化学 1834

电学实验研究·第七辑

迈克尔·法拉第

化学变化，由电量来计量。

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In depth · the introduction

让电流穿过一种化学物，它就会分解——而法拉第发现，究竟分解出多少，竟精确到毫克，全由你送进去多少电来决定。

核心想法

让电流穿过一种含有溶解化合物的液体，化合物便会裂开，碎片聚到浸在其中的两片金属上——一把便宜的叉子镀上一层真银，靠的正是这个。法拉第的发现是：这个过程并不含糊，而是精确的——电量加倍，得到的金属就恰好加倍。析出的量，与流过的总电荷成正比。

他还发现了第二条规则。同一份电量析出的不同元素，其质量按各自的「当量」成比例。银——它每需一份电荷都很「重」——在同一电流下析出的质量，远多于轻盈的铝。电荷，就此成了化学家能够称量的东西。

它是如何诞生的

1830 年代初，法拉第——曾是装订学徒，那时已是伦敦皇家研究院的明星实验家——正在理清：来自电池、火花机、磁铁与电鱼的电，是不是同一回事。他造了一些小电池，用它们析出的金属之重、或产生的气体之量，来记录通过了多少电。

一只接一只，一种溶液接一种溶液，只有一条规则始终成立：化学变化只随电荷而变，别无其他。为了描述所见，他需要旧理论尚未占有的词，于是写信给剑桥学者威廉·休厄尔，请他造词——离子、阳极、阴极、电极、电解质——这些词如今在每一堂化学课上都被说起。

它为何重要

它让电可以用化学家的天平来测量，把电镀、金属精炼与电池制造，从凭经验的猜测变成精确的工艺。而那份奇异的精确——每一「当量」物质对应一份固定的电——是一个早期的暗示：电本身也是按可数的单位出现的。几十年后，这个暗示，化作了电子。

一个可以想象的画面

把它想成一台「原子自动售货机」。每一枚硬币，是一份电荷，由一个电子携带。释放一个银离子要一枚硬币，一个铜离子要两枚，一个铝离子要三枚。投进固定数目的硬币，你便能预先准确说出：每种金属会掉出多少个原子——而因为不同元素的原子轻重不同，同样的硬币，买到的质量大不相同。法拉第定律，就是这台机器精确的价目表。

它的位置

伏打 1800 年的电池让稳定电流成为可能，戴维曾用它把化合物撕开、分离出钠和钾。法拉第——戴维当年的助手——把这种定性的本领变成了一条定律。他那份固定的「每当量对应的电荷」，向前指向斯托尼与汤姆孙的电子，也指向能斯特（1889），后者将为这些同样的离子所产生的电压标上数字——而法拉第所造的词，至今仍在标注你手中每一节电池的阳极、阴极与电解质。

The original document

Original source text

Michael Faraday · Philosophical Transactions of the Royal Society of London 124 (1834): 77–122 · Royal Institution, London

The Seventh Series is the paper in which electrolysis became quantitative. Faraday had spent the previous year showing that the electricity of a voltaic battery, a friction machine, a magnet and an electric fish are one and the same agent; here he asks how much chemical work a given amount of it performs — and answers: a definite, weighable amount.

1 · A meter made of chemistry

First he needed to measure electricity itself. His “volta-electrometer” is a cell in which the gas evolved, or the metal deposited, records the total quantity of electricity that has passed. Placed in series in a circuit, identical cells always showed identical amounts — the measure did not depend on the size of the plates, the strength of the solution, or the kind of generator.

2 · Definite electro-chemical action

From cell after cell he drew a single law of definite action: the chemical effect is fixed by the quantity of electricity, and by nothing else.

…the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes.

And when the same current was sent through different electrolytes, the weights set free stood in the ratio of what Faraday called their electro-chemical equivalents — numbers that proved to coincide with the ordinary chemical equivalents already known to chemists. In modern symbols the whole result is m = (Q·M)/(z·F), with the Faraday constant F = 96485 coulombs per mole.

3 · A new language for electrolysis

To describe a process he pictured as matter travelling through the liquid to the plates — not as an attraction reaching out from the “poles” — Faraday wanted words free of the old theory. With the Cambridge polymath William Whewell he coined them: the plates became the electrode, the anode and the cathode; the decomposed substance an electrolyte; and the migrating particles ions, dividing into anions and cations.

I propose to distinguish these bodies by calling those anions which go to the anode of the decomposing body; and those passing to the cathode, cations; and when I have occasion to speak of these together, I shall call them ions.

[ … ]

Faraday measured one fixed dose of electricity for each “equivalent” of matter, but did not claim what carried it. Half a century later Helmholtz read in these very numbers the existence of “atoms of electricity,” and the electron — the coin Faraday had unknowingly been counting — was named by Stoney and found by J. J. Thomson.

Royal Institution, London · 1834