By Kevin Beck Updated Aug 30, 2022
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Even the most science‑averse among us encounter the term “pH” in everyday life—from shampoo ads to your home aquarium. The pH scale is a chemist’s tool for quantifying how acidic or alkaline a solution is, and it’s indispensable in fields ranging from medical diagnostics to environmental monitoring.
The abbreviation pH stands for “potential of hydrogen ion.” Danish biochemist Søren Sørensen coined the term in 1909, defining it as the negative base‑10 logarithm of the hydrogen‑ion concentration, [H⁺]. Mathematically:
pH = -\log₁₀[H⁺]
This logarithmic relationship means each unit change on the scale corresponds to a ten‑fold change in [H⁺]. A solution with a pH of 5.0 contains ten times the hydrogen ions of one with a pH of 6.0.
In chemistry, the number of particles—not their mass—determines reactivity. One mole equals Avogadro’s number (6.02 × 10²³) of entities. The molar mass of an element, listed in its periodic‑table “box,” tells how many grams one mole weighs.
For example, sodium chloride (NaCl) has a molar mass of 58.5 g/mol. Dissolving 5.85 g of NaCl in 1 L of water yields a 0.10 M solution:
5.85 g ÷ 58.5 g/mol = 0.10 mol
Such a concentration corresponds to 0.10 mol L⁻¹ of dissolved ions.
A logarithm compresses wide numerical ranges into manageable values. In the pH context, each decade (factor of ten) in hydrogen‑ion concentration shifts the pH by one integer unit. This scale is why a “neutral” pH of 7 indicates that [H⁺] equals [OH⁻] in pure water.
Laboratory pH meters use a glass electrode that responds to the potential difference between the test solution and a 1 M hydrogen‑ion reference. The electrode’s voltage is converted into a pH value via calibration curves.
Typical pH values illustrate its importance:
Blood contains bicarbonate (HCO₃⁻), a natural buffer that neutralizes excess H⁺ ions and maintains pH near 7.4. Antacids, which accept protons, mitigate stomach acidity by forming water molecules from hydroxyl groups.
Example 1: What is the pH of a solution with [H⁺] = 4.9 × 10⁻⁷ M?
pH = −log₁₀(4.9 × 10⁻⁷) = 6.31
Example 2: What is [H⁺] in a solution with pH = 8.45?
8.45 = −log₁₀[H⁺] ⇒ [H⁺] = 10⁻⁸.⁴⁵ = 3.5 × 10⁻⁹ M
Use the Online pH Calculator to explore how acid identity and concentration affect pH. Experiment with different acids in the drop‑down menu and observe how a weaker acid at a higher molarity can yield a lower pH than a dilute strong acid.
