Chlorine

This is a study entry. Chlorine is the third of the daguerreotype’s halogen vapours, it is the oldest light-sensitive chemistry in this course’s history, and it is the one substance in this batch that an ordinary darkroom can produce by accident. The page gives the chemistry, the hazard data and the historical role, and gives no quantity, no apparatus and no sequence, because the course classifies daguerreotype sensitising at Level D and a Level D page is not a procedure.

In photography

It was the last of the three accelerators, and the priority is disputed. Eder’s account is worth following because it corrects a story that is still repeated. Antoine Claudet, who had bought the English patent rights from Daguerre, first mentioned iodo-chloride sensitising in May 1841 and read a paper before the Royal Society on 10 June 1841 comparing it with bromo-iodide. But Eder insists that Franz Kratochwila in Vienna “was also the first to introduce the use of chlorine vapor in addition to that of bromine”, and rejects Potonniée’s claim that Claudet introduced iodo-chloride at the end of 1840 as undocumented and probably wrong, on the ground that Claudet’s own publication did not appear until the following May. The Natterer brothers, also in Vienna, were using a mixture of all three halogens — iodine, bromine and chlorine — in 1840 and 1841, and getting what Eder calls instantaneous exposures with a Petzval lens.

Eder’s own figure belongs to a mixture, not to chlorine alone, and the distinction is worth keeping. What Kratochwila measured was that an iodised silver plate gained at least five times in sensitivity when subjected to a mixture of bromine and chlorine vapours containing at least 50 per cent bromine. No source read here separates chlorine’s contribution from bromine’s. The same caution applies to the mechanism: the plate carries a mixed halide rather than a single salt, mixed-halide surfaces are more sensitive, and the course does not offer a crystal-level account of this system that it has not read. The size of the whole effect can be read backwards from the AIC’s note that the Becquerel process, which uses no bromine or chlorine sensitising at all, is about ten times slower.

⚗Chlorine and silver, and chlorine and water

The reaction with the plate is the same one iodine performs, one row up the group:

2 Ag + Cl₂ → 2 AgCl

Chlorine converts metallic silver to silver chloride

Silver chloride is the compound the whole of Part I begins with — Schulze’s darkened chalk, Scheele’s blackened precipitate, Wedgwood’s failed fixing — so a daguerreotype plate fumed with chlorine and one of Talbot’s salted papers are, at the surface, the same substance reached by two completely different routes.

In water chlorine does not simply dissolve; it disproportionates. HSDB’s speciation is the clearest statement of it the course has read: of the 0.092 M that goes into water at 25 °C, 0.062 M remains as aqueous Cl₂ and 0.030 M each has become hypochlorous acid and chloride ion. One chlorine atom is oxidised and the other reduced:

Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻

Chlorine water is a disproportionation, not a solution

That is why chlorine water bleaches, why it is more soluble in alkali — which pulls the equilibrium right — and why “chlorine water” and “chlorine” are not interchangeable names for one reagent.

And the darkroom can make it without being asked. Princeton’s environmental health guidance for photography states that hypochlorite bleaches can release highly toxic chlorine gas, and the pairing sits on the course’s incompatibility matrix with the reaction written out. This is the one entry in the Level D batch whose hazard a reader may actually meet, and it will not arrive as an element in a cylinder. It will arrive as a bottle of household bleach and a stop bath.

Properties

A greenish-yellow gas, pungent, two and a half times as dense as air, liquefying at −29 °F under its own pressure of 6.8 atmospheres. Slightly soluble in water — 0.7 per cent — and partly reacted with it when it is.

It is a strong oxidiser that does not itself burn. NIOSH puts both halves in one line: non-flammable gas, but a strong oxidiser; CAMEO makes the comparison explicit by saying it supports combustion like oxygen. Its incompatibility list is accordingly a list of things that are ordinarily inert to each other: acetylene, ether, turpentine, ammonia, fuel gas, hydrogen and finely divided metals, with all of which NIOSH records that it reacts explosively or forms explosive compounds.

The density is worth one more sentence, because it is what makes the accidental release dangerous indoors. A gas at a relative density of 2.47 does not rise to a ceiling extractor. It fills the low part of a room, which in a darkroom is where the trays and the person are.

Handling

The harmonised classification is signal word Danger with an oxidiser statement, two irritation statements, toxic if inhaled, respiratory irritation and aquatic toxicity. The notifiers behind it split 53 to 47 on whether the correct acute statement is fatal or toxic if inhaled, and the front matter above records that split rather than smoothing it.

The exposure limits are all written as ceilings, which is the regulators’ way of saying that a fifteen-minute average would already be too generous. NIOSH sets a ceiling of 0.5 ppm (1.45 mg/m³) over fifteen minutes; OSHA sets a ceiling of 1 ppm; and HSE’s EH40 gives chlorine a short-term limit only, 0.5 ppm or 1.5 mg/m³, with no long-term figure at all. NIOSH’s immediately-dangerous-to-life value is 10 ppm — twenty times the working ceiling, which is a narrow margin for a gas that pools at floor level.

NIOSH’s symptoms are burning of the eyes, nose and mouth, lacrimation and rhinorrhoea, cough, choking and substernal pain, nausea and vomiting, headache, dizziness and fainting, then pulmonary oedema, pneumonitis and hypoxaemia, with dermatitis, and frostbite from the liquid; the target organs are the eyes, skin and respiratory system.

⚠Why Level D

The Level D policy argues the daguerreotype family in full and is cited here rather than restated. What is specific to chlorine is that the substance cannot be held by a domestic reader at all — it is supplied as a liquefied compressed gas, and cylinder handling is outside everything this course teaches — and that the historical operation deliberately released it into a small closed space.

The one operation that would take this entry out of Level D does not exist for the element, but it does exist for the hazard, and that distinction is the useful part of this page. The chlorine a reader is genuinely at risk from is generated in a sink, from two ordinary bottles, and the control for it is not a classification but a rule: acids and hypochlorite bleaches are stored apart and never share a drain. That control belongs on the incompatibility matrix, and it is a Level A control on a Level D substance — which is only a contradiction if you forget that a level is assigned to an operation.

Waste

Nothing here produces waste. The rule that matters is the one about the drain, and it is stated on the disposal page: a trap holding household bleach and a rinse carrying acid are a chlorine generator in the one place in a room that cannot be ventilated. Check your local regulations; they govern.

History

Chlorine is where photochemistry starts, and it starts before photography. Eder credits Scheele with isolating the element, and then records a second, less famous discovery: in 1785 Claude Louis Berthollet found that chlorine water is light-sensitive, decomposing under light with the liberation of oxygen. Eder quotes Berthollet’s own account of a bottle totally filled with chlorine water — dephlogisticated marine acid, in the language of the day — left in the light, and of the gas that appeared in it. Berthollet had also discovered bleaching by chlorine, which Eder calls one of the discoveries of the greatest consequence.

Take those two together and something worth noticing appears. Berthollet’s light-sensitive system does not contain silver. It is water and a halogen, and the light does not build an image; it drives an ordinary reaction. That is a useful thing to have met before reading about silver halides, because it separates two ideas the rest of this course keeps welded: light causes chemistry, which was known in the eighteenth century, and light can be made to leave a picture, which took another fifty years and a metal that holds still.