Chlorohydroquinone

Take hydroquinone, put a chlorine atom on the ring, and three of its four inconveniences go away. That is the whole of chlorohydroquinone’s photographic argument, and it was made in 1899.

In photography

A faster, warmer-blooded hydroquinone. Wall’s 1912 entry is four sentences and every one of them is a comparison: Adurol “is a clean working developer somewhat akin to hydroquinone, but develops more quickly and is less affected by cold. It is also more soluble and keeps better.” Set that against hydroquinone, whose temperature coefficient Wall elsewhere gives as 2.25 — the factor by which development time changes over 10 °C — and whose alkaline solutions brown within hours. The chlorine buys speed, cold tolerance, solubility and shelf life at once.

What it does not buy is a different curve. Wall’s 1924 table of development factors, the multiple of the appearance time that gives full development, puts Adurol at 5 — exactly where it puts hydroquinone with bromide, and a sixth of metol’s 30. It is still a contrast agent that shows nothing for a while and then builds. The 1911 Encyclopædia Britannica groups it with glycin and pyrocatechin as later organic developers that “all give a black in lieu of that dark olive-green deposit of silver which is so often found with pyrogallol developers”, and notes that all of them are alkaline developers — a reminder that the chlorine relaxes hydroquinone’s demand for a caustic alkali without removing the need for alkali.

The formulas. Wall’s concentrated single solution is 400 g of sodium sulphite and 300 g of potassium carbonate in a litre of water, with 50 g of Adurol dissolved once the rest has gone in; one part to three of water for portraits and snapshots, one to five for full outdoor exposures. His two-solution version is A, 19.5 g of Adurol with 175 g of sodium sulphite in a litre; B, 125 g of potassium carbonate in a litre; equal parts for portraits, and a third part of water added for time exposures. Carbonate, not hydroxide — which is the practical difference from a hydroquinone developer of the same period, where Kodak’s own high-contrast formula reached for 52.5 g of sodium hydroxide per litre.

On paper. Wall lists Adurol with rodinal and azol among the “clean-working developers” that can be used for bromide papers, and gives it one further job with an honest caveat attached: brown or warm tones on bromide paper can be got “by greatly increasing the exposure and using a very dilute and highly restrained adurol developer, but the method is uncertain”.

⚗What one chlorine should do, and what the course can prove it does

Sourced. Wall states four differences from hydroquinone — faster, less affected by cold, more soluble, keeps better — and prints formulas in which the alkali is carbonate rather than caustic. Wall elsewhere gives hydroquinone’s temperature coefficient as 2.25 against 1.9 for a metol–quinol developer, so “less affected by cold” is a claim against a measured baseline even though Wall gives no coefficient for Adurol itself.

Not sourced, and named as such. The obvious explanation is that an electron-withdrawing chlorine lowers the ring’s first dissociation constant, so a larger fraction of the molecule is ionised at any given pH and carbonate will do what hydroquinone needs hydroxide for. The IUPAC dissociation-constant compilation gives hydroquinone pK_a1 about 9.9 to 10.0 and pK_a2 about 11.5 to 11.7 at 13 to 22 °C. It carries no entry for 2-chlorobenzene-1,4-diol at all, and no other source this course holds measures one. The course therefore states the behaviour Wall observed and declines to supply the constant that would explain it.

Properties

A white to light tan solid, or a light brown powder depending on which sheet you read; 2-chlorobenzene-1,4-diol, C₆H₅ClO₂, relative molecular mass 144.55, CAS 615-67-8. Very soluble in water and in alcohol, slightly soluble in ether — HSDB gives those words and no figure, so the only quantitative statement the course can make about its solubility is Wall’s comparative one, that it dissolves more readily than hydroquinone.

One trade name, two substances. This is the entry’s trap. Eder records that in 1899 Dr Luppo-Cramer, then at the Schering works in Berlin, observed that “a substitution bromo- or chloro-hydroquinone increased the strength of the developer over the hydroquinone in alkaline developer, and he called his product ‘Adurol’”. The name was given to a pair of compounds: the chloro one on this page, CAS 615-67-8 at 144.55, and the bromo analogue, bromohydroquinone, at 189.0. Wall’s formulas say “Adurol” and do not say which, and the two differ by 31 per cent in molar mass. Where a historical formula is reproduced anywhere in this course, it is reproduced as a mass of whatever the author’s supplier sold, and the course does not convert between the two.

Handling

The aggregated ECHA notifications are the only classification in the record — no harmonised CLP entry exists — and they give Danger with the skull-and-crossbones and exclamation-mark pictograms. The statement that matters is H311, toxic in contact with skin, at 87.8 per cent of notifiers. Hydroquinone’s dermal statement is H312, harmful, and a minority one at that; here the majority puts the substance a whole category worse by the route a darkroom worker actually meets it. H317, may cause an allergic skin reaction, follows at 81.6 per cent, with H315, H319 and H335 in the mid-eighties.

There is no workplace exposure limit: neither EH40 nor the NIOSH Pocket Guide lists the substance under any name searched, and EH40’s paragraph 6 states that absence from the list does not indicate that a substance is safe. HSG262 supplies why the sensitisation clause matters at tray dilutions — once a person is sensitised, tiny amounts provoke the reaction, and the only remedy left is to prevent further exposure.

Why Level B. Level A of the course rubric admits substances classified “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. Toxic by skin contact is past that, and Level B explicitly names sensitisers and fine powders. Level C’s test is whether a fume cupboard or specialist disposal is the recognised control, and for manual development it is not: HSE’s own COSHH essentials sheet prescribes general ventilation, nitrile gloves, eye protection and overalls. The operation that would take it out of Level B is making up Wall’s concentrated single solution at its published strength — 50 g of an H311 sensitiser dissolved into a litre already carrying 300 g of potassium carbonate, a splash from which lands on skin as both a caustic and a toxic exposure. At the 1+3 and 1+5 tray dilutions the same page is squarely Level B.

⚠Safety

Gloves are the control here, not the courtesy. Weigh the powder over a tray in still air with extraction or an enclosure, wear goggles while it is open, and treat any contact with the concentrated stock as a wash-immediately event rather than a wipe-later one. Keep it away from oxidisers on the shelf and from alkalis in store, and remember that the alkali you add in the developer starts the same oxidation on purpose and on a clock.

Waste

Spent developer, bottled separately. No notifier reports an aquatic hazard for chlorohydroquinone, so the reason to keep it out of the fixer bottle is Kodak’s: J-300 records that developer holds negligible silver, so mixing the two recovers nothing and contaminates a stream that does. ILFORD’s advice to United Kingdom domestic users is to bottle each waste chemical separately, label it, and leave it in the chemical cupboard at a Household Waste and Recycling Centre. Check your local regulations; they decide.

History

Adurol belongs to the second wave of organic developers, the one that followed the first by substitution rather than by discovery. Eder’s account puts it in 1899, ten years after Andresen’s eikonogen and paramidophenol and six after metol reached practice, and the method is different in kind: Luppo-Cramer did not find a new developing molecule, he modified one that already worked and measured what the modification bought. That is why the entry reads as a list of comparisons — every claim Wall makes for it is a claim relative to hydroquinone.

It did not last. By 1949 Kodak Limited’s handbook of chemicals and formulae names Elon, hydroquinone, Kodurol, pyro and Dolmi among its developing agents and no chlorohydroquinone at all, and no current manufacturer sheet in this course’s corpus mentions it. What displaced it was phenidone, which achieved by a wholly different ring system what Adurol had achieved by a substituent: more development for less agent, with less regard for temperature.