Overview Data Input Data Output DIC Charge Balance Physico-Chem Params Reactions Redox Mineral Phases

Calcium Carbonate Saturation State

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 T_eval, 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 T_eval):

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

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 T_eval):

• 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.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