Flying high: thoughts on oxygen supply at altitude

I am not a frequent flier, but I have spent enough time flying at altitude to start thinking about the difference in pressure between the air outside the window and the air inside the cabin. Except I haven’t, until now. I like looking at the screen tracking the progress of the flight and showing the altitude, temperature, groundspeed etc so I know how cold it gets at 37,000 feet. But it doesn’t show the pressure, and for whatever reason I suddenly became interested in finding out.
An internet search for ‘pressure at altitude’ yielded this formula:

P = pressure at altitude h [any unit so long as same as P_b]
P_b = pressure at sea level [any unit so long as same as P]
h = height above sea level [m]
h_b = height at ground level [m]
T_b = temperature at sea level [K]
L_b = temperature lapse rate = -0.0065 Km^-1
R = universal gas constant = 8.31432 kg.m²s^-2K^-1mol^-1
g₀ = gravitational acceleration constant = 9.80665 ms^-2
M = molar mass of atmosphere = 0.0289644 kg.mol^-1

Plugging in P_b = 1 atm, T_b = 293K, h = 11,277 m (37,000 ft) and taking h_b as sea level gave me an answer I honestly didn’t expect. The outside pressure at 37,000 feet is only 0.22 atm – barely a fifth of its value at sea level!

That got me thinking about the structural stresses at altitude if the cabin is maintained at ground-level pressure, and I soon discovered that aeronautical engineers had been there long before me. In order to reduce these stresses, the cabin pressure is programed to reduce gradually during ascent from the airport of origin to a regulatory cabin altitude of 8,000 ft (2,438 m) and then increase gradually during descent until the cabin pressure matches the air pressure at the destination. So what is the pressure at 8,000 ft? I plugged h = 2,438 m into the equation and discovered that the cabin pressure at cruising altitude is 25% less than at sea level. A significant difference.

This in turn got me thinking about the reduction in oxygen availability, given that passengers do not appear to be distressed by it. Applying the ideal gas equation brought me to the conclusion that the reduction must be in proportion to the pressure difference, other things like cabin temperature and breathing rate and tidal volume being equal, since under these circumstances:
n₁ = oxygen availability at ground level, n₂ = oxygen availability at cruising altitude
P₁ = cabin pressure at ground level, P₂ = cabin pressure at cruising altitude
n₁RT/P₁V = n₂RT/P₂V
n₂/n₁ = P₂/P₁

In other words, the cabin air I am breathing at altitude contains 25% less oxygen per unit volume than it did at takeoff. I suspect the reason we don’t notice the change is that we are doing nothing more physically demanding than sitting in our seats. If we were all riding exercise bicycles we would probably notice soon enough.

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Checking cabin pressure at altitude

Watches like the SKMEI 1358 and Casio Pro Trek have a barometer function. I used one on a recent flight; at take-off the cabin pressure was 1007 hPa and at a cruising altitude of around 35,000 feet (10.7 km) the cabin pressure was 760 hPa, exactly 25% less than at take-off. Given the big fall in pressure it is remarkable how little we seem affected by it, although some online writers ascribe the tiredness and lassitude some passengers experience to depleted oxygen levels.

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Breathing in, breathing out

The type of airplane I usually fly in has around 270 passengers and crew. Each of us inhales an average 7½ liters of air per minute of which 20% is oxygen, so we inhale around 400 liters of oxygen per minute of which we utilize something like 5%. That corresponds to consuming 1.2 cubic meters of oxygen per hour.

We use this oxygen to sustain vital cellular processes, and biochemistry tells us that the oxygen we consume is replaced with equal amounts of exhaled carbon dioxide and water vapor. These gases cannot be allowed to build up in the cabin airspace since increased levels of carbon dioxide can have adverse physiological effects while water vapor carries a risk to the aircraft of condensation and corrosion. Again, aeronautical engineers have long known this and have solved both this and the oxygen depletion problem by continuously replacing the cabin air with air from outside which fortunately has the same nitrogen-oxygen composition as air at sea level. This air is compressed to the required cabin altitude using either bleed air from the jet engine compressor or purpose-built electrical compressor systems.

Water vapor is a different matter. At cruising altitude the outside temperature is -76°F (-60°C) or thereabouts. The vapor pressure of water at this temperature is extremely low so there can be only tiny quantities of water vapor in the air outside – this reference proves the point with a graph showing how the Mixing Ratio decreases exponentially with altitude.

Given this fact, it is a bit of a mystery to me where any replacement water vapor comes from if not from the breath of passengers and crew. In any event, cabin air has a reputation for dryness with relative humidity often as low as 20%. This explains why bottles of water are supplied as a courtesy service on longer-haul flights.

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“If the cabin air system should fail …”

When the safety video plays and it gets to the oxygen mask part, I often wonder if I will be able to breathe normally as the voice asks me to do once I have put the mask on as quickly as I can and tightened the elastic bands before helping others. I am not the excitable type but I think normal breathing is a tall order in this situation, especially since I know that the flow of oxygen is coming from a chemical generator usually designed to last not much more than 15 minutes. Enough time for the airplane to descend to a safe height I have read. However I have also read that the cockpit crew have the use of compressed oxygen cylinders which last somewhat longer, which makes me wonder what I would do in the meantime if they needed that extra capacity.

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When flying is a pain in the ear

I thought I would add a few words on this topic as I have experienced problems with this myself. If you have read this far, you will know that cabin air undergoes decompression on the way up to cruising altitude and compression on the way down. This can cause some passengers to experience ear pain – I have noticed that children seem more vulnerable to this than adults, especially during descent.

The problem centers on the middle ear cavity, which has the eardrum and ear canal on one side and a tube connected to the nasal canal on the other called the Eustachian tube. Its purpose is to ensure equal pressure on either side of the eardrum. Trouble is that the Eustachian tube is not always up to the job and when it doesn’t function properly, pain from pressure imbalance on the eardrum can result. This can be relieved by yawning or swallowing but if these techniques don’t work there are a number of proprietary products that might be worth trying, including pressure-controlling ear plugs and nasal balloons.

Nasal balloons encourage a closed Eustachian tube to open and allow pressure on both sides of the eardrum to equalize.

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P Mander December 2022, additions February 2023

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Carlisle, Nicholson and the thermodynamics of electrolysis

May 1800: Carlisle (left) and Nicholson discover electrolysis

The two previous posts on this blog concerning the leaking of details about the newly-invented Voltaic pile to Anthony Carlisle and William Nicholson, and their subsequent discovery of electrolysis, are more about the path of temptation and birth of electrochemistry than about classical thermodynamics. In fact there was no thermodynamic content at all.

So by way of steering this set of posts back on track, I thought I would apply contemporary thermodynamic knowledge to Carlisle and Nicholson’s 18th century activities, in order to give another perspective to their famous experiments.

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The Voltaic pile

Z = zinc, A = silver

In thermodynamic terms, Alessandro Volta’s fabulous invention – an early form of battery – is a system capable of performing additional work other than pressure-volume work. The extra capability can be incorporated into the fundamental equation of thermodynamics by adding a further generalised force-displacement term: the intensive variable is the electrical potential E, whose conjugate extensive variable is the charge Q moved across that potential

hence

At constant temperature and pressure, the left hand side identifies with dG. For an appreciable difference therefore

where E is the electromotive force of the cell, Q is the charge moved across the potential, and ΔG_rxn is the free energy change of the reaction taking place in the battery.

For one mole of reaction, Q = nF where n is the number of moles of electrons transferred per mole of reaction, and F is the total charge on a mole of electrons, otherwise known as the Faraday. For a reaction to occur spontaneously at constant temperature and pressure, ΔG_rxn must be negative and so the EMF must be positive. Under standard conditions therefore

The redox reaction which took place in the Voltaic pile constructed by Carlisle and Nicholson was

ΔG⁰_rxn for this reaction is –146.7 kJ/mole, and n=2, giving an EMF of 0.762 volts.

We know from Nicholson’s published paper that their first Voltaic pile consisted of “17 half crowns, with a like number of pieces of zinc”. We also know that Volta’s method of constructing the pile – which Carlisle and Nicholson followed – resulted in the uppermost and lowest discs acting merely as conductors for the adjoining discs. Thus there were not 17, but 16 cells in Carlisle and Nicholson’s first Voltaic pile, giving a total EMF of 12.192 volts.

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External work

On May 1st, 1800, Carlisle and Nicholson set up their Voltaic pile, gave themselves an obligatory electric shock, and then began experiments with an electrometer which showed “that the action of the instrument was freely transmitted through the usual conductors of electricity, but stopped by glass and other non-conductors.”

Electrical contact with the pile was assisted by placing a drop of water on the uppermost disc, and it was this action which opened the path to discovery. Nicholson records in his paper that at an early stage in these experiments, “Mr. Carlisle observed a disengagement of gas round the touching wire. This gas, though very minute in quantity, evidently seemed to me to have the smell afforded by hydrogen”.

The fact that gas was formed “round the touching wire” indicates that the contact was intermittent: when the wire was in contact with the water drop but not the zinc disc, a miniature electrolytic cell was formed and hydrogen gas was evolved at the wire cathode, while at the anode the zinc conductor was immediately oxidised as soon as the oxygen gas was formed.

In thermodynamic terms, the electrochemical cells in the pile were being used to do external work on the electrolytic cell in which the decomposition of water took place

ΔG⁰_rxn for this reaction is +237.2 kJ/mole. So it can be seen that the external work done by the pile consists of driving what is in effect the combustion of hydrogen in a backwards direction to recover the reactants.

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Intuitive

Carlisle and Nicholson were intuitive physical chemists. They knew that water was composed of two gases, hydrogen and oxygen, so when bubbles which smelled of hydrogen were observed in their first experiment, it immediately set them thinking. Nicholson wrote of being “led by our reasoning on the first appearance of hydrogen to expect a decomposition of water.”

William Nicholson (1753-1815)

Nicholson used the term decomposition, so it seems safe to assume they formed the notion that just as water is composed from its constituent gases, it can be decomposed to recover them. That is a powerful conception, the idea that the combustion of hydrogen is a reversible process.

Whether Carlisle and Nicholson extended this thought to other chemical reactions, or even to chemical reactions in general, we do not know. But their demonstration of reversibility, beneath which the principle of chemical equilibrium lies, was an achievement of perhaps even greater moment than the discovery of electrolysis by which they achieved it.

Anthony Carlisle (1768-1840)

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Redox reactions

Carlisle and Nicholson’s discovery of electrolysis was made possible by the fact that the decomposition of water into hydrogen and oxygen is a redox reaction. In fact every reaction that takes place in an electrolytic cell is a redox reaction, with oxidation taking place at the anode and reduction taking place at the cathode. The overall electrolytic reaction is thus divided into two half-reactions. In the case of the electrolysis of water, we have

These combined half-reactions are not spontaneous. To facilitate this redox process requires EQ work, which in Carlisle and Nicholson’s case was supplied by the Voltaic pile.

Redox reactions also take place in every voltaic cell, with oxidation at the anode and reduction at the cathode. The difference is that the combined half-reactions are spontaneous, thereby making the cell capable of performing EQ work.

The spontaneous redox reactions in voltaic cells, and the non-spontaneous redox reactions in electrolytic cells, can best be understood by looking at a table of standard oxidation potentials arranged in descending order, such as the one shown below. Using such a list, the EMF of the cell is calculated by subtracting the cathode potential from the anode potential.

[Note that if you use a table of standard reduction potentials, the signs are reversed and the EMF of the cell is calculated by subtracting the anode potential from the cathode potential.]

For voltaic cells, the half-reaction taking place left-to-right at the anode (oxidation) appears higher in the list than the half-reaction taking place right-to-left at the cathode (reduction). The EMF of the cell is positive, and so ΔG will be negative, meaning that the cell reaction is spontaneous and thus capable of performing EQ work.

The situation is reversed for electrolytic cells. The half-reaction taking place left-to-right at the anode (oxidation) appears lower in the list than the half-reaction taking place right-to-left at the cathode (reduction). The EMF of the cell is negative, and so ΔG will be positive, meaning that the cell reaction is non-spontaneous and that EQ work must be performed on the cell to facilitate electrolysis.

The half-reactions of Carlisle and Nicholson’s Voltaic pile, and their platinum-electrode electrolytic cell, are indicated in the table below.

Table of standard oxidation potentials

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The advent of the fuel cell

Anthony Carlisle and William Nicholson

If Carlisle and Nicholson had disconnected their platinum-wire electrolytic cell after bubbles of hydrogen and oxygen had formed on the respective electrodes, and then connected an electrometer across the wires, they would have added yet another momentous discovery to that of electrolysis. They would have discovered the fuel cell.

From a thermodynamic perspective, it is a fairly straightforward matter to comprehend. Under ordinary temperature and pressure conditions, the decomposition of water is a non-spontaneous process; work is required to drive the reaction shown below in the non-spontaneous direction. This work was provided by the Voltaic pile, the effect of which was to increase the Gibbs free energy of the reaction system.

Upon disconnection of the Voltaic pile, and the substitution of a circuit wire, the reaction would spontaneously proceed in the reverse direction, decreasing the Gibbs free energy of the reaction system. This system would be capable of performing EQ work.

The reversal of reaction direction transforms the electrolytic cell into a voltaic cell, whose arrangement can be written

H₂(g)/Pt | electrolyte | Pt/O₂(g)

As can be seen from the above table, the EMF of this voltaic cell is 1.229 volts. We know it today as the hydrogen fuel cell.

Carlisle and Nicholson most surely created the first fuel cell in May 1800. They just didn’t apprehend it, nor did they operate it as a voltaic cell – at least we have no record that they did. So we must classify Carlisle and Nicholson’s fuel cell as an overlooked actuality; an unnoticed birth.

It would take another 42 years before a barrister from the city of Swansea in Wales, William Robert Grove QC, developed the first operational fuel cell, whose essential design features can clearly be traced back to Carlisle and Nicholson’s original.

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Mouse-over link to the original papers mentioned in this post

Nicholson’s paper (begins on page 179)

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P Mander September 2015

Carlisle, Nicholson and the discovery of electrolysis

Anthony Carlisle (left) and William Nicholson, London, May 1800

The rise of physical chemistry in the 19th century has at its root two closely connected events which took place in the final year of the 18th century. In 1800, Alessandro Volta in Lombardy invented an early form of battery, known as the Voltaic pile, which Messrs. Carlisle and Nicholson in England promptly employed to discover electrolysis.

Carlisle and Nicholson’s discovery that electricity can decompose water into hydrogen and oxygen caused as big a stir as any scientific discovery ever made. It demonstrated the existence of a relationship between electricity and the chemical elements, to which Michael Faraday would give quantitative expression in his two laws of electrolysis in 1834. Faraday also introduced the term ‘ion’, a little word for a big idea that Arrhenius, Ostwald and van ‘t Hoff would later use to create the foundations of modern physical chemistry in the 1880s.

About this post

The story of Carlisle and Nicholson’s discovery properly begins with a letter that Volta wrote on March 20th, 1800 to the President of the Royal Society in London, Sir Joseph Banks. The leaking of that letter (which contained confidential details of the construction of the Voltaic pile) to among others Anthony Carlisle, forms the narrative of my previous post “The curious case of Volta’s leaked letter”.

This post is concerned with the construction details themselves, which have their own story to tell, and the experimental activities of Messrs. Carlisle and Nicholson after they had seen the letter, which were reported in July 1800 by Nicholson in The Journal of Natural Philosophy, Chemistry & the Arts – a publication that Nicholson himself owned.

The Voltaic pile

“The apparatus to which I allude, and which will no doubt astonish you, is only the assemblage of a number of good conductors of different kinds arranged in a certain manner.”
Alessandro Volta’s letter to Joseph Banks, introducing the Voltaic pile

Volta’s arrangement comprised a pair of different metals in contact (Z = Zinc, A = Silver), followed by a piece of cloth or other material soaked in a conducting liquid; this ‘module’ could be repeated an arbitrary number of times to build a pile in the manner illustrated below.

The Voltaic Pile: Volta’s own illustration enclosed with the letter to Banks

Volta believed the electrical current was excited by the mere contact of two different metals, and that the liquid-soaked material simply conducted the electricity from one metal pair to the next. This explains why Volta’s illustration shows the metals always in pairs – note the silver disc below the zinc at the bottom of the pile and a zinc disc above the silver at the top.

It was later shown that these terminal discs are unnecessary: the actual electromotive unit is zinc-electrolyte-silver. Volta’s arrangement can therefore be seen as a happy accident, in that his mistaken belief regarding the generation of electromotive force led him to the correct arrangement of repeated electrochemical cells, in which the terminal discs act merely as connectors for the external circuit wires.

Volta’s pile thus contained one less generating unit than he thought; it also caused the association of the two metals with the positive and negative poles of the battery to be reversed.

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Enter Mr. Carlisle

London’s Soho Square in the early 19th century. Animals were often driven to market through the square.

The president of the Royal Society, Sir Joseph Banks, lived in a house at No.32 Soho Square. Here he entertained all the leading members of the scientific establishment, and it was here in April 1800 that he yielded to temptation and disclosed the contents of Signor Volta’s confidential letter to certain chosen acquaintances. Among them was another resident of Soho Square, the fashionable surgeon Anthony Carlisle, who had just moved in at No.12.

Volta’s announcement of his invention made an instant impression on Carlisle, who immediately arranged for his friend the chemist William Nicholson to look over the letter with him, after which Carlisle set about constructing the apparatus according to the instructions in Volta’s letter.

Nicholson records in his paper that by 30th April 1800, Carlisle had completed the construction of a pile “consisting of 17 half crowns, with a like number of pieces of zinc, and of pasteboard, soaked in salt water”. Using coinage for the silver discs was smart thinking by Carlisle – with a diameter of 1.3 inches (3.3 cm), the half crown was an ideal size for the purpose, and was made of solid silver.

Silver half crown, diameter 1.3 inches

From Nicholson’s account, it seems likely that Carlisle obtained a pound (approx. ½ kilo) of zinc from a metal dealer called John Tappenden who traded from premises just opposite the church of Saint Vedast Foster Lane, off Cheapside in the City of London. A pound of zinc was enough to make 20 discs of the diameter of a half crown.

Having constructed the pile exactly according to Volta’s illustration above, Carlisle and Nicholson were ready to begin their experiments. But before describing their work, it is pertinent to draw attention to the way in which they approached their program of research, which was quite unlike that of Volta.

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Differences in approach

Alessandro Volta’s letter to Joseph Banks, apart from briefly detailing the construction of the pile, comprises a lengthy account of electric shocks administered to various parts of the human anatomy and the nature of the resulting sensations.

Volta does first prove with a charging condenser that the pile generates electricity, but having ascertained this fact, he makes no further observations on the pile, other than asserting that the device has “an inexhaustible charge, a perpetual action” and later commenting: “This endless circulation of the electric fluid (this perpetual motion) may appear paradoxical and even inexplicable, but it is no less true and real;”

One of Volta’s arrangements, using electrodes dipped in bowls of water for delivering electric shocks to the hands. If Volta had just put both electrodes in one bowl, he would have discovered electrolysis.

Volta appears not to have observed that the zinc discs quickly oxidise during operation; perhaps it was because he enclosed the pile in wax to prevent it from drying out. But nonetheless it seems strange that Volta did not discover during the course of his many experiments that the zinc discs do not have an unlimited lifetime.

William Nicholson also found it strange, commenting in his paper, “I cannot here look back without some surprise and observe that … the rapid oxidation of the zinc should constitute no part of his [Volta’s] numerous observations.”

Reading Volta’s communication to Banks, one is struck by the brevity of the text pertaining to his fabulous invention, and contrarily, the abundant descriptions of the shocks he administered with it. Volta is demonstrably more occupied with how humans experience the shocks that the pile delivers, than with the pile itself.

With Carlisle and Nicholson, the situation is very much the reverse. Having given themselves an obligatory shock with their newly-built machine, the attention immediately shifts to the pile itself. Their experiments and attendant reasoning show an approach that is more analytical in character.

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The path to discovery

On May 1st, 1800, Carlisle and Nicholson set up their pile – most likely in Carlisle’s house at 12 Soho Square – and began by forming a circuit with a steel wire and passing a current through it. To assist contact with the wire, a drop of river water was placed on the uppermost disc. As soon as this was done, Nicholson records

“Mr. Carlisle observed a disengagement of gas round the touching wire. This gas, though very minute in quantity, evidently seemed to me to have the smell afforded by hydrogen”

It is amazing that Nicholson was able to identify hydrogen from such a minute sample. But even more amazing was the thought that occurred to him next

“This [release of hydrogen gas], with some other facts, led me to propose to break the circuit by the substitution of a tube of water between two wires.”

Nicholson does not say what those other facts are, but he does record that on the first appearance of hydrogen gas, both he and Carlisle suspected that the gas stemmed from the decomposition of water by the electric current. Following that wonderfully intuitive piece of reasoning, Nicholson’s suggestion can be seen as a natural next step in their investigation.

William Nicholson (1753-1815)

On 2nd May, Carlisle and Nicholson began their experiment using brass wires in a tube filled with river water. A fine stream of bubbles, identifiable as hydrogen, immediately arose from the wire attached to the zinc disc, while the wire attached to the silver disc became tarnished and blackened by oxidation.

This was an unexpected result. Why was the oxygen, presumably formed at the same place as the hydrogen, not evolved at the same wire? Why and how does the oxygen apparently burrow through the water to the other wire where it produces oxidation of the metal? This finding, which according to Nicholson “seems to point at some general law of the agency of electricity in chemical operations” was to occupy physical chemists for the next 100 years…

Meanwhile, Carlisle and Nicholson responded to their new experimental finding with another intuitive piece of reasoning. What would be the effect, they asked, of using electrodes made from a metal that resisted oxidation, such as platinum?

Immediately they set about finding the answer. With electrodes fashioned from platinum wire they observed a plentiful stream of bubbles from the wire attached to the zinc disc and a less plentiful stream from the wire attached to the silver disc. No tarnishing of the latter wire was seen. Nicholson wrote

“It was natural to conjecture, that the larger stream was hydrogen, and the smaller oxygen.”

The conjecture was correct. On a table top in Soho Square, Carlisle and Nicholson had successfully decomposed water into its constituent gases by the use of the Voltaic pile, and had thereby discovered electrolysis – a technique which was to prove of immeasurable importance to industry.

Anthony Carlisle (1768-1840)

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Quantitative analysis

Carlisle and Nicholson realised that the decomposition of water using platinum wires “offered a means of obtaining the gases separate from each other”. This not only provided a new way of producing these gases, but also opened up a new avenue of analysis. By measuring the relative volumes of hydrogen and oxygen evolved from the wires, they could compare their result with known data for water. [It should be noted that Carlisle and Nicholson did not have the benefit of Avogadro’s law, which was not formulated until 1811].

Carlisle and Nicholson subjected water to electrolysis for 13 hours, after which they determined the weight of water displaced by each gas in the respective tubes. The weights were in the proportion 142:72 in respect of hydrogen and oxygen; this is very close to the whole number ratio of 2:1 which was known to be the proportions in which these gases combine to produce water. Here then was quantitative evidence that the hydrogen and oxygen observed in Carlisle and Nicholson’s electrolytic cell originated from the decomposition of water.

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The experimental observations – explained

It was that drop of water placed on the uppermost disc to assist contact with the metal wire that opened the path to discovery. The fact that gas was formed “round the touching wire” indicates that the contact was intermittent: when the wire was in contact with the water drop but not the uppermost disc, a miniature electrolytic cell was formed and hydrogen gas was evolved.

Illustrating this graphically requires some qualifying explanation, since as already mentioned the terminal discs of the Voltaic pile assembled according to Volta’s instructions were unnecessary, and acted merely as conductors. Electrochemically, the uppermost disc of Carlisle and Nicholson’s Voltaic pile was a silver cathode, connected to the water drop via a zinc disc; the lowest disc in the pile was a zinc anode, which via an interposed silver disc was connected to the water drop via a steel wire. The electrochemical processes can be illustrated as follows

Carlisle and Nicholson’s first experiment, May 1st, 1800

The drop of water shown in blue acted as an electrolytic cell supplied by a zinc anode (the uppermost disc) and a steel cathode (the wire). When current was passed through this cell at moments when the wire lost contact with the zinc disc, reduction of hydrogen ions produced bubbles of hydrogen at the cathode, i.e. around the wire, as Carlisle observed. At the anode, the oxygen formed would have immediately oxidised the zinc with no visible evolution of gas.

The evolution of hydrogen gas between each pair of discs in the Voltaic pile, i.e. on the side in communication with the electrolyte, was also noted in Nicholson’s paper, as was the erosion of the zinc anode.

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And so to the experimental set-up with which Carlisle and Nicholson successfully decomposed water into its constituent gases by the use of the Voltaic pile, and thereby discovered electrolysis. Electrochemically, the uppermost disc in the pile was a silver cathode, which via an interposed zinc disc was connected to the water in the tube via a platinum electrode; the lowest disc in the pile was a zinc anode, which via an interposed silver disc was connected to the water in the tube via a platinum electrode. The electrochemical processes can be illustrated as follows

Carlisle and Nicholson’s electrolysis of water, May 1800

The tube of water shown in blue acted as an electrolytic cell supplied by a platinum anode and cathode. When current was passed through this cell, reduction of hydrogen ions produced bubbles of hydrogen at the cathode, while the oxidation of water produced hydrogen ions and bubbles of oxygen at the anode.

The evolution of hydrogen gas between each pair of discs in the Voltaic pile, i.e. on the side in communication with the electrolyte, was also noted in Nicholson’s paper, as was the erosion of the zinc anode.

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Mouse-over links to original papers mentioned in this post

Volta’s letter to Banks (begins on page 289)

Nicholson’s paper (begins on page 179)

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P Mander September 2015