Introduction to Acid-Base Titrations

James Richard Fromm


A titration is a procedure used in analytical chemistry to determine the amount or concentration of a substance. In a titration one reagent, the titrant, is added to another slowly. As it is added a chemical stoichiometric reaction occurs until one of the reagents is exhausted, and some process or device signals that this has occurred. The purpose of a titration is generally to determine the quantity or concentration of one of the reagents, that of the other being known beforehand. In any titration there must be a rapid quantitative reaction taking place as the titrant is added, and in acid-base titrations this is a stoichiometric neutralization. The type of titration is simply the type of chemical reaction taking place, and so in this section we consider acid-base titrations.

Acid-Base Titration Reactions

All acid-base titration reactions, as all acid-base reactions, are simply exchanges of protons. The reaction could be strong acid + strong base rarrow.gif (63 bytes) (neutral) salt, as in the case of

HCl + NaOH rarrow.gif (63 bytes) NaCl + H2O,

although the reaction would be correctly written as

H3O+ + OH- rarrow.gif (63 bytes) H2O

since strong acids and strong bases are totally dissociated to protons and hydroxide ions in water.

For reactions which are

strong acid + weak base rarrow.gif (63 bytes) (acidic) salt,

such as the example

HCl + CH3NH2 rarrow.gif (63 bytes) CH3NH3+Cl-,

or

strong base + weak acid rarrow.gif (63 bytes) (basic) salt,

such as the example

NaOH + CH3COOH rarrow.gif (63 bytes) Na+CH3COO- + H2O,

the cations and anions could be omitted as they do not actually participate in the reaction. (Some chemists call these bystander ions.)

Virtually all acid-base titrations are carried out using a strong acid or strong base. In most cases the strong acid or strong base is used as the titrant. It is less common, but equally feasible, to place the strong acid or strong base in the titration vessel and use the weak acid or weak base as the titrant. A weak acid-weak base titration would have only a small pH change at the equivalence point. This small change is difficult to detect, and for this reason weak acid-weak base titrations are uncommon.

Standards in Acid-Base Titrations

One of the substances involved in a titration must be used as a standard for which the amount of substance present is accurately known. The standard can be present either in the form of a pure substance or as a standard solution, which is a solution whose composition is accurately known. A standard can be prepared in only two ways: use a primary standard or standardize by titration against some previously standardized solution. A primary standard is some substance such as oxalic acid which can be precisely weighed out in pure form, so that the number of moles present can be accurately determined from the measured weight and the known molar mass. For example, we might prepare a 0.1000 molar solution of primary standard oxalic acid by weighing out exactly 0.1 moles of oxalic acid and diluting to one liter in a volumetric flask.

The standard solutions used in an acid-base titration need not always be primary standards. A standard solution which has been prepared by quantitative dilution of a primary standard is an excellent secondary standard solution. Secondary standards can also be prepared by titration against a primary standard solution.


Copyright 1997 James R. Fromm