Glacial acetic acid is a common name for pure or anhydrous acetic acid . It consists of acetic acid with a minimum purity of 99.8% containing a very low proportion of water or other impurities. The term glacial is used to distinguish it from ordinary acetic acid, by which we generally refer to aqueous solutions of this compound.
Acetic acid is an organic compound belonging to the group of carboxylic acids. This family of organic acids are all weak acids, and their properties strongly depend on their structure.
The word glacial comes from the fact that it has a melting point of 16.6 °C, so it solidifies or freezes at relatively high temperatures, forming ice-like crystals.
Structure and nomenclature of acetic acid
Acetic acid is the common name for a two-carbon carboxylic acid. In this sense, its systematic name according to the nomenclature rules of the International Union of Pure and Applied Chemistry (IUPAC, for its acronym in English) is ethanoic acid .
The common name “acetic” comes from the fact that ethanoic acid is, after water, the main component of vinegar, whose Latin name is acetum . Despite being a common name, it is so widespread and in such daily use throughout science that the IUPAC accepts its use as the official name of the compound.
The molecular formula of acetic acid is CH 3 COOH. Its structure is shown in the following figure:
Characteristics of glacial acetic acid
- It is a clear, colorless liquid at room temperature but easily freezes in cold weather.
- It has a very intense and penetrating smell and taste of vinegar, since it corresponds to the main active component of this dressing that is ubiquitous throughout the world.
- Its melting point is 16.6 °C. Below this temperature it is a white crystalline solid.
- Its boiling point is 118 °C, slightly higher than that of water.
- It is a water-miscible liquid, which means that water-glacial acetic acid mixtures can be prepared in any ratio.
- It is a weak acid with an acid ionization constant of K a = 1.75.10 -5 .
How is glacial acetic acid obtained?
There are two classes of processes used to produce acetic acid. The first class is the fermentation of sugars using different types of microorganisms such as yeasts and bacteria; the second consists of the different types of industrial chemical synthesis.
Fermentation
Fermentation is obviously the oldest way to obtain acetic acid, since vinegar, which is nothing more than an aqueous solution of this compound, has been produced this way for hundreds of years. However, this process does not produce glacial acetic acid but ordinary acetic acid which, at most, has a concentration of about 10% by mass. However, this type of solutions can be distilled to obtain more concentrated solutions, although it is difficult to obtain the purity required to be considered glacial acetic acid.
industrial synthesis
Currently, most of the world’s acetic acid is produced industrially through chemical reactions. There are several different reactions that give acetic acid as the main product and that have been used to produce this organic acid. Some examples are the oxidation of acetaldehyde, the oxidation of hydrocarbons in the liquid phase, and the carbonylation of methanol.
In addition to being much faster processes than biosynthesis by fermentation, one of the benefits of industrial synthesis is that it allows the synthesis of acetic acid in the absence of water. Consequently, the purification process of the finished product is much simpler and it is easier to obtain glacial acetic acid.
Glacial acetic acid stalactite formation
A third ingenious way to obtain glacial acetic acid starts from an aqueous solution of acetic acid, which is cooled below 16.6°C (its melting point) and allowed to evaporate slowly until crystals of the acid begin to form. . Thereafter, if we let a cold, concentrated solution of acetic acid drip from these crystals, a part of the acetic acid will crystallize while the rest will drip with water.
Since the crystallization process, if done slowly, incorporates only pure acetic acid molecules, this is a very effective form of acetic acid purification. Then it is enough to separate the stalactites of pure acetic acid in the solid state, dry them well and then melt them again to obtain glacial acetic acid.
Differences between glacial and ordinary acetic acid
Differences in composition
As can be seen, both glacial acetic acid and ordinary acetic acid refer to either ethanoic acid or the two-carbon carboxylic acid. However, the difference lies in the purity and in the presence or not of water. Glacial acetic acid corresponds to said purified and dehydrated compound, devoid of water and practically any other chemical substance. That is to say, that the term glacial is added with the purpose of indicating that it is a pure chemical compound.
On the other hand, when we talk about ordinary acetic acid (or simply acetic acid, dry), we can refer to both the chemical compound itself, or to any mixture or aqueous solution of acetic acid. That is, we do not specify whether it is a pure substance or a mixture.
Differences in application
Most organic synthesis processes require the absence of water for different reasons. In many cases, the water can react with some of the reactants, with the catalyst, or even with the product of the reaction, considerably reducing the yield. Additionally, many organic reactions are carried out in nonpolar solvents that are immiscible (ie, cannot mix) with water. This makes the use of aqueous solutions of acetic acid not convenient because the separation of two phases (the aqueous phase and the organic phase) occurs, in which most of the acetic acid remains distributed in the aqueous phase; therefore, it is not available to react in the organic solvent. In these cases, not any presentation of acetic acid serves as a reagent,
Differences in danger
Although acetic acid is a weak acid and its solutions are safe even to consume, that is not the case with glacial acetic acid. Because it is practically pure, we will not get acetic acid that is more concentrated than glacial, which is why it can become irritating to the skin.
References
Glacial acetic acid . (2017, August 11). chempedia. https://www.vadequimica.com/quimipedia/a/acido-acetico-glacial/
Chang, R. (2021). Chemistry ( 11th ed.). MCGRAW HILL EDUCATION.
Ministry of Labour, Migrations and Social Security. (2018). TOXICOLOGICAL DOCUMENTATION FOR THE VL UPDATE OF THE OCCUPATIONAL EXPOSURE LIMIT OF ACETIC ACID . Documentation Limits Professional Exposure. https://www.insst.es/documents/94886/431980/DLEP+119+%C3%81cido+ac%C3%A9tico++A%C3%B1o+2018.pdf/1d5b5a9a-4438-4105-8b77- 3e68196f2701?version=1.0&t=1551310408920
Sánchez Levoso, A. (2016, June). Modeling of the Acetic Acid Extraction Process with Recovery of Organic Solvent . Polytechnic University of Madrid. https://oa.upm.es/42845/1/TFG_ANA_SANCHEZ_LEVOSO.pdf
Wolfram. (nd). Separation of Acetic Acid from Water Using Ethyl Acetate Entrainer . Wolfram Notebook. https://www.wolframcloud.com/objects/demonstrations/SeparationOfAceticAcidFromWaterUsingEthylAcetateEntrainer-source.nb
