Calcium Carbonate Saturation State
All relevant information and parameters about the calcium-carbonate saturation state are displayed in a separate panel. An example is shown in the right screenshot.
pH values at Calcite Saturation
First of all: The calculated calcite-saturation parameters refer to the evaluation temperature Teval which, in general, can differ from the temperature of the water sample (input water).
The following types of pH values are presented:
||pH value of the input water at water-sample temperature T
||pH value of the input water at evaluation temperature Teval
||calcite saturation pH at Teval (that enters the LSI formula)
||= 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):
- saturation pH after CO2 exchange
- saturation pH according Strohecker/Langelier (this is the pH value when both the Ca and DIC concentrations remain unaltered)
||saturation pH after CO2 exchange
||saturation pH according Strohecker/Langelier (this is the pH value when both Ca and DIC concentrations remain unaltered)
Saturation Index and CCPP
Other fundamental parameters are (valid for Teval):
- the saturation index (SI) of calcite
- the Calcium Carbonate Precipitation Potential (CCPP) in mmol/L
- the amount of precipitated or dissolved calcite in mg/L
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. In the following we reconstruct these traditional quantities from the carbonate speciation.
The total amount of inorganic carbon (DIC) is the sum of all carbonate species:
||DIC = [CO2] + [HCO3-] + [CO3-2] + carbonate complexes
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 Eq.(1), we get a new classification scheme:
||total CO2 = free CO2 + half bound CO2 + firmly bound CO2
with the components:
||half bound CO2
||[HCO3-] + [CaHCO3+] + [MgHCO3+] + …
||firmly bound CO2
||[CO3-2] + [CaCO3(aq)] + [MgCO3(aq)] + …
The half bound CO2 (or semi-bound CO2) comprises all hydrogen carbonates, while the semi 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:
||aggressive CO2 = free CO2 – corresponding CO2
Otherwise we have
||CO2 in deficit = corresponding CO2 – free CO2
The aggressive CO2 causes corrosion.
[last modified: 2015-12-29]