pHpH

水的离子积 Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴（25 °C），所以 pH + pOH = 14。pH 每改变 1 个单位，[H⁺] 变化 10 倍。Kw 随温度升高而增大，100 °C 时纯水 pH 约 6.14，但仍是中性，因为 [H⁺] = [OH⁻]。浓度很高的强酸 pH 可以小于 0。

The ion product of water, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, gives pH + pOH = 14. Each unit of pH is a tenfold change in [H⁺]. Kw grows with temperature, so pure water at 100 °C has a pH of about 6.14 yet is still neutral because [H⁺] = [OH⁻]. Very concentrated strong acids can have a pH below 0.

1909 年丹麦嘉士伯实验室的化学家索伦森在研究酶活性与酸度的关系时提出 pH 标度，最初写作 pᴴ，p 来自德语 Potenz（幂）或拉丁语 pondus。1924 年他根据活度概念修订了定义。现行国际定义以标准缓冲溶液和电动势测量为基础。

In 1909 Sørensen, a chemist at the Carlsberg Laboratory in Copenhagen studying how acidity affects enzymes, introduced the pH scale, first written pᴴ; the p is usually taken from German Potenz (power) or Latin pondus. In 1924 he revised the definition in terms of activity. The current international definition rests on standard buffers and EMF measurements.

粗测用 pH 试纸或酸碱指示剂，精确测量用玻璃电极 pH 计（1934 年贝克曼推出商用仪器）。人体血液 pH 维持在 7.35–7.45，胃液约 1.5–3.5；土壤、饮用水、发酵、电镀和污水处理都需要控制 pH。

pH paper or indicators give rough values; glass-electrode pH meters, commercialised by Beckman in 1934, give precise ones. Human blood is held at pH 7.35–7.45 and gastric juice at about 1.5–3.5; soils, drinking water, fermentation, electroplating and wastewater treatment all require pH control.