The key difference between acid value and saponification value is that acid value gives the mass of potassium hydroxide that is required to neutralize one gram of a chemical substance whereas saponification value gives the mass of potassium hydroxide required to saponify one gram of fat.
Although acid value and saponification value are different from each other, both these values are given as a mass (of potassium hydroxide). Moreover, these terms give the value in the unit milligrams.
CONTENTS
1. Overview and Key Difference
2. What is Acid Value
3. What is Saponification Value
4. Side by Side Comparison – Acid Value vs Saponification Value in Tabular Form
5. Summary
What is Acid Value?
Acid value is the mass in milligrams of potassium hydroxide that is required to neutralize one gram of a chemical substance. In other words, this value determines the amount of acid in the substance. Generally, this value is used as a measure of the amount of carboxylic acid groups in a chemical compound – E.g. fatty acid.
The determination of acid value includes dissolving a known amount of a sample in an organic solvent (usually we use isopropanol as the solvent) followed by titration with a solution of potassium hydroxide. Here, the potassium hydroxide solution should have a known concentration, and the indicator for this titration is phenolphthalein. The color change is colorless to pink.
Figure 01: Color Change at Endpoint of the Titration
The acid number can be used to quantify the acidity of a substance such as biodiesel. The chemical equation for this determination is as follows: Veq is the volume of potassium hydroxide required for the titration that reacts with the sample and 1 mL of spiking solution at the equivalence point, beq is the volume of titrant reacted with 1 mL of spiking solution at the equivalence point. 56.1 g/mol is the molar mass of potassium hydroxide, and Woil is the mass of the sample in grams.
AN=( Veq – beq)N{56.1/ Woil}
What is Saponification Value?
Saponification value is the mass in milligrams of potassium hydroxide required to saponify one gram of fat under specific conditions. This value measures the average molecular weight of all the fatty acids present in a sample.
Figure 02: Saponification
This value allows us to compare the average fatty acid chain length. Actually, this value determines the amount of carboxylic acid groups per fatty acid chain. E.g. a low saponification value of a certain fat indicates that this fat contains a fewer number of carboxylic functional groups per unit mass of that fat compared to short-chain fatty acids.
What is the Difference Between Acid Value and Saponification Value?
Acid value and saponification value are masses. The key difference between acid value and saponification value is that the acid value gives the mass of potassium hydroxide that is required to neutralize one gram of a chemical substance whereas saponification value gives the mass of potassium hydroxide required to saponify one gram of fat. Therefore, acid value determines the acidity of a particular substance while saponification value determines the amount of ester linkages in a fat.
Below infographic summarizes the differences between acid value and saponification value.
Summary – Acid Value vs Saponification Value
Acid value and saponification value are masses. The key difference between acid value and saponification value is that the acid value gives the mass of potassium hydroxide that is required to neutralize one gram of a chemical substance whereas saponification value gives the mass of potassium hydroxide required to saponify one gram of fat.
Reference:
1. “Acid Value.” ScienceDirect Topics, Available here.
2. “Saponification Value.” Wikipedia, Wikimedia Foundation, 29 Feb. 2020, Available here.
Image Courtesy:
1. “Titolazione” By Luigi Chiesa – (Public Domain) via Commons Wikimedia
2. “Saponification triglyceride-fr” By Pinpin – travail personnel from french Wikipedia (CC BY-SA 3.0) via Commons Wikimedia