The Classical Developing Agents: Metol, Hydroquinone, Phenidone

D-76 carries 2 g of metol and 5 g of hydroquinone in a litre. Either agent will develop a film on its own — Kodak’s D-23 is metol alone, and the primer’s D-9 is hydroquinone alone — so the obvious question is why the standard developer of the twentieth century contains both, in that ratio, and not 7 g of one of them.

The answer needs three things this page supplies: what makes a molecule capable of developing at all, what each of these three does that the others do not, and what becomes of each when it has done it. The fourth thing — why the pair outperforms the sum of its parts — is superadditivity, and it is deliberately left to its own page, because the mechanism is contested and this page has enough established chemistry to be getting on with.

Which molecules develop at all

Take the three dihydroxybenzenes. They have the same formula, C₆H₆O₂, the same relative molecular mass of 110.11, and the same two hydroxyl groups on the same benzene ring. Only the positions differ. Eder and Toth reported in 1880, from work on gelatine dry plates, that this made all the difference: the para arrangement, hydroquinone’s, shows a very strong action on silver bromide in an alkaline developer; the ortho arrangement of pyrocatechin gives great developing power; and resorcin, with its hydroxyls meta, has no energy as a developer at all.

Where the groups have to stand, and the molecule that ignores the rule

1. 1Hydroquinone, hydroxyls para — develops strongly in an alkaline bath (Eder and Toth, 1880)
2. 2Catechol, hydroxyls ortho — great developing power; the staining developers page owns it
3. 3Resorcinol, hydroxyls meta — no developing energy — the control that makes the rule a rule
4. 4Metol, hydroxyl and methylamino, para — an amino group standing in for the second hydroxyl
5. 5Phenidone, a pyrazolidinone — develops at a fifth of metol’s weight and fits none of the above

Skeletons, not structures: what the drawing asserts is which positions carry which groups. Bond lengths, angles and hydrogen atoms are not shown and are not the point.

Two electron-donating groups, hydroxyl or amino, standing para or ortho on the ring: that is the Kendall–Pelz rule, and it is the one structural generalisation this course offers about developing agents. Eder records that the rule drawn from the phenols was later found to hold for other derivatives, para-aminophenol among them — the amino group standing in for one of hydroquinone’s two hydroxyls, which is exactly what metol and p-aminophenol are.

⁇Whose rule, and how much of it this course has read

The rule is named for J. Kendall and, in the later and more general form, for Pelz, and it is quoted in that form throughout the photographic literature. This course has read neither Kendall’s statement of it nor Pelz’s. What it has read is Eder’s account of the 1880 experiment that the rule generalises, and Kendall’s 1941 patent, which is a different document about a different molecule.

So the position here is: the ortho-and-para-versus-meta result is sourced, from Eder, in the specific case of three dihydroxybenzenes and one aminophenol. The general rule is named and attributed and is not, in this course, independently verified. That distinction matters because the generalisation has an obvious counterexample sitting in half the developers on the market — phenidone is not a benzene ring with two donor groups on it, and it develops better than metol. Kendall knew that: his patent’s stated object was a new series of compounds, not a variation on a phenol. A rule with a large and commercially dominant exception is a good guide and a poor law, and the course presents it as the first.

Hydroquinone, and the price of its pKa

Kodak’s primer puts the working difference between the two classical agents in one sentence: an image developed with Elon, its name for metol, comes up very quickly and gains density slowly, while the hydroquinone image comes up very slowly but gains density steadily and rapidly. Hydroquinone builds contrast. It is the reason a D-76 negative has highlights.

It is also the fussiest of the three about its alkali. Kodak notes that hydroquinone is often used with caustic alkalis while the other agents manage on the weaker carbonated ones, and that it has so low a reduction potential that it is rarely used alone. The IUPAC dissociation-constant dataset says why, in two numbers: benzene-1,4-diol has pKa₁ between 9.85 and 9.91 at 25 °C and pKa₂ near 11.4.

How much of the hydroquinone is ionised, and where real developers sit

• Fraction as the monoanion

Show the numbers behind this plot

A sigmoid curve rising from left to right, showing the percentage of hydroquinone present as its singly ionised form against the pH of the solution. At pH 7 the figure is 0.1 per cent; at pH 8 it is 1.3 per cent; at pH 9, 11.6 per cent; at pH 9.88, which is the first dissociation constant, exactly half; at pH 10.5, 81 per cent; and by pH 12, 99 per cent. Four working solutions are marked on the curve at their published pH values. ILFORD PERCEPTOL stock at pH 7.75 sits at under one per cent. Kodak XTOL working solution at pH 8.20 sits at two per cent, and it is annotated as containing no hydroquinone at all. ILFORD ID-11 stock at pH 8.65 sits at about six per cent. ILFORD BROMOPHEN paper developer stock at pH 10.40 sits at about seventy-seven per cent. The teaching point is the gap between the film developers, all clustered in the first few per cent at the foot of the curve, and the paper developer, which is three quarters of the way up it, and therefore why hydroquinone is the contrast agent in a paper developer and only a partner in a film developer.

Series	pH of the working solution	Hydroquinone present as the monoanion, per cent
Fraction as the monoanion	7.00	0.13
Fraction as the monoanion	7.50	0.42
Fraction as the monoanion	8.00	1.30
Fraction as the monoanion	8.50	4.00
Fraction as the monoanion	9.00	11.60
Fraction as the monoanion	9.50	29.40
Fraction as the monoanion	9.88	50.00
Fraction as the monoanion	10.00	56.90
Fraction as the monoanion	10.50	80.60
Fraction as the monoanion	11.00	92.90
Fraction as the monoanion	11.50	97.70
Fraction as the monoanion	12.00	99.20

The curve is computed, not measured: it is the Henderson-Hasselbalch fraction 1/(1 + 10^(pKa − pH)) with pKa taken as 9.88, the midpoint of the IUPAC range. The four marked pH values are published measurements from ILFORD's and Kodak's own technical sheets. What the curve does NOT assert is that ionisation equals activity — that the ionised species is the active reducer is the obvious reading and this course has not sourced it.

Read the plot as a design constraint rather than as a mechanism. At the pH of every film developer in the corpus, the great majority of the hydroquinone is sitting there undissociated; at the pH of a paper developer, most of it is not. That is consistent with everything the manufacturers do — hydroquinone dominates paper developers and partners in film developers — and it is why the primer’s maximum-energy formula, D-9, pairs 22.5 g of hydroquinone per litre with a second stock carrying 52.5 g of sodium hydroxide.

The same fraction may lie behind hydroquinone’s other famous vice, though here the course is reading rather than reporting. Wall prints Watkins’s temperature coefficients — the factor by which development time changes over 10 °C — as 2.25 for a tabloid hydroquinone developer against 1.9 for a metol–quinol one. The sources give the coefficients and not the cause; the reading offered here, and labelled as one, is that an agent working at a few per cent ionisation has two temperature-sensitive steps in series, the ionisation and the reduction, where a fully ionised agent has one. Whatever the cause, the consequence is not in doubt: eight minutes at 20 °C is not eight minutes at 15 °C.

Metol, and why it works where hydroquinone cannot

Metol is not a phenol with two hydroxyls. It is the para aminophenol with a methyl group on the nitrogen, sold as the hemisulfate salt, (C₇H₉NO)₂·H₂SO₄, relative molecular mass 344.39. Two molecules of the free base account for 246.3 of that, so weighing the free base against a recipe written for the salt puts about 40 per cent more agent in the tank than intended — the first of the two ways this page sets out to ruin a developer while weighing accurately.

Its acid–base behaviour is the mirror image of hydroquinone’s. The IUPAC dataset gives the free base 4-hydroxy-N-methylaniline a pKaH of 5.9 at 25 °C, from the pH-dependence of its oxidation–reduction potentials. That is the conjugate acid: above about pH 8 essentially none of the molecule remains protonated on the nitrogen. Metol has already finished the acid–base change it needs before hydroquinone has begun its own.

⚗What Kodak's ranking is really measuring, and what it implies here

The primer ranks the agents by what it calls reduction potential, lowest first: hydroquinone, Athenon, pyro, Kodelon, Elon highest. The measurement behind the ranking is bromide tolerance — the higher the potential, the more bromide is needed to produce a given effect — and how-development-works shows why that is a sensible proxy, since bromide moves the silver electrode a known 59 millivolts per decade.

Two behaviours follow, and both are what a photographer notices first. A high-potential agent makes the image flash up all over at once, because it starts development quickly even in the thinly exposed shadows; a low-potential one brings up the highlights first and leaves the shadows until the highlights are established. And a high-potential agent keeps working under adverse conditions — the primer’s phrase — where a low-potential one slows: in the cold, and in a developer that has been used. Kodak’s summary of the asymmetry is exact: a small temperature change affects hydroquinone greatly and Elon very little, and a small quantity of bromide affects hydroquinone and does not affect Elon nearly so much.

So metol supplies shadow speed, tolerance of a cold darkroom, and tolerance of the bromide the film itself releases; hydroquinone supplies density and contrast, and pays for them with sensitivity to everything.

What becomes of them, and what the course cannot tell you

An oxidised developing agent does not leave the tank, and its identity decides whether the negative stains, whether the solution keeps and whether the developer can be revived.

Hydroquinone’s product is known and is a problem. Two electrons and two protons leave, and quinone is what remains:

C₆H₆O₂ + 2 OH⁻ → C₆H₄O₂ + 2 H₂O + 2 e⁻

Hydroquinone oxidised, written for the alkaline bath it works in

Quinone is not inert. Kodak’s 1928 primer describes the oxidation product of a hydroquinone solution as being of the nature of a dye, staining fabric and gelatin, and states that adding carbonate to a hydroquinone solution with no preservative turns it dark brown so fast that a plate developed in it comes out stained and fogged. That is the whole argument for the preservative in three clauses.

Metol’s product is not something this course can name. The primer establishes what it is not: metol’s oxidation product is not deposited in a coloured form with the silver, which is why a sulfite-rich pyro image comes out almost as blue as an Elon one. It adds one physical observation that is easy to test and rarely mentioned — an oxidised Elon or Elon–hydroquinone developer frequently fluoresces strongly. Beyond that, no source in this course’s corpus names metol’s oxidation product, and the metol page says the same. The quinone-imine that the wider literature names is not asserted here.

⚗Sulfite closes the loop, and then closes it permanently

Kodak’s primer gives two different fates for quinone in a sulfite developer, in two different chapters, and the difference between them is the most useful thing on this page.

Route one, regeneration. “If we add sulphite to quinone, the quinone oxidizes the sulphite to sulphate and is itself reduced again to hydroquinone.” The agent comes back; the preservative is spent.

C₆H₄O₂ + SO₃²⁻ + H₂O → C₆H₆O₂ + SO₄²⁻

Route one: the developing agent restored at the sulfite's expense

Route two, sulfonation. In the chapter on the useful life of developers the primer says something else entirely: “In the presence of sodium sulphite, however, the oxidation products of hydroquinone consist of hydroquinone mono- and disodium sulphonates which are colorless.” Here the sulfite adds to the quinone rather than handing back electrons, and what results is a new substance — a sulfonate — not the original agent.

The two routes predict opposite things about a developer’s history, and the primer draws the conclusion that matters: “The fact that an old Elon-hydroquinone developer is colorless is, therefore, no indication that the original developing power is unimpaired.” A brown developer is certainly spent. A colourless one is undiagnosed.

Part III recorded sulfonation as a route it could not source and deferred it here. It is now sourced, from Kodak, in Kodak’s own words. What remains unsourced is the next question: whether hydroquinone monosulfonate is itself a developing agent. The photographic literature says it is; no document this course holds says so; and this page does not claim it. sulfite-and-preservation returns to what that gap costs.

⁇The semiquinone, and where it is being asked to do work

Between hydroquinone and quinone lies a one-electron intermediate, the semiquinone, and it is doing a great deal of load-bearing in the photographic literature: it is the usual explanation of infectious development in lith materials, and it appears in most accounts of superadditivity.

No Tier-1 source in this course’s corpus establishes it for photographic developers. Part IV said so of infectious development and this page says so of the oxidation scheme. The course names the species because its own glossary does and because a reader will meet the word immediately, and it marks the claim rather than burying it. What is established, and is enough for everything on this page, is the pair of end points: hydroquinone in, quinone out, two electrons in between.

Phenidone, and the agent Ilford will not describe

In May 1941 John David Kendall filed a patent for Ilford whose declared objects were to provide a new series of photographic developing compounds and to provide a substitute for metol. The compound was 1-phenyl-3-pyrazolidone, and the claim that made it famous is quantitative: replacing the metol in a metol–hydroquinone developer with about one fifth to one sixth of its weight of the pyrazolidone gives a developer of similar development characteristics.

The patent prints the two cases side by side, and reading them together tells you more than either does alone. Used alone, Example I gives 3 g of the agent with 25 g of crystalline sulfite, 50 g of crystalline carbonate and 0.5 g of potassium bromide in 500 cm³ — 6 g of agent per litre with 1 g/L of restrainer. 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 — 0.3 g of agent per litre, 8 g of hydroquinone, and twice the restrainer. The patent does not comment on that difference and neither will this course; it is two examples, not a study.

What the ratio establishes is that a developing agent can do its work at a molar concentration a small fraction of its partner’s. That is the observation superadditivity has to explain, and the reason MQ gave way to PQ on every packet. Ilford’s Bromophen is a phenidone–hydroquinone paper developer measured at pH 10.30 to 10.50 as stock; Foma’s Fomadon LQN and LQR are phenidone–hydroquinone film concentrates.

!Dimezone-S: what the sheets say, and where the course stops

Ilford’s current Multigrade and PQ Universal developers are described on its own technical sheet as dimezone-s/hydroquinone developers. The sheet names the agent and says nothing else about it: no structure, no proportion, no comparison with phenidone. Kodak’s XTOL sheet names no second agent at all, only “ascorbic acid-based” and “no hydroquinone”.

This course has researched neither substance’s identity and states nothing about it. It gives no structure for dimezone-s, no hazard classification, no substitution ratio and no solubility, because it holds no source for any of them, and the phenidone page takes the same position. Note what that costs and what it does not: you can still read a modern packet correctly, because the class is disclosed — a pyrazolidone-type agent paired with hydroquinone, or an ascorbate with something unnamed — and the behaviour of the class is the subject of this part. What you cannot do is weigh it, and you were never going to; it is not sold to you.

Stability is phenidone’s other selling point and the course’s other gap. The agent is bought as a powder and used in tenths of a gram — which is the second weighing trap, since a balance reading to 0.01 g is already three per cent out on Kendall’s 0.3 g per litre before any other error is counted. The smallest jar sold therefore lasts years and will be old when it is used; no published shelf-life figure for the dry solid appears in this course’s corpus, and none is offered. PubChem holds no solubility record for it either, no CAMEO reactivity datasheet and no safety card. Manufacturers take the question out of your hands by supplying two-bag powders with the order fixed: Ilford’s part A, the smaller bag, dissolved first in warm water at about 40 °C before part B is added gradually, with a note that a few undissolved grains are normal.

Solubility, and the order you dissolve in

Three agents, three solubilities, and one of them is the reason every Kodak formula prints its ingredients in a particular order.

Agent	Water solubility	Source and condition
Metol (hemisulfate)	4.7 g per 100 mL at 15 °C	ILO-WHO Chemical Safety Card 1528, via PubChem
Hydroquinone	about 70 g/L at ordinary temperature	four sources agree; PubChem carries a second series an order of magnitude lower that the course cannot reconcile and does not use
Phenidone	not stated	PubChem returns no solubility for CID 7090 and the course has verified none elsewhere

The interesting number is not any of those. It is that metol is only slightly soluble in a sulfite solution. Kodak’s 1924 primer explains the mechanism: sulfite dissolved first takes the sulfuric acid away from the salt and throws down the sparingly soluble free base as a white precipitate. Carbonate does not do this, because the base combines with it into a soluble sodium salt. Kodak’s 1928 chapter on developer keeping states the consequence for stock solutions bluntly: sulfite stock keeps best at around 10 per cent w/v, and owing to the relative insolubility of Elon in a sodium sulfite solution, it is not possible to prepare such stock solutions with Elon. There is no such thing as a concentrated metol-and-sulfite stock.

Dissolving order for a metol formula, and the reason for each step

1. 1Warm water, well short of the final volumeKodak’s 1928 D-76 directions dissolve the metol in a small volume at about 125 °F (52 °C) and the sulfite portions in hot water at about 160 °F (71 °C). The same primer’s list of causes of developer fog includes mixing the solution too hot, so hotter is not better.
2. 2Metol first, and completelyIt is readily soluble in warm water and only slightly soluble in sulfite solution without alkali. Every later step makes it harder to dissolve, not easier.
3. 3A small part of the sulfite, then the hydroquinoneThe 1928 D-76 directions dissolve about a quarter of the sulfite in hot water and add the hydroquinone to that, so the second agent never meets air in an unprotected alkaline solution.
4. 4The rest of the sulfiteThe preservative has to be in place before the alkali arrives. Kodak Ltd’s 1949 handbook warns that dissolving the agent and then adding the alkali allows considerable aerial oxidation, and coloured oxidation products, before the sulfite is in solution at all.
5. 5The alkali lastBorax here, carbonate or hydroxide elsewhere. This is the step that switches the developer on, and nothing should be undissolved when it happens.
6. 6Bromide whenever you likeKodak Ltd 1949: potassium bromide has no action on the developing agents, so it is immaterial at what stage it is added. Sodium bisulfite is the exception — it goes in with the sulfite.
7. 7Cold water to volume, then cool to 20 °C before useVolume is made up at the end because dissolving changes it; temperature is brought down because every published time assumes it.

The order is Kodak's, from the 1924 and 1928 primers and the 1949 handbook. Every step in it exists to keep an agent out of contact with either air or sulfite at the wrong moment.

✕A white sludge in the bottom of the bottle

Kodak’s own troubleshooting entry is worth knowing before it happens to you. “If a white precipitate settles on standing, this is probably Elon.” It may often be redissolved with five per cent of methanol; if that fails, the formula contains too much metol or too much sulfite, or not enough carbonate. And the reassuring half: if the metol precipitates during mixing when the sulfite goes in, it will usually redissolve when the carbonate is added, and if the final solution is not coloured, no harm will have been done.

Colour is the test throughout. The primer’s general rule is that a developer when mixed should be colourless, and a coloured one should be suspected of giving fog. That single observation catches most mixing errors before a film is lost — with the one exception the sulfonate route creates, where a colourless solution proves nothing about an old developer.

The hazards, and why the industry changed agents

The three agents differ more in their hazard records than in their chemistry, and the difference is most of the reason the modern shelf looks as it does.

Metol carries a harmonised classification under Regulation (EC) No 1272/2008 — agreed at law, not merely notified — with H317, may cause an allergic skin reaction, which appears in every ECHA notification that classifies it, together with H302, H373, H400 and H410. Wall’s 1912 dictionary describes the resulting sensitisation in clinical detail: the skin cracks, the fingers swell until the nails and fingertips are broken, and the affected worker should abandon the developer altogether. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit, and here that absence tells you nothing, because the hazard is on the skin rather than in the air.

Hydroquinone is the most heavily classified thing in an ordinary developer: Danger, with H318 serious eye damage, H341 suspected of causing genetic defects and H351 suspected of causing cancer all at or above 99.9 per cent of 2,485 company reports, plus H317 alongside metol’s. It is also the only one of the three with a workplace exposure limit: HSE’s EH40 lists it at 0.5 mg m⁻³ as an eight-hour average. That limit describes airborne dust sustained over years, which is why the powder is the dangerous form and why weighing happens with extraction or inside an enclosure.

Phenidone is classified Warning, with H302 and H411 and nothing else — and the honest reading of that is not that it is benign but that it is thinly investigated. Those figures rest on 62 reports from two notifications, against 21 for metol and 36 for hydroquinone; PubChem holds no CAMEO reactivity datasheet, no international chemical safety card and no NIOSH entry. What can be said with confidence is the negative that changed the industry: phenidone is not classified as a skin sensitiser where metol and hydroquinone both are.

⚠Controls, and the one that is not negotiable

All three are handled at Level B, and the mixed bath is governed by its most heavily classified component, which in an MQ or PQ developer is the hydroquinone. Nitrile gloves and eye protection whenever a solution is handled; tongs rather than fingers in the tray; general ventilation at the level HSE’s COSHH essentials sheet P1 describes for manual film development; and the powders weighed with extraction or inside an enclosure, never where a draught can lift them.

The non-negotiable one is what you do at the first sign of skin trouble. HSG262 explains why: sensitisation may take weeks, months or years to appear, and once it has, tiny amounts trigger the reaction and the only remedy left is to prevent further exposure. Treat the first episode as the end of your exposure to that agent, not as something to work through. A PQ developer exists partly so that you have somewhere to go.

That is also, in one paragraph, the history. Kendall was looking for a metol substitute in 1940 and found one at a fifth of the weight. Kodak Limited printed formula D-173 in 1949 under a heading naming the reason — an Elon-free paper developer, to eliminate the risk of discomfort to persons prone to metol dermatitis. Ilford now uses an agent it does not describe, and Kodak sells a developer whose headline benefit is the absence of the other one. MQ became PQ, and PQ is quietly becoming something with no quinol in it at all, and at every step the driver was the skin of the person mixing it.

Recap

• The Kendall–Pelz rule asks for two donor groups, hydroxyl or amino, standing para or ortho on the ring. The course’s sourced case is Eder and Toth’s 1880 comparison of hydroquinone, catechol and resorcinol, three molecules of one formula of which only two develop, extended by Eder to para-aminophenol. Phenidone obeys none of it and works better than metol.
• Hydroquinone’s pKa₁ is 9.85 to 9.91. At every film-developer pH in the corpus it is a few per cent ionised; at paper-developer pH it is three quarters ionised. It builds contrast, needs the strongest alkali, and has the steepest temperature coefficient — 2.25 against metol–quinol’s 1.9.
• Metol is the hemisulfate, 344.39, and the free base is a different substance: weigh the wrong one and you are 40 per cent over. Its amine pKaH is 5.9, so it is ready to work two pH units below hydroquinone, and Kodak’s bromide-tolerance ranking puts it highest of the five agents it sold.
• Hydroquinone oxidises to quinone, which stains. Sulfite then does two different things to the quinone — reduces it back, or adds to it as a colourless sulfonate — and the primer’s warning follows from the second: a colourless old MQ developer is not evidence of unimpaired power.
• Metol’s oxidation product is not named by any source this course holds. What is sourced is that it is not deposited coloured, and that an oxidised Elon developer often fluoresces strongly.
• Phenidone replaces metol at one fifth to one sixth of its weight, on Kendall’s own claim, and is the only one of the three not classified as a skin sensitiser — on a notably thin evidence base. Dimezone-S is named by Ilford and researched by nobody here.
• Metol dissolves first, always, because it is only slightly soluble in sulfite solution without alkali, and there is therefore no such thing as a metol-and-sulfite stock solution. Preservative before alkali; alkali last; bromide whenever.

Check your understanding

Question 1. Resorcinol has the same formula as hydroquinone and catechol, C₆H₆O₂, and the same two hydroxyl groups on the same ring. Why does it not develop?

• It is insoluble in alkali
• Its hydroxyls are meta to one another rather than para or ortho, and Eder and Toth found in 1880 that the meta arrangement has no developing energy
• It oxidises too quickly to reach the film
• It is not a reducing agent at all

Show the answer and why

Answer: Its hydroxyls are meta to one another rather than para or ortho, and Eder and Toth found in 1880 that the meta arrangement has no developing energy

This is the cleanest demonstration in photographic chemistry that a formula is not a substance. Eder and Toth compared the three bivalent phenols on gelatine dry plates in 1880 and reported that the para arrangement acts very strongly on silver bromide in an alkaline developer, the ortho arrangement has great developing power, and resorcin has none. The generalisation from that result is the Kendall-Pelz rule, which asks for two electron-donating groups para or ortho. Note the limits of the rule as well as its content: phenidone satisfies none of it and develops at a fifth of metol’s weight, which is why the course calls it a good guide and a poor law.

Question 2. Taking hydroquinone’s pKa₁ as 9.88, roughly what fraction of it is present as the monoanion in ILFORD ID-11 stock at pH 8.65, and what does that suggest about its role in a film developer?

• About half, so it is doing most of the developing
• About 6 per cent, so its contribution in a film developer is small and it earns its place through contrast and through whatever it does for its partner rather than by bulk reduction
• Essentially none, so it could be left out
• Essentially all of it, because sulfite ionises it

Show the answer and why

Answer: About 6 per cent, so its contribution in a film developer is small and it earns its place through contrast and through whatever it does for its partner rather than by bulk reduction

The Henderson-Hasselbalch fraction is 1/(1 + 10^(9.88 − 8.65)) = 1/(1 + 17.0) = 5.6 per cent. Set that beside ILFORD BROMOPHEN paper developer at pH 10.40, where the same arithmetic gives 77 per cent, and the division of labour on the manufacturers’ own shelves makes sense: hydroquinone dominates paper developers and partners in film developers. Two cautions. The calculation says nothing about the third agent in the pairing, and it assumes the ionised form is the active reducer, which is the obvious reading and is not sourced in this course.

Question 3. A litre of MQ developer mixed six months ago is perfectly clear and colourless. What does that tell you about its developing power?

• That it is unimpaired, since oxidised developer is brown
• Nothing useful: in the presence of sulfite the oxidation products of hydroquinone are its colourless mono- and disodium sulfonates, so a clear solution is undiagnosed rather than healthy
• That the sulfite has been used up
• That it has been kept in the dark

Show the answer and why

Answer: Nothing useful: in the presence of sulfite the oxidation products of hydroquinone are its colourless mono- and disodium sulfonates, so a clear solution is undiagnosed rather than healthy

Kodak’s 1928 primer states it directly, and it is the most useful practical consequence of the sulfonation route: the fact that an old Elon-hydroquinone developer is colourless is no indication that the original developing power is unimpaired. The inference works one way only. Brown means certainly spent, because an unprotected oxidation product is coloured; colourless means the sulfite was still doing its job while the agent was being consumed. The primer offers one further tell for this particular case — an oxidised Elon or Elon-hydroquinone developer frequently fluoresces strongly.

Question 4. You dissolve the sulfite for a D-76 batch before the metol, and a white precipitate appears. What is it, and what happens next?

• Sodium sulfate, and the batch is ruined
• The metol free base, thrown down because the sulfite has taken the sulfuric acid from the salt; it will usually redissolve when the alkali is added, and if the final solution is colourless no harm has been done
• Silver bromide
• Borax, which is only sparingly soluble cold

Show the answer and why

Answer: The metol free base, thrown down because the sulfite has taken the sulfuric acid from the salt; it will usually redissolve when the alkali is added, and if the final solution is colourless no harm has been done

Metol is sold as the hemisulfate and behaves in solution as an acid. Kodak’s 1924 primer explains that sulfite dissolved first removes the acid and precipitates the sparingly soluble free base, while carbonate does not, because the base combines with it into a soluble sodium salt. The 1928 troubleshooting entry gives the recovery: if the precipitate appeared when the sulfite went in, it will usually redissolve on adding the carbonate, and if the final solution is not coloured no harm will have been done. The general rule the episode illustrates is the one worth carrying: every later ingredient makes metol harder to dissolve, so it goes in first.

Question 5. Kendall’s patent claims that a fifth to a sixth of the weight of metol, replaced by 1-phenyl-3-pyrazolidone, gives similar development characteristics. Which of these is a legitimate conclusion from that claim alone?

• That phenidone regenerates itself catalytically at the grain surface
• That a developing agent can do its work at a small fraction of its partner’s molar concentration, which is the observation any theory of superadditivity has to explain
• That phenidone is five times as strong a reducing agent as metol
• That phenidone developers keep five times as long

Show the answer and why

Answer: That a developing agent can do its work at a small fraction of its partner’s molar concentration, which is the observation any theory of superadditivity has to explain

The patent states a substitution ratio and gives two worked examples; it offers no mechanism and this course does not supply one. What the ratio establishes is a fact about proportions, and that fact is the whole puzzle: 0.3 g of agent per litre transforms the behaviour of 8 g of hydroquinone. The usual explanation — that the pyrazolidone reduces at the grain while the hydroquinone regenerates it in solution — appears in no source in this course’s corpus, and both the phenidone encyclopaedia page and this page decline to assert it. It belongs to the superadditivity page, with the same caveat attached.

Question 6. Why does the course say that phenidone’s mild hazard classification should be read as thin evidence rather than as reassurance?

• Because manufacturers under-report deliberately
• Because the classification rests on 62 reports from two ECHA notifications, against 21 notifications for metol and 36 for hydroquinone, and PubChem holds no reactivity datasheet, no safety card and no NIOSH entry for it
• Because phenidone is newer than the other two
• Because it is used in such small quantities that nobody has measured it

Show the answer and why

Answer: Because the classification rests on 62 reports from two ECHA notifications, against 21 notifications for metol and 36 for hydroquinone, and PubChem holds no reactivity datasheet, no safety card and no NIOSH entry for it

Absence of a finding is not a finding of absence. The two hazard statements phenidone carries — harmful if swallowed, and toxic to aquatic life with long lasting effects — are agreed at law through a harmonised entry, so what is thin is the depth of investigation rather than the agreement. Set against that, one negative can be stated with confidence and is the reason the substance exists commercially: phenidone is not classified as a skin sensitiser, where both metol and hydroquinone are. HSE’s own note on EH40 makes the general point that absence from a list of exposure limits does not indicate a substance is without risk.

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