Looking at Secondary Data – Chlorinating Water | 解读二手数据——水的氯化消毒

📚 Looking at Secondary Data – Chlorinating Water | 解读二手数据——水的氯化消毒

In science, you will often meet data that you did not collect yourself. Water companies publish measurements of chlorine levels in drinking water, environmental agencies publish the results of monitoring programmes that run for years, and textbooks print tables copied from research papers. When you use this kind of information, you are working with secondary data. The skill is not just reading the numbers but also judging how reliable and how useful they are for the question you are trying to answer.

在科学中，你会经常遇到并非自己收集的数据。自来水公司会公布饮用水中氯含量的测量结果，环境机构会发布持续多年的监测项目结果，教科书里也会印着从研究论文中摘录的表格。使用这类信息时，你就是在处理二手数据。这项技能不只是读取数字，还要判断这些数据对你想要回答的问题有多可靠、多有用。

1. Why Drinking Water Is Chlorinated | 为什么饮用水要氯化消毒

Raw water taken from rivers, lakes and boreholes contains bacteria and viruses that cause diseases such as cholera, typhoid and dysentery. Treating water with chlorine kills these microorganisms and has saved millions of lives since the early 1900s. Chlorine is added as chlorine gas, as sodium hypochlorite solution or as calcium hypochlorite granules. In water, chlorine reacts to form hypochlorous acid:

Cl₂ + H₂O → HCl + HOCl

Hypochlorous acid HOCl is the active disinfectant. It is a weak acid, so it partly dissociates in water:

HOCl ⇌ H⁺ + OCl⁻

从河流、湖泊和井中取来的原水里含有细菌和病毒，会引起霍乱、伤寒和痢疾等疾病。用氯处理水可以杀死这些微生物，自 20 世纪初以来已经挽救了数以百万计的生命。氯以氯气、次氯酸钠溶液或次氯酸钙颗粒的形式加入水中，与水反应生成次氯酸。次氯酸 HOCl 是真正的消毒活性物质，它是一种弱酸，在水中会部分电离，生成氢离子和次氯酸根离子 OCl⁻。

Both HOCl and the hypochlorite ion OCl⁻ kill microbes. A small amount, called the residual or free chlorine, is deliberately left in the water so that it stays safe all the way from the treatment works to the kitchen tap. Too little residual chlorine and bacteria may grow again inside the pipes; too much and the water tastes and smells of a swimming pool, and more chlorination by-products such as trihalomethanes can form. Water companies therefore monitor chlorine constantly and keep it inside a narrow permitted band.

HOCl 和次氯酸根离子 OCl⁻ 都能杀死微生物。水中会被刻意保留少量氯，称为余氯或游离氯，使水从水厂一路到厨房水龙头都保持安全。余氯太少，细菌可能在管道里重新繁殖；余氯太多，水就会有游泳池的味道和气味，还会生成更多氯化副产物，例如三卤甲烷。因此自来水公司会持续监测氯含量，把它控制在一个很窄的允许范围内。

2. Primary and Secondary Data | 一手数据与二手数据的区别

Secondary data is data collected by someone else. It can come from reports, websites, textbooks, newspapers or databases. Primary data, in contrast, is collected by you through your own experiment or observation. In a school laboratory you might test a water sample with a chlorine test strip and read the colour against a chart. That is primary data, because you measured it yourself and you know exactly how it was obtained.

二手数据是由其他人收集的数据，它可以来自报告、网站、教科书、报纸或数据库。相反，一手数据是你通过自己的实验或观察收集到的。在学校实验室里，你可能会用氯测试条检测水样，再根据色卡读出结果。那是一手数据，因为是你自己测的，你清楚它是怎么得来的。

If you then look at a water company's table of chlorine readings from many treatment works, that is secondary data. The numbers were produced by other people, using instruments you have never seen, at times you did not choose. Secondary data is not automatically worse than primary data. It is often far larger, covers years instead of minutes, and has been checked by professional analysts. The problem is that it was collected for someone else's purpose, not for yours.

如果你随后查看自来水公司从许多水厂收集的氯含量读数表格，那就是二手数据。这些数字是别人用你从未见过的仪器、在你没有选择的时间测出来的。二手数据并不一定比一手数据差：它往往规模大得多，覆盖的是数年而不是几分钟，而且经过专业分析人员的核查。问题在于，它是为别人的目的而收集的，不是为你的目的。

3. How Chlorine Is Measured | 氯含量是怎样测量的

Most field measurements of chlorine use the DPD method. DPD is a colourless reagent; when it is added to water containing free chlorine, the sample turns pink. The intensity of the pink colour is proportional to the chlorine concentration, so the analyst compares the sample with a colour disc or measures its absorbance in a colorimeter or spectrophotometer. Test strips work in the same way but are much less precise and are easily misread.

野外测量氯含量最常用的方法是 DPD 法。DPD 是一种无色试剂，加入含游离氯的水样后，水样会变成粉红色。粉红色的深浅与氯浓度成正比，因此分析人员可以把水样与比色盘比较，或者用比色计、分光光度计测量其吸光度。测试条的原理相同，但精度低得多，也很容易读错。

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Large treatment works use online analysers that sample the water every few minutes and log the result automatically. That is why a single water company can publish thousands of readings a year. Two details matter when you evaluate such data: whether the value is free chlorine or total chlorine, and whether the sample was taken before or after the water passed through the pipes. Free chlorine and total chlorine are different quantities, so mixing them in one table makes comparison meaningless.

大型水厂使用在线分析仪，每隔几分钟取样一次并自动记录结果，所以一家自来水公司一年可以公布成千上万个读数。评价这类数据时有两个细节很重要：数值是游离氯还是总氯，以及水样是在水流经管道之前还是之后取的。游离氯和总氯是不同的量，把它们混在同一张表里比较就失去了意义。

4. Units, Concentrations and Standards | 单位、浓度与标准

Chlorine in drinking water is measured in milligrams per cubic decimetre, written mg dm⁻³. Because 1 dm³ is exactly 1 litre, mg dm⁻³ and mg/L mean the same thing. In very dilute aqueous solutions, 1 mg dm⁻³ is almost exactly 1 part per million, or 1 ppm, so the older unit ppm is still seen in reports.

饮用水中的氯用毫克每立方分米表示，写作 mg dm⁻³。由于 1 dm³ 正好等于 1 升，mg dm⁻³ 与 mg/L 含义完全相同。在很稀的水溶液中，1 mg dm⁻³ 几乎正好是百万分之一，即 1 ppm，所以报告中仍能见到 ppm 这个旧单位。

Water quality regulations in England and Wales require a residual of at least 0.5 mg dm⁻³ leaving the treatment works and at least 0.1 mg dm⁻³ at the consumer's tap, while the World Health Organization gives a guideline maximum of 5 mg dm⁻³ for free chlorine. These numbers are useful anchors: a published reading of 0.2 mg dm⁻³ at a tap is normal, but a reading of 12 mg dm⁻³ would be far outside the guideline and worth questioning.

英格兰和威尔士的饮用水法规要求水厂出水的余氯至少为 0.5 mg dm⁻³，用户水龙头处至少为 0.1 mg dm⁻³；世界卫生组织给出的游离氯指导值上限是 5 mg dm⁻³。这些数字是有用的参照：公布的水龙头读数 0.2 mg dm⁻³ 属于正常，而 12 mg dm⁻³ 则远远超出指导值，值得追问原因。

Always carry the unit through your working. A table headed simply 'chlorine' is ambiguous, and a value written as 0.5 could mean 0.5 mg dm⁻³, 0.5 μg dm⁻³ or 0.5 mmol dm⁻³, which differ by factors of a thousand or more.

计算时一定要始终带着单位。一张只写 'chlorine' 的表是有歧义的，写成 0.5 的数值可能是 0.5 mg dm⁻³、0.5 μg dm⁻³ 或 0.5 mmol dm⁻³，彼此相差一千倍甚至更多。

5. Reading a Water Company's Data Table | 阅读自来水公司的数据表

The table below shows free chlorine readings published by one treatment works over five days. Samples were taken at the works itself and at two points along the distribution main. Read the column headings first, then look for the pattern before you calculate anything.

下表是某水厂连续五天公布的游离氯读数，取样点分别是水厂本身和配水干管上的两个位置。先读列标题，再看规律，之后才做计算。

Two patterns stand out. First, the works outlet is remarkably steady, staying between 0.48 and 0.52 mg dm⁻³. Second, the concentration falls as the water travels further from the works, from about 0.50 mg dm⁻³ to about 0.22 mg dm⁻³ over 8 km. Chlorine is used up reacting with organic matter and pipe surfaces, and it also escapes slowly as gas.

两个规律很明显。第一，水厂出水非常稳定，始终在 0.48 至 0.52 mg dm⁻³ 之间。第二，随着水离水厂越来越远，浓度不断下降，8 km 之外从约 0.50 mg dm⁻³ 降到约 0.22 mg dm⁻³。氯会与有机物和管壁反应而被消耗，也会缓慢以气体形式逸出。

6. Judging Reliability | 判断数据的可靠性

Reliability means that the data would give the same picture if the measurement were repeated. For secondary data you cannot repeat the measurement yourself, so you judge reliability from the way the data is presented. Ask who collected it, when, with what method, and how many samples were taken.

可靠性指的是：如果重复测量，数据会给出同样的结果。对于二手数据，你无法自己去重复测量，所以只能根据数据的呈现方式来判断可靠性。要问：是谁收集的？什么时候收集的？用什么方法？取了多少个样本？

• Who produced the data? A regulator, a university group or an accredited laboratory is usually more reliable than an anonymous web page.
  数据是谁产生的？监管机构、大学研究组或经认可的实验室，通常比匿名网页更可靠。
• How many samples? A single reading is an anecdote; hundreds of readings across a year show a real pattern.
  有多少个样本？单个读数只是奇闻轶事，一年中数百个读数才能显出真实规律。
• Is the method stated? A described method can be checked and criticised; a missing method cannot.
  方法
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