Bicarbonate Calculator

Alkalinity and bicarbonate are not the same thing — though in many natural waters they are nearly equivalent. Alkalinity measures a water's total capacity to neutralize acids (contributed by bicarbonate, carbonate, hydroxide, and other bases), while bicarbonate concentration quantifies the specific HCO₃⁻ species. At the pH range of most natural waters (6.5–8.5), bicarbonate dominates alkalinity so completely that the two are nearly interchangeable; but in swimming pools, industrial process water, or unusual natural settings, the distinction matters significantly. The bicarbonate calculator resolves this using the full carbonate equilibrium system.

Carbonate Equilibrium: The Three Species

In water, dissolved carbon dioxide exists as three interconverting species depending on pH:

CO₂(aq) + H₂O ⇌ H₂CO₃* ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺

The relative proportions of these three species depend entirely on pH, through the two carbonate equilibrium constants:

• pKa₁ = 6.35 at 25°C: equilibrium between CO₂(aq) and HCO₃⁻; below pH 6.35, CO₂ dominates; above pH 6.35, HCO₃⁻ begins to dominate
• pKa₂ = 10.33 at 25°C: equilibrium between HCO₃⁻ and CO₃²⁻; above pH 10.33, CO₃²⁻ dominates

In the pH range 6.5–8.5 (virtually all natural freshwaters and drinking waters), bicarbonate (HCO₃⁻) is the dominant carbonate species. The fraction as bicarbonate: f_HCO₃ = [H⁺]Ka₁ / ([H⁺]² + [H⁺]Ka₁ + Ka₁Ka₂). At pH 7.5: f_HCO₃ > 99.9%. Use this online calculator to convert your alkalinity measurement to bicarbonate concentration. The alkalinity calculator performs the complementary conversion.

Alkalinity Measurement and Units

Alkalinity is measured by acid titration — typically to an endpoint of pH 4.3–4.5 (the "total alkalinity" or "M alkalinity" in water analysis). Common units and conversion factors:

• mg/L as CaCO₃: the most common reporting unit in the US; divide by 50.04 to convert to meq/L
• meq/L (milliequivalents per liter): the dimensionless chemical unit; multiply by 61 to get mg/L HCO₃⁻ at pH where HCO₃⁻ dominates
• mmol/L (millimoles per liter): numerically equal to meq/L for bicarbonate (monovalent); equals 0.5 × meq/L for carbonate (divalent)
• German hardness degrees (°dH): 1 °dH = 17.85 mg/L as CaCO₃

Aquaculture and Aquarium Applications

Bicarbonate concentration (KH — "carbonate hardness" in the aquarium hobby) is critical for fish and coral health:

• Freshwater fish: optimal KH 4–12 °dH (70–215 mg/L HCO₃⁻); below 3 °dH, pH crashes are possible from CO₂ accumulation or acidic fish waste without adequate buffering
• Marine aquariums: target alkalinity 8–12 dKH (140–215 mg/L HCO₃⁻); coral calcium carbonate skeleton deposition requires both adequate calcium and alkalinity
• Trout farming: minimum 50 mg/L alkalinity as CaCO₃; below this, CO₂ toxicity risk increases as the water's ability to buffer dissolved CO₂ is reduced

The water hardness calculator and water chemistry calculators provide complementary aquatic chemistry tools.

Medical Context: Blood Bicarbonate and Acid-Base Status

In clinical medicine, bicarbonate is the primary metabolic acid-base buffer in blood. Normal serum bicarbonate: 22–26 mEq/L (mmol/L). Bicarbonate deviates in acid-base disorders: metabolic acidosis (HCO₃⁻ below 22 mEq/L) — diabetic ketoacidosis, lactic acidosis, renal failure; metabolic alkalosis (HCO₃⁻ above 26 mEq/L) — prolonged vomiting, diuretic use, primary hyperaldosteronism. The Henderson-Hasselbalch equation relates pH, bicarbonate, and partial CO₂ pressure in blood: pH = 6.1 + log([HCO₃⁻] / (0.0307 × pCO₂_mmHg)), enabling calculation of any one variable from the other two in clinical acid-base interpretation.