Alkalinity

Alkalinity is the defined as a body of water’s ability to react with the acidic proton. The alkalinity is like buffer capacity in that higher alkalinity is associated with a body of water’s ability to "soak up" proton without altering the pH. Alkalinity is an important concept in understanding the effects of acid rain.

Alkalinity is different from basicity, which is directly related to the pH. The higher the pH, the more basic the water.

Like acidity, there are different ways to measure and report alkalinity;

The first is to titrate the water with acid titrant to the phenolphthalein end point. This is called the phenolphthalein alkalinity. Since phenolphthalein changes color at pH~8.3, this corresponds to a pH where all the CO32- present would be protonated.

Second, acid titration to a methyl orange end point, pH~4.3, further converts the bicarbonate to aqueous carbon dioxide. At this end point, some of the weaker conjugate bases are protonated. The methyl orange end point titration indicates total alkalinity.

Alkalinity is typically defined in terms of the equivalent weight, in milligrams (mg), of CaCO3 per liter of water. In other words, assuming that the alkalinity is due only to the presence of CaCO3. The mg of CaCO3 per liter is determined assuming all HCO3- comes from CaCO3.

The standard method used to determine alkalinity is to titrate 100 mL of water with 1.0 10-2 mole L-1 H2SO4 (sulfuric acid) titrant to a methyl orange end point. In this case, the alkalinity, expressed in mg CaCO3 per liter, is simply 10 mL titrant. The factor-of-10 comes from the various volume fractions and the formula weight of 100 g mole-1 for CaCO3.

The alkalinity of a body of water is important in the context of the acid rain problem. The more alkaline the water, the less susceptible it is to acidification. The EPA classifies water susceptibility to acid rain based on the following

Classification

Alkalinity (mg CaCO3 /L)

critical

below 2

endangered

between 2 and 5

highly sensitive

between 5 and 10

sensitive

between 10 and 20

not sensitive

above 20

In addition, a body of water is designated as acidified if the alkalinity classification is "critical" and the pH is less than 5.

Using the model that all proton activity comes from the presence of CaCO3 results in a neat expression for the alkalinity

[alk] = [HCO3-] + 2[CO32-] + [OH-] – [H+]

where [alk] is the effective alkali concentration (in equivalents or moles per liter). The 2[CO32-] factor takes into account that each CO32- reacts with 2 H+ to form CO2 (aq).

It might also be recognized that [OH-] and [H+] are always related through the water equilibrium

H2O H+ + OH-

which has the equilibrium constant

KW = [H+][OH-] = 10-14 @ 25oC

In addition, the relative amounts of total CO2 (aq) will be a function of pH.

We examine the pH-dependent solubility of CO2 before addressing the issue pH-dependent alkalinity.

This page edited Thursday, December 21, 2006

This page was last edited Thursday, December 21, 2006