In chemistry, the terms volatile acid and volatile acidity (VA) are used somewhat differently in various application areas.

Wine

wine chemistry Wine is a complex mixture of chemical compounds in a hydro-alcoholic solution with a pH around 4. The chemistry of wine and its resultant quality depend on achieving a balance between three aspects of the berries used to make the wine: their sugar ...

, the volatile acids are those that can be separated from wine through steam distillation . Many factors influence the level of VA, but the growth of spoilage bacteria and yeasts are the primary source and consequently VA is often used to quantify the degree of wine oxidation and spoilage .

Acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

is the primary volatile acid in wine, but smaller amounts of lactic, formic, butyric,

propionic acid Propionic acid (, from the Greek words πρῶτος : ''prōtos'', meaning "first", and πίων : ''píōn'', meaning "fat"; also known as propanoic acid) is a naturally occurring carboxylic acid with chemical formula CH3CH2CO2H. It is a liq ...

, carbonic acid (from carbon dioxide), and

sulfurous acid Sulfurous acid (also sulfuric(IV) acid, sulphurous acid (UK), sulphuric(IV) acid (UK)) is the chemical compound with the formula . There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase. ...

(from sulfur dioxide) may be present and contribute to VA ; in analysis, measures may be taken to exclude or correct for the VA due to carbonic, sulfuric, and sorbic acids . Other acids present in wine, including malic and

tartaric acid Tartaric acid is a white, crystalline organic acid that occurs naturally in many fruits, most notably in grapes, but also in bananas, tamarinds, and citrus. Its salt, potassium bitartrate, commonly known as cream of tartar, develops naturally i ...

are considered ''non-volatile'' or ''fixed acids''. Together volatile and non-volatile acidity compromise total acidity . Classical analysis for VA involves distillation in a Cash or Markham still, followed by titration with standardized

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

, and reporting of the results as acetic acid . Several alternatives to the classical analysis have been developed. While VA is typically considered a wine flaw or fault, winemakers may intentionally allow a small amount of VA in their product for its contribution to the wine's sensory complexity . Excess VA is difficult for winemakers to correct . In the some countries, including the United States, European Union, and Australia, the law sets a limit on the level of allowable VA .

Wastewater

wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environme ...

, the volatile acids are the short chain

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

s (1-6 carbon atoms) that are water soluble and can be steam distilled at atmospheric pressure - primarily acetic, proprionic, and butyric acid . These acids are produced during

anaerobic digestion Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the ferm ...

. In a well functioning digester, the volatile acids will be consumed by the methane forming bacteria . Volatile acid/

alkalinity Alkalinity (from ar, القلوي, al-qaly, lit=ashes of the saltwort) is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of ...

ratio is often measured as one indicator of a digester's condition .

Physiology

In physiology, volatile acid (or ''respiratory acid'') refers to carbonic acid, a product of dissolved carbon dioxide. In this context, ''volatile'' indicates that it can be expelled as a gas through the lungs . Carbonic acid is the only physiologically volatile acid; all other acids are physiologically

nonvolatile acid A nonvolatile acid (also known as a fixed acid or metabolic acid) is an acid produced in the body from sources other than carbon dioxide, and is not excreted by the lungs. They are produced from e.g. an incomplete metabolism of carbohydrates, fats ...

s (also known as a ''fixed'' or ''metabolic acids''). Volatile acid results from the aerobic oxidation of substances such as carbohydrates and fatty acids .

Butter

Volatile acid concentration can be used to detect adulteration of

butter Butter is a dairy product made from the fat and protein components of churned cream. It is a semi-solid emulsion at room temperature, consisting of approximately 80% butterfat. It is used at room temperature as a spread, melted as a condiment ...

with less expensive fats.

Butterfat Butterfat or milkfat is the fatty portion of milk. Milk and cream are often sold according to the amount of butterfat they contain. Composition Butterfat is mainly composed of triglycerides. Each triglyceride contains three fatty acids. Butt ...

has uncommonly high levels of volatile butyric and caproic acids, and mixing with fats from other sources dilutes the volatile acids. A measurement of the volatile acids is known as the Reichert Meissel value .

Industrial hygiene

In workplace air samples, concentrations of

hydrochloric Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digestiv ...

, hydrobromic, and

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

may be monitored as hazardous volatile acids .

References