⟵ 05 Carbonated Water (Problem Set) · Contents · 06 Destructive Oxidation ⟶
Carbonated Water: Answer Key
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Ionic solids are pulled into solution - water’s polarity dismantles the crystal lattice and hydration shells stabilize the freed ions. Gases are pushed into solution by pressure - they occupy gaps in the Hydrogen-Bond Network but are not electrostatically stabilized. When pressure drops or temperature rises, gases leave because nothing is holding them in place.
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CO₂(g) ⇌ CO₂(aq) ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺. pH determines which species dominates: low pH favors CO₂/carbonic acid, neutral pH favors bicarbonate, and high pH favors carbonate.
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Bicarbonate can absorb hydrogen (becoming carbonic acid) when acid is added, or donate hydrogen (becoming carbonate) when base is added. This dual role allows it to resist pH change in both directions, which is the definition of a buffer.
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The tower continuously strips CO₂ from the water through warm temperature, massive airflow, and enormous surface area. As CO₂ leaves, equilibrium shifts to replace it: carbonic acid decomposes, bicarbonate consumes H⁺. The decrease in hydrogen increases the pH.
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(a) 250 / 50 = 5.0 meq/L. (b) 250 / 50,000 = 0.0050 eq/L.
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½M = 120. Since P (80) < ½M (120), bicarbonate + carbonate are present. Carbonate alk = 2P = 160 ppm as CaCO₃. Bicarbonate alk = M − 2P = 240 − 160 = 80 ppm as CaCO₃.
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CO₂ ≈ 15 × 0.79 = 11.85 ppm. Amine demand ≈ 2.5 × 11.85 = 29.6 ppm active amine.
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½M = 225. Since P (400) > ½M (225), hydroxide + carbonate are present. OH– alk = 2P − M = 800 − 450 = 350 ppm as CaCO₃. [OH⁻] = 350 / 50,000 = 0.0070 eq/L = mol/L. pOH = −log10(0.0070) = 2.15. pH = 14 − 2.15 = 11.85.
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Upstream treatment, such as reverse osmosis or dealkalization, removes bicarbonate before it enters the boiler. This greatly reduces alkalinity-derived CO₂ formation and helps control the condensate corrosion pathway. Downstream treatment uses neutralizing amines that volatilize with steam and condense with water, raising condensate pH and neutralizing acidity before carbonic acid can attack metal. Either way, alkalinity must be controlled because bicarbonate can thermally decompose into CO₂ inside the boiler, and that CO₂ can travel with steam, re-dissolve in condensate, lower pH, and create a corrosive environment.
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When acid feed stopped, CO₂ stripping continued unchecked. As CO₂ left, equilibrium shifted: carbonic acid decomposed, bicarbonate consumed H⁺, and pH rose. Once pH passed ≈8.3, carbonate was no longer suppressed - it was permitted to exist. With calcium already present and 60,000 sq ft of wetted surface area, CaCO₃ precipitated naturally and relentlessly. pH did not cause the scale - it gave carbonate permission.
⟵ 05 Carbonated Water (Problem Set) · Contents · 06 Destructive Oxidation ⟶