pK Scale

Fig. 3.39  Chemical reactivity of aqueous solutions

pH = 1/2 (pKa − log C) With C for Concentration of the acid or base.

Fig. 3.40  pH of a weak acid

pH = 7 + 1/2 (pKa + log C)

Fig. 3.41  pH of a weak base

pH = 1/2 (pKa - log C) = 1/2 (3.2 - log 0.1)

= 1/2 (3.2 + 1) = 2.1)

Fig. 3.42  pH of HF in a water solution

pH = 7 + 1/2 (pKa + log C) = 7 + 1/2 (9.2 + 0) = 11.6

Fig. 3.43  pH of 1N ammonia solution

pH = -log [C] for a strong acid and

pH = 14 + log [C] for a strong base

Fig. 3.44  pH of strong acid and base

In the following table, we shall find examples of acids, which illustrate, using the pKa value, their more or less important strength (Fig. 3.45).

3.2.4.2  Prediction of the Irritant

Power of Acids or Bases

Given the concentration and the pK of the involved corrosive substance, it is possible to predict if a solution of an acid or basic chemical is not aggressive or can be irritant or corrosive to the eye. So we define the notion of threshold concentration as developed below in Sect. 3.4.3.2.

The intermediate zone of the scale – pK between 5 and 9 – is considered as having no aggressive effect for the physiological balance of the cornea. Out of this zone, we can notice either an irritation when the substance is diluted or corrosion when the substance is concentrated.

Below 0.2N, there is no reaction of the tissues. In the intermediate cases, either with a limit pK in the intervals 5–4 and 9–10 or for weakly concentrated solutions (from 0.2 to 1N), there is only an irritation. Only the solutions with a pK lower than 4 or superior to 10 and a concentration of 1N or more are corrosive (Fig. 3.46).

3.2.4.3  Scales of Energy Level

It is easy to understand that every type of reactive chemical function causing an irritation or a corrosion corresponds to a type of elementary reaction (Sect. 3.2).

The intensity of reaction of the chemical aggressor can be represented on a scale of energy, the functioning of which is governed by a simple rule.

•Thus, in an acid–base solution, there is an exchange of protons and the relation is (Fig. 3.47). The reaction develops until equilibrium reaching a specific

pH. Acid1 reacts on base2 when pK1 < pK2. The reaction will move even more to the right side when pK1–pK2 is big.

•For oxidizing and reducing agents, the intrinsic potential E°, measured in volts (V) or millivolts (mV), is the expression of their reactivity and the general rule to apply is: “An oxidizing agent oxidizes every reducing agent with a weaker potential than its own.”

The applicable relation is (Fig. 3.48)