什么是通用指标？

酸是溶解在水中时会产生游离氢离子（H+ 离子）的化合物。当碱溶解在水中时，它们会形成羟基离子（OH-离子）。氢离子在酸性溶液中含量丰富，在碱性溶液中有限。 pH标度用于确定酸或碱的酸度或碱度。 pH值范围从0到14。由几种化合物组成的 pH 指示剂被称为通用指示剂。

酸的 pH 值

酸或酸性溶液的 pH 值小于 7。pH 值小于 7 的溶液称为酸性溶液。例如，pH 值为 4 表示溶液呈酸性。应该记住，溶液的 pH 值越低，酸性越强。例如，pH 1 溶液的酸性明显高于 pH 4 溶液。换句话说，pH 1 的溶液比 pH 4 的溶液强得多。强酸通常定义为 pH 值为 0、1、2 或 3 的溶液。弱酸溶液的 pH 值为 4、5 或 6。很明显，物质的酸度与其 pH 成正比。强酸性化学品的 pH 值非常低。事实上，酸越强，pH 值越低。

Since all solutions with a pH less than 7 are acidic in nature, they turn blue litmus red. In addition, they cause the methyl orange indicator to turn red.

碱基的 pH 值

碱或碱性溶液的 pH 值大于 7。当溶液的 pH 值大于 7 时，称为碱性溶液。例如，pH 值为 12 的溶液本质上是碱性的或碱性的。应该记住，溶液的 pH 值越高，其碱性越强。例如，pH 值为 14 的溶液比 pH 值为 11 的溶液碱性明显更强。换句话说，pH 值为 14 的溶液比 pH 值为 11 的酸强得多。强碱通常定义为具有a pH 值为 11、12、13 或 14。弱碱定义为 pH 值为 8、9 和 10 的溶液。强碱性化合物的 pH 值极高。事实上，碱越强，pH 值就越高。

Since all solutions with a pH greater than 7 are basic in nature, they turn red litmus blue. In addition, they make the phenolphthalein indicator pink.

中性物质的 pH 值

中性物质的pH值正好是7。纯水既不酸性也不碱性；它是一种中性物质。因此，纯水的 pH 值为 7。糖溶液和氯化物溶液都是中性的，pH 值为 7。因此，任何 pH 值为 7 的溶液都被认为是中性的。石蕊试纸以及甲基橙和酚酞等其他常见指示剂对 pH 值为 7 的材料没有影响。

通用指标

石蕊是确定化学物质是酸还是碱的常用指标。他们无法确定化学物质是强酸、弱酸、强碱还是弱碱。换句话说，典型的指标不能告诉我们酸或碱有多强。例如，石蕊可以确定硫酸和醋都是酸性的，但不能确定硫酸是比醋溶液更强的酸还是更酸。然而，我们可以使用一种称为“通用指示剂”的特定形式的指示剂，它通过检测溶液的 pH 值来确定酸性或碱性溶液的强度。

A universal indicator is a common method of measuring the pH of a solution in a school laboratory. A universal indicator is a mixture of several different indicators or dyes that produce varied colours at various pH levels across the pH scale. 

我们还可以注意到，由于溶液的 pH 值取决于氢离子浓度，因此通用指示剂在溶液中不同浓度的氢离子时会显示不同的颜色。当将酸或碱溶液加入通用指示剂时，它会呈现出新的颜色。通过将通用指示剂的颜色与 pH 比色图上的颜色进行比较，可以确定酸或碱溶液的 pH 值。我们可以通过查看 pH 值来判断溶液是强酸、弱酸、强碱还是弱碱。通用指示剂在各种 pH 值下产生的颜色如下所示。

pH	Colour
0	Dark red
1	Red
2	Red
3	Orange-red
4	Orange
5	Orange-yellow
6	Greenish-yellow
7	Green
8	Greenish blue
9	Blue
10	Navy blue
11	Purple
12	Dark purple
13	Violet
14	Violet

通用指示剂制造商给出了一个 pH 比色表，可用于比较加入酸或碱溶液确定 pH 值后在试纸或指示溶液上产生的颜色。通用指示剂和石蕊一样，可以溶液的形式或通用试纸的形式使用。

How a universal indicator paper is used to measure the pH value of a solution?

On a strip of universal indicator paper, a drop of the solution to be tested is put. The colour of the indicator paper will change where the solution has been applied. The colour on the indicator paper is then compared to a colour on the standard pH colour chart. The pH value corresponding to this colour may be found on the pH colour chart. We can tell whether a solution is a strong acid, a weak acid, a strong base, or a weak base based on its pH value.

If a drop of a solution is placed on universal indicator paper and the paper turns dark red, the pH of the solution is near zero, indicating that it is a strong acid. However, if an orange colour is generated, the pH will be about 4 and the acid will be weak. Different colours can also be created using a different base solution. Weakly basic solutions are blue, whereas extremely basic solutions are violet with a universal indicator. If a solution turns universal indicator green, it is a neutral solution that is neither acidic nor basic. As a result, water will take on a green colour when tested with a universal indicator.

示例问题

问题1：蓝色石蕊的颜色被溶液变成红色。溶液的 pH 值是多少？

回答：

A solution that changes the colour of blue litmus to red is acidic. Acidic solutions are those that have a pH of less than 7. As a result, the solution’s pH will be less than 7.

问题2：鲜奶的pH值是6。牛奶变成凝乳的pH值是多少？

回答：

When milk turns into curd, it becomes acidic, and its pH drops below 6.

问题 3：如果用通用指示剂测试溶液的 pH 值为 1，那么该溶液是强酸还是弱酸？

回答：

A pH of 1 denotes an extremely acidic solution. As a result, this solution is a strong acid.

问题 4：解决方案将红色石蕊的颜色变为蓝色。溶液的 pH 值是多少？

回答：

In nature, a solution that turns red litmus blue is basic. All solutions with a pH greater than 7 are considered basic. As a result, the pH of the solution will exceed 7.

问题5：哪种溶液的氢离子浓度更大？溶液 A 的 pH 值为 4，而溶液 B 的 pH 值为 9。

回答：

The concentration of hydrogen ions in a solution is inversely proportional to its pH. This means that a solution with a lower pH will have a higher concentration of hydrogen ions. Since solution A has a lower pH than solution B, solution A will have a higher hydrogen ion concentration.