Storage, Incompatibilities and Secondary Containment

An opened tub of sodium sulfite is not the same substance a year later, and nothing about it will look different. That is the problem this page exists to solve: storage failures are almost all silent, and the first evidence usually arrives as a bad negative you blame on something else.

By the end you should be able to sort your own shelf into groups, name the reaction behind each forbidden adjacency, size a containment tray, decide whether an undated bottle can be used, and explain why a half-empty bottle of developer fails before a full one.

What destroys a chemical on a shelf

Kodak’s 1928 teaching primer opens its section on storage with one sentence that still organises the whole subject: chemicals should be stored in well-stoppered jars in a cool, dry place, because most chemicals are affected by air — which contains oxygen, carbon dioxide and moisture. Add light and heat and you have the five agents.

Oxygen

Oxygen attacks sodium sulfite, especially in the presence of moisture, converting it to sodium sulfate, which is useless as a preservative.

2 Na₂SO₃ + O₂ → 2 Na₂SO₄

Aerial oxidation of sulfite to sulfate

The primer writes it in the period style, with atomic oxygen; the equation above is the same reaction balanced with molecular oxygen as it is written today. It is the single most consequential shelf reaction in photography, because sulfite is the preservative in nearly every developer you will make. What makes it dangerous is that it is invisible: Kodak notes that in the crystalline form the sulfate appears as a powder on the outside of the crystals and can be washed off, but that it is not easy to detect sodium sulfate in desiccated sulfite except by chemical tests. An anhydrous sulfite that has partly oxidised looks exactly like one that has not. The identification page gives the two CAS numbers, 7757-83-7 and 7757-82-6; the shelf does not label them.

Sodium bisulfite, potassium metabisulfite and every developing agent oxidise the same way, and those at least announce themselves. Kodak’s primer says the developing agents turn more or less brown, and that the extent of the colour roughly indicates the degree of oxidation. A jar of metol that has gone from white to beige to tan has been telling you something.

Carbon dioxide

Carbon dioxide converts the caustic alkalis — sodium and potassium hydroxide — into their carbonates, which are less reactive. PubChem carries the same statement from the European food-additive specification: sodium hydroxide solutions are strongly caustic and hygroscopic, and when exposed to the air they absorb carbon dioxide, forming sodium carbonate. A hydroxide that has been sitting open is therefore part carbonate, and a formula that specified hydroxide because it wanted a high pH has quietly been given something weaker.

Kodak’s 1928 primer adds the detail that anyone who has fought a hydroxide bottle will recognise: a stopper in a sodium hydroxide bottle usually becomes cemented fast by the sodium carbonate that forms, which is why the period practice was a waxed cork. The modern answer is a plastic screw cap with a chemically resistant liner, wiped clean before it goes back on.

Moisture: deliquescent and efflorescent, and why both are a weighing problem

Deliquescence is a solid with such an affinity for atmospheric water that it dissolves in the water it absorbs. Kodak’s primer names potassium carbonate among the familiar examples, and PubChem carries independent descriptions calling it a “white, very deliquescent powder” and hygroscopic crystals. Sodium carbonate anhydrous is hygroscopic; sodium hydroxide is hygroscopic.

Efflorescence is the opposite: a hydrated crystal giving up its water of crystallisation to the air, losing its crystal shape and falling to a powder. Sodium carbonate decahydrate — the “crystals” or “washing soda” of older formulae — is the case that matters, and it is documented directly: PubChem carries the Merck Index description that it is a transparent solid which effloresces in air, melting at 30 °C, and the food-additive specification states the pairing exactly: the anhydrous form is hygroscopic, the decahydrate efflorescent.

Both are weighing problems, in opposite directions. A deliquescent salt has absorbed water, so a weighed mass is partly water and you are under-dosing. An effloresced hydrate has lost water, so a weighed mass is more concentrated than the formula assumed and you are over-dosing. Kodak’s 1928 primer says the obvious thing about the first case — it is difficult to prepare a solution of definite percentage strength from a chemical that has deliquesced — and the obvious thing about storage: a very dry atmosphere suits deliquescent salts and not efflorescent ones, and the only real answer is to isolate the chemical from the air by sealing it properly.

🧮The size of the efflorescence error

Sodium carbonate decahydrate has a relative molecular mass of 286.14; the anhydrous salt, 105.99. The ratio is 2.70, so a formula written for the anhydrous salt needs 2.70 times as much of the decahydrate. Kodak Ltd’s 1949 book tells you to multiply by 2½ instead. That is not the stoichiometric figure, and the book does not say why it differs. The most likely explanation is that commercial “crystals” had already partly effloresced and were not the full decahydrate — which is exactly the reason this course prefers the anhydrous salt and states the hydrate every time. Where you must use crystals, the correction depends on how long the jar has been open, and nobody can tell you that number but your own balance. The general arithmetic of hydrates, in both directions and with the water counted into the final volume, belongs to the concentration and dilution page later in this part; what belongs here is that storage is what makes the number uncertain.

Light

Kodak’s primer names the photographic cases and they are still the right list: silver nitrate above all, whose crystals darken in light and whose solutions darken quite rapidly; potassium iodide solutions, which turn deep yellow as free iodine is liberated; nitric acid, which yellows on long standing in a clear bottle; and potassium ferricyanide solution, which turns blue as Prussian blue forms. PubChem’s independent descriptions agree on the first: silver nitrate becomes black on exposure to light or organic material, and the ICSC card says it turns grey on exposure to light.

The remedy the primer gives is to store both the solid and the solution in dark brown bottles — and the reason it names four specific cases rather than saying “everything” is that not everything needs amber glass. Sodium sulfite, sodium carbonate, sodium thiosulfate, potassium bromide and the ordinary alkalis are not light-sensitive in any way that matters on a shelf. Buying amber bottles for all of them does no damage, but it wastes money and, worse, blurs the distinction, so that the bottles that genuinely need amber stop looking special.

Kodak’s primer also notes that chemicals decomposed by light usually change faster in solution than as a solid, which is the general rule behind the whole of the next section but one.

Heat, and time

Kodak’s guidance for amateurs asks for unmixed concentrates to be stored dry, between 5 and 30 °C, away from direct sunlight and away from sources of heat. The lower bound matters as much as the upper: Kodak Ltd’s 1949 book warns that solubilities fall as temperature falls, so a concentrated stock stored cold may crystallise out, and that the precipitate often contains the most important constituents of the solution — so it is redissolved by warming rather than discarded. A garage in January is a real hazard to a developer stock, and an unheated shed can go below freezing.

Separation groups

A home laboratory holds six groups. Sorting by these rather than alphabetically is the single change that prevents the accidents on this page.

Six groups, and the adjacencies that are forbidden

1. 1Acids — glacial acetic acid, stop bath concentrate, any mineral acid
2. 2Alkalis — sodium and potassium hydroxide, carbonates, metaborate, ammonia solution
3. 3Oxidisers — silver nitrate; persulfates, permanganate and dichromate in later parts
4. 4Reducing and developing agents — sulfite, bisulfite, metabisulfite, metol, hydroquinone, phenidone, ascorbate
5. 5Silver salts — silver nitrate and every silver-bearing residue
6. 6Thiosulfates — sodium and ammonium thiosulfate, and all spent fixer
7. 7Acid + sulfite or thiosulfate — sulfur dioxide
8. 8Oxidiser + reducing agent or organic material — fire and violent decomposition
9. 9Silver salt + ammonia — silver nitride, a contact explosive; never store such a mixture

Sort by group and the forbidden pairs become impossible rather than merely discouraged.

Notice that silver nitrate belongs to two groups. When a substance does, it takes the stricter treatment of the two, and it gets a shelf of its own. That is the argument the silver nitrate page makes at length.

The five pairs, and the reaction behind each

Acid with sulfite, bisulfite or thiosulfate: sulfur dioxide

Acidify a sulfite and you re-form sulfurous acid, which is unstable and gives up sulfur dioxide. Kodak’s 1928 primer sets out the underlying chemistry — sulfur dioxide dissolving in water gives sulfurous acid, and sodium sulfite is the salt of that acid — so running the reaction backwards is what an acid does to a sulfite.

Na₂SO₃ + 2 CH₃COOH → 2 CH₃COONa + H₂O + SO₂

A stop bath meeting a sulfite

Thiosulfate does the same and also throws down sulfur, which is why spent fixer that has met an acid goes cloudy and smells. Kodak’s motion-picture processing manual gives the industrial version of the rule: acid cleaning solution is collected separately to prevent the emission of sulfur dioxide when it mixes with developer and some processing solutions. Princeton’s guidance identifies the everyday domestic version: sodium bisulfite in a fixing bath releases sulfur dioxide if the bath contains boric acid, or if acetic acid is carried in on the surface of a print.

The consequences are ranked. On a shelf: a spilled acid finding a tub of sulfite is a room full of an irritant gas. In a waste container: mixing spent stop into spent fixer does the same in a sealed bottle, which is worse. Sodium metabisulfite is worth singling out. PubChem’s physical description, from CAMEO Chemicals, calls it a corrosive acid when mixed with water and notes that it may decompose to emit toxic oxides of sulfur when heated; the description attributed to NIOSH records that it smells of sulfur dioxide before you have done anything to it at all.

Acid with hypochlorite bleach: chlorine

Household bleach is a hypochlorite solution, and acid liberates chlorine from it. Princeton’s guidance states it directly: hypochlorite bleaches can release highly toxic chlorine gas when acid is added, or if heated. This is a domestic hazard rather than a photographic one, and it belongs here because a darkroom is often a kitchen and the bleach is under the sink.

Kodak’s handling guidance gives the plumbing version, which is the one people actually meet: never pour photographic processing chemicals into a drain where chlorine-containing cleaning agents are present unless the drain has been thoroughly rinsed, and run plenty of water down before and after. A stop bath poured onto bleach sitting in a trap is the same reaction in a place you cannot ventilate.

Silver nitrate with ammonia: silver nitride

This is the one genuinely explosive possibility in a photographic laboratory, and it is worth stating precisely rather than dramatically.

Add ammonia to a silver nitrate solution and you first precipitate silver oxide, which redissolves in excess ammonia to give the colourless diammine silver complex. Mike Ware, whose account this course follows, gives the cautionary note in full: solutions of diammine silver can, with time, deposit a black solid precipitate of silver nitride — also called “fulminating silver” — which is a highly sensitive contact explosive, detonating at a touch and sometimes even when wet. Storing such solutions is not recommended. Reilly’s history of albumen and salted-paper printing records the historical route by which photographers actually met it: nineteenth-century manuals recommended “boiling down” an exhausted ammoniacal silver bath to rejuvenate it, and he explicitly declines to recommend that operation to anyone.

The rules that follow are simple and absolute. Ammonia solution is never stored next to silver nitrate. No ammoniacal silver solution is ever stored at all, or allowed to dry out. Where a later part uses an ammoniacal silver step, it is made immediately before use and quenched immediately after, and the page that does so says so. NIOSH’s own pocket-guide card for silver lists ammonia second among its incompatibilities, after acetylene, which is a useful independent confirmation that this is not a piece of alternative-process folklore.

Oxidisers with reducing agents and organic material

Silver nitrate carries H272, may intensify fire; oxidizer, in its harmonised classification and in 97.4 per cent of ECHA notifications, and CAMEO’s physical description records that it blackens on contact with organic material as readily as in light. An oxidiser stored above a shelf of paper, cloth or a wooden bench is a fire question as well as a stain question, and the silver nitrate page takes it further.

Within the darkroom’s own inventory the relevant pairing is oxidiser against developing agent. The NIOSH card for hydroquinone lists its incompatibilities as strong oxidisers and alkalis; the acetic acid card lists strong oxidisers, naming chromic acid, sodium peroxide and nitric acid, and strong caustics. Persulfates and permanganates arrive in later parts as reducers and bleaches, and they belong on their own shelf from the day they arrive.

Concentrated alkali with aluminium

NIOSH’s card for sodium hydroxide lists its incompatibilities as water, acids, flammable liquids, organic halogens, metals such as aluminium, tin and zinc, and nitromethane, with the note that it is corrosive to metals. Concentrated alkali attacking aluminium evolves hydrogen. Nothing in this course asks you to put alkali in an aluminium vessel, and the reason this pair is on the list is that aluminium is everywhere in a kitchen: foil, a baking tray pressed into service as a drip tray, a saucepan used as a water bath. Use plastic.

Note also the first item on that NIOSH list. Water is an incompatibility for solid sodium hydroxide, because dissolving it releases a great deal of heat, which is the basis of the rule that alkali and acid alike are added to water and never the reverse. Princeton’s guidance states it as a rule for photographic work: always add acid to the water when diluting.

Secondary containment and shelf elevation

How the shelf is arranged, and why

1. 1Above head height: nothing — you cannot see what you are reaching for, and it falls onto your face
2. 2Upper shelf: light, dry solids — sealed jars, labels facing out, lightest at the top
3. 3Bench height: working stocks — in a containment tray; the shelf you actually use
4. 4Low: heavy and corrosive — concentrated acid and alkali, large concentrates — a breakage splashes low
5. 5Containment tray — sized to hold the entire contents of the largest container standing in it
6. 6Silver nitrate, separately and locked — oxidiser and silver salt at once; also the most expensive thing you own

Princeton's rule is the one to remember: corrosives go low, so that a breakage does not splash at face height.

The three rules the drawing encodes are each sourced. Corrosives low: Princeton’s guidance is to store concentrated acids and other corrosive chemicals on low shelves so as to reduce the chance of face or eye damage in the event of a breakage and splash. Within reach: Kodak asks that containers be positioned where they can be reached without stretching, in a designated area away from heavy traffic where they can be identified and inventoried. Nothing above head height follows from both: a shelf you have to reach up into is a shelf you retrieve from blind, and gravity takes anything you drop towards your face.

Secondary containment is the tray. Size it by the largest single container, not by the total, because bottles do not all fail at once — and a tray that would hold everything at once is usually so deep that you cannot see the labels.

Containers

Materials

Material	Where it works	Where it does not	Source of the judgement
Amber glass	light-sensitive solutions: silver nitrate, potassium iodide, nitric acid, ferricyanide	anywhere it can be knocked off a shelf; strongly alkaline solutions frost the inside	Kodak 1928 names the light-sensitive cases and the frosting
Clear glass	measuring and mixing, where you need to see what is happening	long-term storage of anything light-sensitive	—
High-density polyethylene (HDPE, resin code 2)	the default for stock solutions and concentrates	—	Kodak’s own resin-code table lists chemical bottles under HDPE
PET (resin code 1)	—	this is the beverage-bottle plastic; Kodak’s table lists it under beverage containers	Kodak resin codes; ILFORD’s rule against soft-drink bottles

!A compatibility table this course does not have

The right way to settle a container question is a supplier’s chemical-resistance table for the specific polymer against the specific solution at the specific concentration, the way the protective-equipment page uses a glove manufacturer’s permeation data. This course searched its corpus and found no such table covering HDPE, PET and glass against concentrated acetic acid, sodium hydroxide solution and silver nitrate solution. The table above is therefore built from what the sources do say — Kodak’s resin codes, Kodak’s 1928 note on glass, ILFORD’s rule about soft-drink bottles — and the objection to PET in it is one of identity rather than chemistry: nobody in this course’s corpus says PET is attacked by these solutions, and everybody says a bottle that looks like a drink is the wrong bottle. If you need to know how a polymer stands up to a particular solution, ask the bottle’s supplier for their resistance data and record the answer.

Two of those rows need their reasoning stated rather than tabulated.

Glass and caustic alkalis. Kodak’s 1928 primer records that the solvent action of caustic alkalis on glass frosts the inside of a bottle holding caustic or strongly carbonated solutions, and adds, drily, that the quantity of glass dissolved will usually do no harm. It is a useful observation for a different reason: a frosted bottle is telling you what has been in it.

Glass and breakage. Princeton’s guidance goes further than this course does and says not to store photographic solutions in glass containers at all. The reasoning is breakage, and it is sound in a shared teaching studio where many people reach onto the same shelf. This course’s position is narrower: use HDPE by default, use amber glass where a solution is genuinely light-sensitive, and keep the glass low and inside a tray. Where you disagree with a source, say so and say why — that is what this paragraph is doing.

And never a bottle that looks like a drink. ILFORD’s leaflet for beginners says do not store chemicals in soft-drink bottles. Kodak’s version is stronger still: store processing chemicals only in the containers they were delivered in, and do not remove the labels that came on them. The practical compromise, when you must decant for weight or for volume, is on the previous page: the new label carries everything the old one did, plus the dates.

Caps, liners and headspace

A cap has a liner, and the liner is the part that fails. Wipe the neck and the cap threads before closing a bottle, because a crust of dried alkali is what cements a stopper shut, and a cap that has been cross-threaded once will not seal again. HSE’s control sheet is blunt on the general habit: put lids on containers immediately after use, even empty ones.

Headspace is the storage variable people underestimate, and Kodak Ltd’s 1949 book both explains it and measures it. The explanation first: a mixed solution should be stored in a tightly corked bottle, and stock is best kept in small bottles, because when stock is drawn from a large bottle the air space increases every time it is opened and the chances of aerial oxidation are greatly increased. A small air space should still be left, so that a temperature change does not loosen the stopper or burst the bottle.

Useful life of a stored developer, full bottle against half-full

• Kodak D-8
• Kodak D-16
• Kodak D-19b

Show the numbers behind this plot

Three Kodak developers, each with two published keeping figures, plotted against how full the bottle is. Kodak D-8 keeps two months in a full stoppered gallon bottle and one month in a half-full one. D-16 keeps six months full and two months half full. D-19b keeps six months full and three months half full. In every case the half-full bottle keeps between a third and a half as long as the full one, at 65 to 70 degrees Fahrenheit. Only those two published points exist for each developer; the lines joining them show the direction, not a measured shape in between.

Series	How full the bottle is (per cent of its volume)	Useful life in store (months)
Kodak D-8	50.00	1.00
Kodak D-8	100.00	2.00
Kodak D-16	50.00	2.00
Kodak D-16	100.00	6.00
Kodak D-19b	50.00	3.00
Kodak D-19b	100.00	6.00

Points from Kodak Ltd's 1949 table of keeping properties, for a stoppered gallon bottle at 65–70 °F (18–21 °C). Two published points per developer, joined for legibility; nothing is claimed about the shape between them. D-23 and D-25 carry the same pair of figures as D-16.

Halving the contents of the bottle costs you between half and two-thirds of its remaining life. The practical answers are all cheap. Decant a large stock into several small bottles on the day you mix it — that one is Kodak’s own advice, in the same paragraph. Use a collapsible bottle that can be squeezed to expel the air before capping, or drop clean glass marbles into a part-used bottle to bring the liquid back up to the neck; those two are ordinary darkroom practice rather than cited recommendations, and they follow from the same reasoning. For an open tank rather than a bottle, Kodak’s 1949 answer is a floating lid, with the surface skimmed with a clean blotter before use to remove any scum.

🔬Why the half-full bottle loses so much

Oxidation is a reaction between the dissolved oxygen in the solution and the developing agent or sulfite, and the dissolved oxygen is replenished from the headspace. A bottle opened weekly is refreshed with a bottle’s worth of air each time. From that mechanism it follows — this step is the course’s own reasoning rather than a published measurement — that the damage goes with the air volume multiplied by the number of times the bottle is opened, so the same solution split between five small bottles opened once each should outlive one large bottle opened five times. Kodak’s 1949 remedy for developers that are particularly prone to it is the same reasoning applied inside the formula: split the developer into two or three solutions and keep the developing agent separate from the alkali until use.

Solids against solutions

Shelf life is far more a property of a solution than of a powder. A sealed jar of anhydrous sodium sulfite in a dry place changes slowly; a made-up 10 % w/v stock of the same salt is meeting dissolved oxygen continuously. Kodak Ltd’s 1949 book notes the same preference at the point of purchase: anhydrous sodium sulfite is specified in its formulae because its advantages over the crystalline form are greater stability, ease of solution and smaller bulk in storage.

This course found no manufacturer statement of a shelf life for sodium sulfite powder and does not invent one. What the sources give is the mechanism — oxidation to sulfate, accelerated by moisture — and one diagnostic, which is that the sulfate is detectable on crystalline material as a surface powder and effectively undetectable in the anhydrous form without a chemical test. That is enough to act on: date the jar, keep it sealed and dry, buy the quantity you will use.

Three dates matter and they are different clocks.

Date	What it starts
Purchased or received	the manufacturer’s own shelf life, if the container states one
First opened	the real clock for a solid: from here it meets air every time you use it
Mixed	the clock for a solution, and the one that runs fastest

Kodak’s environmental guidance for amateurs closes the loop on the economics: buy only the supplies you need, so that you are not storing chemicals past their recommended shelf life and then having to dispose of them, and mix only the amount you will use. Waste avoided at the point of purchase is the cheapest waste there is, which is the argument the waste page opens with.

Where not to store, and locking up

Away from food, which Kodak states directly for concentrates. Not in a bedroom, where you spend eight unconscious hours. Not in an unheated shed or an uninsulated garage if the temperature can leave the 5 to 30 °C band. Not on the floor of a cupboard that also holds bleach or drain cleaner.

Out of reach of children and animals: ILFORD’s leaflet says to store chemicals safely and out of reach of children and not to let children use photochemicals unsupervised, and its general guidance asks for all processing chemicals to be kept out of reach of children and animals. A dedicated cupboard that locks is the answer if there is any doubt, and it has a second benefit — it is a boundary, so the inventory is the contents of one enclosure rather than a memory of where things are.

Decanting and transfer

Four rules, each protecting against a specific failure.

Use a funnel, and one funnel per family. A funnel is a small vessel with a large surface and a narrow neck that is difficult to inspect, which makes it the worst possible thing to share between a fixer and a developer.

Never return surplus to a stock bottle. What you poured out has been exposed to air, to the graduate, and possibly to a splash. Returning it contaminates the whole stock with the worst thing that happened to the portion. Measure what you need; what is left over is either used now or becomes waste.

Never top up an old bottle with fresh solution. This is the failure that hides itself. The bottle now contains a mixture of two ages with a single date on it, and every subsequent top-up makes the date less true. If the label says one thing and the bottle contains another, the label is worse than useless because it is believed.

Label before you fill. A bottle filled first gets labelled in a minute, and a minute is how long the telephone takes to ring.

The stock check, and retiring a chemical honestly

Once a quarter, or at the start of any part of this course that introduces new chemistry, go through the shelf with the inventory in hand and ask five questions of each container.

1. Is it labelled completely? If not, it is waste. That is not a punishment; it is the only honest answer, because you cannot state what it is, what it must not stand beside, or how to dispose of it.
2. Is it in the right group, in a tray, at the right height?
3. Has it changed? Caked or damp where it should be free-flowing. Browner than it was. Cloudy, or with a deposit, or with a crust round the neck. A frosted bottle. A cap that no longer seals.
4. Is it past a date the manufacturer stated, or past a date your own experience has set?
5. Will you actually use it before the next check? If not, that is information for the next time you order.

The honest criterion for retiring a chemical is this: use it only if you would be willing to record, in the notebook, that you used it and why you judged it sound. A chemical you would rather not write down is a chemical you should not be using, because the alternative — using it and wondering — poisons every result that follows and costs more than the replacement.

📓Lab notebook

When you retire something, write down what it was, how old it was, what made you retire it, and what you replaced it with. Ten of those entries are worth more than any published shelf life, because they are measurements of your own storage conditions rather than somebody else’s.

Recap

• Five agents destroy a stored chemical: oxygen, carbon dioxide, moisture, light and heat, with time as the multiplier. Sulfite oxidises to sulfate and becomes useless as a preservative, and the change is invisible in the anhydrous form.
• Deliquescent solids absorb water and effloresced hydrates lose it, and both make a weighed mass a lie. Potassium carbonate deliquesces; sodium carbonate decahydrate effloresces.
• Only some solutions need amber glass. Silver nitrate, potassium iodide, nitric acid and ferricyanide are the cases Kodak names; the rest of the shelf does not need it.
• Sort by group, not alphabetically. Acid with sulfite or thiosulfate gives sulfur dioxide; acid with hypochlorite gives chlorine; silver with ammonia can deposit silver nitride, a contact explosive that is never stored; oxidisers meet organic material at their own risk; and concentrated alkali attacks aluminium.
• Corrosives low, nothing above head height, everything liquid in a tray sized to the largest bottle in it.
• Headspace decides the life of a stock. Kodak’s own figures give a half-full bottle a third to a half the life of a full one, so decant into small bottles or exclude the air.
• Shelf life belongs to solutions far more than to powders, and three dates matter: received, opened and mixed.
• Do not return surplus to a stock bottle, do not top up an old bottle with fresh, and treat an unlabelled bottle as waste.

Check your understanding

Question 1. Two bottles of the same stock developer, one full and one half full, are opened weekly for a month. Which fails first, and by what mechanism?

• The full one, because it contains more solution to oxidise
• The half-full one, because the air in the headspace supplies dissolved oxygen that oxidises the developing agent and the sulfite, and it is replenished at every opening
• Neither; oxidation depends on time alone
• The half-full one, because evaporation concentrates the solution

Show the answer and why

Answer: The half-full one, because the air in the headspace supplies dissolved oxygen that oxidises the developing agent and the sulfite, and it is replenished at every opening

Kodak Ltd's 1949 table of keeping properties measures it: D-16 keeps six months in a full stoppered gallon bottle and two months half full; D-19b, six months against three. The mechanism is aerial oxidation, and the oxygen comes from the headspace and is renewed every time the bottle is opened, which is why the 1949 text advises storing stock in small bottles rather than drawing repeatedly from a large one. The practical change is to decant on the day you mix, not on the day you notice the developer is weak.

Question 2. You find a jar on your shelf with no date and a label that reads only "sulfite". How do you decide whether to use it?

• Dissolve a little and smell it; if it does not smell of sulfur dioxide it is sound
• Use it, but double the quantity to compensate for any oxidation
• You cannot decide: an undated container of a substance that oxidises invisibly to a useless product is retired, and the incident is recorded
• Weigh it; if the mass matches the label it is unchanged

Show the answer and why

Answer: You cannot decide: an undated container of a substance that oxidises invisibly to a useless product is retired, and the incident is recorded

Kodak's 1928 primer is explicit that sodium sulfate is not easy to detect in desiccated sulfite except by chemical tests, so there is no bench test available to you. Smell tells you nothing useful about the degree of oxidation, mass tells you nothing because sulfite and sulfate have different molar masses but the jar was never weighed when new, and doubling the quantity substitutes one unknown for another while changing the solvent action of the sulfite in the developer. Retire it, and write down what happened so the labelling habit improves.

Question 3. Why does this course say silver nitrate takes a shelf of its own rather than joining either the oxidisers or the silver salts?

• Because it is the most expensive chemical in the laboratory
• Because it belongs to both groups at once, so it takes the stricter rule of the two, and it also must never be near ammonia
• Because it is the only chemical that is light-sensitive
• Because its waste is collected separately

Show the answer and why

Answer: Because it belongs to both groups at once, so it takes the stricter rule of the two, and it also must never be near ammonia

Silver nitrate carries H272, may intensify fire, alongside H314, so it is an oxidiser and a corrosive at once, and it is also the substance whose contact with ammonia can leave silver nitride behind. Any one of those would justify separation; together they make a shared shelf a set of coincidences waiting to line up. The cost is a real reason for care too - it is the most expensive material on the bench - but cost is not why it is segregated, and the difference between those two arguments is the difference between a habit and a control.

Question 4. A student stores their spent stop bath and their spent fixer in the same waste container to save space. What is the specific hazard?

• The mixture will corrode the container
• Acid on thiosulfate releases sulfur dioxide and precipitates sulfur, inside a container that may be sealed
• The silver in the fixer will plate out onto the container walls
• There is no hazard; both are waste and will be treated together

Show the answer and why

Answer: Acid on thiosulfate releases sulfur dioxide and precipitates sulfur, inside a container that may be sealed

Acidifying a thiosulfate reverses it towards sulfurous acid, which gives up sulfur dioxide, and throws down free sulfur as well. Kodak's own motion-picture processing manual gives the industrial version of the rule - acid cleaning solution is collected separately to prevent sulfur dioxide emission when it meets developer and some processing solutions - and Princeton records the domestic version, where acetic acid carried in on a print releases sulfur dioxide from a bisulfite-containing fixer. A sealed container makes it worse rather than better, because the gas has nowhere to go until you open it. One container per stream is the rule the waste page enforces, and this is the reaction behind it.

Question 5. Sodium carbonate decahydrate has a relative molecular mass of 286.14 and the anhydrous salt 105.99. A formula written for 30 g of the anhydrous salt needs how much of the decahydrate, and what makes the answer uncertain in practice?

• 11 g; the uncertainty is the balance's resolution
• 81 g; the uncertainty is that the decahydrate effloresces in storage, so an opened jar has lost an unknown amount of its water
• 30 g; hydrates do not change the mass required
• 300 g; the uncertainty is the purity of the crystals

Show the answer and why

Answer: 81 g; the uncertainty is that the decahydrate effloresces in storage, so an opened jar has lost an unknown amount of its water

The ratio is 286.14 / 105.99 = 2.70, so 30 × 2.70 = 81 g. The uncertainty is real and is documented: PubChem carries the Merck Index description that sodium carbonate decahydrate effloresces in air, and the food-additive specification pairs the hygroscopic anhydrous form with the efflorescent decahydrate. An old open jar is somewhere between the decahydrate and the monohydrate and nobody can tell you where. Kodak Ltd's 1949 book gives the conversion as 2½ rather than 2.70, which is very likely an allowance for exactly this - and is why the course specifies anhydrous salts wherever it can.

Question 6. Which of these best describes why "never top up an old bottle with fresh solution" is a stronger rule than it sounds?

• The fresh solution is contaminated by the old one and immediately becomes useless
• The bottle now holds a mixture of two ages under a single date, so the label states something false and is believed
• Topping up increases the headspace and accelerates oxidation
• Mixing two batches of the same solution causes a precipitate

Show the answer and why

Answer: The bottle now holds a mixture of two ages under a single date, so the label states something false and is believed

The physical harm is modest - some partly oxidised solution diluted with fresh - and that is exactly what makes the rule easy to break. The real damage is to the record. Every date on that bottle is now an average of two unknowns, and the next top-up compounds it, so the one piece of information you would use to judge the solution has quietly stopped being true. A label that is wrong is worse than a label that is missing, because a missing label makes you cautious. Topping up in fact reduces headspace, which is why the move is tempting.

Check my answersClear