By Chris Deziel Updated Mar 24, 2022
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Immediately after distillation, water is neutral (pH 7). Within a few hours, it absorbs CO₂ from the air and drops to a slightly acidic pH of about 5.8.
pH is the negative logarithm of the hydrogen‑ion concentration in a solution. A higher pH means fewer free protons, while a lower pH indicates a higher proton concentration. The scale ranges from 0 to 14: values below 7 are acidic, 7 is neutral, and values above 7 are basic.
In the Bronsted–Lowry framework, an acid donates protons (H⁺) in water, and a base accepts them. Strong acids like HCl dramatically lower pH, whereas strong bases such as NaOH raise it. When an acid and a base meet, they neutralise each other and form a salt; for example, HCl + NaOH → NaCl + H₂O.
Distillation removes virtually all dissolved solids and volatile impurities. In an ideal setup—boiling the water, condensing the vapor, and collecting the condensate—the resulting liquid should be chemically pure and, in theory, have a pH of exactly 7.
Even after distillation, water is not entirely immune to atmospheric gases. Carbon dioxide (CO₂) readily dissolves in water, forming carbonic acid (H₂CO₃). This acid dissociates to release hydronium ions (H₃O⁺), which lower the pH:
2 H₂O + CO₂ → H₂O + H₂CO₃ → H₃O⁺ + HCO₃⁻
It typically takes about two hours for a freshly distilled sample to equilibrate with the ambient CO₂ concentration and reach its final pH of roughly 5.8.
