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K. Saruhashi — Meteorological Research Institute (May 31, 1955)
This study is directly relevant to how ECO2MIX works: carbonic acid, bicarbonate, and carbonate do not exist in fixed proportions in water. Their ratios shift with pH and temperature in predictable ways. Understanding these equilibrium relationships is why we know that lowering water pH with dissolved CO₂ converts bicarbonates into carbonic acid — rather than raising them.
Citation: Saruhashi, K. (1955). On the Equilibrium Concentration Ratio of Carbonic Acid Substances Dissolved in Natural Water. Meteorological Research Institute, Japan.
Abstract
It is of importance to study the behavior of carbonic acid substances dissolved in natural water to know the chemical properties of the water. Acidity or alkalinity method has been generally used for this purpose for a long time. But this method often leads to erroneous results.
The present author proposed that a better way is to determine first the amount of total carbonic acid, pH, and temperature, and then to calculate theoretically the amount of H₂CO₃, HCO₃⁻ and CO₃²⁻ using equilibrium constants between them.
However, such a calculation is too laborious for every analyst, and therefore the author completed tables convenient for use containing calculated values of molar fractions of each carbonic acid substance for different temperature and pH in fresh water and sea water. In these tables, one can find any necessary values of molar fraction when the total amount of carbonic acid, temperature and pH of waters are given. Using this method, exact determination of the amount of each carbonic acid substance in water is made possible.
Conclusion
To obtain reasonable values of concentrations of H₂CO₃, HCO₃⁻ and CO₃²⁻ dissolved in natural water, we can NOT rely on the determination of the alkalinity and the acidity as has usually been done for a long time, because such a method often leads to erroneous results.
An alternative and better way consists in the determination of the total carbonic acid followed by calculation of each molar fraction of H₂CO₃, HCO₃⁻ and CO₃²⁻, taking the change of dissociation constants with temperature and ionic intensity of the solution etc. into consideration.
However, such a calculation being rather laborious for every analyst to do, complete tables convenient for use containing calculated values of molar fractions of each carbonic acid substance for different pH and temperature would be desirable. The present author completed such tables using the most reliable data up to now in the case of fresh water as well as sea water. One can easily find any necessary value of molar fraction in these tables, by which exact determination of the amount of each carbonic acid substance is made possible.
Percentage Molar Fraction Table
Percentage molar fraction of free carbonic acid (F), bicarbonate ion (B) and carbonate ion (C) at different temperature and pH (Fresh water).
| pH | F (24°C) | B (24°C) | C (24°C) | F (26°C) | B (26°C) | C (26°C) | F (28°C) | B (28°C) | C (28°C) | F (30°C) | B (30°C) | C (30°C) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6.0 | 70.0 | 30.0 | 0.0 | 69.5 | 30.5 | 0.0 | 69.2 | 30.8 | 0.0 | 69.0 | 31.0 | 0.0 |
| 6.1 | 65.0 | 35.0 | 0.0 | 64.2 | 35.8 | 0.0 | 64.0 | 36.0 | 0.0 | 63.8 | 36.2 | 0.0 |
| 6.2 | 59.9 | 40.1 | 0.0 | 59.2 | 40.8 | 0.0 | 58.9 | 41.1 | 0.0 | 58.5 | 41.5 | 0.0 |
| 6.3 | 54.1 | 45.9 | 0.0 | 53.2 | 46.8 | 0.0 | 53.0 | 47.0 | 0.0 | 52.7 | 47.3 | 0.0 |
| 6.4 | 48.1 | 51.9 | 0.0 | 47.6 | 52.4 | 0.0 | 47.2 | 52.8 | 0.0 | 47.0 | 53.0 | 0.0 |
| 6.5 | 42.4 | 57.6 | 0.0 | 42.0 | 58.0 | 0.0 | 41.5 | 58.5 | 0.0 | 41.2 | 58.8 | 0.0 |
| 6.6 | 36.9 | 63.1 | 0.0 | 36.4 | 63.4 | 0.0 | 36.1 | 63.9 | 0.0 | 35.9 | 64.1 | 0.0 |
| 6.7 | 31.8 | 68.2 | 0.0 | 31.3 | 68.7 | 0.0 | 31.0 | 69.0 | 0.0 | 30.5 | 69.5 | 0.0 |
| 6.8 | 27.4 | 72.6 | 0.0 | 26.9 | 73.1 | 0.0 | 26.5 | 73.5 | 0.0 | 26.0 | 74.0 | 0.0 |
| 6.9 | 23.0 | 77.0 | 0.0 | 22.3 | 77.7 | 0.0 | 22.2 | 77.8 | 0.0 | 21.8 | 78.2 | 0.0 |
| 7.0 | 18.9 | 81.1 | 0.0 | 18.5 | 81.5 | 0.0 | 18.4 | 81.6 | 0.0 | 18.2 | 81.8 | 0.0 |
| 7.1 | 15.8 | 84.2 | 0.0 | 15.3 | 84.7 | 0.0 | 15.0 | 85.0 | 0.0 | 14.8 | 85.2 | 0.0 |
| 7.2 | 13.0 | 87.0 | 0.0 | 12.7 | 87.3 | 0.0 | 12.5 | 87.5 | 0.0 | 12.3 | 87.7 | 0.0 |
| 7.3 | 10.6 | 89.4 | 0.0 | 10.3 | 89.7 | 0.0 | 10.1 | 89.9 | 0.0 | 10.0 | 90.0 | 0.0 |
| 7.4 | 8.4 | 91.5 | 0.1 | 8.3 | 91.6 | 0.1 | 8.2 | 91.7 | 0.1 | 8.1 | 91.8 | 0.1 |
| 7.5 | 6.9 | 92.9 | 0.2 | 6.8 | 93.0 | 0.2 | 6.6 | 93.2 | 0.2 | 6.6 | 93.2 | 0.2 |
| 7.6 | 5.4 | 94.4 | 0.2 | 5.4 | 94.4 | 0.2 | 5.3 | 94.5 | 0.2 | 5.3 | 94.5 | 0.2 |
| 7.7 | 4.5 | 95.2 | 0.3 | 4.4 | 95.3 | 0.3 | 4.4 | 95.3 | 0.3 | 4.4 | 95.3 | 0.3 |
| 7.8 | 3.6 | 96.1 | 0.3 | 3.5 | 96.2 | 0.3 | 3.5 | 96.2 | 0.3 | 3.5 | 96.2 | 0.3 |
| 7.9 | 2.9 | 96.7 | 0.4 | 2.9 | 96.7 | 0.4 | 2.9 | 96.7 | 0.4 | 2.8 | 96.8 | 0.4 |
| 8.0 | 2.3 | 97.3 | 0.4 | 2.2 | 97.3 | 0.5 | 2.2 | 97.3 | 0.5 | 2.1 | 97.4 | 0.5 |
F = free carbonic acid (H₂CO₃), B = bicarbonate ion (HCO₃⁻), C = carbonate ion (CO₃²⁻). Values are percentage molar fractions.
What this table shows clearly: at pH 6.5 (ECO2MIX’s target irrigation range), roughly 42% of the dissolved carbon is free carbonic acid and 58% is bicarbonate. As pH rises toward 8.0, bicarbonate dominates at over 97%. When ECO2MIX lowers irrigation water from pH 8.0 to 6.5, it shifts the equilibrium — converting bicarbonates into carbonic acid, not increasing them. This is the chemistry that answers the most common skeptic question about our system.
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