Acids

Infobox on Acids
Example of Acids
Facts
Origin	Various
Stowage factor (in m3/t)	-
Humidity / moisture	-
Ventilation	-
Risk factors	See text

Acids

Description

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH under 7, with acidity increasing the lower the pH. Chemicals or substances having the property of an acid are said to be acidic.

Common examples of acids include acetic acid (in vinegar), sulfuric acid (used in car batteries), and tartaric acid (used in baking). As these three examples show, acids can be solutions, liquids, or solids. Gases such as hydrogen chloride can be acids as well. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.

There are three common definitions for acids: the Arrhenius definition, the Brønsted-Lowry definition, and the Lewis definition. The Arrhenius definition states that acids are substances which increase the concentration of hydronium ions (H3O+) in solution. The Brønsted-Lowry definition is an expansion: an acid is a substance which can act as a proton donor. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, and these two definitions are most relevant. The reason why pHs of acids are less than 7 is that the concentration of hydronium ions is greater than 10−7 moles per liter. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acids thus have pHs of less than 7. By the Brønsted-Lowry definition, any compound which can easily be deprotonated can be considered an acid. Examples include alcohols and amines which contain O-H or N-H fragments.

In chemistry, the Lewis definition of acidity is frequently encountered. Lewis acids are electron-pair acceptors. Examples of Lewis acids include all metal cations, and electron-deficient molecules such as boron trifluoride and aluminium trichloride. Hydronium ions are acids according to all three definitions. Interestingly, although alcohols and amines can be Brønsted-Lowry acids as mentioned above, they can also function as Lewis bases due to the lone pairs of electrons on their oxygen and nitrogen atoms.

Acids may be solid, liquid or gaseous, and are usually classified as Inorganic or Mineral Acids and Organic Acids.

Generally speaking all mineral acids are more or less corrosive, as are many organic acids. Some organic acids are completely harmless.

Corrosive acids are those which readily attack other substances, especially metal and timber, and will cause considerable damage should leakage occur. Such acids should never be handled as they are also corrosive to skin and other human tissue. Poisonous acids are those which are poisonous in themselves or evolve poisonous fumes, which are a danger to life.

Reference is made to the relevant IMO regulations of hazardous cargo.

Applications

There are numerous uses for acids. Acids are often used to remove rust and other corrosion from metals in a process known as pickling. They may be used as an electrolyte in a wet cell battery, such as sulfuric acid in a car battery.

Strong acids, sulfuric acid in particular, are widely used in mineral processing. For example, phosphate minerals react with sulfuric acid to produce phosphoric acid for the production of phosphate fertilizers, and zinc is produced by dissolving zinc oxide into sulfuric acid, purifying the solution and electrowinning.

In the chemical industry, acids react in neutralization reactions to produce salts. For example, nitric acid reacts with ammonia to produce ammonium nitrate, a fertilizer. Additionally, carboxylic acids can be esterified with alcohols, to produce esters.

Acids are used as additives to drinks and foods, as they alter their taste and serve as preservatives. Phosphoric acid, for example, is a component of cola drinks. Acetic acid is used in day to day life as vinegar. Carbonic acid is an important part of some cola drinks and soda. Citric acid is used as a preservative in sauces and pickles.

Tartaric acid is an important component of some commonly used foods like unripened mangoes and tamarind. Natural fruits and vegetables also contain acids. Citric acid is present in oranges, lemon and other citrus fruits. Oxalic acid is present in tomatoes, spinach, and especially in carambola and rhubarb; rhubarb leaves and unripe carambolas are toxic because of high concentrations of oxalic acid.

Ascorbic acid (Vitamin C) is an essential vitamin required in our body and is present in such foods as amla, lemon, citrus fruits, and guava.

Certain acids are used as drugs. Acetylsalicylic acid (Aspirin) is used as a pain killer and for bringing down fevers.

Acids play very important roles in the human body. The hydrochloric acid present in our stomach aids in digestion by breaking down large and complex food molecules. Amino acids are required for synthesis of proteins required for growth and repair of our body tissues. Fatty acids are also required for growth and repair of body tissues. Nucleic acids are important for the manufacturing of DNA, RNA and transmission of characters to offspring through genes. Carbonic acid is important for maintenance of pH equilibrium in the body.

Shipment/storage

Acids are (mostly) categorised as Dangerous Goods.

Dangerous substances are classified according to their main characteristics and properties into 9 classes of the IMDG Code (International Maritime Dangerous Goods Code). Some substances have properties fitting them for inclusion in more than one Class: these have been placed in the class appropriate to the most dangerous property when carried in ships.

The IMDG Code provides an index which lists all the substances. It should not, however, be assumed that if a particular item is omitted it is either non-hazardous or forbidden for shipment. Non-inclusion of any substance possessing dangerous properties does not relieve the shipper from responsibility for declaring the nature of the substance, and under common law for any damage caused through default.

In the IBC Code, dangerous chemicals in bulk are listed with standards and guidelines on board ships.

Additionally, the Material Safety Data Sheet(s) should be consulted for chemical product identification, composition and information on ingredients, hazard identification, handling, storage and transport information etc.

Reference is also made to the Merchant Shipping (Dangerous Goods) Rules.

Note: No (packaged) dangerous goods shall be taken on board any ship to which these Rules apply for carriage in that ship unless the shipper of the goods has furnished the shipowner or master of the ship with a dangerous goods declaration. Such declaration shall indicate with the correct technical name, the identity of the goods and the United Nations number (whenever such a number exists) and shall indicate to which of the classes the goods belong.

Risk factors

Refer to the applicable IMDG/IBC Codes, Material Safety Data Sheets and Merchant Shipping (Dangerous Goods)Rules.