1
Name: _______________________________________ Date:_______________________________
Lab Partners: ____________________________________________________________
Acid-Base Titrations
By Thomas Cahill, Arizona State University, New College of Interdisciplinary Arts and Sciences.
Background:
One of the most common reactions in chemistry is the reaction of an acid with a base. This type
of reaction is also called neutralization. The mechanism of this process is the combination of hydroxide
ions and hydronium to create water:
H3O+(aq) + OH−(aq) → 2H2O(l) (1)
It is important to note that the hydronium ion (H3O+) is often simply represented as H+. The hydronium
ion representation is technically a more accurate representation of association between a hydrogen ion
and a water molecule, but either representation is acceptable and interchangeable in this class.
The acid-base reaction is a second order reaction, thus the rate of the reaction is dependent on the
concentrations of both of the reactants. The concentration of hydronium ion is most commonly
expressed as the pH of the solution. The equation to calculate pH is:
pH= −Log[H+] (2)
where [H+] is the concentration of the hydronium ion.
The pH of a solution is a measure of the hydronium ion concentration in a solution. The pH scale ranges
from 0 to 14. Any pH value lower then 7 indicates an acidic solution, a pH value above 7 denotes a
basic solution, and a pH value of 7 indicates a neutral solution. The pH for several familiar substances
can help give an idea of what the capricious pH values represent. Gastric juice has a pH of 1 while wine
has a pH of around three. Conversely, household bleach has a pH of 12 and baking soda has a pH of
eight. Human blood and tears are very close to the neutral pH of seven.
One important thing to note about the pH scale is that it is based on the logarithmic scale. This
means that for every single unit of change in pH there is actually a tenfold difference in the hydronium
ion concentration. For example, if the pH of a solution drops from 6 to 5, the hydronium ion
concentration has increased by a factor of 10, from 10−6 M to 10−5 M.
The most accurate method of measuring the pH of a solution is utilizing a device called a pH
meter. A specialized electrode, which measures the concentration of hydronium ions, is simply placed
into the solution. A few seconds after the probe is submerged, the pH of the solution will be displayed
on the pH meter’s screen. Colored pH paper and indicator dyes can also be used to roughly estimate the
pH of the solution.
Another important concept in acid-base chemistry is that of acid-dissociation constants. An acid
dissociation constant, denoted by Ka, is an equilibrium constant for the dissociation of a weak acid.
HA H++ A−
HA H++ A−
(3)
