Understanding the pH Scale and Its Applications | 理解pH标度及其应用

📚 Understanding the pH Scale and Its Applications | 理解pH标度及其应用

The pH scale is one of the most practical and widely used concepts in science. It connects chemistry with biology, environmental science, and everyday life. This article will guide you through the definition of pH, how it is measured, the chemistry behind acids and alkalis, and why pH matters in living organisms and industrial processes.

pH（酸碱度）标度是科学中最实用、应用最广泛的概念之一。它将化学与生物学、环境科学以及日常生活联系起来。本文将带你了解pH的定义、测量方法、酸碱背后的化学原理，以及为什么pH在生物体和工业过程中如此重要。

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1. What is pH? | 什么是pH？

pH is a numerical scale used to specify the acidity or alkalinity of an aqueous solution. The term “pH” stands for “potential of hydrogen” and is related to the concentration of hydrogen ions (H⁺) in the solution. In pure water at 25 °C, the concentration of H⁺ is 1 × 10⁻⁷ mol/dm³, which gives a neutral pH of 7.

pH是用于表示水溶液酸性或碱性的数值标度。“pH”代表“氢离子浓度指数”，与溶液中氢离子（H⁺）的浓度有关。在25°C时，纯水中H⁺的浓度为1 × 10⁻⁷ mol/dm³，对应的pH为7，即中性。

The pH scale is logarithmic, which means that each whole number change represents a tenfold change in H⁺ concentration. A solution with pH 3 has ten times more H⁺ ions than a solution with pH 4, and one hundred times more than a solution with pH 5.

pH标度是对数标度，这意味着每变化一个整数，H⁺浓度就变为原来10倍。pH为3的溶液中H⁺离子浓度是pH为4溶液的10倍，是pH为5溶液的100倍。

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2. The pH Scale | pH标度

The pH scale usually ranges from 0 to 14. Values from 0 to 6 are considered acidic, 7 is neutral, and values from 8 to 14 are alkaline (also called basic). The lower the pH, the more acidic the solution; the higher the pH, the more alkaline it is.

pH标度通常范围在0到14之间。0到6为酸性，7为中性，8到14为碱性（也称为碱性溶液）。pH越低，酸性越强；pH越高，碱性越强。

Range	Category	Example
0–2	Strong acid	Stomach acid
3–6	Weak acid	Vinegar, lemon juice
7	Neutral	Pure water
8–11	Weak alkali	Baking soda
12–14	Strong alkali	Drain cleaner

Substances that do not dissolve in water to form ions are often neither acidic nor alkaline. However, many substances can be tested as solutions in water. The pH of a solution can be measured using a pH meter or by using indicators that change colour at different pH values.

不溶于水形成离子的物质通常既非酸性也非碱性。然而，许多物质可以在水中配成溶液进行测试。溶液的pH可以使用pH计或在不同pH值下变色的指示剂来测量。

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3. Acids and Alkalis | 酸和碱

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. For example, hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl⁻. An alkali is a soluble base that produces hydroxide ions (OH⁻) in water. For example, sodium hydroxide (NaOH) dissociates to form Na⁺ and OH⁻.

酸是溶于水时产生氢离子（H⁺）的物质。例如，盐酸（HCl）在水中解离形成H⁺和Cl⁻。碱（可溶性碱）是溶于水时产生氢氧根离子（OH⁻）的物质，其溶水时形成OH⁻。例如，氢氧化钠（NaOH）解离形成Na⁺和OH⁻。

In aqueous solution, the concentration of H⁺ and OH⁻ are linked. If you add an acid to water, the H⁺ concentration increases and the OH⁻ concentration decreases. The pH value falls. If you add an alkali, the OH⁻ concentration increases and the H⁺ concentration decreases, so the pH value rises.

在水溶液中，H⁺和OH⁻的浓度是相互关联的。如果将酸加入水中，H⁺浓度增大，OH⁻浓度减小，pH值下降。如果加入碱，OH⁻浓度增大，H⁺浓度减小，pH值上升。

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4. Measuring pH: Indicators and pH Meters | 测量pH：指示剂和pH计

There are several ways to measure pH. Universal indicator is a mixture of dyes that gives a distinct colour at different pH values. A few drops are added to the test solution, and the colour is compared with a colour chart. This method is quick and convenient, but not very precise.

测量pH有几种方法。万能指示剂是多种染料的混合物，在不同pH下呈现不同颜色。在待测溶液中滴加几滴，并将颜色与比色卡比较。这种方法快速方便，但不够精确。

A pH meter is an electronic instrument that measures the voltage between two electrodes immersed in the solution. It provides a numerical pH reading to one decimal place or more. pH meters require calibration with buffer solutions of known pH before use.

pH计是一种电子仪器，通过测量浸入溶液中的两个电极之间的电压来工作。它可提供精确到一位或更多小数的pH数值。使用前需要用已知pH的缓冲溶液进行校准。

• Universal indicator gives a rough pH (whole number).

  万能指示剂只能给出大致pH（整数）。

• Litmus paper only tells you whether a solution is acidic, alkaline, or neutral.

  石蕊试纸只能判断溶液是酸性、碱性还是中性。

• pH meter gives a precise reading.

  pH计可给出精确读数。

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5. Neutralisation Reactions | 中和反应

Neutralisation is the reaction between an acid and a base (or alkali) to form salt and water. The H⁺ ions from the acid combine with the OH⁻ ions from the alkali to form water. The general equation is:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The ionic equation above shows the essential change. For example, when hydrochloric acid reacts with sodium hydroxide:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Neutralisation also occurs when an acid reacts with an insoluble base, such as copper(II) oxide, to form a salt and water. This is important in the preparation of soluble salts.

中和反应是酸与碱（或碱性物质）反应生成盐和水的过程。酸中的H⁺与碱中的OH⁻结合形成水。上述离子方程式展示了本质变化。例如，盐酸与氢氧化钠反应时：NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)。中和反应也发生在酸与不溶性碱（如氧化铜）反应生成盐和水时，这在可溶性盐的制备中很重要。

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6. pH and Enzymes | pH与酶

Enzymes are biological catalysts that speed up chemical reactions in living organisms. Each enzyme has an optimum pH at which it works fastest. For most enzymes in the human body, the optimum pH is close to neutral (around pH 7), but there are important exceptions.

酶是生物体内加速化学反应的生物催化剂。每种酶都有一个最适pH，在这个pH下活性最高。人体中大多数酶的最适pH接近中性（约pH 7），但也有重要例外。

For example, pepsin is a digestive enzyme in the stomach that works best at pH 2. In contrast, trypsin, an enzyme in the small intestine, works best at pH 8. If the pH moves too far from the optimum, the enzyme’s active site changes shape and the enzyme becomes denatured. This means it can no longer catalyse its reaction.

例如，胃蛋白酶是胃中的消化酶，最适pH为2。相反，小肠中的胰蛋白酶最适pH为8。如果pH偏离最适值太远，酶的活性位点形状改变，酶就会变性，从而失去催化功能。

Many organisms regulate pH inside their cells and body fluids. Human blood has a pH of about 7.40, and even small changes can be dangerous. The body uses buffer systems to keep blood pH within a narrow range.

许多生物体调节细胞和体液内部的pH。人体血液的pH约为7.40，稍有变化就可能危险。人体使用缓冲体系将血液pH维持在狭窄范围内。

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7. Strong and Weak Acids | 强酸和弱酸

In the IGCSE course, you need to distinguish between strong and weak acids. A strong acid is fully ionised in water, meaning that all acid particles dissociate to release H⁺ ions. Hydrochloric acid, sulfuric acid, and nitric acid are common strong acids.

在IGCSE课程中，你需要区分强酸和弱酸。强酸在水中完全电离，即所有酸颗粒都解离释放H⁺离子。盐酸、硫酸和硝酸是常见的强酸。

A weak acid is only partially ionised in water. Ethanoic acid, citric acid, and carbonic acid are examples. For the same concentration, a weak acid has a higher pH than a strong acid because fewer H⁺ ions are produced.

弱酸在水中仅部分电离。乙酸、柠檬酸和碳酸是例子。在相同浓度下，弱酸的pH比强酸高，因为产生的H⁺离子较少。

It is important to note that concentration (mol/dm³) and strength (fully vs. partially ionised) are different. A dilute strong acid can have a higher pH than a concentrated weak acid, but still be a strong acid because it is fully ionised.

注意，浓度（mol/dm³）和强度（完全电离与部分电离）是不同的概念。稀的强酸可能比浓的弱酸pH更高，但它仍是强酸，因为它完全电离。

pH = -log₁₀[H⁺]

This formula allows you to calculate pH from hydrogen ion concentration. In IGCSE, you may be asked to interpret pH values qualitatively rather than perform calculations.

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Published by TutorHao | IGCSE Science Revision Series | aleveler.com

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