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REACH will present significant challenges to all of us. There’s still much water to flow under the bridge before the detailed requirements of REACH are set in tablets of stone.
However, there is one thing we can anticipate and that is that REACH will demand much greater supply chain co-operation on the use of chemicals.
From the chemical manufacturer, through the formulator, to the product manufacturer to the final retailer we will all have to work together far more, understand each others challenges, languages and contribution to a mutual value chain.
Today we want to share with you some ideas about how such partnerships can work. We don’t offer a panacea, nor do we imagine that our relationship is necessarily representative of those that you in this room may have, but we feel much of the learning from our experience can be interpreted constructively in your supply chains.
Nonaqueous titration
Nonaqueous titration is the titration of substances dissolved in nonaqueous solvents.
It is the most common titrimetric procedure used in pharmacopoeial assays and serves a double purpose:
it is suitable for the titration of very weak acids and very weak bases, and it provides a solvent in which organic compounds are soluble.
Theory
The theory is that water behaves as both a weak acid and a weak base; thus, in an aqueous environment, it can compete effectively with very weak acids and bases with regard to proton donation and acceptance, as shown below:
H2O + H+ ⇌ H3O+
Competes with RNH2 + H+ ⇌ RNH3+
or
H2O + B ⇌ OH- + BH+
Competes with ROH + B ⇌ RO- + BH+
The effect of this is that the inflection in the titration curves for very weak acids and very weak bases is small, because they approach the pH limits in water of 14 or 0 respectively , thus making endpoint detection relatively more difficult.
A general rule is that bases with pKa < 7 or acids with pKa > 7 cannot be determined accurately in aqueous solution.
Substances which are either too weakly basic or too weakly acidic to give sharp endpoints in aqueous solution can often be titrated in nonaqueous solvents. The reactions which occur during many nonaqueous titrations can be explained by means of the concepts of the Brønsted-Lowry theory. According to this theory an acid is a proton donor, i.e. a substance which tends to dissociate to yield a proton, and a base is proton acceptor, i.e. a substance which tends to combine with a proton. When an acid HB dissociates it yields a proton together with the conjugate base B of the acid: base proton acid B- + H+ ⇌
HB
base proton acid B- + H+ ⇌ HB Alternatively, the base B will combine with a proton to yield the conjugate acid HB of the base B, for every base has its conjugate acid and, every acid has its conjugate base.
It follows from these definitions that an acid may be either:
* an electrically neutral molecule, e.g. HCl, or
* a positively charged cation, e.g. C6H5NH3+, or
* a negatively charged anion, e.g. HSO4-.
A base may be either:
* an electricially neutral molecule, e.g. C6H5NH2, or
* an anion, e.g. Cl-.
Substances which are potentially acidic can function as acids only in the presence of a base to which they can donate a proton. Conversely basic properties do not become apparent unless an acid also is present.
Nonaqueous solvents used
Aprotic solvents are neutral, chemically inert substances such as benzene and chloroform.
They have a low dielectric constant, do not react with either acids or bases and therefore do not favor ionization.
The fact that picric acid gives a colorless solution in benezene which becomes yellow on adding aniline shows that picric acid is not dissociated in benzene solution and also that in the presence of the base aniline it functions as an acid, the development of yellow color being due to formation of the picrate ion.
Protophilic solvents are basic in character and react with acids to form solvated protons.
HB + Sol. ⇌ Sol.H+ + B-
Acid + Basic solvent ⇌ Solvated proton + Conjugate base of acid
weakly basic solvent has less tendency than a strongly basic one to accept a proton. Similarly a weak acid has less tendency to donate protons than a strong acid. As a result a strong acid such as perchloric acid exhibits more strongly acidic properties than a weak acid such as acetic acid when dissolved in a weakly basic solvent. On the other hand, all acids tend to become indistinguishable in strength when dissolved in strongly basic solvents owing to the greater affinity of strong bases for protons. This is called the leveling effect. Strong bases are leveling solvents for acids, weak bases are differentiating solvents for acids.
Protogenic solvents are acidic substances, e.g. sulfuric acid. They exert a leveling effect on bases.
Amphiprotic solvents have both protophilic and protogenic properties.
Examples are water, acetic acid and the alcohols. They are dissociated to a slight extent. The dissociation of acetic acid, which is frequently used as a solvent for titration of basic substances, is shown in the equation below:
CH3COOH ⇌ H+ + CH3COO-
Here the acetic acid is functioning as an acid. If a very strong acid such as perchloric acid is dissolved in acetic acid, the latter can function as a base and combine with protons donated by the perchloric acid to form protonated acetic acid, an onium ion:
HClO4 ⇌ H+ + ClO4-
CH3COOH + H+ ⇌ CH3COOH2+ (onium ion)
Since the CH3COOH2+ ion readily donates its proton to a base, a solution of perchloric acid in glacial acetic acid functions as a strongly acidic solution.
When a weak base, such as pyridine, is dissolved in acetic acid, the acetic acid exerts its levelling effect and enhances the basic properties of the pyridine. It is possible, therefore, to titrate a solution of a weak base in acetic acid with perchloric acid in acetic acid, and obtain a sharp endpoint when attempts to carry out the titration in aqueous solution are unsuccessful.
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