Here is what appears in Volume 3 Number 1 of the Journal of the Swimming
Pool and Spa Industry:
General:
An Introduction to the Journal
Articles:
John A. Wojtowicz
Chemcon
Swimming Pool Water Balance Part 5: Factors Affecting Precipitation
of Calcium Carbonate
Laboratory tests with clear solutions showed that precipitation of calcium
carbonate does not occur in the pH range 7.5 to 8.0 at alkalinities of 80
to 160 ppm and saturation indexes as high as 1.5. However, when the alkalinities
are increased to very high levels, i.e., ~460 to ~325 ppm over the same
pH range, evidence of precipitation was observed in the 0.5 to 1.1 saturation
index (SI) range. At typical swimming pool pH and alkalinity, seed crystals
are necessary to initiate precipitation of calcium carbonate supersaturation.
Suspended particulate matter can serve as seed crystals. Results of laboratory
studies on precipitation of calcium carbonate in the presence of seed crystals
are in general agreement with predictions based on the calcium carbonate
precipitation potential (CCPP) model discussed in a previous article (Wojtowicz
1996). The results can be summarized as follows: a) at a given initial pH
and alkalinity, the extent of precipitation increases with increasing SI,
b) at a given initial pH and SI, the extent of precipitation increases with
increasing alkalinity, c) at a given initial alkalinity and SI, the extent
of precipitation decreases with increasing pH, and d) at a given initial
pH, SI, and carbonate alkalinity, the extent of precipitation increases
with increasing cyanuric acid concentration due to increased buffer intensity.
John A. Wojtowicz
Chemcon
Swimming Pool Water Balance Part 6: Applicability of The Langelier Saturation
Index to Swimming Pools
It is a common misconception that the Langelier Saturation Index applies
only to closed systems because it was developed for water in distribution
lines. Since it is based on calcium carbonate solubility equilibria, the
Langelier Saturation Index is applicable to both open and closed systems
containing dissolved calcium carbonate. The main difference is that in closed
systems the alkalinity can vary at a given pH whereas in equilibrated open
systems alkalinity is fixed at a given pH. In addition, since alkalinities
are much lower in equilibrated open systems at comparable pH values, saturation
hardness is much higher. Another common misconception is that swimming pools
are equilibrated open systems. Although swimming pools are open in a physical
sense, they are not open in a thermodynamic (i.e., chemical equilibrium)
sense. Swimming pools exhibit the characteristics of closed systems since
they show the expected range and variability of alkalinity which is also
typical of many public water supplies. If swimming pools were equilibrated
open systems (i.e., in equilibrium with atmospheric carbon dioxide), they
would contain only 4 to 18 ppm alkalinity over the 7.2 to 7.8 pH range.
At a given pH and alkalinity, swimming pools have the same concentration
of dissolved CO2 as a closed system. In order to attain equilibrium with
the atmosphere, swimming pools would have to lose the excess carbon dioxide
that they contain above the equilibrium value of 0.45 ppm. This will cause
an increase in pH where higher alkalinities are allowed. However, although
swimming pools are open to the atmosphere, they never achieve equilibrium
with the atmosphere because of acid addition, which in combination with
continual carbon dioxide loss causes the pH to vary with time resembling
a sawtoooth pattern.
John A. Wojtowicz
Chemcon
Swimming Pool Water Balance Part 7: A Revised and Updated Saturation
Index Equation
At a given temperature, swimming pool water chemistry must be balanced
by adjusting pH, carbonate alkalinity, and calcium hardness in order to
maintain the proper saturation with respect to calcium carbonate to avoid
etching of concrete, plaster, and tile grout, scaling, and cloudy water.
Water balance is determined by means of the calcium carbonate saturation
index (SI), which was originally proposed to provide corrosion control for
iron pipes in public water distribution systems by means of deposition of
thin films of CaCO3 (Langelier 1936). The current saturation index equation
is based on calcium carbonate solubility data published in 1929. This paper
discusses revisions to the saturation index equation due to more accurate
values for the calcium carbonate solubility product constant and its temperature
dependence and more realistic ionic strength corrections. The revised equation
is: SI = pH + Log [Hard] + Log [Alk] + TC + C where both hardness and alkalinity
are expressed in ppm CaCO3, TC is the temperature correction, and C = –11.30
– 0.333 Log TDS. The equation requires a reasonably accurate value
of total dissolved solids (TDS). At 1000 ppm TDS, C is equal to 12.3. Above
1000 ppm TDS, this equation yields significantly lower values for SI than
the current equation.
John A. Wojtowicz
Chemcon
Swimming Pool Water Balance Part 8: The Thermodynamic Basis of the Saturation
Index
Thermodynamics (i.e., the laws governing the conversion of heat to and
from other forms of energy) is the logical discipline for the mathematical
treatment of chemical reactions in homogeneous and heterogeneous systems.
The sign of the free energy change as an indicator of the direction of a
reaction was known in 1886 (Van’t Hoff) under the name “reaction
isotherm”. The free energy change is a measure of the useful energy
available from a system. When applied to solutions of calcium carbonate,
the expression for the free energy change for a reaction readily and naturally
leads to the calcium carbonate saturation index. The equation that bears
Langelier’s name, derived from ionic equilibria, is not novel. Therefore,
a more appropriate name would be calcium carbonate saturation index as Langelier
originally named it (Langelier 1936).
John A. Wojtowicz
Chemcon
Swimming Pool Water Balance Part 9: Corrections, Potential Errrors,
and Significance of the Saturation Index
Calculation of the saturation index requires a knowledge of the water
temperature and the concentrations of total alkalinity, calcium hardness,
and cyanuric acid. Total alkalinity must be corrected for cyanuric acid
present as cyanurate ion as well as the concentrations of other significant
alkaline species. In addition, the concentrations of complex forming ions
other than bicarbonate such as sulfate and magnesium are required. Although
these ions decrease the saturation index by reducing the concentrations
of calcium hardness and carbonate alkalinity through ion pair formation,
the effect is small except at very high levels of these ions. Cumulative
errors in typical swimming pool test kit analysis can result in a potential
deviation in the calculated saturation index of ±0.14 for water with
120 ppm total alkalinity, 300 ppm calcium hardness, and 100 ppm cyanuric
acid. The saturation index is not a corrosion index but rather a scaling
index, ie, it is an indicator of the calcium carbonate scaling or scale
dissolving tendency of water and not of corrosion.