Fig. 3.2  Organic structure of corrosive agents

An irritant or a corrosive agent is, most of the times:

•A simple mineral entity: HCl, HF, HNO3, H2O2, H2SO4, NaOH, KOH, etc.

•An organic structure with a low molecular weight, based on a carbonated structure, for instance, formic acid, acetic acid, or propanoic acid (Fig. 3.2), etc.

Thus, the irritant and corrosive agents can be easily considered as molecular entities. In most cases, they are small or even very small structures (like hydrofluoric acid – HF, for instance).

3.2.2  Reactive Functional Groups

of Irritant or Corrosive Agents

In a simplistic vision, molecular structures have one or, more scarcely, several functional groups. These groups, of diverse nature, provide the expression of the reactivity of a molecule. They are functions such as:

3.2.2.1  Acidic Function

H+ results either from mineral acids or from carboxylic acids (–COOH function) or from alcohol function (–OH) or thiol function (–SH) (Fig. 3.3).

Fig. 3.4  Examples of basic molecules

3.2.2.2  Basic Function

The basic function defines the substances called bases. Either mineral or organic, these substances have the following properties:

•To release an OH− ion

•The ability to capture a proton (H+ ion), for instance,

an amine group (–NH2), or a function including a triple bond: ≡N (Fig. 3.4)

Energy Scale of Acid–Base Reactions:The pK Notion

When an acid dissolves in water, a certain proportion of the molecule dissociates:

AH ® A- + H+

For any given molecule, the dissociated fraction in water is always equal. It is an intrinsic property of the molecule in relation with the connection forces between its constitutive atoms in water.

This constant is called Ka. The bigger the Ka constant, the bigger the dissociated fraction and the stronger the acidity.

Ka =[A-] ´[H+ ] / [AH]

The same reasoning works for the dissociation of bases with a constant of basicity called Kb. To make things easier, it is the logarithmic value of K, called pK, that is commonly used: pKa = –log10 Ka.

3.2.2.3  Oxidizing Function

Fig. 3.5  Examples of oxidizing function

oxidizers. They are either mineral, like KMnO4 or CrO3, or organic, like peracids (Fig. 3.5).

3.2.2.4  Reduction Function

Some reagents can release one or several electrons and be part of a reduction reaction. They are called reducing agents. Reducing agents are mineral, like Li, Na, or AlLiH4, or organic, like hydrazine (Fig. 3.6).

The redox potential is the energy scale of the redox reaction. It evaluates the importance of the oxidizing or the reducing property. Pure water is taken as a reference. Therefore, the value of the redox couple of water is said to be zero.

H2N NH2

Hydrazine

Fig. 3.6  Example of reduction function

CI C CI

CI

Carbon tetrachloride

Fig. 3.7  Examples of apolar solvents

Fig. 3.8  Examples of protic polar solvents

3.2.2.5  Solvent Function

The solvation expresses according to a set of physicochemical characteristics. We distinguish several big families of solvents with varied structures or varied functional groups [6]:

•Apolar solvents, hydrocarbonated structure that are electronically symmetric molecules (Fig. 3.7).

•Protic polar solvents, like water – omnipresent in the body – or functions such as alcohol (–OH) or thiol (–SH) (Fig. 3.8). A molecule is said to be protic when it can release a proton. The connection between the hydrogen and another atom is weak enough. A molecule that cannot release a proton is

called aprotic. A molecule is said to be polar when, without any electronic field, the center of gravity of the negative charges is different from the center of gravity of the positive charges.

•Aprotic polar solvents, with functions such as ketone

(>C=O), nitrile (–CN), amide (O=C–NH2), sulfoxide (>S=O), ether (–O–), and halogenated and electronically asymmetric solvents (Figs. 3.9 and 3.10).

The partition coefficient of solvents is also a reactional energy scale.

A substance has more or less of affinity and it thus dissolves more easily in a type of solvent than in the other one. We call partition coefficient the constant