The Concept of Alkalinity
Alkalinity is “one of the most central but perhaps not the best understood concept in aquatic chemistry.”1 As a newcomer, you are sometimes confronted with several different types of alkalinity (e.g. M-alkalinity, P-alkalinity, caustic alkalinity), as well as several different definitions of alkalinity (experimental vs. mathematical).
Advantages of the alkalinity concept are:
- it is a conservative quantity which remains unaffected by changes in temperature, pressure or pH
- it is easily experimentally accessible (by titration with strong acids until an endpoint)
- it is a measure of water’s ability to resist pH changes (due to addition of strong acids and bases)
Addition or removal of weak acids or bases does not change the alkalinity.
Experimental Definition
The alkalinity of a solution is its “Acid Neutralizing Capacity” (ANC) when the endpoint of the titration is the CO2 (or H2CO3*) equivalence point, which is typically at pH 4.2 … 4.5:
| (1) | alkalinity = ANC to pH 4.2 … 4.5 |
The following terms are used interchangeably for alkalinity (all are the same or have the same numerical value):
| (2) | alkalinity = total alkalinity = M alkalinity = Carbonate Hardness (CH) |
Mathematical Definition (Main Idea)
Let us consider a NaCl-CO2-H2O system. Electroneutrality dictates:2
| (3) | [Na+] – [Cl-] – [HCO3-] – 2 [CO3-2] + [H+] – [OH-] = 0 |
Now, bring all conservative ions to the left, i.e. ions that do not change their amount of protons H+ when titrated, and all other ions to the right-hand side:
| (4) | [Na+] – [Cl-] = [HCO3-] + 2 [CO3-2] – [H+] + [OH-] |
The left-hand side is a constant. Thus, the right-hand side must also be a constant. The constant on the right-hand side is called alkalinity, more precisely, total alkalinity:
| (5) | alkalinity = [HCO3-] + 2 [CO3-2] – [H+] + [OH-] |
This is the definition which is valid for most natural waters. It comprises the three main ions (HCO3-, CO3-2, and OH-) that consume protons H+.
The General Case
The physical definition of alkalinity (based on the principle of electro-neutrality) is the charge difference between conservative cations and anions:
| (6) | alkalinity = ∑ conservative cations – ∑ conservative anions |
Conservative ions are ions that remain unaffected by changes of pH: Na+, K+, Ca+2, Mg+2, Cl-, SO4-2, NO3- and others. According to 6, total alkalinity is expressed as
| (7) | alkalinity = [Na+] + [K+] + 2 [Ca+2] + 2 [Mg+2] + … – [Cl-] – 2 [SO4-2] – [NO3-] |
This general approach to calculate alkalinity is used in PhreeqC and aqion. (In addition, the program accepts alkalinity as an input parameter for DIC.)
Units of Alkalinity
Alkalinity is expressed in units of meq/L (or mval/L) which is the modern and more fundamental choice (favored by aqion). In practice, however, the use of ppm CaCO3 is also preferred. A conversion chart is given here.
References & Remarks