Of much interest to biochemists is the behavior of weak acids and weak bases -those not completely ionized when dissolved in water- because they are common in biological systems and play important roles in metabolism and its regulation.

The behavior of aqueous solutions of weak acids and bases is what this unit is all about and will be better understood if we first define some terms.

Acids may be defined as proton donors and Bases as proton acceptors. Each acid has a characteristic tendency to lose its proton in an aqueous solution. The stronger the acid, the greater its tendency to lose its proton. Such tendency can be measured quantitatively by the dissociation constant, designated as Ka, and also by the pKa [analogous to pH], which is defined by the equation

pKa = - log Ka

The stronger the tendency to dissociate a proton, the stronger the acid and the lower its pKa.

We shall see in subsequent parts of this unit the pKa of any weak acid can be determined quite easily with titration and by plotting its titration curve.

Titre [n] the concentration of specific antibodies, antigens, virus particles, etc. in a sample, often presented in terms of the maximum dilution at which the sample still gives a reaction in an immunological test with a standard preparation of an appropriate antigen or antibody or in other biological or biochemical assays.

Source: Henderson's dictionary of biological terms. Edited by E. Lawrence. 11th ed. John Wiley & Sons Publsihers. 1995.

(Acid-base) Titration, often referred to as titration, is used to determine the amount of an acid in a given solution. A measured volume of the acid is titrated with a solution of a strong base, usually sodium hydroxide (NaOH) of known concentration. The NaOH is added in small increments until de acid is consumed (neutralized), as determined with an indicator dye or a pH-meter. The concentration of the acid in the original solution can be calculated from the volume and the concentration of the NaOH added.

Source: Lehninger Principles of Biochemistry. D. Nelson and M. Cox. 3rd ed. Woth Publishers, 2000.

Titration Curve. A plot of the pH against the amount of NaOH added during the acid-base titration procedure is usually referred to as the titration curve (see figure below); it allows us to deduce the quantities of acidic and basic components in a mixture, and what their relative pK values are. In the plot, the independent variable [in the x-axis] is the volume of the titrant, while the dependent variable [in the y-axis] is the pH of the solution (which changes depending on the composition of the two solutions).

Endpoint in a titration. Also referred to as the equivalence point. The endpoint is where the titration ends; that is, the point at which all the unknown has been reacted with the standard titrant and some kind of chemical indicator must let us know when that point has been arrived at.

There are many ways of measuring the endpoint of a titration; in this unit we will use the ph indicator, the pH meter, and the color change during the experiment and we will also measure it by analytical means during the quantitative analysis steps.

Doing a Titration. To perform a titration in a chemistry lab, follow the instructions in this url: Source: www.dartmouth.edu/~chemlab/techniques/titration.html

A pH titration performed in a wet lab is certainly a good handsom experiment for students to be exposed to. However, due to time constraints, lack of equipment (volumetric glassware, pH meters, burets, etc.), safety concerns, and waste disposal issues, some instructors may prefer to conduct a 'virtual titration experiment' with the Virtual Lab Simulator provided here in subsequent parts of this unit.