酸硷值(pH)
溶液中酸（和硷）的强度的定量量度，其定义为用克分子／升表示的氢离子（H+）浓度的自然对数的负数：pH = -log10H+〕。其名源自这样的事实，即pH是氢离子浓度（H）的幂数（p）。水中H+和OH-（氢氧化物离子）的浓度的乘积总在10～14左右。最强的酸溶液约有1克分子／升的H+（约10～14的OH-），对应於pH=1。最强的硷溶液约有10～14克分子／升的H+（约1的OH-），对应pH=14。中性溶液的H+和OH-的浓度大约都是10～7克分子／升，对应pH=7。用酸硷度计、滴定或指示剂（例如石蕊）条来测定pH值，可以帮助化学家了解物质的性质、组分或反应的程度，帮助生物学家了解有机体或它们的各部分或流体的组成和环境，帮助医生了解整个身体系统的功能情况，帮助农学家们了解土壤对作物的适应性，是否需要作任何处理等。现在已用电化学的表示方法来定义pH了。
English version：
pH
Quantitative measure of the strength of the acid (and base) in a solution, defined as the negative common logarithm of the concentration of hydrogen ions (H+) in moles/liter (written in square brackets): pH = −log10{lsqb}H+{rsqb}. Its name comes from the fact that pH is the absolute value of the power (p) of the hydrogen ion concentration (H). The product of the concentrations in water of H+ and OH− (the hydroxide ion) is always about 10−14. The strongest acid solution has about 1 mole/liter of H+ (and about 10−14 of OH−), for a pH of 1. The strongest basic solution has about 10−14 moles/liter of H+ (and about 1 of OH−), for a pH of 14. A neutral solution has about 10−7 moles/liter of both H+ and OH−, for a pH of 7. The pH value, measured by a pH meter, titration, or indicator (e.g., litmus) strips, helps inform chemists of the nature, composition, or extent of reaction of substances, biologists of the composition and environment of organisms or their parts or fluids, physicians of the functioning of bodily systems, and agronomists of the suitability of soils for crops and any treatments needed. The pH is now defined in electrochemical terms (see electrochemistry).