Discussion Examples for Vitamin C Determination in Foods | Redox Reactions and the Effects of Storage Conditions

What Is the Determination of Vitamin C in Foods?

Determination of vitamin C in foods is an analysis in which the amount of ascorbic acid contained in foods and beverages is measured.
Vitamin C is a water-soluble vitamin and is abundant in fruits, vegetables, fruit-juice beverages, nutritional supplements, and other products.
While it is important as a nutrient, it is also readily decomposed by oxygen, heat, and light.

Because vitamin C has reducing properties, it reacts with oxidizing agents and is oxidized.
By using this redox reaction, the amount of vitamin C can be determined from the amount of oxidizing agent consumed or from a color change.
Representative methods include iodine titration and the DCPIP method using 2,6-dichlorophenolindophenol.

Main Items to Include in the Results

In the results of vitamin C determination, organize the type of food sample, sample amount, extraction method, dilution factor, measurement method, concentration of the standard solution, titration volume, endpoint color change, vitamin C content, and other information.
Because extraction conditions and storage conditions greatly affect measured values for food samples, how the sample was handled must also be recorded.

Main Items to Include in the Results

• Type of food sample
• Mass or volume of the sample
• Storage conditions of the sample
• Presence or absence of heat treatment
• Type of extraction solution
• Extraction time
• Presence or absence of filtration or centrifugation
• Dilution factor
• Measurement method
• Concentration of the standard solution
• Titration volume or absorbance
• Endpoint color change
• Blank value
• Vitamin C content
• Converted value per 100 g or per 100 mL
• Average value from multiple measurements
• Comparison with the food-label value
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Vitamin C Determination

Here, for an experiment in which vitamin C in foods is determined by redox titration, titration volume, vitamin C content, and changes caused by storage conditions and heating are organized using reference experimental values.

Vitamin C is a reducing component and is oxidized by reacting with an oxidizing agent.
Therefore, the amount of vitamin C in foods can be determined using oxidizing agents such as iodine solution and DCPIP.
However, vitamin C is readily decomposed by heat, light, oxygen, metal ions, and other factors, so measured values change depending on storage and cooking conditions.

Reference Experimental Conditions

Concept of the Redox Reaction

Vitamin C is oxidized by iodine, while iodine is reduced to iodide ions.
While all of the iodine is being consumed in the reaction with vitamin C, the blue-violet color of starch does not remain.
Once the vitamin C has finished reacting, excess iodine reacts with starch and produces a blue-violet color.

Confirmation Titration of a Standard Vitamin C Solution

First, an example is shown in which a vitamin C standard solution of known concentration is titrated to confirm its relationship with the iodine solution.
Here, 10.00 mL of the vitamin C standard solution is assumed to contain 1.76 mg of vitamin C.

Because the titration volumes of the standard solution were approximately 10.00 mL, the concentration of the titrant and the endpoint determination were considered generally appropriate under these conditions.

Titration Results for Different Foods

The following shows examples in which 10.00 mL of sample solution prepared from each food was titrated with 0.00100 mol/L iodine solution.
The calculation assumes that vitamin C and iodine react in a 1:1 ratio.

Example Calculation of Vitamin C Amount

Because vitamin C and iodine react in a 1:1 ratio, the amount of iodine consumed corresponds to the amount of vitamin C.

Amount of vitamin C = Iodine-solution concentration × Iodine-solution titration volume

For orange juice, the iodine-solution concentration is 0.00100 mol/L and the titration volume is 4.85 mL = 0.00485 L.

Amount of vitamin C = 0.00100 mol/L × 0.00485 L = 4.85 × 10−6 mol

Because the molar mass of vitamin C is 176.1 g/mol, the mass is calculated as follows.

Mass of vitamin C = 4.85 × 10−6 mol × 176.1 g/mol = 0.000854 g

0.000854 g = 0.854 mg

Therefore, 10.00 mL of the orange-juice sample solution was calculated to contain 0.854 mg of vitamin C.

Example Conversion to Amount per 100 mL

If 10.00 mL of the sample solution contains 0.854 mg of vitamin C, the content per 100 mL is determined by multiplying by 10.

Vitamin C amount = 0.854 mg × 10 = 8.54 mg/100 mL

However, in this reference example, the fruit juice is assumed to have been diluted fivefold before titration.
Therefore, the amount of vitamin C in the original juice is further multiplied by 5.

Vitamin C amount in the original juice = 8.54 × 5 = 42.7 mg/100 mL

Therefore, the vitamin C content of the orange juice is determined to be 42.7 mg/100 mL.

Calculation Formula Including the Dilution Factor

When a food sample is diluted before titration, the dilution factor is included in the calculation.

Vitamin C content in food (mg/100 mL) = Vitamin C amount in 10 mL of sample solution × 10 × Dilution factor

For solid foods, the mass of food used for extraction and the final extraction volume are considered when converting the result to an amount per 100 g.

Vitamin C content in food (mg/100 g) = Vitamin C amount in the entire extract ÷ Food mass × 100

Changes in Vitamin C Amount Depending on Storage Conditions

Because vitamin C is readily oxidized, its content changes depending on the storage conditions.
Here, a reference example is shown for orange juice stored for 24 hours under different conditions.

Example Calculation of Remaining Percentage

The amount of vitamin C remaining after storage can be expressed as a remaining percentage.

Remaining percentage (%) = Vitamin C amount after storage ÷ Vitamin C amount before storage × 100

Under the room-temperature and light-exposure condition, the value before storage is 42.7 mg/100 mL and the value after storage is 28.2 mg/100 mL.

Remaining percentage = 28.2 ÷ 42.7 × 100 = 66.0%

Therefore, after 24 hours of storage at room temperature under light exposure, approximately 66.0% of the vitamin C remained and approximately 34.0% was considered to have been lost.

Changes in Vitamin C Amount Caused by Heating

Vitamin C is sensitive to heat and is more readily decomposed as the heating time becomes longer.
Here, a reference example is shown for heating a green pepper extract.

During boiling, in addition to decomposition caused by heat, water-soluble vitamin C may have leached into the boiling water.
Therefore, even with the same 5-minute heating period, the remaining percentage is lower than with heating alone.

Comparison Between Acidic and Neutral Conditions

The stability of vitamin C may change depending on pH conditions.
In general, it tends to be relatively stable under acidic conditions and may be more readily oxidized under neutral to alkaline conditions.

In this reference example, the amount of vitamin C decreased greatly under slightly alkaline conditions.
This was considered to result from conditions that made vitamin C more readily oxidized.

Example of How to Write the Results

The amount of vitamin C in each food was measured by redox titration with iodine solution.
For orange juice, the titration volume of 0.00100 mol/L iodine solution was 4.85 mL, and the amount of vitamin C in 10.00 mL of sample solution was determined to be 0.854 mg.
Because the fruit juice was diluted fivefold before measurement, the amount of vitamin C in the original juice was 42.7 mg/100 mL.

When the foods were compared, lemon juice contained 54.6 mg/100 mL, orange juice contained 42.7 mg/100 mL, and grapefruit juice contained 34.8 mg/100 mL, with lemon juice showing the highest value.
In addition, the green pepper extract contained 68.7 mg/100 g, confirming that the vegetable sample also contained a large amount of vitamin C.

When orange juice was stored for 24 hours, the vitamin C remaining percentage was 91.8% under refrigerated and light-protected conditions, whereas it was 66.0% under room-temperature and light-exposure conditions and 58.8% in an open container.
From this, vitamin C was considered to have undergone oxidative decomposition through exposure to light and oxygen.

Points for Connecting the Results to the Discussion

In a discussion of vitamin C determination, it is important not only to determine the content from the titration volume but also to explain the effects of redox reactions, storage conditions, heating, pH, and extraction procedures in relation to one another.

• Has the 1:1 reaction between vitamin C and iodine been correctly reflected in the calculation?
• Can the titration volume be converted to amount of substance, mass, and amount per 100 mL or 100 g?
• Have the dilution factor and extraction-solution volume been included in the calculation?
• Can the reason vitamin C decreased depending on storage conditions be explained in relation to oxidation?
• Can the effects of light, heat, oxygen, and pH on the stability of vitamin C be explained?
• Can the decrease in vitamin C during boiling be explained from both thermal decomposition and leaching into water?
• Can endpoint determination, turbidity of the sample, and other reducing components be considered as possible sources of error?

Example Discussion

In this experiment, vitamin C in foods was determined by redox titration with iodine solution.
For orange juice, the iodine-solution titration volume was 4.85 mL, and the vitamin C amount was determined to be 42.7 mg/100 mL.
Lemon juice contained 54.6 mg/100 mL and showed a higher value than orange juice.
This was because the amount of vitamin C contained differs depending on the type of food.

Because vitamin C has reducing properties, it reduces iodine and is itself oxidized.
During titration, while vitamin C remains in the sample, iodine is consumed, so the blue-violet color produced by starch does not readily remain.
At the endpoint, the vitamin C has almost completely reacted, and the excess iodine was considered to have reacted with starch to produce a pale blue-violet color.

In the comparison of storage conditions, the decrease in vitamin C was small under refrigerated and light-protected conditions, whereas it decreased greatly under room-temperature and light-exposure conditions and in an open container.
This was because vitamin C is readily oxidized by light and oxygen.
In particular, in an open container, contact with oxygen in the air is greater and oxidative decomposition was considered to have progressed.

During heat treatment, the vitamin C amount decreased as the heating time increased.
In addition, during boiling, vitamin C may have leached into the boiling water because it is water-soluble, in addition to being decomposed by heat.
Therefore, to retain vitamin C in foods, avoiding prolonged heating and treatment in water is considered effective.

Possible measurement errors include endpoint determination, sample turbidity, insufficient extraction, and errors in dilution procedures.
In addition, foods may contain reducing substances other than vitamin C, and if these also react with iodine, the vitamin C amount may be overestimated.
Therefore, when discussing the results, not only the titration values but also the food components and pretreatment conditions must be considered.

Summary

In vitamin C determination, the reducing properties of vitamin C are used to determine the content from the amount of reaction with an oxidizing agent such as iodine solution.
By correctly handling the titration volume, reaction ratio, dilution factor, and extraction amount, the vitamin C content can be converted to the amount per 100 mL or 100 g of food.

In this reference example, the amount of vitamin C differed depending on the type of food and decreased because of storage conditions and heating.
In a report, it is useful to discuss redox reactions, differences in content among foods, storage conditions, heating, pH, and sources of error in relation to one another.

Redox Reaction of Vitamin C

Ascorbic acid, which is vitamin C, becomes dehydroascorbic acid when oxidized.
In this reaction, ascorbic acid acts as a reducing agent that donates electrons.
Therefore, when oxidizing agents such as iodine or DCPIP are used, the amount of vitamin C can be determined from the quantitative relationship in the redox reaction.

The fact that vitamin C is readily oxidized can be used for determination, but it is also a cause of its decrease during storage.
When oxidation progresses because of oxygen in the air, light, heat, metal ions, and other factors, the amount of vitamin C measured decreases.
Therefore, in quantitative experiments, it is important to carry out operations while preventing oxidation as much as possible.

Ascorbic acid → Dehydroascorbic acid + 2H+ + 2e-

Concept of Iodine Titration

Iodine titration uses the reaction in which vitamin C reduces iodine, I2, and is itself oxidized.
As long as vitamin C remains, the added iodine is immediately reduced, so the color of iodine and the blue-violet color of the starch indicator do not remain.
After all the vitamin C has reacted, excess iodine remains and a color change appears at the endpoint.

When starch indicator is used, the blue-violet color caused by the iodine-starch reaction appears at the endpoint.
However, if too much iodine is added beyond the endpoint, the vitamin C amount may be overestimated.
Near the endpoint, it is important to add the titrant in small amounts and confirm the point at which the color persists.

C6H8O6 + I2 → C6H6O6 + 2I- + 2H+

Concept of the DCPIP Method

The DCPIP method uses the redox dye 2,6-dichlorophenolindophenol.
Oxidized DCPIP shows a blue or reddish color, but it becomes colorless when reduced by vitamin C.
This color change is used to determine the amount of vitamin C in foods.

While DCPIP is being reduced by vitamin C, the color disappears.
Once vitamin C is no longer present, the color of DCPIP remains and this point is taken as the endpoint.
However, when a food sample is colored, it may become difficult to determine the endpoint color.

Discussion of Foods Rich in Vitamin C

Vitamin C may be abundant in citrus fruits, strawberries, kiwifruit, broccoli, green peppers, potatoes, green and yellow vegetables, and other foods.
If larger titration volumes are obtained for these foods, their vitamin C contents are considered to be high.
However, the content changes depending on variety, harvest time, ripeness, storage period, and cooking method.

In fruit-juice beverages and processed foods, vitamin C may be added as an antioxidant in addition to the vitamin C originating from the raw materials.
Therefore, comparing the results with food labels and ingredient lists makes it easier to discuss the meaning of the measured values.

Discussion of Foods Low in Vitamin C

When a food contains little vitamin C, this may be because its original vitamin C content was low or because vitamin C was decomposed during storage, heating, or processing.
Vitamin C is water-soluble and sensitive to heat and oxidation, so it tends to decrease during prolonged heating or storage.
Oxidation may also progress when cut surfaces are exposed to air.

If the measured value is lower than expected, possible causes include not only the vitamin C content of the food itself but also insufficient extraction, oxidation during operation, decomposition during storage, and stopping the titration before the endpoint.
Particularly for low-concentration samples, the titration volume is small, so an error of one drop has a large effect.

Effect of Storage Conditions

Vitamin C is a component that readily decreases depending on storage conditions.
Oxygen in the air, light, temperature, metal ions, pH, and other factors may promote oxidative decomposition.
Refrigerated or light-protected storage may suppress decomposition, whereas leaving the sample at room temperature for long periods tends to reduce the vitamin C amount.

When food is cut or ground, cells are disrupted and come into easier contact with oxygen and enzymes.
As a result, vitamin C may be oxidized by the action of enzymes such as ascorbic acid oxidase.
Relating differences in storage conditions to measured values leads to a deeper discussion.

Effect of Heating

Vitamin C is known to be sensitive to heat and may decrease during cooking by heating.
Heating not only makes oxidation reactions more likely to proceed but may also cause water-soluble vitamin C to dissolve into the cooking water during boiling.
Therefore, the measured vitamin C amount may change greatly depending on the cooking method.

However, heating may also inactivate enzymes and suppress decomposition caused by oxidative enzymes.
The actual amount of decrease changes depending on heating temperature, heating time, amount of water, shape of the food, and standing time after cooking.
Rather than simply writing that “it decreased because it was heated,” it is useful to specifically consider which factors were involved.

Effects of Light and Oxygen

Vitamin C may be oxidized under the influence of light and oxygen.
Beverages stored in transparent containers in bright locations may show a greater decrease in vitamin C than those stored protected from light.
In addition, after a beverage has been opened, it comes into easier contact with oxygen in the air and oxidation progresses more readily.

If a food sample is left for a long period after grinding, its surface area increases and contact with oxygen becomes greater.
As a result, the amount of vitamin C may decrease before measurement.
It is important to perform the measurement promptly after extraction.

Effect of pH

The stability of vitamin C is affected by pH.
In general, it tends to be relatively stable under acidic conditions and may be more readily oxidized under neutral to alkaline conditions.
Therefore, an acidic extraction solution may be used during vitamin C extraction to suppress decomposition.

The pH of the food itself is also related to the storage stability of vitamin C.
Acidic foods such as fruit juices may be relatively stable, whereas oxidation may progress more readily in foods or extraction solutions with higher pH.
It is important to standardize the pH conditions during measurement.

Effect of Metal Ions

Metal ions such as iron ions and copper ions may promote oxidation of vitamin C.
If trace amounts of metal ions are present in the food or equipment, vitamin C may be more readily oxidized and the measured value may become low.
Care is also required regarding metal equipment and prolonged contact with metals.

In experiments, clean equipment is used, and acidic conditions or chelating agents may be used when necessary to suppress oxidation.
However, specific methods should follow the instructions in the laboratory manual.
The effects of metal ions are also important when discussing vitamin C decomposition during storage.

Discussion of Extraction From Food Samples

Because vitamin C is water-soluble, it readily transfers from foods into aqueous extraction solutions.
However, when it is present within food tissues, it may not be completely extracted unless the sample is sufficiently ground or stirred.
If extraction is insufficient, the measured vitamin C amount becomes lower than the actual value.

During extraction, the sample is homogenized and the amount of extraction solution and extraction time are kept constant.
When filtration or centrifugation is performed, loss of liquid and vitamin C remaining in the residue must also be considered.
In food analysis, not only the titration procedure but also the accuracy of extraction is important.

Effects of Filtration, Turbidity, and Coloration

Food extracts may contain fibers, pulp, pigments, starch, proteins, and other substances.
If the extract is turbid, the endpoint color change may be difficult to observe.
In addition, when fruit juice or vegetable extract is strongly colored, it may become difficult to judge the color change in the DCPIP method or iodine-starch reaction.

Removing turbidity by filtration or centrifugation may make endpoint determination easier.
However, if extract is lost during filtration or liquid containing vitamin C remains in the residue, the measured value may become low.
Pretreatment must consider both endpoint visibility and loss of components.

Discussion of Endpoint Determination

In vitamin C determination, it is important to accurately determine the endpoint color change.
In iodine titration, the appearance of a blue-violet color produced by the starch indicator is used as the endpoint, while in the DCPIP method the point at which the color remains is treated as the endpoint.
However, when the food extract is colored, the endpoint may be difficult to observe.

If titration continues beyond the endpoint, too much oxidizing agent is added and the vitamin C amount may be overestimated.
Conversely, if titration is stopped before the endpoint, the amount is underestimated.
Near the endpoint, the titrant is added one drop at a time while the solution is thoroughly mixed and the color change is observed.

Effects of Coexisting Reducing Substances

Foods may contain reducing substances other than vitamin C that react with oxidizing agents.
For example, polyphenols, sulfites, reducing sugars, and other antioxidant components may consume the oxidizing agent.
If these substances react, they cause the vitamin C amount to be overestimated.

The value obtained by redox titration ideally represents the amount of vitamin C, but depending on the sample it may also include contributions from other reducing substances.
Particularly in fruit juices, vegetables, and processed foods, many different components are present, so the effects of coexisting substances must be considered.

Concentration Errors in Standard Solutions

In vitamin C determination, the concentrations of standard solutions such as iodine solution and DCPIP solution serve as the basis for calculations.
If the concentration of the standard solution is inaccurate, systematic errors occur in all measured values.
Because redox reagents may change during storage, they are standardized when necessary before use.

Depending on whether the standard-solution concentration is treated as higher or lower than the actual value, the calculated vitamin C amount also deviates.
In addition, DCPIP solution may deteriorate depending on light exposure and storage conditions.
Preparation and storage of standard solutions are directly related to the reliability of quantitative results.

Comparison With Food-Label Values

Foods and beverages may list vitamin C content in their nutritional information.
Comparing experimental values with the labeled values makes it easier to discuss the validity of the measurement results.
However, the labeled value may be shown as an average or standard value for the product and does not necessarily completely agree with the experimental value.

Possible reasons for differences between labeled and experimental values include vitamin C decomposition during storage, individual variation among samples, insufficient extraction, endpoint-determination errors, coexisting reducing substances, and differences in measurement methods.
Particularly in samples measured after opening or heating, the value may be lower than the labeled value.

Causes of Error in Vitamin C Determination

Causes of error in vitamin C determination include sample nonuniformity, insufficient extraction, oxidation during operation, concentration errors in standard solutions, endpoint-determination errors, coloration or turbidity of food extracts, coexisting reducing substances, errors in the dilution factor, and losses during filtration.
In food analysis, not only the titration procedure but also pretreatment and storage conditions have large effects.

Causes that make the value lower include oxidative decomposition of vitamin C, insufficient extraction, stopping titration before the endpoint, and losses during filtration.
Causes that make the value higher include titration beyond the endpoint, reactions of coexisting reducing substances, and errors in the standard-solution concentration.
Organizing error causes into overestimation and underestimation makes the discussion easier.

When the Results Can Be Considered Good

Results can be considered good in vitamin C determination in foods when multiple titration values are close to one another, the endpoint is clear, the standard-solution concentration is accurate, and the sample extraction conditions are consistent.
In addition, if the obtained vitamin C values do not greatly contradict the type of food or storage conditions, the results are easier to consider valid.

For example, high values for citrus fruits and vitamin C-fortified beverages and low values for samples after heating or long-term storage can be explained in relation to the characteristics of the food and storage conditions.
A trend in measured values that agrees with the experimental conditions leads to a good discussion.

Example Discussion When the Experiment Did Not Go Well

When vitamin C determination does not go well, possible causes can be considered from results such as variation in titration values, difficulty observing the endpoint, a large difference from the labeled value, unnatural differences before and after heating, or trends that cannot be explained by the storage conditions.
Organizing the causes according to sample preparation, extraction, storage, titration, standard solutions, and endpoint determination makes the discussion easier.

How to Write Points for Improvement

In a discussion of vitamin C determination in foods, writing not only the causes of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sample storage, extraction, titration operation, standard solutions, and calculation and analysis.

Improvements to Sample Storage and Pretreatment

• Store samples refrigerated and protected from light
• Measure promptly after opening
• Do not leave samples for long periods after grinding or extraction
• Reduce contact with air as much as possible
• Record the heating conditions and storage time
• Homogenize the sample sufficiently

Improvements to Extraction and Measurement Procedures

• Use the same type and pH of extraction solution
• Keep the extraction time constant
• Stir sufficiently during extraction
• Avoid loss of liquid during filtration
• Consider the effects of coloration and turbidity
• Add the titrant one drop at a time near the endpoint
• Perform multiple measurements and calculate the average value

Improvements to Standard Solutions and Calculations

• Accurately prepare the iodine solution or DCPIP solution
• Standardize the standard solution when necessary
• Store standard solutions protected from light
• Correctly reflect the dilution factor in the calculation
• Clearly state the units per 100 g or per 100 mL
• When comparing with food-label values, use the same units
• Consider the effects of coexisting reducing substances

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of vitamin C determination in foods, simply writing that “there was a lot of vitamin C” or “it was titrated” results in a superficial discussion.
A good discussion relates redox reactions, storage conditions, the effects of heating and oxygen, extraction, coexisting substances, and causes of error.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of vitamin C determination in foods.
Adjust the necessary parts according to your own experimental results.

• Because vitamin C has reducing properties, it can be determined using a redox reaction.
• Ascorbic acid becomes dehydroascorbic acid when oxidized.
• Iodine titration uses the reaction in which vitamin C reduces iodine.
• The DCPIP method uses the property that DCPIP is reduced and decolorized by vitamin C.
• The larger the titration volume, the greater the amount of vitamin C considered to be present in the sample.
• Vitamin C is readily oxidatively decomposed by oxygen, light, and heat.
• During cooking by heating, the vitamin C amount may decrease because of thermal decomposition and leaching into water.
• If extraction is insufficient, the vitamin C amount may be underestimated.
• If other reducing substances in food consume the oxidizing agent, the vitamin C amount may be overestimated.
• The sample must be measured promptly after extraction and the effects of air and light minimized.

Points to Check When Discussing Vitamin C Determination in Foods

Checking the following points before writing the report makes the discussion easier to write.

• Is it explained that vitamin C is a reducing substance?
• Is the principle of the redox reaction described?
• Is the endpoint of iodine titration or the DCPIP method explained?
• Are differences in vitamin C amount among foods considered?
• Are the effects of storage conditions discussed?
• Is decomposition caused by heating, light, and oxygen explained?
• Is the possibility of insufficient extraction considered?
• Are the effects of coloration and turbidity on endpoint determination considered?
• Are the effects of coexisting reducing substances considered?
• Are concentration errors in the standard solution checked?
• When comparing with food-label values, are the units consistent?
• Do the points for improvement correspond to the causes of error?

Summary

Determination of vitamin C in foods is an analysis in which the reducing properties of vitamin C are used to determine its content through a redox reaction.
In iodine titration, the reaction in which vitamin C reduces iodine is used, while the DCPIP method uses the property that DCPIP is reduced and decolorized by vitamin C.
The amount of vitamin C can be calculated from the titration volume and color change.

Vitamin C is readily oxidized under the influence of oxygen, light, heat, metal ions, and pH conditions, and its content changes depending on storage conditions.
Vitamin C tends to decrease during heating, prolonged storage, and standing after opening.
On the other hand, decomposition may be suppressed under refrigerated, light-protected, and acidic conditions.

In a report, rather than simply writing that “the vitamin C amount was high or low,” organize and discuss the principle of redox reactions, storage conditions, the effects of heating and oxygen, extraction procedures, endpoint determination, coexisting reducing substances, causes of error, and points for improvement.
In vitamin C determination, it is important to understand the relationship between the nutritional components of foods and redox reactions.