Therefore, acid value is an important quality-control parameter for fats and oils.

Ascorbic acid, commonly known as vitamin C, is a water-soluble vitamin and a potent reducing agent. Its molecular formula is .

Important biological functions include:

• It acts as an antioxidant and helps protect cells from reactive oxygen species.
• It is required for collagen synthesis and therefore supports skin, blood vessels, cartilage, bone, and wound healing.
• It enhances the absorption of non-heme iron by reducing ferric iron, , to ferrous iron, .
• It supports normal immune function.
• It participates in the synthesis of certain hormones and neurotransmitters.
• It helps maintain connective tissue and protects other biological molecules from oxidation.

Deficiency of vitamin C can cause scurvy, characterized by bleeding gums, poor wound healing, and weakness.

The determination is based on the reducing property of ascorbic acid. In a commonly used method, ascorbic acid reduces a colored oxidizing reagent such as 2,6-dichlorophenolindophenol, abbreviated as .

The reduced form of the reagent is colorless, while the oxidized form is colored. Ascorbic acid reduces the colored reagent until all the ascorbic acid in the sample has reacted. The first permanent faint color indicates a slight excess of reagent and represents the endpoint.

The reaction can be represented generally as:

The volume of standardized used is proportional to the amount of ascorbic acid present. The concentration is calculated from the standardization factor and the volume of fruit juice titrated.

Reagents:

• Fresh fruit juice or a suitably diluted fruit juice sample.
• Standard ascorbic acid solution for standardizing the dye.
• Standard 2,6-dichlorophenolindophenol solution, or .
• Metaphosphoric acid or another suitable stabilizing acid, when specified by the method.
• Distilled water.

Apparatus:

• Burette for delivering the solution.
• Pipette for accurately transferring standard and sample solutions.
• Conical flasks for titration.
• Volumetric flask for preparing dilutions.
• Filter paper or centrifuge, if clarification of the juice is necessary.
• Analytical balance for preparing the standard ascorbic acid solution.

The sample should be protected from light and analyzed quickly because ascorbic acid is readily oxidized by air, heat, light, and metal ions.

Procedure:

1. Prepare a fresh standard ascorbic acid solution of known concentration.
2. Fill the burette with the solution.
3. Pipette a known volume of standard ascorbic acid into a conical flask.
4. Titrate with until a faint pink color persists for approximately – seconds.
5. Record the titre and calculate the amount of ascorbic acid equivalent to of dye. This is the dye factor.
6. Filter the fruit juice if it contains pulp or suspended particles.
7. Pipette a known volume of the juice, usually after appropriate dilution, into a conical flask.
8. Titrate the sample with standardized to the same permanent faint-pink endpoint.
9. Record the sample titre and perform replicate titrations.
10. Calculate the ascorbic acid content, correcting for dilution and sample volume.

The analysis should be carried out promptly because oxidation of ascorbic acid before titration lowers the measured value.

A known concentration of pure ascorbic acid is used to determine the amount of ascorbic acid equivalent to each milliliter of .

Steps:

• Prepare a standard ascorbic acid solution of known concentration.
• Pipette a known volume, , of this solution into a conical flask.
• Titrate it with the solution.
• Record the volume of dye used, .
• Calculate the mass of ascorbic acid present in the aliquot.

The dye factor is:

where is expressed as milligrams of ascorbic acid per milliliter of dye.

After standardization, the ascorbic acid in a sample aliquot is calculated by:

Standardization is essential because the dye may deteriorate during storage and its exact concentration may change.

Let:

• = dye factor in mg ascorbic acid per mL of dye
• = volume of dye used for the juice aliquot in mL
• = volume of juice aliquot titrated in mL
• = total volume of diluted juice prepared in mL
• = original volume of juice used to prepare the diluted sample in mL

The mass of ascorbic acid in the aliquot is:

The total mass of ascorbic acid in the diluted sample is:

This mass corresponds to mL of the original juice. Therefore, the ascorbic acid content per mL of original juice is:

The result is expressed as mg of ascorbic acid per mL of fruit juice. If no dilution is made, the dilution factor is omitted.

Given:

• Volume of juice aliquot,
• Volume of dye used,
• Dye factor,

Mass of ascorbic acid in the aliquot:

Therefore, the amount per is:

Both methods require accurate weighing or pipetting, standardized reagents, proper endpoint detection, and suitable calculations.

Ascorbic acid is easily oxidized to dehydroascorbic acid and may undergo further degradation. Important factors include:

• Exposure to oxygen: Oxygen promotes oxidation, especially during vigorous shaking or prolonged storage.
• Light: Light accelerates degradation; samples should be kept in dark or amber containers.
• Heat: High temperature increases the rate of oxidation and decomposition.
• pH: Ascorbic acid is generally more stable in an acidic medium than in neutral or alkaline conditions.
• Metal ions: Copper and iron can catalyze oxidation reactions.
• Storage time: Delayed analysis may produce a lower result.
• Sample preparation: Excessive blending, aeration, or filtration delay can increase losses.
• Microbial activity: Microorganisms may consume or degrade nutrients in poorly stored juice.

For accurate analysis, use fresh samples, acidify or stabilize them when appropriate, protect them from light, minimize air exposure, and titrate promptly.

Important precautions:

• Use freshly prepared or properly stored ascorbic acid standards.
• Protect the standard and juice samples from direct light.
• Analyze the sample immediately after preparation.
• Use clean glassware free from oxidizing or reducing contaminants.
• Standardize the solution before use.
• Avoid excessive shaking, which introduces oxygen into the sample.
• Filter pulpy samples carefully without unnecessary delay.
• Perform titration in an acidic medium when required by the method.
• Add the dye slowly near the endpoint.
• Use the same endpoint criterion for standardization and sample titration.
• Carry out replicate titrations and use concordant values.
• Apply the correct dilution factor in the final calculation.

These precautions reduce oxidation losses, endpoint errors, and variation between replicate results.