Carbonate System -- Main Parameters

aqion output panel: carbonate-system parameters

All relevant information about the carbonate system (buffer capacities, alkalinity, water hardness) is displayed in the right screenshot.1

Buffer Capacities

In the upper part of the panel, four types of buffer capacities (in meq/L) are listed:

ANC to pH 4.3 (≈ M alkalinity)
ANC to pH 8.2 (≈ P alkalinity)
BNC to pH 4.3 (≈ –M alkalinity)
BNC to pH 8.2 (≈ –P alkalinity)

For each type of buffer capacity two values are displayed:

  • first value – refers to the case when there is no precipitation or dissolution of minerals

  • second value (in parentheses) – refers to the case when minerals precipitate or dissolve2


Two types of alkalinity are shown (in meq/L):

Both quantities are interrelated via the total dissolved inorganic carbon (DIC):

(1) DIC  =  m – p

Water Hardness

TH total hardness in meq/L CaCO3
CH carbonate hardness (temporary hardness) in meq/L CaCO3
NCH non-carbonate hardness (permanent hardness) in meq/L CaCO3

Based on the calculated TH value the water is classified into four categories:

  • soft
  • moderately hard
  • hard
  • very hard

Carbonate Species

The total dissolved inorganic carbon (DIC) is decomposed into four parts:

•   CO2 = CO2(aq) + H2CO3 composite carbonic acid H2CO3*
•   HCO3- bicarbonate (hydrogen carbonate)
•   CO3-2 carbonate
•   carbonate complexes CaCO3(aq) + CaHCO3+ + MgCO3(aq) + …

Thus, for the molar concentrations we have:3

(2) DIC  =  [CO2] + [HCO3-] + [CO3-2] + carbonate complexes

CO2 Partial Pressure

pCO2 is a measure of the amount of CO2(g) dissolved in water. For more details see here.


  1. This panel is the left-side part of a larger window shown here

  2. If there is no mineral precipitation or dissolution both values coincide, and the second value won’t be displayed. The distinction becomes especially relevant for (amorphous) minerals such like Fe(OH)3 and Al(OH)3

  3. This relation is valid only for molar concentrations (mol/L or mmol/L). Adding mass concentrations (mg/L) will result in nonsense. 

[last modified: 2015-12-19]