Discussion Examples for Saponification Value of Fats and Oils | Fatty Acid Chain Length and Properties of Fats and Oils

What Is the Saponification Value of Fats and Oils?

The saponification value is the number of milligrams of KOH required to completely saponify 1 g of fat or oil.
Fats and oils are composed mainly of triglycerides in which glycerol and fatty acids are linked by ester bonds.
When these compounds are hydrolyzed with a strong base such as KOH, glycerol and potassium salts of fatty acids are formed.
This reaction is called saponification.

The saponification value is related to the amount of ester bonds contained in the fat or oil.
When fats and oils of the same mass are compared, fats and oils with smaller molecular weights contain more molecules and therefore more ester bonds, so a larger amount of KOH is required for saponification.
For this reason, the saponification value provides a clue for estimating the average chain length of the fatty acids that make up the fat or oil.

Main Items to Include in the Results

In the results of saponification-value measurement, organize the type of fat or oil sample, sample amount, concentration and amount of ethanolic KOH solution added, heating and reflux conditions, titration volume in the blank test, titration volume in the sample test, concentration of hydrochloric acid or acid standard solution, saponification value, and other information.
Because the saponification value is calculated from the difference from the blank test, the blank-test value must always be clearly stated.

Main Items to Include in the Results

• Type of fat or oil sample
• Mass of the fat or oil sample
• Concentration of the ethanolic KOH solution
• Amount of ethanolic KOH solution added
• Heating and reflux time
• Heating temperature or heating conditions
• Type of titrant used
• Concentration of the titrant
• Type of indicator
• Titration volume in the blank test
• Titration volume in the sample test
• Difference from the blank test
• Saponification value
• Average value from multiple measurements
• Comparison with literature values and other fats and oils
• Causes of error and points for improvement

Reference Experimental Values and Calculation Examples for Saponification-Value Measurement

Here, reference experimental values and calculation examples are organized for measuring the saponification value of fats and oils and discussing its relationship with fatty acid chain length and the properties of fats and oils.

The saponification value is expressed as the number of milligrams of potassium hydroxide, KOH, required to saponify 1 g of fat or oil.
In general, fats and oils containing fatty acids with smaller molecular weights have more ester bonds per unit mass, so a larger amount of KOH is required for saponification and the saponification value becomes higher.

Reference Experimental Conditions

Concept of Saponification-Value Measurement

In saponification-value measurement, an excess amount of KOH is added to the fat or oil and heated to saponify it.
The KOH remaining after the reaction is back-titrated with hydrochloric acid, and the amount of KOH used to saponify the fat or oil is determined from the difference from the blank.

Titration Results for Different Types of Fats and Oils

An example is shown in which different fats and oils were saponified under the same conditions and the remaining KOH was back-titrated with 0.500 mol/L hydrochloric acid.
The blank titration volume is assumed to be 24.80 mL, and the sample mass is 2.000 g in all cases.

Example Calculation of the Difference From the Blank

The difference between the blank titration volume and the sample titration volume corresponds to the amount of KOH used for saponification of the fat or oil.

Difference from blank = Blank titration volume − Sample titration volume

For olive oil, the blank titration volume is 24.80 mL and the sample titration volume is 11.30 mL.

Difference from blank = 24.80 − 11.30 = 13.50 mL

The larger this difference, the more KOH was consumed in the saponification of the fat or oil.

Example Calculation of Saponification Value

The saponification value can be determined using the following equation.

Saponification value (mg KOH/g) = (Blank titration volume − Sample titration volume) × HCl concentration × 56.1 ÷ Sample mass

Here, the titration volume is expressed in mL, the HCl concentration in mol/L, 56.1 is the coefficient corresponding to the molar mass of KOH, and the sample mass is expressed in g.
For olive oil, the difference from the blank is 13.50 mL, the HCl concentration is 0.500 mol/L, and the sample mass is 2.000 g.

Saponification value = 13.50 × 0.500 × 56.1 ÷ 2.000

Saponification value = 189.3 mg KOH/g

Therefore, in this reference example, the saponification value of olive oil is determined to be 189.3 mg KOH/g.

Relationship Between Saponification Value and Fatty Acid Chain Length

The saponification value is related to the average molecular weight of the fatty acids in the fat or oil.
Fats and oils containing shorter fatty acid chains have more fatty acid molecules in the same 1 g, so they also contain more ester bonds and require more KOH for saponification.

In this reference example, coconut oil and butter fat had high saponification values, while olive oil, canola oil, and soybean oil had values around 190.
This is considered to be because coconut oil and butter fat contain relatively large amounts of shorter fatty acids.

Example Calculation for Estimating Average Molecular Weight

If the fat or oil is regarded as a simple triacylglycerol, the approximate average molecular weight of the fat or oil can be estimated from the saponification value.
Three moles of KOH are required to saponify 1 mol of triacylglycerol.

Approximate average molecular weight of the fat or oil = 3 × 56.1 × 1000 ÷ Saponification value

When the saponification value of olive oil is 189.3 mg KOH/g, the approximate average molecular weight is calculated as follows.

Average molecular weight = 3 × 56.1 × 1000 ÷ 189.3 = 889

For coconut oil, because the saponification value is 228.6,

Average molecular weight = 3 × 56.1 × 1000 ÷ 228.6 = 736

In this way, fats and oils with higher saponification values are considered to have smaller average molecular weights and a tendency toward shorter fatty acid chains.

Comparison With Deteriorated and Hydrolyzed Oils

When fats and oils undergo hydrolysis, free fatty acids and partial glycerides increase.
The saponification value is used as an indicator of average fatty acid chain length, but deterioration and hydrolysis may cause the measured value to change slightly.

In oils in which deterioration or hydrolysis has progressed, other indicators such as acid value must also be examined.
It should be noted that saponification value alone cannot sufficiently determine oxidative deterioration or the amount of free fatty acids.

Comparison With Iodine Value

The saponification value is mainly related to the average chain length of fatty acids, while the iodine value is related to the number of double bonds.
Therefore, comparing the two values makes it easier to organize the properties of fats and oils.

For example, coconut oil has a high saponification value and a low iodine value, so it is considered to contain many relatively short fatty acids while having few double bonds.

Example of Checking Reproducibility

The same fat or oil is measured multiple times to check how closely the titration volumes and saponification values agree.
Here, an example is shown in which olive oil was measured three times.

The three saponification values ranged from 188.5 to 190.0 and agreed relatively well.
By standardizing the heating time, titration endpoint, sample mass, and concentration control of the KOH solution, highly reproducible results can be obtained more easily.

Example of How to Write the Results

When the saponification values of various fats and oils were measured, the values were 228.6 mg KOH/g for coconut oil, 216.0 mg KOH/g for butter fat, 189.3 mg KOH/g for olive oil, 185.8 mg KOH/g for canola oil, and 190.7 mg KOH/g for soybean oil.
These results showed that coconut oil and butter fat had higher saponification values than olive oil and canola oil.

For olive oil, the sample titration volume was 11.30 mL compared with a blank titration volume of 24.80 mL, giving a difference of 13.50 mL.
Using a 0.500 mol/L hydrochloric acid standard solution and a sample mass of 2.000 g, the saponification value was calculated to be 189.3 mg KOH/g.

When the approximate average molecular weight was calculated from the saponification value, it was approximately 889 for olive oil and approximately 736 for coconut oil.
Coconut oil, which had the higher saponification value, was considered to have a smaller average molecular weight and to contain a larger proportion of relatively short fatty acids.

Points for Connecting the Results to the Discussion

In a discussion of saponification value, it is important not only to compare the numerical values but also to relate them to fatty acid chain length, average molecular weight, properties of the fat or oil, and differences from other indicators.

• Has the difference between the blank titration volume and sample titration volume been correctly determined?
• Is it understood that the unit of saponification value is mg KOH/g?
• Can it be explained that fats and oils with higher saponification values tend to have shorter average fatty acid chain lengths?
• Can it be explained that fats and oils with lower saponification values tend to have larger average molecular weights?
• Can it be explained that, unlike iodine value, saponification value is not an indicator that directly represents the number of double bonds?
• Can what can and cannot be learned from saponification value be organized by comparison with acid value and peroxide value?
• Can insufficient heating, endpoint judgment in back titration, and moisture absorption or deterioration of the KOH solution be discussed as causes of error?

Example Discussion

In this experiment, the saponification values of fats and oils were measured and their relationship with fatty acid chain length was discussed.
The saponification value of coconut oil was 228.6 mg KOH/g, which was higher than the values of 189.3 mg KOH/g for olive oil and 185.8 mg KOH/g for canola oil.
Because the saponification value is the number of milligrams of KOH required to saponify 1 g of fat or oil, a higher value indicates that more ester bonds are present in the same mass.

When fats and oils of the same mass are compared, shorter fatty acid chains result in smaller molecular weights and therefore a greater number of molecules, increasing the amount of KOH required for saponification.
Therefore, coconut oil and butter fat, which had high saponification values, are considered to contain relatively large amounts of shorter fatty acids.
On the other hand, olive oil and canola oil had saponification values around 190 and were considered to be close to the values of common vegetable oils composed mainly of long-chain fatty acids.

When the approximate average molecular weights were calculated from the saponification values, the values were approximately 736 for coconut oil and approximately 889 for olive oil.
This result is consistent with the relationship that fats and oils with higher saponification values have smaller average molecular weights.
However, because actual fats and oils are mixtures composed of multiple fatty acids, these average molecular weights must be treated only as approximate values.

In comparison with iodine value, coconut oil had a high saponification value but a low iodine value.
This indicates that coconut oil contains many short- to medium-chain fatty acids but has few double bonds.
Therefore, the saponification value is an indicator related to fatty acid chain length and average molecular weight and has a different meaning from the iodine value, which evaluates degree of unsaturation.

Possible causes of error include insufficient heating during saponification, changes in the concentration of the KOH solution, deviations in endpoint determination during back titration, and errors in weighing the sample.
In particular, because KOH readily absorbs moisture and carbon dioxide, its concentration is likely to change, so it is important to perform a blank test and compare the sample under the same conditions.

Summary

The saponification value is an indicator representing the amount of KOH required to saponify 1 g of fat or oil.
The amount of KOH consumed in saponification of the fat or oil is determined from the difference between the blank titration volume and sample titration volume and calculated in mg KOH/g.

In this reference example, coconut oil and butter fat had high saponification values, while vegetable oils had values around 190.
In a report, it is useful to discuss the saponification value, fatty acid chain length, average molecular weight, differences from iodine value, and causes of error in relation to one another.

Principle of the Saponification Reaction

Triglycerides, the main components of fats and oils, are compounds in which three fatty acids are ester-linked to glycerol.
When KOH is added and the mixture is heated, the ester bonds are hydrolyzed, producing glycerol and potassium salts of fatty acids.
Potassium salts of fatty acids correspond to components of soap.

Because one triglyceride molecule normally contains three ester bonds, 3 mol of KOH are required per molecule for complete saponification.
However, actual fats and oils are mixtures of fatty acids and do not have a constant molecular weight.
Therefore, the saponification value represents an average property of the entire fat or oil.

Triglyceride + 3KOH → Glycerol + Potassium salts of fatty acids

Relationship Between Fatty Acid Chain Length and Saponification Value

The saponification value is closely related to the average chain length of the fatty acids that make up the fat or oil.
In fats and oils containing shorter fatty acid chains, the molecular weight of each triglyceride molecule becomes smaller.
Because more molecules are contained in the same 1 g of fat or oil, more ester bonds are also present and the amount of KOH required for saponification increases.

In contrast, fats and oils containing longer fatty acid chains have larger triglyceride molecular weights.
Because fewer molecules are contained in the same 1 g, fewer ester bonds are present and less KOH is consumed.
Therefore, in general, fats and oils with shorter fatty acid chains have higher saponification values, while fats and oils with longer fatty acid chains have lower saponification values.

Discussion of Fats and Oils With High Saponification Values

Fats and oils with high saponification values may contain relatively large amounts of short- or medium-chain fatty acids.
The more triglycerides with small molecular weights are present, the more molecules are contained in the same mass and the more KOH is required for saponification.
Therefore, a high saponification value provides a clue for considering the fatty acid composition of the fat or oil.

However, the types of fatty acids cannot be completely identified from the saponification value alone.
Fats and oils are mixtures composed of multiple fatty acids, and degree of unsaturation, free fatty acids, oxidation products, and other factors may also affect the measured value.
The saponification value should be treated only as an indicator for estimating average molecular weight and chain length.

Discussion of Fats and Oils With Low Saponification Values

Fats and oils with low saponification values may contain relatively large amounts of long-chain fatty acids.
Triglycerides containing long-chain fatty acids have large molecular weights, so fewer molecules are present in the same 1 g.
As a result, the total number of ester bonds being saponified is smaller and less KOH is consumed.

A low saponification value may indicate that the molecular weight of the fat or oil is large, but experimental errors can also cause the value to appear low.
For example, if saponification is incomplete, KOH consumption becomes smaller than it should be and the saponification value is underestimated.
Therefore, the reaction conditions must also be considered when interpreting the value.

Relationship With the Molecular Weight of Fats and Oils

The saponification value can also serve as an indicator for considering the average molecular weight of fats and oils.
When one triglyceride molecule is saponified, it basically consumes 3 mol of KOH.
Therefore, when more KOH is consumed by the same 1 g of fat or oil, more fat or oil molecules are present and the average molecular weight is considered to be smaller.

Conversely, when less KOH is consumed, fewer molecules are contained in the same 1 g and the average molecular weight is considered to be larger.
From this relationship, the saponification value is used to compare the average chain length and molecular weight of the fatty acids that make up fats and oils.
However, it must be noted that actual fats and oils are mixtures rather than single compounds.

Difference From Degree of Unsaturation

The saponification value is mainly related to fatty acid chain length and molecular weight and does not directly indicate the degree of unsaturation of fats and oils.
Iodine value is commonly used as an indicator for evaluating degree of unsaturation.
The iodine value reflects the number of double bonds and has a different meaning from the saponification value.

For example, even if fats and oils have similar fatty acid chain lengths, their saponification values may not differ greatly even when one contains many saturated fatty acids and the other contains many unsaturated fatty acids.
In contrast, iodine value is strongly affected by the amount of unsaturated bonds.
In a report, it is important not to confuse saponification value with iodine value.

Difference From Acid Value

In analysis of fats and oils, acid value is also commonly measured in addition to saponification value.
Acid value represents the amount of KOH required to neutralize the free fatty acids contained in the fat or oil.
In contrast, saponification value includes the amount of KOH required to hydrolyze the ester bonds in the fat or oil.
In other words, acid value and saponification value evaluate different targets.

Fats and oils with high acid values may contain increased amounts of free fatty acids because of hydrolysis or deterioration.
Saponification value reflects the average molecular weight and fatty acid chain length of the fat or oil.
Therefore, acid value is important when considering deterioration of fats and oils, while saponification value is important when considering fatty acid chain length and molecular weight.

Why Ethanolic KOH Solution Is Used

Because fats and oils are poorly soluble in water, they may not come into sufficient contact with KOH in an aqueous solution.
Therefore, an ethanolic KOH solution may be used to create conditions under which both the fat or oil and KOH can react more readily.
Ethanol helps bring the fat or oil and alkali into contact and promotes the saponification reaction.

However, ethanolic KOH solution may absorb CO2 from the air or undergo changes in concentration.
Therefore, storage conditions and standardization after preparation are important.
If the concentration is inaccurate, systematic errors occur in the calculation of the saponification value.

Why Heating Under Reflux Is Performed

The saponification reaction may not proceed sufficiently rapidly simply by mixing fats and oils with KOH.
Therefore, the reaction is promoted by heating under reflux.
Reflux allows the reaction to proceed while heating for a fixed period without losing the solvent through evaporation.

If the heating time is too short, the fat or oil is not completely saponified and less KOH is consumed.
As a result, the saponification value is determined to be too low.
On the other hand, if the solution splashes during heating or the reflux is insufficient and solvent is lost, errors may occur in the concentration and titration volume.

Concept of Back Titration

In saponification-value measurement, an excess amount of KOH is commonly added to the fat or oil to carry out the saponification reaction, and the KOH remaining after the reaction is titrated with an acid standard solution.
This is back titration.
Instead of directly measuring the amount of KOH consumed by the fat or oil, the consumption is determined from the difference between the amount of KOH initially added and the amount remaining.

Back titration is used because the saponification reaction of fats and oils is not a reaction that immediately shows an endpoint and instead proceeds over time while being heated.
Adding excess KOH makes it easier to completely saponify the fat or oil, after which the remaining amount can be accurately titrated.
Comparison with a blank test is important.

Importance of the Blank Test

A blank test is an operation in which the same procedure is performed without adding the fat or oil sample to determine how much of the ethanolic KOH solution is neutralized by the acid.
The titration volume in the blank test corresponds to the amount of KOH initially added.
In the sample test, KOH is consumed in saponification of the fat or oil, so the amount of KOH remaining is smaller than in the blank test.

Without a blank test, the amount of KOH consumed by the fat or oil cannot be accurately determined.
In addition, changes in the concentration of the KOH solution and effects originating from the reagents can also be corrected using the blank test.
Because the saponification value is calculated from the difference between the blank test and sample test, the accuracy of the blank test is extremely important.

Discussion of Saponification-Value Calculations

The saponification value is calculated as the number of milligrams of KOH consumed per 1 g of fat or oil.
The amount of KOH used to saponify the fat or oil is determined from the difference between the titration volumes in the blank test and sample test.
Dividing this amount by the mass of the fat or oil sample gives the KOH consumption per 1 g of fat or oil.

In the calculation, the concentration of the titrant, titration volume, formula weight of KOH, sample mass, and unit conversions must be handled correctly.
When the sample mass is small, even a small weighing error greatly affects the saponification value.
Care must also be taken to avoid mistakes in conversions between mL and L and between g and mg.

Saponification value = Number of milligrams of KOH required to saponify 1 g of fat or oil

Differences Among Types of Fats and Oils

The saponification value of fats and oils differs depending on the type of fat or oil, such as vegetable oil, animal fat, milk fat, fish oil, or coconut oil.
Fats and oils containing large amounts of short- or medium-chain fatty acids tend to have higher saponification values, while fats and oils containing large amounts of long-chain fatty acids tend to have lower values.
Therefore, comparing types of fats and oils allows differences in fatty acid composition to be discussed.

However, actual fats and oils contain many different types of fatty acids.
Saponification value does not directly identify individual fatty acids and instead reflects differences in average molecular weight and chain length.
To determine the fatty acid composition in greater detail, another analysis such as gas chromatography may be necessary.

Effects of Old or Deteriorated Fats and Oils

Fats and oils may undergo oxidation and hydrolysis during storage.
When hydrolysis progresses, free fatty acids increase and the acid value may become higher.
The saponification value is mainly related to the amount of ester bonds, but changes in the components of the fat or oil caused by deterioration may affect the measured value.

Oxidized fats and oils may contain peroxides and decomposition products that affect titration and endpoint determination.
In addition, if volatile components or low-molecular-weight decomposition products increase, the properties of the fat or oil change.
When deteriorated fats and oils are measured, it is useful to discuss the results together with other indicators such as acid value and peroxide value.

Discussion When Saponification Is Incomplete

When saponification is incomplete, some ester bonds in the fat or oil remain unreacted.
In this case, KOH consumption becomes smaller than the actual amount required and the saponification value is underestimated.
Possible causes include insufficient heating time, insufficient temperature, insufficient stirring, incomplete dissolution of the fat or oil, and insufficient KOH.

The saponification reaction proceeds when the fat or oil and alkali come into sufficient contact.
If the fat or oil remains attached to the wall of the flask or the solution is not uniform, the reaction becomes difficult to proceed.
The mixing condition during heating under reflux also affects the result.

Discussion of Endpoint Determination

In saponification-value measurement, the KOH remaining after the reaction is titrated with acid, so the color change of the indicator must be accurately judged.
When phenolphthalein is used, the solution is red under basic conditions and becomes colorless as it approaches the acidic side.
If the endpoint judgment is shifted, the calculation of the remaining KOH amount is also shifted and the saponification value is directly affected.

If too much acid is added beyond the endpoint, it may cause the amount of remaining KOH to be overestimated or underestimated.
In addition, if the solution is cloudy because of fats, oils, or soap components, the color change may be difficult to observe.
Near the endpoint, the titrant is added one drop at a time while the solution is mixed thoroughly and observed.

Error in Sample Amount

Because the saponification value is expressed as KOH consumption per 1 g of fat or oil, weighing errors in the sample amount greatly affect the result.
If the sample mass is recorded as smaller than the actual value, the saponification value is calculated as higher.
Conversely, if the sample mass is recorded as larger, the saponification value is calculated as lower.

Fats and oils may be highly viscous and tend to adhere to equipment.
If part of the sample remains during transfer from the weighing container to the flask, the amount of fat or oil that actually reacts may differ from the recorded value.
It is important to accurately weigh the sample and avoid losses during transfer.

Concentration Error of the KOH Solution

If the concentration of the KOH solution is inaccurate, an error occurs in the calculation of the saponification value.
KOH is hygroscopic and may absorb CO2 from the air to form carbonate.
Therefore, the concentration of the KOH solution may change over time.

If the KOH concentration is actually higher or lower than expected, the calculation of the amount of KOH consumed by the fat or oil is also shifted.
In addition, if the concentration of the acid standard solution is inaccurate, an error also occurs in the amount of remaining KOH determined by back titration.
Standardization and storage conditions of the standard solutions are important.

Causes of Error in Saponification-Value Measurement

Causes of error in saponification-value measurement include errors in weighing the sample, concentration errors in the ethanolic KOH solution, concentration errors in the acid standard solution, insufficient heating, poor reflux, insufficient stirring, incomplete saponification, endpoint-determination errors, variation in the blank test, splashing of the solution, and adhesion of the fat or oil.
Because the saponification value is determined from differences among many operations, errors at each stage accumulate.

Causes that lower the value include incomplete saponification, incomplete transfer of the fat or oil sample to the reaction vessel, and underestimation of KOH consumption.
Causes that raise the value include deviations in endpoint determination, underrecording of the sample mass, and errors in the blank test or titration values.
Organizing the causes into overestimation and underestimation makes the discussion easier.

When the Results Can Be Considered Good

Saponification-value measurements can be considered to have produced good results when the titration values in the blank and sample tests are stable, variation among multiple saponification-value measurements is small, and the results do not contradict the trends expected from the types of fats and oils.
It is also important that heating under reflux was sufficient, the reaction solution was uniform, and the endpoint was clear.

If the saponification value falls within a reasonable range when compared with literature values and other fats and oils, the measurement procedure can be considered generally appropriate.
However, exact agreement with literature values is not necessary, because differences may arise from the place of origin, variety, degree of refining, and storage condition of the fat or oil.

Example Discussion When the Experiment Did Not Go Well

When saponification-value measurement does not go well, possible causes are considered from results such as variation in titration values, an excessively small difference from the blank test, a saponification value far from literature values, a nonuniform reaction solution, or difficulty observing the endpoint.
Organizing the causes according to sample amount, heating under reflux, KOH solution, titration, blank test, and calculation makes the discussion easier.

How to Write Points for Improvement

In a discussion of saponification-value measurement, 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 weighing, saponification reaction, titration operation, standard solutions, and calculation and analysis.

Improvements to Sample and Reaction Operations

• Accurately weigh the fat or oil sample
• Completely transfer the fat or oil to the reaction vessel
• Accurately add the ethanolic KOH solution
• Follow the specified heating and reflux time
• Mix the reaction solution sufficiently
• Prevent the fat or oil from remaining on the flask wall
• Prevent splashing of the solution and evaporation of the solvent

Improvements to Titration and Standard Solutions

• Store the KOH solution in a tightly sealed container
• Standardize the KOH solution as necessary
• Confirm the concentration of the acid standard solution
• Perform the blank test under the same conditions
• Remove air bubbles from the burette
• Add the titrant one drop at a time near the endpoint
• Carefully observe the indicator color change

Improvements to Calculation and Analysis

• Correctly use the difference between the titration volumes in the blank and sample tests
• Accurately reflect the sample mass in the calculation
• Check the conversion to the number of milligrams of KOH
• Check unit conversions
• Perform multiple measurements and calculate the average value
• Compare with literature values and other fats and oils
• Distinguish the meanings of saponification value, acid value, and iodine value

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of saponification-value measurement of fats and oils, simply writing that “the saponification value was high” or “the fat or oil was saponified” results in a superficial discussion.
A good discussion relates KOH consumption, fatty acid chain length, molecular weight of the fat or oil, blank testing, incomplete saponification, and titration errors.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of saponification-value measurements of fats and oils.
Adjust the necessary parts according to your own experimental results.

• The saponification value is the number of milligrams of KOH required to saponify 1 g of fat or oil.
• Triglycerides in fats and oils are hydrolyzed by KOH to produce glycerol and potassium salts of fatty acids.
• One triglyceride molecule normally contains three ester bonds and requires 3 mol of KOH for saponification.
• Fats and oils with shorter fatty acid chains contain more molecules in the same mass and tend to have higher saponification values.
• Fats and oils with longer fatty acid chains have larger average molecular weights and tend to have lower saponification values.
• Saponification value does not directly indicate degree of unsaturation; degree of unsaturation is evaluated using iodine value.
• Acid value indicates the amount of free fatty acids and has a different meaning from saponification value.
• If saponification is incomplete, KOH consumption decreases and the saponification value may be underestimated.
• The amount of KOH consumed by the fat or oil is determined from the difference between the titration volumes in the blank and sample tests.
• Changes in KOH-solution concentration and deviations in endpoint determination are causes of error in saponification-value measurement.

Points to Check When Discussing the Saponification Value of Fats and Oils

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

• Is the definition of saponification value explained?
• Is the saponification reaction of fats and oils written?
• Is the reaction ratio between triglycerides and KOH understood?
• Is the relationship between fatty acid chain length and saponification value explained?
• Is the relationship with the average molecular weight of the fat or oil considered?
• Are saponification value, acid value, and iodine value distinguished?
• Is the reason for using ethanolic KOH solution described?
• Is the necessity of heating under reflux explained?
• Is the meaning of the blank test explained?
• Is the possibility of incomplete saponification considered?
• Are errors in titration and standard-solution concentration considered?
• Do the points for improvement correspond to the causes of error?

Summary

The saponification value of fats and oils is a value representing the number of milligrams of KOH required to completely saponify 1 g of fat or oil.
Triglycerides in fats and oils are hydrolyzed by KOH to produce glycerol and potassium salts of fatty acids.
From the amount of KOH consumed in this reaction, the amount of ester bonds and average molecular weight of the fat or oil can be discussed.

In general, fats and oils containing shorter fatty acid chains have smaller average molecular weights and more molecules in the same 1 g, so their saponification values tend to be higher.
Conversely, fats and oils containing longer fatty acid chains have larger average molecular weights and fewer molecules in the same 1 g, so their saponification values tend to be lower.
However, fatty acid composition cannot be completely identified from the saponification value alone.

In a report, rather than simply writing that “the saponification value was high or low,” organize and discuss the principle of the saponification reaction, fatty acid chain length, average molecular weight of the fat or oil, KOH consumption, blank testing, heating under reflux, incomplete saponification, and titration errors.
For saponification value, it is important to understand the connection between the molecular structure of fats and oils and titration procedures in analytical chemistry.