Is carbonate and carbon the same

Carbonic acid / lime-carbonic acid balance


The lime-carbonic acid balance is of purely technical importance.

The uptake of carbon dioxide from the atmosphere occurs through rainwater and when it passes through the soil and is exchanged with the soil air.

Only about 0.7% of the dissolved CO2 reacts with water to form carbonic acid, H2CO3, which in the first stage in H+ and HCO3-Ions and in the second stage in H+ and Co32- Ions dissociated.

Definition:

  • The chemical equilibrium between the ions of carbonic acid - carbon dioxide and calcium carbonate is known as the "lime-carbonic acid balance".
  • It essentially determines the lime-removing character of the water or the lime-dissolving

Dissociation of carbonic acid

  • If CO2 in water, it reacts with water molecules (H.2O)
  • Carbonic acid H is produced2CO3
    • The carbonic acid dissociates, i.e. it disintegrates
    • in hydronium ions H3O+ (= H+)
    • in hydrogen carbonate ions HCO3 - (Bicarbonate)
    • in carbonate ions CO32-

    CO2 + H2O <--> H2CO3

    H2CO3 <--> H + + HCO3-

The formation and dissociation of carbonic acid takes place in 2 stages and is strongly dependent on the pH value:

  • 1st dissociation stage:
    • CO2 + H2O <--> H+ + HCO3-

  • 2nd stage of dissociation
    • HCO3-   <--> H + + CO32- 

    • pH decrease (H. + - increase) -> more CO2
    • pH increase (H. + - less) -> more HCO3- and Co32-

The reaction equilibrium depends on the pH value:

 
Fig. 1: Proportion of carbonic acid forms depending on the pH value

Fig. 1 shows the distribution of the components of inorganic carbon (DIC - Dissolved Inorganic Carbon, see below) along the pH axis. The principles shown here apply regardless of the cations involved, for example also for the solution of sodium carbonate and bicarbonate. From Fig. 1 it can be seen that at every pH value that usually occurs in natural water, two components of the inorganic carbon always coexist in proportions of more than one percent. This coexistence of two components is usually referred to in connection with the calcite saturation as the “associated” carbonic acid to the other component.
In the past, the designation of the concentration of hydrogen carbonate as “carbonate hardness” was taken as a starting point and the CO2 treated as “related”. This is how the term “associated carbonic acid” came about.

Note:

The increase in the concentration of the associated CO2 accelerates with increasing concentration of hydrogen carbonate (= approximately acid capacity up to pH 4.3). This has a number of important consequences:

  • - As the concentration of hydrogen carbonate increases, every further increase in the concentration becomes more and more complex, because the additional need for the associated CO2 increases disproportionately. this creates a natural barrier against extremely high concentrations of hydrogen carbonate in hard water.
     
  • - Two waters A and B, which are in the state of calcite saturation, but differ significantly in terms of their hydrogen carbonate gel, result in a mixture of waters that are no longer in the state of calcite saturation. These mixed waters contain excess CO2. If such water forms, e.g. in a distribution system, it must be checked whether its calcite dissolving capacity is still within the specified tolerance range or whether the uncontrolled formation of mixed water must be avoided.

Types of carbonic acid

Free carbon dioxide lies almost exclusively as CO2 before, only about 0.7% as hydrated carbonic acid H 2CO3

Carbonic acid can be calcium carbonate CaCO3 Dissolve and convert to calcium hydrogen carbonate [Ca (HCO3)2]. The CO bound in carbonates2 is bound carbonic acid, it breaks down into carbonates and CO when cooked2

  • firmly bound carbonic acid is CaCO in carbonates3 bound
  • half-bound carbonic acid is in hydrogen carbonate [Ca (HCO3)2] bound
    • Hydrogen carbonates would break down if the so-called free associated carbonic acid would exist

If there is no more carbon dioxide, the water is in:

      • Lime-carbonic acid equilibriumt
         
  • If there is more carbon dioxide, this is it free excess carbonic acid
    • -> there is no longer a lime-carbonic acid balance!

Aggressive water: in the presence of free excess carbon dioxide, Dissolution of lime (Anti-rust layer)

  • Metal aggressive is total free carbon dioxide, so also free associated Carbonic acid,
  • The preparation of water in lime-carbonic acid equilibrium is necessary in the treatment!
  • Lime aggressive is only the part of the free excess carbonic acid, since after this carbonic acid has attacked lime again Ca (HCO3)2 arises, which needs a further amount of free associated carbonic acid to stabilize it.
  • The protective layer that arises when processing is carried out in the lime-carbonic acid equilibrium in metallic pipelines consists of calcium carbonate crystals with deposits of magnesium and iron compounds
  • The amount of carbon dioxide is very important from a technical point of view, as questions of material and water treatment are touched upon.

Types of carbonic acid and their properties

Free carbon dioxide

Bound carbonic acid

Associated
carbonic acid

Excess
carbonic acid

totally bound
carbonic acid

Half bound
carbonic acid

(harmless to
Pipe networks and concrete)

(aggressive)

e.g. CaCO3
in carbonates

in hydrogen carbonates
e.g. Ca (HCO3)2

also aggressive to metal!

- total surplus
Anti-rust layer
preventing; Pipe attack;
Metal corrosion (Cu, lead, zinc, Fe, ..)
- Partial surplus
lime aggressive;
Concrete attack

not aggressive
 

 

 

 

 

 

 

 

 

The lime-carbonic acid equilibrium is reached when:

Amount of free carbon dioxide = amount of the associated (bound) carbon dioxide
  • If the content of the associated free carbon dioxide is too low, limescale can precipitate
  • If there is more free carbon dioxide than the associated carbon dioxide, then water is aggressive to lime and strives for a state of equilibrium again by dissolving the lime.

In the case of water of different composition, it should be noted that two equilibrium waters can result in one water that is not in equilibrium.

Summary

Water is aggressive

Deficit water

Equilibrium water

with excess carbonic acid =
limescale and attacks metal

of associated carbonic acid =
lime separating

There is water in the
Lime-kolenic acid balance = protective layer formation
 

pH is lower than equilibrium pH

pH is higher than that
Equilibrium pH

pH value = equilibrium pH

Calculation of the carbon dioxide approximately:

      for the pH range 4.3 ... 8.2 the following applies:

      "Free" carbonic acid - free CO2
      c (CO2) in mmol / l = p = KB8.2
      in mg / l = KB8.2 * 44

      half-bonded carbonic acid
      c (HCO3-) in mmol / l = m = KS4.3 – 0,05

      fully bound carbonic acid
      c (CO32-) in mmol / l = 0

      Inorganically bound carbon (DIC - Dissolved Inorganic Carbon) can be roughly calculated from the acid and base capacity:

      DIC = KS4.3 - 0.05 + KB8.2 0 =c [CO2] + c [HCO3] + c [CO3] in mmol / l

Definitions / determinations of carbonic acids (from Walter Kölle: Judging water analyzes correctly, Wiley-VCH, 2010):

Unless otherwise stated, the unit mg / l applies to all concentrations in the following.

  • - Free carbon dioxide: dissolved, free CO2, corresponds approximately to the base capacity up to pH 8.2 (mmol / l)
  • - Associated carbon dioxide: dissolved, free CO2which must be present in equilibrium with hydrogen carbonate. Corresponds to the concentration of free CO2 that of the associated CO2, the water is in a state of calcite saturation.
  • - Excess carbon dioxide: Free CO2 less associated CO2 (underlying deacidification method: outgassing of CO2).
  • - Aggressive carbon dioxide: Excess CO2.
  • - Lime-aggressive carbon dioxide: Free CO2 less associated CO2 (underlying deacidification method: deacidification using calcite).
  • - Carbon dioxide to prevent rust protection: Excess CO2.
  • - Carbonic acid attacking the assault: Free CO2 exceeds 20% of the “bound carbonic acid” in the presence of oxygen.
  • - Bound carbonic acid: hydrogen carbonate and carbonate after stoichiometric conversion into CO2.
  • - Half-bonded carbonic acid: hydrogen carbonate after stoichiometric conversion into CO2.
  • - Fully bound carbonic acid: carbonate after stoichimetric conversion into CO2.
  • - Carbon dioxide deficit: shortage of CO2 in a supersaturated water. Required CO2 dosage to bring a supersaturated water into the state of calcite saturation.