Phenidone

Where a metol–hydroquinone developer calls for a gram of metol, the phenidone version of the same developer calls for a fifth or a sixth as much. That ratio is not a curiosity of formulation; it is the whole reason phenidone exists, and it is the claim made in the patent that introduced it.

In photography

It replaces metol, and it is the P of PQ. John David Kendall, working for Ilford Ltd, filed a patent whose declared objects were to provide a new series of compounds capable of acting as photographic developers and to provide a substitute for metol. The compound he settled on was 1-phenyl-3-pyrazolidone, described in the patent as a particularly valuable developing agent whose properties resemble those of metol. The quantitative claim is the one that stuck: replacing the metol in a metol–hydroquinone developer with about one fifth to one sixth of its weight of 1-phenyl-3-pyrazolidone yields a developer of similar development characteristics.

The patent prints both cases. Used alone, Example I gives 3 g of the agent with 25 g of crystalline sodium sulfite, 50 g of crystalline sodium carbonate and 0.5 g of potassium bromide in 500 cm³. Used as a metol substitute, Example II gives 0.15 g of the agent against 4 g of hydroquinone, 75 g of sulfite, 50 g of carbonate and 1 g of bromide in the same volume — a ratio of about 27 parts of hydroquinone to one of phenidone.

Where you meet it. Ilford’s Bromophen is a phenidone–hydroquinone powder developer for papers, made to a stock and diluted 1+3 for use, which Ilford’s sheet measures at pH 10.30 to 10.50 as stock. Foma’s Fomadon LQN and LQR are phenidone–hydroquinone film concentrates, and its Fomatol LQN, Foma GD-L and Foma Universal Developer are phenidone–hydroquinone paper and general developers. Ilford now uses Dimezone-S rather than phenidone in Multigrade and PQ Universal; its sheet names the agent and says no more. That is a different substance, and nothing on this page transfers to it.

⚗What the ratio implies, and what it does not

A developing agent working at 0.15 g per 500 cm³ is doing its job at a small fraction of the molar concentration of the hydroquinone beside it. Two things follow, and only the first is sourced.

Sourced. Kendall’s patent states that these compounds may be the sole active developing agent, may replace part only of the metol in a metol-type developer, or may be added to developers generally, and that the substitution ratio for metol is a fifth to a sixth by weight. It gives no explanation of why so little is needed. Kodak’s 1928 primer supplies the frame the patent assumes: a developer has four parts — developing agent, alkali, preservative, restrainer — and the agent will not develop at all without the alkali, the developer’s energy following the amount of alkali present.

Not sourced by anything this course holds. The standard modern account — that in a superadditive pair the pyrazolidone does the reducing at the grain and the hydroquinone regenerates it in the bulk solution, so a catalytic quantity suffices — appears in no document in the course’s research corpus, and this page does not assert it. What the sources establish is the shape of the behaviour: a very small quantity of one agent transforms the performance of a much larger quantity of another, and the manufacturers who print their formulations describe the result as one developer type, “phenidone–hydroquinone”, rather than as two agents in a tank.

What more or less of it does. In a PQ developer, phenidone controls how quickly the image appears and how far into the shadows development reaches, in the role metol played; hydroquinone controls density and contrast. Because the phenidone is present in tenths of a gram, a weighing error that would be trivial for hydroquinone is a large proportional error here. That is why the course’s measurement page uses 0.2 g of phenidone as its worked example of a balance being asked for more than it can give: 0.01 g of resolution on 0.20 g is 5 per cent before any other source of error is counted.

Properties

C₉H₁₀N₂O, relative molecular mass 162.19, CAS 92-43-3, PubChem’s IUPAC name 1-phenylpyrazolidin-3-one. The molecule is a five-membered ring containing two adjacent nitrogen atoms and a ketone, with a phenyl group on one nitrogen — structurally unlike metol and hydroquinone, which are both substituted benzene rings, and that is the point: Kendall was not looking for a variation on a phenol.

Form. Haz-Map records the Merck Index as giving a solid and an Alfa Aesar safety data sheet as a white powder, and that is the whole of what the course can source: there is no CAMEO datasheet, no International Chemical Safety Card and no NIOSH Pocket Guide entry, and so no independent description of colour, melting point, decomposition products or reactivity.

Solubility is a gap this page will not paper over. PubChem’s solubility heading returns no values for CID 7090, and the course has verified no figure from anywhere else. It therefore states none. What it can say is that the manufacturers take the mixing order out of your hands: Ilford’s powder developers arrive as two bags, part A the smaller, to be dissolved first in warm water at about 40 °C before part B is added gradually, with the note that a few grains remaining undissolved is normal; Foma’s two-bag powders name which bag goes first and give 40 °C for Fomadon P and 50 to 70 °C for the Universal Developer. Follow the packet, and do not assume a phenidone powder will simply dissolve because a metol one did.

pH behaviour. Phenidone works, like every developing agent in this family, in alkaline solution. The working figures the course can quote are the finished developers rather than the molecule: Ilford measures Bromophen stock at pH 10.30 to 10.50 and PQ Universal at 1+9 at pH 10.48 to 10.58. No dissociation constant for phenidone appears in the IUPAC dataset the course uses for the other agents on these pages, so the ionisation behaviour is not stated here.

Handling

The aggregated ECHA notifications classify phenidone as Warning with the irritant and environmental pictograms, and two statements: H302, harmful if swallowed, in 98.4% of reports, and H411, toxic to aquatic life with long lasting effects, in all of them.

That much is agreed at law. The harmonised entry for 1-phenyl-3-pyrazolidone under Regulation (EC) No 1272/2008 gives the same signal word, the same two pictograms and the same two statements, and Safe Work Australia’s HCIS matches. What is thin is not the agreement but the investigation. The notified figures rest on 62 reports from two notifications, where metol’s rest on 21 and hydroquinone’s on 36; there is no exposure limit in HSE’s EH40 and no NIOSH Pocket Guide entry, and EH40 states that absence from its list does not indicate a substance is without risk. Princeton University’s guidance on photography says only that phenidone is slightly toxic by skin contact, which is a comparative remark in a list of developers rather than a hazard assessment, and this page does not use it as one. What can be said with confidence is that phenidone is not classified as a skin sensitiser where metol and hydroquinone both are, and that this, along with Kendall’s substitution ratio, is why it displaced metol from so many formulations.

Why Level B. The classification alone — harmful if swallowed, hazardous to the aquatic environment — would sit inside Level A of the course rubric, whose first criterion admits substances classified no further than irritant, harmful if swallowed, or corrosive at the concentrations handled. Two things move it up. First, Level B names fine powders that must not be inhaled, and phenidone is bought as a powder and weighed in tenths of a gram, where Princeton’s requirement for an enclosure or local exhaust when mixing powdered developers applies in full. Second, this course only ever meets phenidone in a bath that also contains hydroquinone, and the mixed bath is governed by the more heavily classified component. Level B here is partly a judgement about the state of the evidence: with no reactivity datasheet and no safety card to consult, the course is not willing to place phenidone at its most permissive level on two notifications, and says so rather than implying a confidence it does not have.

⚠Safety

Nitrile gloves and eye protection for the solution; tongs rather than fingers in the tray; ventilation at the level HSE’s sheet P1 describes for manual development. Weigh the powder with extraction or inside an enclosure, and take particular care not to raise dust from a substance dispensed in tenths of a gram. In a PQ developer the more heavily classified component is the hydroquinone beside it, and its controls govern the mixed bath: eye protection is mandatory because of hydroquinone’s H318, not because of the phenidone.

Incompatibilities: an empty list, and why. The course has found no sourced incompatibility for phenidone — no CAMEO reactivity profile, no safety card, no NIOSH entry. A prudent shelf keeps it with the other developing agents and away from oxidisers, which is where the course’s storage page puts the class as a whole, but that is a general precaution for reducing agents rather than a documented reaction, and this page records it as such rather than inventing an entry.

Waste

Spent developer, bottled separately from fixer and stop bath. Kodak’s J-300 guidance records that developer carries negligible silver, so there is nothing to recover from it and no reason to combine it with a fixer that does carry silver.

The classification to work from is H411, toxic to aquatic life with long lasting effects — one category milder than the H410 that metol carries and that a minority of hydroquinone’s notifiers give. In practice a phenidone–hydroquinone developer’s waste is a hydroquinone problem and an alkali problem before it is a phenidone problem, simply because of the proportions: Kendall’s own example holds 4 g of hydroquinone to 0.15 g of phenidone. 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, never mixing different chemicals for disposal and never using a septic tank. Check your local regulations; they decide what is permitted where you are.

History

Kendall filed the patent for Ilford Ltd on 28 May 1941, claiming priority from 10 July 1940; it was published as United States patent 2,289,367 on 14 July 1942. The date matters: this is a wartime British patent from a company whose principal developing agent had come out of a German dye works — Eder credits metol to Bogisch at J. Hauff, near Stuttgart — and whose customers had been getting metol dermatitis since Wall was printing remedies for it in 1912. The patent does not mention dermatitis. It states two objects — a new series of developing compounds, and a substitute for metol — and lets the second speak for itself.

What followed is visible on any modern packet. The abbreviation MQ, metol and quinol, gave way to PQ, phenidone and quinol, as the ordinary description of a general-purpose developer, and Foma’s catalogue now lists four phenidone–hydroquinone products against a single surviving metol–hydroquinone one. The family did not stop with phenidone either: Ilford’s current Multigrade and PQ Universal are given on its sheet as dimezone-S/hydroquinone, a developing agent this course has not researched and about which it makes no claim, and Foma’s Fomatol P pairs phenidone with isoascorbate instead of hydroquinone. What began as a 1940 attempt to get metol out of the tray is now most of the shelf.