Titration Standards
With regard to titrations, a standard is a solution of known concentration. This solution is used as a reference to determine the concentration of a species in another solution. Standards can be classified as either primary or secondary .

A primary standard is essentially a standard that does not decompose over time. Consequently, its concentration is always known. Sodium oxalate is such a standard. Regardless of when a solution of sodium oxalate was made, its concentration will always remain the same.

A secondary standard is simply a solution that deteriorates over time but which has been standardised by titration with a primary standard immediately prior to its use as a standard. Potassium permanganate is just such a solution. Over time its concentration decreases as MnO 2 precipitates out of the solution. Hence, whenever a permanganate solution is used for a titration, it needs to be freshly made and standardised against a primary standard.

General Acid/Base Theory

Acids

A typical weak acid, HA, reacts with water according to the following equation:

HA + H₂O

H₃O⁺ + A^-

The equilibrium constant for this reaction is called the acid dissociation constant and is given by:

The stronger acids have the higher values of K_a. Indeed, for strong acids the reaction proceeds essentially 100% to the right and thus K_a is very large. The vast majority of organic acids, however, have K a values smaller than 10^-3 M. In order to avoid dealing with these small numbers, it is common practice to tabulate acid strengths as pK_a values, where:

pK_a = -log₁₀ K_a .

The weakest acids have the smallest K_a values. Hence, the weakest acids have the largest pK_a values and the strongest acids have the smallest (or negative) pK_a values.

Sodium hydroxide is a strong base and dissociates completely in water to produce OH^- ions. These OH^- ions react with a weak acid essentially to completion, as follows:

HA + OH^-

H₂O + A^-

Bases

A typical weak base, B, reacts with water according to the following equation:

B + H₂O

HB + OH ^-

The equilibrium constant for this reaction, called the base dissociation constant , is given by:

The stronger bases have the higher values of K_b. The vast majority of organic bases have K_b values smaller than 10^-3 M, and it is common practice to tabulate base strengths as pK_b values, or even more commonly to merely tabulate the pK_a values of the conjugate acids, from which the K_b can be deduced:

pK_b = -log₁₀ K_b .

For a conjugate acid base pair at 298 K, HA and A^-:

K_a (HA) x K_b (A^-) = 10^-14

and

pK_a (HA) + pK_b (A^-) = 14

The strongest bases have the smallest pK_b values and hence their conjugate acids have the largest pK_a values.

Hydrochloric acid is a strong acid and dissociates completely in water to produce H⁺ and Cl^- ions. The H⁺ ions react, essentially to completion, with a base as follows:

B + H⁺

HB⁺

Back to E28

E28 Appendix 1

Academic Resources

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General Acid/Base Theory

Acids

Bases