chemistry

Mercury coulometer

Mercury coulometer is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mercury coulometer rather than just read about it. In short: In electrochemistry, a mercury coulometer is an analytical instrument which uses mercury to perform coulometry (determining the amount of matter transformed in a chemical reaction by measuring electric current) based on the following reaction: Hg 2 + + 2 e − ↽ − − ⇀ Hg ∘ {\displaystyle {\ce {Hg^{2}+}}+{\ce {2e^{-}<=>Hg^{\circ }}}} These oxidation/reduction processes have 100% efficiency within a wide range of curren…

Key takeaways

Mercury coulometer belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mercury coulometer to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mercury coulometer from memory before moving on to harder problems.

Reference excerpt

In electrochemistry, a mercury coulometer is an analytical instrument which uses mercury to perform coulometry (determining the amount of matter transformed in a chemical reaction by measuring electric current) based on the following reaction:

Hg 2 + + 2 e − ↽ − − ⇀ Hg ∘ {\displaystyle {\ce {Hg^{2}+}}+{\ce {2e^{-}<=>Hg^{\circ }}}}

These oxidation/reduction processes have 100% efficiency within a wide range of current densities. Measuring the quantity of electricity (coulombs) is conducted by measuring changes in the mass of the mercury electrode. The mass of the electrode can be increased during cathodic deposition of the mercury ions or decreased during the anodic dissolution of the metal.

Q = 2 Δ m F M Hg , {\displaystyle Q={\frac {2\,\Delta m\,F}{M_{{\ce {Hg}}}}},}

where Q is the quantity of electricity; Δm is the change in mass; F is the Faraday constant; and MHg is the molar mass of mercury.

Use as Hour Meter Before the development of solid-state electronics, coulometers (electrochemical hour meters, elapsed time indicators) were used as long-period (up to 25,000 hours) elapsed hour meters in electronic equipment and other devices, including on the Apollo Program space vehicles. By passing a constant, calibrated current through the coulometer, the movement of a gap between mercury droplets provides a visual indication of elapsed time. Brands included HP's Chronister and Curtis' Indachron.

Construction This coulometer has different constructions but all of them are based on mass measurements. The device consists of two reservoirs connected by a thin graduated capillary tube containing a solution of mercury(II) ions as an electrolyte. Each of the reservoirs contains an electrode immersed in a drop of mercury. Another small drop of mercury is inserted into the capillary. When the current is turned on, it initiates dissolution of the metallic mercury on one side of the drop in the capillary and deposition on the other side of the same drop. This drop starts to move. Because of the high efficiency of the deposition/dissolution of the mercury under the influence of current, the mass or volume of this small drop is constant and its movement is linearly correlated with the passed charge. If the direction of the current is changed, the drop moves in the opposite direction. The sensitivity of this type of coulometer depends on the diameter of the capillary. Another construction is to only have a capillary filled with mercury and electrodes on both sides, with the mercury being separated into two areas by a small electrolyte bubble with a glass tube in the bubble, parallel to the capillary. The capillary is graduated to act as a time meter.

Illustrations

Worked examples

Example 1 — a first encounter with Mercury coulometer

Start with the simplest possible case. Write down what Mercury coulometer claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mercury coulometer before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mercury coulometer ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mercury coulometer

In research: Mercury coulometer appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mercury coulometer in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mercury coulometer is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coulometers, Electroanalytical chemistry devices, Mercury (element), so understanding it makes those chapters shorter.
In everyday life: Look for Mercury coulometer outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Mercury coulometer in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mercury coulometer means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mercury coulometer out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mercury coulometer in simple terms?

Why does Mercury coulometer matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mercury coulometer?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mercury coulometer.