Experiment: Watching a Developer Oxidise

Kodak’s 1928 primer contains a sentence that ought to worry anybody who judges a developer by looking at it: “The fact that an old Elon-hydroquinone developer is colorless is, therefore, no indication that the original developing power is unimpaired.” An hour at a bench is enough to find out how much of that is true in your own darkroom.

Purpose

To watch an alkaline developing-agent solution oxidise in air, with and without a preservative, and then to find out whether the colour you can see is a reliable report of the developing power you cannot.

Hypothesis. Three statements, each one testable in a tube.

1. An unpreserved alkaline hydroquinone solution left open to the air develops a visible yellow and then brown colour within a session, and an otherwise identical solution carrying sodium sulfite does not, or does so far more slowly.
2. Air is the agent of the change, not time or alkali alone: a tube of the same unpreserved solution filled to the brim and stoppered stays much paler over the same period than one left open and swirled.
3. Colour is a good proxy for hydroquinone’s oxidation and a poor one for metol’s. The metol tubes will lose activity without turning nearly as dark, so a reader who judges by colour will over-estimate a metol solution’s remaining power.

The control. Two, doing different jobs. The capped tubes are the internal control on air: same solution, same alkali, same time, minimal air. A fresh reference solution, mixed at the very end from the same weighings, is the control on everything that could have drifted — your eyes adapting, the light changing, the strips coming from a different part of the roll. Without it, “it looks darker than it did” is a statement about the observer.

The one variable that changes in the main arm is the presence of sulfite: 45 g/L or none, with the agent, the alkali, the temperature, the volume, the glass and the handling identical between the pair. A second arm changes air only, holding sulfite constant. A third changes the agent only, holding both. Never more than one at a time, which is the discipline Part IX will spend a whole part on.

!This experiment has not been piloted, and the page says so

The course has not yet run this procedure to confirm that the colour change appears within a single session at the concentrations below. Every concentration is taken from a published Kodak formula, so no number here is invented — but the timing is a prediction, not a measurement.

That makes the first useful datum of the session how long it actually took, which is a measurement nobody has published and which you can make. If nothing has visibly changed at 90 minutes, that is a result: record it, leave the open tubes standing overnight in a place where nothing will be spilled, and read them again. A negative result recorded honestly is worth more than a positive one half-remembered.

Learning objectives

By the end of this session you will be able to:

• Prepare four solutions of stated composition from solids, and say where every concentration in them came from.
• Design a comparison in which exactly one thing differs between two tubes, and name what each control licenses you to claim.
• Describe the colour change of an oxidising hydroquinone solution over time, in words that another person could match against their own tube.
• Test the developing activity of a solution with a fogged film strip, and say what that test does and does not measure.
• Explain why a colourless developer is undiagnosed rather than good, in terms of the two fates of quinone in a sulfite solution.
• Decide, from your own data, whether the bottle on your shelf should be trusted.

Prerequisites

• Aerial oxidation, for why oxygen is the oxidising agent you did not add, and for the three lifetimes of a solution.
• Sulfite: preservative, buffer, silver solvent, for the two routes quinone can take in a sulfite solution and why they predict different futures.
• Concentration and dilution, for making up a solution of stated grams per litre and knowing your own accuracy.
• Laboratory safety and PPE, for the weighing discipline this page depends on.

Safety classification

Level A: a standard home darkroom. The criteria of the rubric that decided it, and the one that came closest to deciding otherwise.

• Substances. Four solutions, made from four solids. Hydroquinone carries a harmonised CLP classification of Danger with skin sensitisation, serious eye damage, suspected mutagenicity and suspected carcinogenicity; metol carries Warning with skin sensitisation and organ toxicity on repeated exposure; sodium carbonate and sodium sulfite are irritants. Handled here as solutions of a few tens of millilitres at concentrations taken from a paper developer, with gloves and eye protection, they sit inside Level A.
• The step that argues for Level B, and the control that answers it. Level B’s criteria include “fine powders that must not be inhaled”, and weighing hydroquinone and metol powder is exactly that. The encyclopaedia classifies both substances at Level B for that reason. What keeps this page at A is that the weighing is a single operation of a few grams, done once, in still air, in a tray, with no film or paper open in the room, with gloves, eye protection and a particulate mask, and with the jar closed between scoops. If you cannot weigh under those conditions, do not weigh: the alternative route below reaches most of the learning with no powder at all.
• Energies. Nothing is heated above hand-warm, nothing is under pressure, and no mains equipment is modified. Warm water from the tap dissolves everything here.
• Procedures. Tubes of solution at room temperature, stoppered and swirled. The failure mode is a spilled tube, not a burn.
• Waste. Spent developer solutions in one labelled container; the fixer and its rinse, which carry silver from the fogged strips, in the silver-bearing container.

What is not a hazard here, and why. There is no sulfur dioxide risk, although sulfite is in two of the tubes and sulfite does release the gas with acid. Nothing here is acid: the tubes are alkaline throughout, so the reaction has no reagent. That is worth saying rather than passing over, because the hazard is real one bench along — a stop bath poured into a sulfite-bearing waste bottle is exactly how it happens, which is why the two waste containers are separate and labelled.

And once the solids are in solution there is no dust hazard: the sensitiser risk becomes a contact risk, met by gloves, and the aquatic classification both agents carry is a disposal hazard rather than a handling one, which is why the waste section is not a formality.

Hazards

Hydroquinone: sensitisation, eye damage and a suspected mutagen. The harmonised CLP entry gives Danger, and the aggregated notifications put skin sensitisation and suspected carcinogenicity above 99.9 per cent, serious eye damage and suspected mutagenicity at 99.9 per cent. Sensitisation is the hazard that ends darkroom careers, because it is cumulative and does not reverse: small repeated contacts over months can produce a dermatitis that makes further work impossible. Gloves are not optional and neither is keeping fingers out of the tubes.

Metol: a skin sensitiser at a hundred per cent of notifiers. Metol dermatitis is the classic occupational complaint of the twentieth-century darkroom. Same control, same reasoning.

Both are very toxic to aquatic life, at or near unanimity among notifiers, which is why the waste goes into a container and not a sink — and it applies to the whole quantity you mixed, because an agent that has developed nothing is entirely still there.

Sodium carbonate: eye and skin irritation. At 67.5 g/L it is alkaline enough to be unpleasant in an eye and hard to notice on skin until it has been there a while, so eye protection is worn while pouring and not only while weighing.

Fine powders. All four solids are powders, and the weighing is the only step where inhalation is a live route: a tray, still air, the container closed between scoops, and a particulate mask. Never weigh in a room where film or paper is open — a settled speck of developing agent becomes a black spot on a print.

Glassware. Tubes crack when knocked, and a rack standing in a shallow tray is the difference between a spill and a mess.

Required PPE

• Single-use nitrile gloves, 0.2 mm, which is the splash-resistant grade HSE’s COSHH essentials sheet P1 specifies for manual film development where the safety data sheet gives no more specific advice. Changed if contaminated, and thrown away at the end rather than reused.
• Eye protection, worn for the whole session and not only for the weighing. The alkaline tubes are the reason.
• A particulate mask for the weighing step only. This is the control on the one criterion that argued for a higher level, and it is not a substitute for the tray and the still air.
• An apron or an overall kept for laboratory work, in line with the same HSE sheet’s instruction to minimise clothing contamination.
• Dedicated utensils, labelled and never used for anything else.

Ventilation

Nothing here evaporates and nothing generates a vapour, so extraction is not what the ventilation is for. It is for the weighing, where a fine powder can become airborne, and for the general standard HSE sets for manual development work: good general ventilation, which the COSHH essentials sheet quantifies as greater than five air changes per hour with a through draught. An openable window and an internal door left open meets that in most rooms.

The window is opened after the weighing, not during it: a draught across an open balance pan is how powder gets distributed around a room. Weigh in still air, close the jars, then ventilate.

Materials

• Six test tubes of at least 30 mL, in a rack, with stoppers or caps that seal.
• Four small bottles of 100 mL or more with tight closures, for the stock solutions, labelled A, B, C and D.
• A shallow tray large enough to stand the rack and the bottles in.
• A 100 mL measuring cylinder and a 25 mL one, or graduated syringes.
• A balance reading to 0.01 g, and a spatula.
• A white card, matt, as the background for every colour reading, and a second one to shade the rack from direct light.
• A length of scrap film, fully fogged, cut into about twenty strips of roughly 10 × 40 mm.
• Tongs or tweezers for the strips.
• A timer, a thermometer, and the notebook.

Chemicals

Chemical	Quantity	Form
Hydroquinone	2.44 g	Solid, for 200 mL of solution at 12.2 g/L
Metol	1.50 g	Solid, for 200 mL of solution at 7.5 g/L
Sodium carbonate, anhydrous	27.0 g	Solid, for 400 mL of solution at 67.5 g/L
Sodium sulfite, anhydrous	9.0 g	Solid, for 200 mL of solution at 45 g/L
Rapid fixer	about 200 mL of working solution	Proprietary ammonium thiosulfate fixer, at the maker’s own dilution for film

🧮Where every concentration on this page comes from

None of these four numbers is invented, and none of the four solutions is a developer. Each concentration is lifted from a published Kodak formula, and the page says which:

• Hydroquinone at 12.2 g/L and sodium carbonate anhydrous at 67.5 g/L and sodium sulfite anhydrous at 45 g/L are the hydroquinone, alkali and preservative of Kodak D-72, the Velox, Azo and bromide paper developer printed in the 1928 primer.
• Metol at 7.5 g/L is the metol of Kodak D-23, printed in Kodak Limited’s 1949 handbook — the concentration Kodak uses when metol is asked to work as the only agent.

The combinations are the course’s, not Kodak’s, and that is deliberate. D-72 contains metol and hydroquinone and bromide, so a tube of D-72 would tell you about four things at once. Removing everything except the agent, the alkali and (in half the tubes) the preservative is what makes the comparison mean something. The cost of that choice is that these solutions are not developers anybody sells, and their behaviour is not a prediction about D-72 or about D-23.

Scale for 100 mL of each stock:

m = c × 0.100 L

Mass for 100 mL at a stated concentration

Hydroquinone at 12.2 g/L gives 1.22 g; metol at 7.5 g/L gives 0.75 g; sodium carbonate at 67.5 g/L gives 6.75 g; sodium sulfite at 45 g/L gives 4.50 g. Each solid appears in two solutions, which is where the totals in the table above come from.

Equipment

The balance is the only instrument that has to be any good, and 0.01 g resolution is enough: the smallest quantity weighed is 0.75 g, so a 0.01 g balance reads it to better than two per cent. The thermometer matters because oxidation is a reaction like any other and its rate rises with temperature; record the room temperature at the start and the end rather than trying to control it.

Everything else is glassware, a rack, a tray and a card.

Estimated cost

Cost band £. The consumables are a few grams of four common solids and a strip of scrap film; the fixer you already have. Test tubes and a rack are a one-off purchase the rest of the course reuses. Dated prices are in the laboratory planner, which is where they can be kept current.

Estimated consumables cost

A few grams of four common solids and a strip of scrap film. This is the cheapest chemistry session in Part VIII, and the test tubes, the rack, the bottles and the tray are all one-off purchases the rest of the course reuses.

Consumed	This session	Sourced price	Cost this session
Hydroquinone	2.44 g, for 200 mL at 12.2 g/L	£11.89 per 50 g (£0.24 a g)	£0.58
Metol	1.50 g, for 200 mL at 7.5 g/L	£16.20 per 50 g (£0.32 a g)	£0.49
Sodium carbonate, anhydrous	27.0 g, for 400 mL at 67.5 g/L	£7.20 per 500 g, anhydrous	£0.39
Sodium sulfite, anhydrous	9.0 g, for 200 mL at 45 g/L	£13.68–£19.98 per 1 kg, anhydrous	£0.12–£0.18
Rapid fixer, working solution	about 200 mL, from a bath you already have	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£0.84–£1.04
Fogged scrap film	about twenty strips of 10 × 40 mm	A by-product: leaders, ends and outdated stock	—
Disposable FFP3 respirator	1, for the weighing	None. A named price gap: an FFP3 disposable respirator	—

The priced rows come to £2.42 to £2.67 for one run of this session, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 1 of the 7 rows carries no dated price, so it is counted as nothing here and is certainly not free. A priced entry is a dated range to plan against, never a quotation.

The fixer row prices the 40 mL of concentrate that 200 mL of working solution at 1+4 represents, not the working solution itself. The respirator is the only unpriced row and it is consumed one per weighing session, so it recurs.

Waste streams

Two streams, kept apart.

1. Spent developer solutions, which is everything from the six tubes and any stock left over. Metol and hydroquinone are both classified very toxic to aquatic life with long-lasting effects, and the whole quantity you weighed is still in the bottle at the end, because an agent that has reduced almost no silver has been consumed almost not at all. Bottle it, label it with the substances and the date, and take it to a household hazardous waste route.
2. The fixer and its first rinse, which are silver-bearing — the fogged strips are almost pure silver halide and the fixer dissolves it — and go into the silver-bearing container that Part II established.

Do not combine them. Kodak’s guidance for amateurs records that developer solutions carry negligible silver, so pouring the developer waste into the fixer recovers nothing and contaminates a stream that could have been recovered.

Preparation

1. Label everything before you weigh anything. Four bottles, six tubes, two waste containers. The tubes are labelled by their contents and their treatment: A-open, A-capped, B-open, B-capped, C-open, D-open.
2. Fog the film. Pull a length of scrap film out into room light, leave it for a minute, and cut about twenty strips of roughly 10 × 40 mm. They must all come from the same length, because you are going to compare the density they reach. Keep them in an envelope so they stay together and stay dry.
3. Set up the tray with the rack in it, the white card behind it, and the waste containers beside it but not in it.
4. Weigh the four solids, in still air, in a tray, with the mask on, one jar open at a time. Record every weight actually obtained rather than the one you were aiming for.
5. Make the four stocks, each in about 80 mL of warm tap water, then made up to 100 mL. Dissolve in this order and note why it is this order: for solutions B and D, the agent goes in first, then the sulfite, then the carbonate last. Kodak Limited’s 1949 handbook gives the reason — dissolving the agent first and then adding the alkali “before the preservative, sodium sulphite, is dissolved” allows “considerable aerial oxidation and formation of coloured oxidation products”. Solutions A and C have no preservative to add, so they cannot follow that rule, and the moment their carbonate goes in the clock has started.
6. Note the time the carbonate went into solution A, to the minute. That is t = 0 for the whole experiment.

Stock	Contents	Role
A	hydroquinone 12.2 g/L + sodium carbonate 67.5 g/L	hydroquinone, unpreserved
B	A, plus sodium sulfite 45 g/L	hydroquinone, preserved
C	metol 7.5 g/L + sodium carbonate 67.5 g/L	metol, unpreserved
D	C, plus sodium sulfite 45 g/L	metol, preserved

Procedure

Six tubes, three comparisons, one variable in each

1. 1A-open, B-open — the sulfite arm: identical but for 45 g/L of sodium sulfite
2. 2A-open, A-capped — the air arm: identical but for the headspace
3. 3A-open, C-open — the agent arm: identical but for which agent is dissolved

The arrangement, not the apparatus. Any six tubes that seal and stand together will do; what matters is that each pair differs in exactly one thing.

1. Charge the tubes, t = 0. Put 25 mL of each stock into its tube. The two capped tubes are filled to the brim so that the enclosed air is a bubble rather than a column, and stoppered at once. The four open tubes take 25 mL and stay open. Stand all six on the same card, out of direct sunlight, and note the room temperature.
2. Take the t = 0 activity readings immediately. Dip one fogged strip into each of the six tubes for exactly 60 seconds, agitating gently, then lift it out with tongs, rinse it in water for 30 seconds, fix it for 2 minutes, wash it and set it aside to dry. Label each strip with its tube and its time in pencil on the back, or notch a corner: six strips look identical thirty minutes later. Re-stopper the capped tubes at once.
3. Read the colour every 10 minutes for the whole 90 minutes, all six tubes at once, against the white card, in the same light and from the same distance. Write a colour and an intensity each time — faint straw, clear yellow, amber, brown, opaque brown — never just “darker”. Swirl the four open tubes for five seconds after each reading and leave the capped ones alone.
4. Take activity readings at 30, 60 and 90 minutes from the four open tubes only, exactly as in step 2. The capped tubes are opened only at 90 minutes, for their second and last strip.
5. At 90 minutes, mix the fresh reference. Make 25 mL of solution A from your own stocks or from a fresh weighing, in a seventh tube, and stand it beside the others. Read its colour against them and develop one more fogged strip in it for 60 seconds.
6. Wash all the strips together for five minutes, dry them, and lay them out in a grid: tubes down the page, times across it, with the fresh reference at the end of the top row.
7. Bottle the waste by stream, cap and label the containers, and wash everything twice.

Expected observations

These are predictions, not results. The course has not run this, and the point of writing them down first is that a prediction you recorded is falsifiable while one you remember is not.

• Tube A-open should be the fastest to change, since it holds the agent whose oxidation product is strongly coloured and no preservative. Expect a straw colour early and a deepening yellow to amber over the session.
• Tube B-open should stay far paler for far longer. If the sulfite is doing what Kodak describes, the colour is being intercepted rather than merely delayed.
• Tube A-capped should sit somewhere between them, and its position is the whole of the air arm. If it tracks A-open, then air was not the limiting factor and something else is oxidising the agent; if it stays near B-open, air was almost the whole story.
• Tubes C-open and D-open should change much less visibly than A-open, whatever happens to their activity. Metol’s oxidation products are not the strongly coloured deposit that pyrogallol’s and hydroquinone’s are.
• The strips should show the tubes losing power over the session; the interesting question is whether they lose it in the same order as the colour did.
• The fresh reference should be the palest tube and its strip the densest. If it is not, something has drifted and every other comparison needs re-reading.

=The result that would be most interesting

If a tube’s colour and its activity come apart — if B-open stays nearly water-clear while its strips get visibly weaker — you have reproduced Kodak’s warning at the bench, and it is the most useful thing this session can teach.

Kodak’s 1928 explanation is that in a sulfite solution the oxidation products of hydroquinone are the mono- and disodium sulphonates, which are colourless. On that route the agent is removed permanently and the solution never announces it. The competing route in the same book has the sulfite reducing quinone back to hydroquinone, which returns the agent. The two predict different futures for the same clear bottle, and this experiment is the cheapest way anyone has of finding out which one your bottle is on.

What is happening chemically

Oxygen is the oxidising agent nobody added. Part III put it at +1.229 V, above everything in the bottle, and pointed out that a developing agent is by design a substance chosen for its affinity for oxygen. Alkali is what makes an agent active, so it is also what makes it vulnerable: the same ionisation that lets hydroquinone give electrons to a silver ion lets it give them to oxygen.

C₆H₆O₂ → C₆H₄O₂ + 2 H⁺ + 2 e⁻

Hydroquinone oxidised to quinone: the electrons go to silver when there is silver, and to oxygen when there is not

Quinone is coloured, and it is also an oxidising agent in its own right, which is why an oxidising developer accelerates: its own product attacks what is left.

Sulfite intervenes in two ways, and Kodak’s own books give both. The 1924 primer states the first plainly: “if we add sulphite to quinone, the quinone oxidizes the sulphite to sulphate and is itself reduced again to hydroquinone.”

C₆H₄O₂ + SO₃²⁻ + H₂O → C₆H₆O₂ + SO₄²⁻

Regeneration: the agent comes back and the sulfite is spent instead

The second is the sulfonate route, which the 1928 primer names as the actual product in a sulfite developer and which removes the agent for good while leaving the solution colourless. The sulfite lesson sets out why the course does not reconcile the two and treats the second as the reason a clear developer is undiagnosed.

Sulfite also simply consumes the oxygen, before the agent gets to it:

2 Na₂SO₃ + O₂ → 2 Na₂SO₄

Sacrificial oxidation: the preservative spending itself, and the sulfate that results is not a preservative

The fogged strip is a crude but honest activity test. A fully fogged film carries developable halide everywhere, so the density a strip reaches in a fixed time at a fixed temperature depends only on how fast the solution delivers electrons. It measures rate, not capacity, and a strip that blackens fully in 60 seconds will keep doing so until a good deal of power has gone — so expect it to be insensitive early and to move fast once it moves.

Why the capped tube is not a sealed one. A stoppered tube still holds the oxygen dissolved in the water it was made with, and Wratten and Wainwright’s measurements — reported in the acutance lesson — lengthened a dilute developer’s time by nearly a quarter on dissolved air alone. It is a less-air control, not a no-air one.

Data to record

Copy this into the notebook before you start, and fill it in as you go rather than afterwards.

Session header. Date; room temperature at start and end; the light you read colours by; the balance used; who did it.

Weighings. The mass actually obtained for each of the four solids, to 0.01 g, and the volume each was made up to. Not the target — the reading.

Colour log. A row per reading time, a column per tube, and a written colour in every cell. Ten-minute intervals for ninety minutes is ten rows. Add a column for anything you notice that is not colour: a film on the surface, a smell, a precipitate.

Activity log. A row per strip: tube, time, seconds in the solution, temperature, and a description of the developed density once dry. If you have a densitometer, the reading; if you do not, a rank order against the other strips of the same time, which is a legitimate measurement as long as you say that is what it is.

The fresh reference. Its colour against the others, and its strip’s density against theirs.

What went wrong. Every session has something. A tube knocked, a strip dropped, a reading missed, a stopper found loose. Write it down at the time; it is the only way the anomaly in the data has an explanation three weeks later.

Analysis

1. Plot colour against time. Convert your written colours to a rank — 0 for water-clear, rising to 5 or 6 for the darkest you saw — and plot rank against minutes for all six tubes on one pair of axes. Say in the notebook that the vertical scale is your own ordinal scale and not a measurement, because it is, and because that is what stops somebody quoting it later.
2. Plot activity against time on the same time axis, with density or rank on the vertical.
3. Answer the sulfite arm. Compare A-open with B-open. How long did B-open take to reach the colour A-open showed at 10 minutes? That ratio is your measurement of what the preservative bought.
4. Answer the air arm. Where did A-capped sit? If it tracked B-open, air was nearly the whole story. If it tracked A-open, it was not, and the next question is what else was available to oxidise the agent.
5. Answer the agent arm. Compare A-open with C-open on colour and then on activity. If metol lost activity while staying pale, you have shown that colour is agent-specific, and you have shown why Kodak’s warning about a colourless developer is a warning and not a curiosity.
6. Cross the two plots. Did colour and activity fall together, or did one lead? This is the question the session exists for, and the answer will be different for the hydroquinone tubes and the metol ones.
7. State your uncertainty. How different did two tubes have to look before you would call them different? Half a rank? A whole one? Write the number down; it is the resolution of the whole experiment, and any difference smaller than it is not a result.

Troubleshooting

What you see	Likely cause	What to do
Nothing has changed at 90 minutes	The solutions are more stable than predicted at this temperature, or the room is cold	Record it as the result, leave the open tubes standing safely, and read again at 24 hours. Note the temperature; this is a rate, and rates depend on it
Every tube looks the same, including the fresh reference	You are comparing against a memory rather than against the card, or the light has changed	Read all seven side by side on the same card at once, always. Colour comparison is only reliable simultaneously
The sulfite tubes went cloudy	Undissolved solid, or hard water	Note it and carry on; a suspension is not an oxidation product, but say in the log that the tube was cloudy, because it affects how you judge its colour
All the strips are equally black	The solutions are still far stronger than the test needs, so the test is saturated	Shorten the dip to 20 or 30 seconds and start again from the next reading. Say in the log where you changed it
All the strips are equally clear	The fixer got into the developer tube, or the strips were fixed before they were developed	Fresh tongs per tube, and rinse them between tubes. Contamination in this direction is the commonest single spoiler
The capped tubes darkened as fast as the open ones	The stopper leaked, the tube was not full, or something other than air is oxidising the agent	Check the fill and the seal first, since those are the mundane explanations, and only then treat it as a finding

Clean-up

Pour each tube into its waste container as you finish with it, rinse it twice into the same container and only then into the sink. Wash and dry the rack, tray, cylinders and tongs. Throw the gloves away rather than reusing them, which is HSE’s own instruction, and put the wiping cloth into the waste rather than a laundry basket.

Storage

Do not keep the four stock solutions. They are the subject of the experiment, which is that unpreserved alkaline agent solutions do not keep, and there is no reason to have them on a shelf.

Keep the strips. Mounted on a card in their grid and labelled with the date, they are a physical record that will still be readable when a later part asks what a spent developer does. Dry, and dark.

Keep the solids properly. Metol and hydroquinone go back into tightly closed labelled containers in a cool dark cupboard, away from oxidisers, with the date of opening on the jar. Kodak Limited’s 1949 handbook makes the point this whole page is about: the air space grows every time a container is opened, and so do the chances of aerial oxidation.

Disposal considerations

Two containers, kept separate, both labelled with their contents and the date.

The developer waste is the one that matters. Metol and hydroquinone both carry aquatic hazard statements at or near unanimity — very toxic to aquatic life, with long-lasting effects — and essentially all of what you weighed is still in the bottle, because almost no silver was reduced. The route is not the course’s to decide: ILFORD’s advice to domestic users in the United Kingdom is to bottle each waste chemical separately, label it and take it to a Household Waste and Recycling Centre, and the disposal page sets out why the answer is jurisdictional. Check your local regulations; they govern, and they differ between authorities in one country.

The fixer and its first rinse are silver-bearing and go to the silver stream. Kodak’s guidance records that developer solutions carry negligible silver, which is exactly why combining the two streams gains nothing and spoils one of them.

Questions

1. Your B-open tube stayed pale for the whole session but its strips got steadily weaker. Which of Kodak’s two accounts of what sulfite does to quinone does that support, and what would you predict about the same bottle a week later?
2. The capped tube darkened almost as fast as the open one. Name three explanations, in the order you would check them, and say what evidence would separate them.
3. You want to know whether temperature matters. Design the smallest experiment that would tell you, name the control, and say how many tubes it needs.
4. Kodak’s 1928 primer says that adding about five per cent of old developer to a freshly mixed one reduces aerial fog, and that this works better than adding more bromide. What does that suggest an oxidised developer contains, and how would you test it with the equipment on this page?
5. Why is the fogged-strip test insensitive at the start of the session and increasingly sensitive later? What does that imply about when you should be taking readings?
6. If you repeated this with pyrogallol instead of hydroquinone, what would you expect to change, and why is the course not asking you to?

Further experiments

The headspace series. Four identical bottles of the same preserved solution, filled to 100, 75, 50 and 25 per cent, stoppered, read weekly for two months. Kodak publishes two points — six months full against two months half-filled for D-76 stock — and nothing between them. You would be measuring the curve, and no published version of it exists.

The temperature arm. The same pair of tubes, A-open and B-open, run at room temperature and in a water bath ten degrees warmer. Part III’s kinetics predicts roughly a doubling of rate for ten degrees; whether aerial oxidation obeys the same rule as development is a question worth two tubes.

The surface-area arm. The same volume of unpreserved solution in a tall narrow tube and in a shallow dish, both open. If the dish darkens far faster, air is the limiting factor; if it does not, the reaction is not limited by how fast oxygen arrives.

The bisulfite bleach. Kodak’s 1924 primer records that adding a little sodium bisulfite to a browned hydroquinone solution bleaches the colour out. Take your darkest tube at the end of the session and try it. Then develop a strip in the bleached solution, and find out whether the colour came back as developing power or only as a colour. That single follow-up is the sharpest test on this page of whether looking at a bottle tells you anything at all.

Check your understanding

Question 1. What does the capped tube control for, and what does it <em>not</em> control for?

• It controls for temperature, but not for alkali
• It controls for the air the solution meets, but not for the oxygen already dissolved in the water it was made with
• It controls for the agent, but not for the sulfite
• It controls for everything, which is why it is the reference

Show the answer and why

Answer: It controls for the air the solution meets, but not for the oxygen already dissolved in the water it was made with

A tube filled to the brim and stoppered meets far less atmospheric oxygen than an open one, so it isolates the air variable — but the water it was made with carried dissolved oxygen into the tube at the start. Wratten and Wainwright measured that effect at a 24 per cent lengthening of development time in a dilute developer. So the capped tube is a less-air control and not a no-air one, and a change in it does not prove the stopper leaked.

Question 2. Why does this page insist that the fresh reference solution is mixed at the <em>end</em> of the session rather than kept from the start?

• Because it saves chemicals
• Because a reference kept from the start would itself have oxidised, and because the drift being controlled for includes the observer’s eyes and the changing light, which only a simultaneous comparison catches
• Because the fresh solution needs to be warm
• Because Kodak specifies it

Show the answer and why

Answer: Because a reference kept from the start would itself have oxidised, and because the drift being controlled for includes the observer’s eyes and the changing light, which only a simultaneous comparison catches

Two different problems are being solved at once. A reference solution that sat on the bench for ninety minutes is not a zero-age reference, so it has to be made fresh. And the reason to make it at the end rather than to trust your memory of how tube A looked at t = 0 is that colour judgements are only reliable side by side: eyes adapt, daylight shifts, and "it looks darker than it did" is a claim about the observer unless something unchanged is standing beside it.

Question 3. Sodium sulfite is in two of the tubes and sulfite releases sulfur dioxide with acid. Why does this page nevertheless state that there is no sulfur dioxide hazard here?

• Because the quantities are too small to matter
• Because the tubes are alkaline throughout and no acid is introduced at any point, so the reaction that would liberate the gas has no reagent
• Because sulfur dioxide is not classified as hazardous
• Because the ventilation removes it

Show the answer and why

Answer: Because the tubes are alkaline throughout and no acid is introduced at any point, so the reaction that would liberate the gas has no reagent

A hazard assessment is about the procedure as written, not about the substances in the abstract. Nothing on this page is acid — carbonate holds the solutions alkaline from start to finish — so the gas-liberating reaction never has the reagent it needs. Naming the absent hazard is worth doing precisely because it is real one bench along: it is what happens when a stop bath meets a sulfite-bearing waste bottle, which is why the two waste streams stay in separate labelled containers.

Question 4. The four test solutions are not developers and are not any published formula. Why is that a strength of the design rather than a weakness?

• Because published formulas are proprietary
• Because a real developer contains several ingredients that change together, so a tube of it would confound the variable being tested; every concentration here is still taken from a published Kodak formula, so no number is invented
• Because the tubes are cheaper to make
• Because the course is not allowed to print D-72

Show the answer and why

Answer: Because a real developer contains several ingredients that change together, so a tube of it would confound the variable being tested; every concentration here is still taken from a published Kodak formula, so no number is invented

Kodak D-72 contains metol, hydroquinone, sulfite, carbonate and bromide, and superadditivity means the two agents do not behave independently. Testing it would tell you about a mixture. Stripping it down to agent, alkali and optionally preservative is what makes "the only difference is the sulfite" a true statement. The discipline that keeps it honest is that each concentration is lifted from a named published formula and the page says which, so the combination is the course’s and the numbers are Kodak’s.

Question 5. Your hydroquinone tube with sulfite stays nearly water-clear all session, but the strips developed in it get weaker each time. What have you shown, and what should you do differently with the bottle on your shelf?

• The experiment failed, since a preserved solution should not lose power
• That colour and activity can come apart, so a clear developer is undiagnosed rather than known to be good, and the bottle should be tested with a fogged strip rather than looked at
• That the sulfite has been used up
• That the strips were from a different part of the roll

Show the answer and why

Answer: That colour and activity can come apart, so a clear developer is undiagnosed rather than known to be good, and the bottle should be tested with a fogged strip rather than looked at

This is Kodak’s own warning reproduced at a bench: an old metol-hydroquinone developer being colourless is no indication that its developing power is unimpaired. The 1928 primer’s explanation is that in a sulfite solution the oxidation products are colourless sulfonates, which removes the agent permanently while removing the visible symptom too. The practical conclusion is the one the course keeps returning to: a brown developer is certainly spent, a clear one has told you nothing, and the test is a fogged clip, not a look.

Question 6. The metol tubes lose activity while staying much paler than the hydroquinone ones. What general lesson does that carry for judging developers by eye?

• Metol is more stable than hydroquinone and lasts longer
• Colour is a proxy for oxidation only for those agents whose oxidation products happen to be strongly coloured, so the reliability of a visual check depends on which agent is in the bottle
• Metol solutions should always be discarded after an hour
• Colour is never related to oxidation

Show the answer and why

Answer: Colour is a proxy for oxidation only for those agents whose oxidation products happen to be strongly coloured, so the reliability of a visual check depends on which agent is in the bottle

The result is about the visibility of the oxidation product, not about the rate of oxidation. Kodak’s graded pyrogallol demonstration makes the same point from the other end: pyrogallol’s product is yellow and is deposited in the film, which is why pyro shows the preservative’s action best of all the agents; metol’s is not deposited in a coloured form. Nothing here licenses a claim that metol oxidises more slowly — only that it does so more quietly, which is a different and more dangerous thing.

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