Aerial Oxidation: Why Chemistry Dies in a Half-Empty Bottle

Ilford gives ID-11 stock solution six months in a full container and one month in a half-full one. Same chemistry, same cupboard, same temperature; the only difference is the air above the liquid, and it costs five-sixths of the shelf life. Kodak’s sheet for D-76 says the same thing more bluntly still: the working solution lasts a month in a tank with a floating lid and twenty-four hours in a tray. Nothing about a bottle of developer is mysterious. It is being oxidised, and the numbers on the datasheet are a measurement of how quickly.

Oxygen is the oxidising agent you did not add

Every solution in a darkroom is in contact with an oxidising agent that is present at all times, in unlimited supply, and free. From OpenStax’s table of standard potentials, oxygen reducing to water in acid sits at +1.229 V — well above the silver couple at +0.7996 V, and far above any developing agent. It is thermodynamically capable of oxidising everything in the bottle.

It reaches the solution by three routes, and they are not equally dangerous.

The three reservoirs of oxygen, and how exposed each one is

1. 1Dissolved oxygen — came in with the mixing water; a fixed, small amount, and once it is spent it is gone
2. 2Headspace oxygen — the air trapped above the liquid; a fixed but much larger amount, delivered slowly across the surface
3. 3The renewing surface — an open tray draws on the whole room; the supply never runs out and the surface is wide

The published lives beneath each container are Kodak's for D-76 and Ilford's for ID-11; both makers give the same pattern. The drawing is schematic, and the liquid volumes are drawn equal to make the point that only the air has changed.

Kodak’s 1928 primer, which had no shortage of confidence about most things, is careful about this one: air “contains 20% of oxygen”, and sodium sulfite left in contact with it “for long periods” is oxidised to sulfate. Half a litre of headspace therefore contains about 100 mL of oxygen gas, waiting. The amount actually dissolved in the liquid is much smaller — the course has no solubility table for oxygen in water that it has read, so it will not put a figure on the ratio — but the manufacturers’ shelf lives settle the question empirically. A full bottle has spent its dissolved oxygen and is then finished with the subject. A half-full bottle has a reservoir above it.

What oxygen attacks, and why that is not bad luck

Kodak’s 1928 primer connects the two halves of the problem in a single sentence, and it is the sentence this whole page turns on. Developers, it says, are “necessarily substances which have a great affinity for oxygen”, and since the air contains oxygen, a solution of developing agent and alkali alone “would be rapidly spoiled from oxidation by the air”.

Read necessarily. A developing agent is chosen because it gives electrons away to silver ions readily. Oxygen wants electrons more than silver ions do. The property that makes a developer work is the property that makes it die, and no formulation can separate them; it can only buy time.

Three things in a photographic solution are on oxygen’s list.

• The developing agent. Hydroquinone goes to quinone; pyrogallol goes to a yellow product that the 1928 primer says is deposited in the film along with the silver; metol and phenidone go to their own oxidation products, less visibly.

• Sulfite. Sulfur at +4 goes to +6, and sulfate is not a preservative.

  2 SO₃²⁻ + O₂ → 2 SO₄²⁻

  The preservative spending itself, which is the point of it

• Iron(II), where a process uses it. The Fe³⁺/Fe²⁺ couple sits at +0.771 V, below both oxygen couples, so air can take a ferrous solution back to ferric. That matters in Parts XXI, XXIV and XXV rather than in a film developer, but it is the same reaction.

Why pH matters, and the part of it the course will not overstate

A developing agent works in alkali. Kodak’s primer is explicit: with one exception among the agents it lists, “developing agents, in order to do their work, must be in an alkaline solution, and the energy depends upon the amount of alkali present.” The reason is that alkali removes a proton from the agent, and the deprotonated form is the one that gives electrons away.

The accepted account is that the same deprotonated form is also the readier target for oxygen, so a developer at pH 10.5 dies faster than the same chemistry at pH 8.6. The course states that as the generally held model rather than as a measured result, because it has not read a source that measures the oxidation rate of a developing agent against pH. What it can put on the table is sourced and suggestive: Ilford’s own figures give its film developer stocks a pH of 7.68 to 7.82 (Perceptol), 8.60 to 8.70 (ID-11) and 8.67 to 8.93 (Microphen), and its paper developers at working strength a pH of 10.30 to 10.58 — and it is the paper developers that get “not more than one working day”.

!A trap in the potentials table

It would be easy to assume that a higher pH makes oxygen a stronger oxidising agent. It does not. The two oxygen entries in OpenStax Appendix L go the other way: O₂ + 4 H⁺ + 4 e⁻ → 2 H₂O is +1.229 V, while O₂ + 2 H₂O + 4 e⁻ → 4 OH⁻ is only +0.401 V. In alkali, oxygen is thermodynamically the weaker oxidant. If an alkaline developer nonetheless dies faster, the explanation has to be on the other side of the transaction — the developing agent becoming a better electron donor — and not in oxygen becoming hungrier. This is a good example of why a potentials table is read for direction and not waved at.

What sets the rate

Oxidation by air is a reaction between a dissolved species and a gas, so it happens at the surface. OpenStax’s chapter on the factors affecting reaction rates makes the general point: a reaction between two phases “takes place only at the interface between the phases”, and goes faster where that interface is larger. Four levers follow.

1. Surface area against volume. The dominant one. A litre in a tall bottle has a small surface; a litre spread over a 12 × 16 inch tray has an enormous one. Ilford’s cleanest evidence isolates this term exactly: the same ID-11 stock in the same deep tank lasts four months with a floating lid and one month without one. Nothing changed but the area of liquid touching air.
2. Temperature. Ilford states plainly that for paper developers “high temperatures will reduce the effective solution life considerably”, and specifies cool storage for its own products: 5–20 °C for liquid concentrates and 4–20 °C for unopened powders. Why temperature does this to any rate is the next page.
3. Agitation. Stirring, pouring and splashing all carry fresh surface into contact with air and mix the dissolved oxygen inwards. Pouring a developer back and forth between two jugs to “mix it” is aerating it.
4. Catalysis by trace metals. This is very commonly asserted, and the course has not found a source it has read that establishes it for photographic developers, so it does not assert it either. What is sourced is adjacent and interesting: the 1924 primer records that ferric salts oxidise hydroquinone to quinone and are themselves reduced to ferrous, and Appendix L says oxygen can take ferrous back to ferric. Those two steps would close a catalytic cycle, and that is a plausible model rather than a verified mechanism, and it is labelled as such here. Kodak’s own 1928 chapter on water supply, meanwhile, is sceptical of water-quality folklore: the only impurities it says are “liable to cause serious trouble with developers” are hydrogen sulfide and soluble metallic sulfides.

Sulfite: the preservative that is spent on purpose

Sulfite protects a developing agent in two ways at once, and the 1924 and 1928 primers describe both.

It is oxidised instead. Sulfite gives its two electrons to oxygen readily, so while there is sulfite in the bottle much of the arriving oxygen is consumed by something whose loss does not matter.

It puts the developing agent back. Add sulfite to quinone and, in the primer’s words, “the quinone oxidizes the sulphite to sulphate and is itself reduced again to hydroquinone.”

C₆H₄O₂ + SO₃²⁻ + H₂O → C₆H₄(OH)₂ + SO₄²⁻

Regeneration: quinone reduced back to hydroquinone at the sulfite's expense

Both routes consume sulfite and produce sulfate, and sulfate does nothing. The primer draws the practical conclusion for the dry salt as well: sulfite that has effloresced badly on the shelf is to be viewed with suspicion, “since the sulphate is not a preservative”.

A third route is often described in the photographic literature: sulfonation, in which sulfite adds directly to the quinone to give a sulfonate rather than simply handing back the electrons. If it happens, it removes the quinone permanently instead of recycling it, which would matter, because the two accounts predict different things about how long the protection lasts and what accumulates. The course has not read a source that establishes sulfonation for photographic developers, and so does not assert it. What is sourced is the account above: sacrificial oxidation and regeneration of the developing agent, from Kodak’s own primers, in their own words. Part VIII, where the developing agents are the subject, is where the organic mechanism belongs.

Sacrificial protection, and the cliff at the end of it

• Sulfite remaining
• Developing agent remaining

Show the numbers behind this plot

Two curves against time. The sulfite curve starts at 1.0 and falls steadily and almost in a straight line, reaching zero at 0.8 of the way along the axis. The developing agent curve starts at 1.0 and stays almost flat, dropping only from 1.00 to about 0.93 over the same stretch, because the sulfite is absorbing the oxidation and handing the agent back. At the point where the sulfite reaches zero the agent's curve turns sharply downward and falls from 0.93 to 0.55 and then to 0.2 in the last fifth of the axis. A vertical guide marks the moment the sulfite is exhausted and is labelled the cliff. The teaching point is that a developer protected by sulfite shows almost no sign of deterioration until the protection runs out, and then deteriorates very quickly, which is why an old bottle can appear to work and then fail within a session.

Series	Time in a part-full bottle, arbitrary units	Fraction of the original amount remaining
Sulfite remaining	0.00	1.00
Sulfite remaining	0.10	0.88
Sulfite remaining	0.20	0.76
Sulfite remaining	0.30	0.64
Sulfite remaining	0.40	0.52
Sulfite remaining	0.50	0.40
Sulfite remaining	0.60	0.28
Sulfite remaining	0.70	0.15
Sulfite remaining	0.80	0.00
Developing agent remaining	0.00	1.00
Developing agent remaining	0.20	0.98
Developing agent remaining	0.40	0.96
Developing agent remaining	0.60	0.95
Developing agent remaining	0.80	0.93
Developing agent remaining	0.85	0.78
Developing agent remaining	0.90	0.55
Developing agent remaining	0.95	0.34
Developing agent remaining	1.00	0.20

Drawn to show the shape of sacrificial protection, not measured from any developer: the course has no time-series data of its own or from a source. What the shape asserts, and what the manufacturers' shelf lives support, is that the failure is sudden rather than gradual. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

This is why more sulfite is not a free improvement. It buys time against oxidation, but sulfite is also a mild solvent for silver halide — the 1928 primer says exactly that of D-76’s high sulfite concentration, which “actually dissolves a small quantity of each grain” and so reduces graininess. Change the sulfite and you have changed the developer’s grain and its effective speed as well as its keeping. Part VIII is where that trade is made deliberately.

The rest of the darkroom ages too, and for related reasons

Aerial oxidation is not only a developer’s problem, and the manufacturers’ figures show the same pattern everywhere.

A fixer. Ilford gives its Rapid Fixer working solution 6 months in a full tightly capped bottle and 1 month in a half-full one — the same shape, a smaller factor. Thiosulfate is not a powerful reducing agent, so it is not attacked as eagerly as a developing agent is; what ages an acid fixing bath is partly oxidation and partly the developer carried into it. Kodak’s 1928 primer describes that second mechanism precisely: films are usually transferred with very little rinsing, “so that a good deal of developer is carried over into the fixing bath, and this soon oxidizes in the bath, turning it brown, and staining negatives or prints.” The remedy the primer gives is the one still in use — sodium sulfite as a preservative in the fixer, whose “preservative action is, of course, greater if the bath is kept in a slightly acid state”. That is one of the reasons a fixer is acid, and one of the reasons a stop bath earns its place: it removes the alkaline developer before it can be carried in.

A wash aid. Ilford’s Washaid concentrate keeps 4 years in a full airtight bottle and 6 months in a half-full one. The absolute figures are long because the chemistry is not especially vulnerable; the ratio is eight to one, which is the largest full-against-half-full factor in any of the sheets read here.

A stop bath. Ilford gives its working strength 7 working days, and its exhaustion is a different subject altogether — acid reserve rather than oxidation — which the buffers page has already covered.

Read across the whole set and the lesson is that every bottle in the darkroom has two clocks running on it: one for the chemistry that ages on its own, and one for the air you leave in the bottle. The first is fixed by what the substance is. The second is yours.

Recognising a solution that has died

Some symptoms are chemistry you can see; others are inferences. The table separates them.

What you observe	What it means	How sure
The solution has gone yellow, then brown	the developing agent’s oxidation products have accumulated; sulfate is colourless, so this is the agent itself	direct; Kodak describes the coloured oxidation products of pyro and their deposition in the film
Prints will not reach a full black at the usual time	fewer electrons available per litre	inference from the stoichiometry, but a reliable one
A developer works for the first sheet and then fails quickly	the sulfite has gone and the agent is now unprotected in an open tray	consistent with the shape above; the course states it as a model
Stain in the highlights and the film base	the oxidised agent is being deposited along with the silver	direct, for pyro; Kodak’s account of the yellow negative
Rising base fog	a general symptom with several causes, of which an over-aged developer is one	weak on its own; check against a fresh batch before concluding
A sharp smell from a fixer or a stop bath	sulfur dioxide from acidified sulfite or bisulfite, which is a warning about ventilation rather than a keeping test	direct; the reason the storage page keeps acids away from sulfite salts

The last row is a hazard, not a diagnosis. Sulfur dioxide is an irritant gas, and a solution that smells sharply of it should be handled where the air moves, in line with the safety and protective equipment page.

✕The old bottle that looks fine

The commonest error is treating clarity as evidence. A stock developer well protected by sulfite can be almost water-clear on the day its protection runs out. Colour is a late symptom, which is why the practice below is built on dates and volumes rather than on appearance, and why the one honest test is a strip of film through the old bottle beside a strip through a fresh one.

The practice that follows from the chemistry

Every line here is a consequence of the surface-area argument, and the manufacturers’ own numbers are given so that none of it has to be taken on trust.

Solution and how it is kept	Published life	Source
Kodak D-76 stock, full tightly closed bottle	6 months	Kodak J-78
Kodak D-76 stock, half-filled bottle	2 months	Kodak J-78
Kodak D-76 working solution, tank with a floating lid	1 month	Kodak J-78
Kodak D-76 working solution, tray	24 hours	Kodak J-78
Kodak D-76 diluted 1+1 (the sheet writes it 1:1)	discard after one batch; do not reuse or replenish	Kodak J-78
Ilford ID-11 stock, full capped container	6 months	Ilford
Ilford ID-11 stock, half-full capped container	1 month	Ilford
Ilford ID-11 stock, deep tank with a floating lid	4 months	Ilford
Ilford ID-11 stock, deep tank without a floating lid	1 month	Ilford
Ilford ID-11 at 1+1 or 1+3	not more than 24 hours	Ilford
Ilford DD-X concentrate, full / half-full bottle	24 months / 4 months	Ilford
Ilford working-strength paper developer, open dish	not more than one working day	Ilford
Ilford Bromophen stock, full / half-full container	6 months / 3 months	Ilford
Ilford Rapid Fixer working solution, full / half-full bottle	6 months / 1 month	Ilford
Ilford Washaid concentrate, full / half-full bottle	4 years / 6 months	Ilford

Read down the pairs. Every one of them is the same liquid in the same room, differing only in how much air is in the container with it. The smallest penalty in the table is Bromophen stock at half the life; the largest is Washaid concentrate at seven-eighths; the developers cluster around two-thirds to five-sixths.

Fill the bottle. A full, tightly capped bottle is the single most effective thing you can do, and it is free.

Divide the batch on the day you mix it. Five 200 mL bottles, each filled to the neck, keep as five full bottles. One litre bottle drawn down 200 mL at a time keeps as a bottle that is half empty by the third session. This is the practical reason to own small bottles.

Squeeze the air out. Collapsible containers, and glass marbles or clean water displacement in a rigid bottle, all attack the same term. The course has no manufacturer figure for how much they gain; what it can say is that the mechanism they address is the one the datasheets are measuring.

Float a lid on a tank. Ilford’s four months against one month is the evidence, and it is unusually clean evidence because nothing else changed.

Treat an inert gas spray as a claim, not a control. The chemistry is plausible: a layer of heavier inert gas would displace the air above the liquid. The course has found no published measurement of the effect on developer life, from a manufacturer or anyone else, and manufacturer marketing is a claim rather than data. If you use one, keep the bottle full as well, and do not count the spray as a reason to keep a half-empty bottle.

Store cool, but know the second failure. Ilford specifies 5–20 °C for its liquid concentrates and 4–20 °C for unopened powder, which it says will then keep indefinitely. A cold garage, however, brings its own problem, and it is not oxidation: the solubility page shows with published solubility figures why carbonate and borax crystallise out of a stock solution in the cold, and why the liquid left above the crystals is then no longer the formula you mixed.

Mix the powder when you open the packet. Ilford’s instruction is explicit: once a packet is opened, “prepare stock solutions immediately”. A part-used packet of powder is a large surface area of solid in a bag of air.

One-shot, and the three lifetimes people confuse

The honest answer for a darkroom that runs intermittently is to make the working solution up on the day and pour it away afterwards. This is not a compromise: Ilford recommends it outright, saying that one-shot processing “eliminates or greatly reduces” the problems of reuse, and “is recommended when image quality, reliability and consistency are more important than economy”. Kodak’s D-76 sheet says the 1+1 dilution — which its own sheet writes as 1:1 — is to be diluted just before use and discarded after one batch. Neither maker is apologising for the advice.

That leaves three different numbers, and confusing them is the source of most of the arguments about whether a developer “keeps”.

1. Stock or concentrate life — an unopened or full, capped bottle of undiluted solution. Months to years. D-76 stock, 6 months full. DD-X concentrate, 24 months full.
2. Working-solution life — solution at use dilution, kept in a closed bottle or a covered tank between sessions. Hours to a month. D-76 in a covered tank, 1 month; ID-11 at 1+1, 24 hours.
3. Tray life — solution in an open dish. One session. D-76 in a tray, 24 hours; Ilford’s paper developers, one working day.

A datasheet figure quoted without saying which of the three it is has told you nothing. And every one of them assumes the container the maker had in mind: Kodak’s 1-month tank figure is stated with a floating lid, and Ilford’s 6-month stock figure is stated full.

🧪A test you can run in one session

Keep 100 mL of a freshly mixed stock developer in a completely full, capped bottle, and 100 mL of the same batch in a 500 mL bottle, capped, in the same place. After a fortnight develop two identical test strips, one in each, at the same time and temperature, and compare the maximum black and the base fog on a light box. Record the mixing date, both bottle volumes, the storage temperature and the development conditions in the notebook. It is one variable, the headspace, and it is the experiment the datasheets have already run for you — worth doing once so that the number on the sheet stops being an abstraction. Part VIII runs the fuller version, following a developer’s oxidation over time.

Where this page is used again

The mechanism here is assumed by three later parts. Part VIII designs developers around it, and runs the experiment that watches one oxidise. Part XVIII takes it into keeping and rotating a working darkroom’s chemistry. Part XXVIII catalogues the faults it produces, so that a stained negative or a print that will not reach black can be traced back to a bottle rather than to bad luck.

Ascorbate developers deserve a note, since the family is named on this page and Foma still sells a phenidone-and-isoascorbate paper developer. Ascorbic acid is described by PubChem as having a role as a food antioxidant, which is to say that being oxidised readily is what it is for. Whether ascorbate developers oxidise by a different route, or at a materially different rate, is a specific claim the course has no source for, and it is not made here; Part VIII returns to it.

Recap

Oxygen is an oxidising agent that is always present, and it reaches a solution as dissolved oxygen from the mixing water, as a fixed charge of headspace air, and — in an open tray — from an atmosphere that never runs out across a surface that is far larger. It attacks the developing agent, the sulfite and any iron(II) present. A developing agent is vulnerable because it is a good reducing agent: Kodak’s 1928 primer calls developers “necessarily substances which have a great affinity for oxygen”. The rate rises with the ratio of surface to volume, with temperature and with agitation; catalysis by trace metals is widely claimed and is not asserted here. Sulfite protects by being oxidised in place of the agent and by handing the agent back, producing sulfate, which is inert — so the protection is consumed, and when it goes the failure is abrupt rather than gradual. The storage practice that follows is not folklore but arithmetic on the surface term: full bottles, small bottles, divided batches, floating lids, cool storage with an eye on crystallisation, and a working solution mixed on the day. Three lifetimes exist — stock, working solution and tray — and a published number means nothing until you know which one it is and what container it assumed.

Next: why the same developer at 24 °C does in seven minutes what it does in ten at 20 °C, and why that relationship is not a straight line.

Check your understanding

Question 1. Two identical litres of stock developer are mixed on the same day. One goes into a full one-litre amber bottle; the other into a two-litre clear bottle, half full. Both are opened weekly. Predict the order and manner of failure, and name what is at work.

• They fail together, since the chemistry is identical
• Neither fails within six months, because sulfite protects indefinitely
• The full bottle fails first, because the solution is in contact with the cap
• The half-full clear bottle fails first: it holds a large charge of headspace oxygen that is renewed each time it is opened, and every withdrawal enlarges the headspace further; the full bottle has only its dissolved oxygen until it is first opened

Show the answer and why

Answer: The half-full clear bottle fails first: it holds a large charge of headspace oxygen that is renewed each time it is opened, and every withdrawal enlarges the headspace further; the full bottle has only its dissolved oxygen until it is first opened

Three mechanisms, and the question is really asking you to name them: the fixed charge of oxygen in the headspace, its renewal every time the cap comes off, and the growing headspace as solution is drawn down. Kodak and Ilford both quantify the first two — D-76 stock is 6 months full and 2 months half-filled, ID-11 6 months and 1 month. Clear against amber is a separate variable and matters to light-sensitive components rather than to oxidation. Note also the manner of failure: protected by sulfite, both bottles will look almost unchanged until the sulfite is spent, and then go quickly.

Question 2. Ilford gives ID-11 stock in a deep tank 4 months with a floating lid and 1 month without one. What does that pair of numbers isolate, and what does it rule out?

• It isolates the area of liquid in contact with air, and rules out temperature, volume, concentration and pH, because none of those changed between the two figures
• It isolates temperature, and rules out surface area
• It isolates agitation, since a lid prevents stirring
• It isolates light, since the lid excludes it

Show the answer and why

Answer: It isolates the area of liquid in contact with air, and rules out temperature, volume, concentration and pH, because none of those changed between the two figures

This is the cleanest natural experiment in the manufacturer literature: same tank, same solution, same room, same everything, with one variable changed. A factor of four in life for a change in exposed surface is direct evidence that aerial oxidation is a surface process, which is what OpenStax says of any reaction between two phases — it happens at the interface, and goes faster where the interface is larger. It is also the argument for why a tray is the worst container of all: not just a large surface, but one open to an unlimited atmosphere.

Question 3. A print developer gave good blacks yesterday and gives muddy greys today, with no obvious change in colour. What are the possible causes, and what is the cheapest test for the leading one?

• It must be exhausted by the prints put through it; no test is needed
• Oxidation with the sulfite now spent, exhaustion by throughput, dilution error, contamination or a temperature drop; the cheapest test is a fresh batch beside the old one, with an identical test strip in each
• The paper has fogged; test by exposing a fresh sheet
• The safelight has failed; test by covering it

Show the answer and why

Answer: Oxidation with the sulfite now spent, exhaustion by throughput, dilution error, contamination or a temperature drop; the cheapest test is a fresh batch beside the old one, with an identical test strip in each

Several causes give the same symptom, so the value of the test is that it separates the developer from everything else in one step. Mix a small fresh batch and run identical strips side by side at the same temperature and time. If the fresh one reaches black and the old one does not, the developer is the problem; if both are flat, look at exposure, paper or safelight. Note the trap in the question: the absence of a colour change is not evidence that the developer is fine. Colour is a late symptom, and a well-buffered sulfite protection keeps a solution looking clear until the moment it fails.

Question 4. A litre of stock developer is to be divided for storage. Which arrangement keeps best, and why?

• One 1 litre bottle, filled to the neck, drawn down 200 mL at a time
• Two 500 mL bottles, each half full, so there is room to shake them
• Five 200 mL bottles, each filled to the neck, opened one at a time
• One 2 litre bottle, so the solution is not in contact with the walls

Show the answer and why

Answer: Five 200 mL bottles, each filled to the neck, opened one at a time

Only one bottle is ever open to air at a time, and the other four remain full. The single-bottle arrangement is full only until the first session, after which it carries an increasing headspace for the rest of its life — which is exactly the half-full case the datasheets measure at a fraction of the full-bottle figure. This is the practical reason to own several small bottles rather than one large one, and it costs nothing but the bottles.

Question 5. Why does adding much more sulfite than the formula calls for fail to be a simple improvement, even though it extends the keeping?

• Sulfite makes the developer acid and stops it working
• Sulfite has no effect on keeping at all
• Sulfite reacts with bromide
• Sulfite is also a mild solvent for silver halide, so raising it changes grain and effective speed as well as keeping — the formula is a compromise and moving one ingredient moves several properties

Show the answer and why

Answer: Sulfite is also a mild solvent for silver halide, so raising it changes grain and effective speed as well as keeping — the formula is a compromise and moving one ingredient moves several properties

Kodak's 1928 primer makes the point about D-76 specifically: its high sulfite concentration is a solvent for silver bromide and iodide, so that as development proceeds "the sulphite actually dissolves a small quantity of each grain", which reduces clumping and so graininess. That is a designed property, not a side effect, and it moves with the sulfite. The general lesson for reading any formula is that ingredients are rarely single-purpose, and "more preservative" is a change to the developer rather than an improvement to its storage.

Question 6. A datasheet says "6 months". What three questions must you answer before that number applies to your bottle?

• Whether it refers to stock, working solution or tray life; whether the container is assumed full; and whether the storage temperature matches the one stated
• The brand, the price and the batch number
• Whether the film is 35 mm or 120, whether it is fresh, and whether the tank is stainless
• The pH, the specific gravity and the colour

Show the answer and why

Answer: Whether it refers to stock, working solution or tray life; whether the container is assumed full; and whether the storage temperature matches the one stated

All three questions have caught people out. Kodak gives D-76 six months as stock in a full bottle and twenty-four hours as working solution in a tray — the same developer, a factor of about 180. Ilford's one-month tank figure for ID-11 assumes no floating lid and its four-month figure assumes one. And both makers state a storage temperature band. A shelf life is a statement about a solution in a container in a place, and quoting the number without the conditions is quoting a third of a sentence.

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