Calcium Carbonate Saturation State

aqion output panel: calcite saturation

All relevant information and parameters about the calcium-carbonate saturation state are displayed in a separate panel. An example for the water gw.sol is shown in the right screenshot.1

pH values at Calcite Saturation

First of all: The calculated calcite-saturation parameters refer to the evaluation tempe­rature Teval, which can usually deviate from the temperature of the water sample (input water).23

The following types of pH values are presented:

pH_0 pH value of the input water at water-sample temperature T
pH pH value of the input water at evaluation temperature Teval
pH_S calcite saturation pH at Teval (that enters the LSI formula)
LSI = pH - pH_S   as the Langelier Saturation Index

Based on the calculated value of LSI the following classification is used:

LSI ≈ 0 The water is in equilibrium with calcite.
LSI < -0.03 The water tends to be corrosive.
LSI >  0.03 The water tends to be scale forming.

Two additional/alternative types of saturation pH are also presented (both at Teval):

pH_A saturation pH after CO2 exchange
pH_B saturation pH according Strohecker/Langelier (this is the pH value when both Ca and DIC concentrations remain unaltered)

Saturation Index and CCPP

Other important parameters are (valid for Teval):

These data are the outcome of chemical equilibrium calculations based on PhreeqC.

Free CO2 and Bound CO2

Well before the advent of modern computing and speciation programs other concepts and vocabulary – such like free, bound and aggressive carbonic acid – were in common use.4 In the following we reconstruct these traditional quantities from the carbonate speciation.

The starting point is the total amount of inorganic carbon (DIC), which is the sum of all carbonate species:

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

and where “carbonate complexes” = [CaCO3(aq)] + [CaHCO3+] + [MgCO3(aq)] + … 

Renaming DIC as “total CO2” = [DIC] × 44.01 g/mol, then, after resorting the individual terms in 1, we get a new classification scheme:

(2) total CO2  =  free CO2 + half bound CO2 + firmly bound CO2

with the components:5

(2a) free CO2 = [CO2]
(2b) half bound CO2 = [HCO3-] + [CaHCO3+] + [MgHCO3+] + …
(2c) firmly bound CO2 = [CO3-2] + [CaCO3(aq)] + [MgCO3(aq)] + …

The half bound CO2 (or semi-bound CO2) comprises all hydrogen carbonates, while the bound CO2 encompasses all carbonates.

Corresponding CO2 and Aggressive CO2

The corresponding CO2 is the amount of free CO2 that refers to the state when the water is in equilibrium with calcite (SI=0). In the program it is named “CO2 in calcite equilibrium”. An alternative name is “related free carbonic acid”.

The aggressive CO2 is the amount of free CO2 that exceeds the equilibrium-state value:

(3) aggressive CO2  =  free CO2 – corresponding CO2

Otherwise we have

(4) CO2 in deficit  =  corresponding CO2 – free CO2

The aggressive CO2 causes corrosion.

Remarks & Footnotes

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

  2. For example: The water sample refers to 20, but we are interested in the calcite saturation at the evaluation temperature 60

  3. In the above example/screenshot the evaluation temperature is equal to the water-sample temperature 11.5

  4. The term “aggressive carbonic acid” was introduced into water analysis by Tillmans and Heublein in 1912. 

  5. Here, the terms carbonic acid and CO2 are used interchangeably. In the program, they are displayed in units of “mg/L CO2”. 

[last modified: 2021-01-10]