Acids
12/12/2016
Acids: these are a group of chemicals known for their caustic nature. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water. Acid comes directly from the Latin acidus, which means sour. Probably the most obvious example of understanding this “sour” characteristic is through tasting vinegar – a dilute form of acetic acid.
Acids are often used to remove rust and other corrosion from metals: a process known as pickling. Acids 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, while zinc is produced by dissolving zinc oxide into sulfuric acid: a process known as electrowinning or electroextraction.
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 beesterified with alcohols, to produce esters.
Acids are used as additives to drinks and foods, as they can modify 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 in many types of soft drinks. Citric acid is used as a preservative in sauces and pickles.
Acids play important roles in the human body. The hydrochloric acid present in the 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 body tissues. Fatty acids are also required for growth and repair of body tissues. Nucleic acids are important for the manufacturing of DNA and RNA and transmitting of traits to offspring through genes. Carbonic acid is important for maintenance of pH equilibrium in the body.
Historically, there have been many industrial accidents involving acids: most prevalently, sulfuric acid during transportation where a train derails, resulting in an uncontrolled spill. However, smaller incidents, involving individuals that handle acids incorrectly, occur on a daily basis. Many times, persons that have been tasked to transfer acids into a container for the purpose of dilution, do not understand that that the sequence of mixing acids and water can be crucial. It is never appropriate to add water to acid: always acid to water! By adding water to acid, an extremely concentrated solution of acid is initially formed. So much heat is released that the solution may boil very violently, splashing concentrated acid out of the container! If you add acid to water, the solution that forms is very dilute and the small amount of heat released is not enough to vaporize and cause spattering.
Without trying to get too complicated, there are two definitions or types of acids: one type is known as Bronsted-Lowery and the other is the Lewis definition. The first type (the more common concept) is a proton donor. These include hydrochloric acid (HCL), nitric acid (HNO3), and sulfuric acid (H2SO4). Note that they all have hydrogen (H). Hydrogen is essentially a proton, which means Bronsted-Lowery acids give up hydrogen. The more readily an acid gives up its hydrogen (proton), the stronger the acid.
Lewis acids, on the other hand, are electron pair acceptors. Common examples of these acids are boron triflouride (BF3) and aluminum fluoride (AlF3) Cations – those materials with a positive charge and attracted to the cathode of a battery or in plating operations – are Lewis acids as they are readily accept electrons. Examples include copper (Cu2) and both forms of iron ions: Fe2+ (ferrous) and Fe3+ (ferric).
What determines the effectiveness of an acid is its ability to supply that proton or accept an electron pair. Molecules that can do this quite readily are referred to as strong acids, which means they dissociate into ions. An example is hydrochloric acid – HCl – dissociating into the positive and negatively charged ions of H+ and Cl−. When the dissociation occurs, it releases a great deal of heat (i.e. exothermic).
So as it is apparent, working with acids requires extreme care and use of proper controls: Engineering controls can be used in processes like electroplating where barriers can be installed, preventing persons from contacting these harsh chemicals. Using ventilation hoods in laboratories is another effective method to protect personnel. Using the sash and bringing it down to the designated elevation is important as that is the mark where annual evaluations –verifying the suction and air velocity – have been conducted. The sash also acts as a physical barrier in case of accidental splashes and spills inside the hood.
Administrative controls are mandatory when working with acids. This includes a chemical-specific hazard communications training so workers know the information contained in the Safety Data Sheet which includes understanding the hazards associated with the chemicals they are using as well as knowing how to handle the specific material. This training needs to be documented such as a tailgate meeting form or an actual training/attendance roster (and maintained with the project files).
Good work practices is another type of administrative control (such as adding acid to water and not water to acid!). During handling operations, such as applying chemical preservatives in a ground water sample requires good work practice techniques. The preservatives are typically an acid, such as hydrochloric, or nitric or sulfuric acid. Working upwind and keeping the preservative vials away from the face and eyes are important practices.
Lastly, there is personal protective equipment – PPE. While one may automatically request nitrile gloves, this could be a mistake as there are so many types of nitrile gloves; each type differing in thickness and components. This means that while one type of nitrile glove may be sufficient for one particular work task, another type may be completely inappropriate. Of course there are many types of acids and each acid has its own specific characteristics; requiring a specific glove type, such as neoprene or butyl rubber.
Coveralls or acid-resistant aprons are typical PPE when handling batteries containing sulfuric acid. And lastly, eye and face protection should always be considered. Goggles or face shields (safety glasses underneath) are commonly used when handling aqueous acid solutions where splashing is a potential hazard. When working with acids, your S&H professional should be involved as this is the person that can prescribe the right type of protective clothing based on the assigned work activity.
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