Hydroquinone

A developer with only hydroquinone in it shows nothing for a long time and then builds density steadily and fast. That behaviour is not a fault; it is the property that makes hydroquinone the contrast half of nearly every general-purpose developer of the last century, from Kodak’s D-76 to Ilford’s Bromophen and Foma’s Fomadon LQN.

In photography

It builds density rather than starting the image. Kodak’s 1928 primer sets the two agents side by side: the hydroquinone image comes up very slowly but gains density steadily and rapidly, where the metol image appears at once and then gains slowly. The primer ranks the agents by reduction potential — how much bromide is needed to hold them back — and hydroquinone sits at the bottom of that list, below Athenon, pyro, Kodelon and metol. A low-potential agent brings up the highlights first and leaves the shadows until the highlights are well established, which is how a high-contrast print or a graphic-arts image behaves.

Two consequences follow, and both are in the primer. Hydroquinone needs more alkali than the others — it is often used with caustic alkalis where the rest manage on carbonate — and it is far more sensitive to temperature than metol, a small change affecting hydroquinone greatly and metol very little. Watkins’s coefficients as Wall prints them in 1912 give a tabloid hydroquinone developer 2.25 against 1.9 for a metol–quinol one, the factor by which development time changes over 10 °C; an eight-minute development at 20 °C is a very different eight minutes at 15 °C. Bromide restrains hydroquinone strongly and metol far less, so a bromide addition bites hard on a hydroquinone-rich developer and barely registers on a metol one.

What more or less of it does. More hydroquinone raises contrast and maximum density and lengthens the time before anything appears; less gives a softer, flatter negative at the same shadow speed. D-76 uses 5 g per litre against 2 g of metol, buffered with borax. The primer’s caustic developer D-9 is the other extreme: 22.5 g of hydroquinone per litre of stock A with an equal 22.5 g of potassium bromide, worked with an equal volume of a second stock carrying 52.5 g of sodium hydroxide per litre. Same molecule, a fine-grain film developer at one end and a hard process developer at the other.

⚗Hydroquinone, quinone, and why sulfite belongs in the bottle

Kodak’s 1928 primer gives the oxidation in one sentence: when hydroquinone is oxidised we get quinone, and if sulfite is added to quinone, the quinone oxidises the sulfite to sulfate and is itself reduced again to hydroquinone.

C₆H₆O₂ → C₆H₄O₂ + 2 H⁺ + 2 e⁻

Hydroquinone is oxidised to quinone; the electrons go to silver, or to dissolved oxygen

C₆H₄O₂ + SO₃²⁻ + H₂O → C₆H₆O₂ + SO₄²⁻

Sulfite reduces the quinone back — the course's balancing of Kodak's sentence

That second reaction is why sulfite is a preservative rather than merely an antioxidant: it does not only mop up dissolved oxygen, it reverses the damage already done, and the primer notes that adding bisulfite to a browned developer bleaches the colour out. The brown matters — the primer describes hydroquinone’s oxidation product as being of the nature of a dye, which stains fabric and gelatin, and says that a hydroquinone solution with alkali but no preservative turns dark brown so fast that a plate developed in it comes out stained and fogged.

Not sourced here: Kodak establishes that hydroquinone is the reducer and quinone the product, but not how the electron reaches the silver, nor why metol and hydroquinone together outperform their separate contributions. This page fills neither gap.

Properties

Light-tan, light-grey or colourless crystals — ChEBI describes the molecule as a benzene ring with two hydroxy groups para to each other, C₆H₆O₂, relative molecular mass 110.11, CAS 123-31-9. CAMEO records that it darkens in air and light and that solutions turn brown from oxidation.

Solubility, with a caveat about the sources. Four independent figures agree on roughly 70 g of hydroquinone per litre of water at ordinary temperatures: HSDB 72 g/L at 25 °C, HMDB 72 mg/mL, NIOSH 7%, and the ILO-WHO safety card 5.9 g per 100 mL at 15 °C. PubChem also carries a temperature series from HSDB and DrugBank that is an order of magnitude lower — 6.72 g/L at 20 °C — and this course cannot reconcile the two; it uses the first group and says so. What is not in doubt is that hydroquinone is far more soluble in alcohol than in water, 46.4 g per 100 g of ethanol at 30 °C against 8.3 g of water, which is the basis of Kodak’s 1924 instruction that up to 10% alcohol in a concentrated developer stops the agents crystallising out on chilling.

pH behaviour is why a hydroquinone developer needs a strong alkali. The IUPAC dissociation-constant dataset gives benzene-1,4-diol a pKa₁ of about 9.85 to 9.91 at 25 °C and a pKa₂ near 11.4. Below pH 9 the molecule is essentially undissociated; a paper developer at pH 10.3 to 10.5, where Ilford’s sheet places Bromophen stock, has a workable fraction of the first ionised form; the caustic developers push past pKa₂ altogether. That the ionised species is the active reducer is the obvious reading and this course has not sourced it, but the alkali dependence Kodak describes and the ionisation constants line up on the same scale.

Stability cuts both ways. Dry, dark and closed, hydroquinone keeps well — the primer notes it is used in tank developers because it does not oxidise as readily as pyro. Wet and alkaline it is the opposite: CAMEO states that oxidation is very rapid in the presence of alkali, and NIOSH lists alkalis among its incompatibilities. Sulfite preservative, closed bottles and minimum air space all exist because of that second fact.

Handling

Hydroquinone is the most heavily classified substance in an ordinary developer. The aggregated ECHA notifications give it Danger, four pictograms and eight statements, and the agreement across 2,485 company reports is close to unanimous: H318, causes serious eye damage, at 99.9%; H341, suspected of causing genetic defects, at 99.9%; H351, suspected of causing cancer, above 99.9%; H317, may cause an allergic skin reaction, above 99.9%. Exactly one report of the 2,485 says it meets no criteria. Nor are these only suppliers’ opinions: the harmonised classification under Regulation (EC) No 1272/2008 carries Danger with H302, H317, H318, H341, H351 and H400, so the hazard is settled at law, and Safe Work Australia and HSDB agree with it.

Unlike metol, it has a workplace exposure limit: HSE’s EH40 lists hydroquinone at 0.5 mg m⁻³ as an eight-hour time-weighted average, and the NIOSH Pocket Guide sets a ceiling of 2 mg m⁻³ over 15 minutes with an immediately-dangerous-to-life-or-health value of 50 mg m⁻³. NIOSH’s symptom list runs from conjunctivitis and keratitis through nausea, dizziness and muscle twitching to sensitisation and dermatitis, and Princeton University’s guidance adds depigmentation and eye injury after five or more years of repeated exposure. That describes airborne dust sustained over years rather than a splash on a glove, which is why the limit exists and why the powder is the dangerous form. HSE’s HSG262 supplies the reason the sensitisation clause still 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 no further than irritant, harmful if swallowed, or corrosive at the concentrations handled, and suspected carcinogenicity is beyond that. The question is B against C, and Level C’s criterion is not simply “carcinogen” but such a substance where a fume cupboard or specialist disposal is the recognised control. Neither applies: HSE’s own sheet for manual film development prescribes general ventilation of more than five air changes an hour, nitrile gloves, eye protection and overalls, and says respiratory protection is not normally needed for routine operations. Level B’s criteria — handling sensitisers, and fine powders that must not be inhaled — describe the weighing step exactly.

⚠Safety

Eye protection is not optional here: the classification is H318, serious eye damage, in 99.9% of notifications, and NIOSH directs that eye contact be prevented and eyewash provided. Handle the dry powder with extraction or inside an enclosure and raise no dust cloud — NIOSH records it as a combustible solid whose dust cloud may explode if ignited in an enclosed area. Nitrile gloves and tongs for the tray. Keep it from strong oxidisers on the shelf and from alkalis in store, and remember that the alkali you add in the developer is the same reaction, run on purpose and on a clock.

Waste

Spent developer, bottled separately. The organic load and the aquatic classification are the issue: H400 appears in above 99.9% of the notifications, H410 in 21.2%, and the precautionary statements are P273, P391 and P501. Kodak’s J-300 guidance records that developer holds negligible silver, so there is nothing to recover and every reason not to tip it into the fixer bottle, where it would spoil a solution that does carry silver. Ilford’s guidance for UK 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, mixing nothing and using no septic tank. Its fallback of flushing small amounts with plenty of water is written for a dilute working solution entering a treated sewer, not for concentrate, and does not sit comfortably beside an H400 classification; the course states the tension rather than resolving it. Check your local regulations; they decide.

History

Hydroquinone is the oldest of the organic developing agents still in daily use. Eder dates the alkaline hydroquinone developer for gelatine silver bromide plates to Abney in 1880, and Abney himself, writing in 1905, recalls that when he first found its developing power the price put on it made it unusable except for experiments, until manufacturing chemists brought the price down. By then he was recording it as a great favourite with many photographers for the colour of its negatives and the clean way they develop.

🕰The isomer rule of the same year

In 1880 Eder and V. Toth found pyrocatechin also developed, and from the comparison drew a rule about the dihydroxybenzenes: the para arrangement, which hydroquinone has, acts strongly on silver bromide in an alkaline developer; the ortho arrangement of pyrocatechin has great developing power; and resorcinol, with its hydroxy groups meta, has none at all. Three molecules of identical formula, C₆H₆O₂, and only two of them develop — as clean a demonstration as photography offers that a formula is not a substance.

Kodak’s 1928 primer describes hydroquinone as made from benzene by way of aniline, less powerful as a reducing agent than pyro but far less inclined to stain, keeping well in tank developers because it does not oxidise as readily, and a constituent of the majority of the better-known commercial developers of the period. That has scarcely changed: Ilford’s Bromophen and Foma’s Fomadon LQN and LQR are phenidone–hydroquinone developers, against a single surviving metol–hydroquinone powder in Foma’s range. What changed is the partner. The Q of MQ became the Q of PQ, and the reasons were the skin of the person mixing it and the quantity of the other agent needed.