Silver Nitrate: The Reagent That Sets the Rules

One bottle on your shelf is worth more than everything else on it put together, and it is also the one that can burn your skin, damage your eyes, set fire to a cloth it dries on, mark your hands until they grow out and poison a river. Those two facts point the same way, which is the most useful thing about silver nitrate: every rule that protects you also protects your money.

Nothing on this page is a procedure. You will not open the bottle here. What you should be able to do by the end is pick it up in Part IV and Part V without having to think — knowing what it does, what it must not meet, how to weigh it, where every drop of it goes, and what to do in the first minute if you spill it.

What it is

Dissolve silver in nitric acid, evaporate, and you have silver nitrate, AgNO₃: colourless or white crystals, CAS 7761-88-8, relative molecular mass 169.87. It is extravagantly soluble in water — the Hazardous Substances Data Bank, via PubChem, gives 122 g in 100 cm³ at 0 °C and 245 g per 100 g of water — which is why photographic solutions of it are made without heat and without difficulty. A supplier’s sheet gives its density as 4.35 times that of water, its melting point as 212 °C and its decomposition temperature as 440 °C, and the pH of the solution as 6 to 7. It is not an acid.

Two physical descriptions on PubChem tell you the rest before any hazard statement does. The international chemical safety card says the crystals turn grey on exposure to light. CAMEO’s datasheet says they become black “on exposure to light or organic material”. Hold on to that “or”: it is the sentence from which half of this page follows.

🕰Lunar caustic

The fused, moulded form of the salt was called lunar caustic, and Towler’s Silver Sunbeam of 1864 records what it was for — surgeons used it to cauterise ulcerated surfaces — and warns photographers off it, because the fused sticks were not reliably pure. His reason is the mechanism in miniature: the caustic sticks “frequently blacken by exposure to light, whilst pure crystallized nitrate of silver does not change by a similar exposure”. Pure silver nitrate needs something to take the electron. The impurities in the surgeon’s stick supplied it.

Four properties, four rules

The classification is not a warning label; it is a list of the four separate things this substance does, each of which forces a different habit.

What silver nitrate does, and the rule each behaviour imposes

1. 1Corrosive — H314 — goggles, gloves, eyewash within reach before the cap comes off
2. 2Oxidiser — H272 — away from paper, cloth, wood, developing agents; spills collected, not left to dry in a rag
3. 3Reduced by light and by organic material — amber glass, dated solutions, stains that appear late
4. 4Very toxic to aquatic life — H400, H410 — every silver-bearing liquid collected; no route to the drain
5. 5With ammonia: silver nitride — never stored, never allowed to dry; see the storage page

The harmonised classification under Regulation (EC) No 1272/2008, read from PubChem's aggregation, carries H272, H314, H400 and H410 with the signal word Danger. Four hazard statements, four different habits.

Where those statements come from

The harmonised entry under Regulation (EC) No 1272/2008, as PubChem reproduces it, gives the pictograms GHS03 (flame over circle, oxidiser), GHS05 (corrosion) and GHS09 (environment), the signal word Danger, and H272, H314, H400 and H410. The ECHA Classification and Labelling Inventory aggregation adds statements that individual notifiers have applied and the harmonised entry does not: H290 (may be corrosive to metals) in 16.9 per cent of 803 reports, H318 (serious eye damage) in 33 per cent, and H360D (may damage the unborn child) in 11.7 per cent.

A supplier’s sheet for the ACS-grade crystal breaks the same hazards into categories, which is more useful than the statements alone: oxidising solids category 2, acute oral toxicity category 4, skin corrosion category 1B, serious eye damage category 1, acute aquatic toxicity category 1. This course has not read the classification criteria that separate the corrosion sub-categories and does not paraphrase them; record 1B as the label the sheet gives and take your instruction from the sheet’s own first-aid section, which is to flush the skin immediately with plenty of water for at least 15 minutes while removing contaminated clothing, and to get medical attention immediately.

✕Read the sheet you have, and read it sceptically

The supplier sheet quoted above is dated 31 August 2012 and its composition table names the component as silver chloride, with a mangled registry number, above a formula and molar mass that are unmistakably silver nitrate’s. It also gives the chronic aquatic hazard as category 4, H413, may cause long lasting harmful effects to aquatic life, where the harmonised entry gives H410, very toxic to aquatic life with long lasting effects. Those are two classifications of the same behaviour and the harmonised one is plainly the stricter. Neither discrepancy makes the sheet worthless: its first-aid, handling and reactivity sections are exactly what you need. Both discrepancies show why the safety data sheet page asks you to cross-check classification against ECHA or PubChem and to record the version and revision date of the sheet you actually read.

What it does to you

Eyes first, because it is the irreversible one. H314 and, in a third of notifications, H318: serious eye damage. Reilly’s history of albumen and salted-paper printing puts the practitioner’s version plainly — silver nitrate can cause permanent, irreversible eye damage, and sheets dripping with silver solution are a particular hazard to eyes. This is why silver nitrate work sits at Level B in this course’s rubric, and why the Level B control is chemical splash goggles that seal against the face rather than safety glasses with an open gap above the cheek.

Skin, second. A splash of solution is not a burn you feel at once, which is why the response is to rinse immediately and at length rather than to wait and see how it develops. NIOSH’s card for silver and its soluble compounds is unusually specific here: alongside “prevent skin contact” and “prevent eye contact” it says remove clothing when wet or contaminated, and names silver nitrate in brackets as the reason. Wet cloth against skin holds the solution there. Its other two instructions are worth reading as a pair — wash skin when contaminated, and change clothing daily.

The nose and throat. The supplier sheet describes the material as extremely destructive to the tissue of the mucous membranes and upper respiratory tract. In practice the route is dust while weighing, which is why the weighing section below is written the way it is.

Ingestion. Acute oral toxicity category 4, H302, harmful if swallowed, with a rat oral LD50 of 1,173 mg/kg on the sheet. The control is the ordinary one: nothing on the bench goes to your mouth, and hands are washed before you leave the room.

Argyria, stated accurately

NIOSH lists the symptoms of exposure to silver and its soluble compounds as “blue-gray eyes, nasal septum, throat, skin; irritation, ulceration skin; gastrointestinal disturbance”, with the nasal septum, skin and eyes as target organs. That blue-grey is argyria: silver deposited in the tissues, where it is reduced and stays. Reilly describes the same effect from the darkroom end — infrequent contact and the resulting skin staining do not appear to be dangerous to health, but prolonged exposure can lead to permanent staining, and silver absorbed through the skin is ultimately deposited around the body, causing irreversible staining of the conjunctivae, blood vessel walls, gums and mucous membranes.

Three things follow, and they are not the same thing.

• Argyria is a permanent discolouration; nothing in these sources describes it as reversible.
• It is a cumulative effect of repeated absorption, which means it is a reason to make good habits automatic in Part IV rather than a reason to be frightened of one bottle.
• It is not the only reported effect, and this course will not tidy that away. Reilly’s own sentence, immediately after the passage about staining, is that consequences more serious than staining of the tissues have also been reported, and that all necessary precautions should be taken to avoid repeated absorption of silver nitrate through the skin. He does not say what those consequences are, and neither does this course, because it has not read a source that specifies them.

What this course could not verify. It found no dose-response data in its corpus: no source it would accept tells you how much silver, absorbed how often, over how long, produces argyria. It therefore quotes no threshold, and states the occupational limits instead, which are the only numbers it has. NIOSH’s recommended exposure limit and the OSHA permissible exposure limit are both 0.01 mg/m³ as Ag over an eight-hour shift, with an IDLH of 10 mg/m³. Those are airborne limits for a working lifetime in an industrial plant. They are not a darkroom measurement, and this course does not pretend they are one; they are quoted because they are the published measure of how seriously silver is taken, and because 0.01 mg/m³ is a very small number.

The stain: what it is, and why it arrives late

The mark is not a dye and not a burn. It is metallic silver, formed where Ag⁺ has been reduced by whatever it landed on. The kit instructions for a published silver process say it exactly: when dilute silver nitrate solutions are spilled on the skin, a brown to brown-black stain results, and “the color is due to silver metal bound to the protein of the skin and cannot be washed off”.

Ag⁺ + e⁻ → Ag

The reduction that makes every silver stain, and every silver image

The electron comes from the organic material — skin protein, cellulose in paper and wood, gelatin, cotton — and light accelerates the same reduction, which is why CAMEO’s description names light and organic material in one breath. The practical consequence is the one that catches people out: the reaction is slow, so a splash you wiped off can still darken into a visible mark long after you wiped it. This course found no source that measures how long, and quotes no figure. What follows from the mechanism alone is enough: you cannot use the appearance of a stain to tell you at the time whether you have been splashed. Assume you have, and rinse.

✦What to do about a stain, and what this course will not tell you to do

Rinse the skin thoroughly with water at once — that is about limiting absorption, not about the colour, because by the time the colour appears the silver is already bound. Then leave it alone. The same kit instructions note that chemical methods to remove the stains exist but that the best procedure is to let them wear off, and this course agrees for a reason it can state: it found no source meeting its standard that evaluates the traditional iodine-and-thiosulfate stain removers for use on skin, and it quotes no timescale for a stain to go, because it found none either. A stain leaves with the skin that carries it. Applying a second and a third reagent to skin that has just been in contact with a corrosive is the worse trade of the two.

Surfaces are a different problem, because they do not shed. Silver nitrate marks worktops, grout, wood, clothing and — importantly — the enamel of a sink. This is one of the arguments the layout page makes for a non-absorbent surface and a tray under everything: the tray is what gets stained.

The ammonia prohibition

Ammonia added to a silver nitrate solution first throws down brown silver oxide, which redissolves in excess ammonia to give the colourless diammine silver complex. Mike Ware’s cautionary note is that such solutions can, with time, deposit a black precipitate of silver nitride — the “fulminating silver” of the old manuals — which is a highly sensitive contact explosive that detonates at a touch and sometimes even when wet, and that storing them is not recommended. The storage page owns the shelf rule that follows: ammonia solution is never stored next to silver nitrate.

What belongs here is the handling rule, which is stricter than the shelf rule and has no exceptions in this course:

1. No ammoniacal silver solution is ever stored, in any quantity, for any length of time.
2. No ammoniacal silver solution is ever allowed to evaporate or dry out. The condition Ware states is time; drying is how a small volume left in a dish spends that time concentrated, and it is also how any solid deposited ends up dry, exposed and able to be struck. The course prohibits both, and says which of the two the source names.
3. Where a later part uses an ammoniacal silver step, it is made immediately before use, used, and quenched immediately after — and the page that does so says so in its own procedure.
4. A vessel that has held one is rinsed at once into the silver waste container, not left on the bench to dry.

NIOSH’s independent list of incompatibilities for silver names ammonia second, after acetylene, which is a useful confirmation that this is chemistry rather than alternative-process folklore.

Oxidiser behaviour, and what it must not meet

H272 — may intensify fire — is the statement people skip, because a white salt in a bottle does not look like a fire risk. It is an oxidiser: it can supply what a fire needs from within the mixture. The kit instructions for a published silver process make the point in one sentence and then draw the conclusion this course draws: never dispose of solid silver nitrate in a wastepaper basket.

That is the real domestic hazard. Not the bottle. The cloth, tissue, kitchen roll or paper towel you wiped a spill with, dropped into a bin and left to dry against more paper.

The full incompatibility list, from two independent sources:

Source	What it names
Supplier safety data sheet, section 10	strong reducing agents, alcohols, ammonia, magnesium, strong bases
NIOSH Pocket Guide, silver card	acetylene, ammonia, hydrogen peroxide, bromoazide, chlorine trifluoride, ethyleneimine, oxalic acid, tartaric acid

Its hazardous decomposition products, on heating, are nitrogen oxides and silver oxides.

⁇Tartaric acid is on the incompatibility list and inside a published kit

NIOSH lists oxalic acid and tartaric acid as incompatible with silver, and both are reducing agents for it. Yet the published instructions for a Van Dyke brown kit have you dissolve 1.5 g of tartaric acid in 33 ml of water, dissolve 3.8 g of silver nitrate in another 33 ml, and combine them, noting that a precipitate may or may not form and that if it does you should disregard it.

Both statements are correct and they are answering different questions. An incompatibility list is written about bulk quantities in storage, where the question is what happens if a container fails. A published process is a designed reaction at a stated dilution, in a stated order, in a vessel somebody has thought about. The way to tell which situation you are in is whether a specific, published procedure tells you to combine them. If one does, follow it. If none does, the incompatibility list governs, and this course will not improvise a mixture that a list names.

Metals

The ECHA aggregation carries H290, may be corrosive to metals, in about a sixth of notifications, and the supplier sheet names magnesium. This course could not verify the behaviour of silver nitrate solutions against stainless steel or aluminium specifically, and states no rule about those two metals. It does not need to, because there is a stronger reason to keep metal out: any metal more reactive than silver displaces silver from solution. That is not a defect, it is an industrial process — Kodak’s silver-recovery publication describes metallic replacement cartridges filled with steel wool, iron filings or steel screen wire, in which “the silver is reduced to its metallic form and stays in the cartridge while the iron is oxidized and passes into solution”, recovering more than 95 per cent of the silver, and it lists the discharge of iron among the drawbacks.

Fe + 2 Ag⁺ → Fe²⁺ + 2 Ag

Metallic replacement, written as bookkeeping for Kodak's sentence

That equation is the arithmetic of Kodak’s words rather than a quotation of them, and it is written for free silver ions — the case you have in a silver nitrate solution. In a spent fixer the silver is held in a thiosulfate complex, and the balance sheet is not the same one; Part XII does that properly.

A steel spatula in a silver nitrate solution is a small, slow, uninvited silver-recovery cell. It plates your silver onto the spatula and puts iron into your sensitiser. Use glass, polyethylene or polypropylene, which the storage page recommends on other grounds anyway.

Photosensitivity, and why the bottle is amber

Pure crystals in a closed bottle are more stable than their reputation — Towler’s observation stands — but a solution in a clear bottle on a lit shelf will slowly reduce, because dust, fingerprints and the label adhesive supply the organic material and daylight supplies the drive. The consequences are practical rather than dramatic:

• Amber glass, capped, in the dark. Not because the salt is fragile, but because a solution that has started to reduce is no longer at the concentration you weighed.
• Date the solution when you make it and when you first open it, by the labelling scheme.
• Grey or black specks, a brown tinge, or a mirror on the glass mean silver has come out of solution. The solution is not what its label says. It goes to the waste container, not into a coating.

Weighing and dissolving

You will do this for real in Part IV. What follows is the discipline that page will assume.

Distilled or deionised water only. Published instructions for silver processes specify distilled or demineralised water for exactly this reason: chloride in the water supply precipitates silver chloride the moment the two meet.

AgNO₃ + NaCl → AgCl + NaNO₃

Why tap water clouds a silver nitrate solution

Silver chloride is essentially insoluble — PubChem records 1.93 mg per litre at 25 °C against 1,220 g per litre for the nitrate at 0 °C — so it drops out as a white curd and stays out. Three things have then gone wrong at once: silver has left the solution, so its concentration is lower than the mass you weighed says it is; the precipitate is itself light-sensitive and will grey; and you cannot correct it by adding more silver nitrate, because you do not know how much left. A clouded solution is a failed batch. It goes to the waste container.

This course quotes no chloride figure for tap water, because the figure is different in every supply and your water company publishes it. The rule does not depend on the number: use distilled or deionised water and the question does not arise.

Weighing, in six habits.

1. Goggles and gloves on before the cap comes off, not after the first spill. Eyewash within reach and reachable with your eyes shut.
2. Weigh into a tared vessel, not onto a paper weighing boat, and certainly not onto a piece of paper you will then fold and tip. Paper is organic material, and this substance stains and oxidises it. A small glass beaker or a plastic weighing dish, tared on the balance, is the whole technique.
3. A spatula that touches nothing else, ever. Silver is displaced from solution by metals more reactive than it, and reduced by organic residues; a spatula that has been in a developer carries a reducing agent. Dedicate one, label it, and keep it with the bottle.
4. Weigh at bench level, slowly, and do not pour from a height. The dust route is the same one the protective equipment page describes for every powder, and the airborne limit for silver is 0.01 mg/m³.
5. Add the solid to the water, in a vessel large enough to swirl, and let it dissolve without heating. At 122 g per 100 cm³ at 0 °C, nothing you will make in this course is anywhere near saturation, and there is no reason to warm it.
6. Rinse the weighing vessel into the mixing vessel with a little of the water you are going to use anyway — the same quantitative transfer the concentration page describes — and then make up to the final volume. Silver nitrate is the most expensive thing you own; the film of solution left in the beaker is money.

Spills

The general small-spill routine is on the protective equipment page: stop, protect, contain, absorb, collect, clean, record. Silver nitrate changes how the contain and collect steps are done, and the change depends on whether what is on the bench is solid or solution.

The supplier’s sheet gives two instructions for a release that pull against each other at domestic scale: pick up and arrange disposal without creating dust, and sweep up and place in suitable, closed containers. A brush is exactly what makes a dust of a fine solid. This course reads the first instruction as governing the second: lift the solid with a card or a scoop into a closed container, and if you do use a brush, use it slowly and close to the surface.

Silver nitrate spill: the two routes, and where both of them end

1. 1Solid: collect dry first — a dedicated card or scoop into a closed container, because the sheet says to pick it up without creating dust
2. 2Solution: absorb from the edge inwards — granules or a mat; a dry cloth pushes it outwards and spreads the stain
3. 3Residue: one damp disposable wipe — after the bulk is lifted, not instead of lifting it
4. 4Everything loaded goes into a sealed bag — oxidiser plus paper or cloth; not into an open bin to dry
5. 5Wash the surface with a small volume of water — and collect that rinse water; a large volume only makes more waste to store
6. 6Silver waste container — the single destination for every silver-bearing liquid and solid from the event
7. 7Notebook entry — what, how much, where, why, and what you would change

The two branches differ only at the start. The reason the solid is lifted dry first is that water turns a contained solid spill into a spreading solution one.

Three things this course tells you not to do, each with its reason.

• Do not wipe a solid spill with a dry cloth. You will spread it, load a cloth with an oxidiser, and put dust into the air. Lift it dry into a container first.
• Do not flood a spill with water to make it disappear. It does not disappear; it becomes ten times as much silver-bearing solution that you now have to collect and store, and it spreads to the edge of the tray and beyond it.
• Do not rinse any of it away. The supplier’s own environmental precaution for this material is to prevent spillage entering drains. That instruction and the aquatic classification behind it are the whole argument of the next section.

⚠On the drain, where two published sources disagree

The kit instructions quoted earlier tell the reader to clean up spilled solid silver nitrate with water and dispose of the excess down the drain. The safety data sheet bound into the same PDF says, in its accidental-release section, to prevent spillage from entering drains. Both were published by people who knew more chemistry than the reader.

This course follows the data sheet, and says why rather than asserting it. Silver nitrate’s harmonised classification is H400 and H410, very toxic to aquatic life with long lasting effects. Kodak’s own disposal publication lists silver among the effluent parameters municipalities most often regulate, with a mean limit of 1.2 mg/L — a figure a single spilled gram of silver nitrate would exceed in half a cubic metre of water. And the silver is recoverable and worth recovering. Where a manufacturer’s minimum and this course’s rule differ, the course states both and takes the stricter one, because a rule you can defend is one you will keep.

Where the silver goes

Every place silver appears in a session, and the one container they all lead to

1. 1Stock bottle, solid — amber glass, dated, stored separately and locked
2. 2Weighing and mixing vessels — rinsed quantitatively into the batch, then into the waste
3. 3The coating itself — the only silver you meant to use
4. 4Rinse water from vessels, brushes and hands — the largest stream by volume and the easiest to forget
5. 5Filters, wipes, loaded absorbent, gloves — solid silver-bearing waste; bagged, not binned loose
6. 6Failed and clouded solutions — a solution that is not what its label says is waste, not stock
7. 7One labelled silver waste container — everything above ends here; nothing here ends at a drain

The stream people lose is number 4. Rinse water looks like water, and it is the reason a silver darkroom keeps a collection container within reach of the sink rather than in a cupboard.

Two numbers make the case for collection rather than discharge. Kodak’s recovery publication gives the silver concentration of spent black-and-white fixer as 3,000 to 7,000 mg/L — grams of silver per litre, which is why fixer is worth processing. Its disposal publication gives the mean municipal sewer limit for silver as 1.2 mg/L. The ratio between those two figures, of order a thousand, is the whole reason a silver-bearing liquid is collected and not discharged.

The chemistry of getting the silver back out — electrolysis, metallic replacement and chemical precipitation, with recovery efficiencies Kodak states as more than 90, more than 95 and more than 99 per cent — belongs to Part XII, and the waste page sets out how the streams are kept apart in the meantime. What belongs here is the collecting habit, and one sentence about the law: waste rules are local, they change, and ILFORD’s own guidance for domestic users tells readers outside the United Kingdom to investigate their own national and local arrangements. Check your local regulations.

Storage, labelling and the standing procedure

The storage page argues that silver nitrate belongs in two separation groups at once — oxidisers and silver salts — and therefore gets a shelf of its own, low, in a tray, locked where there are children or animals. Add the four things that are specific to this substance:

• Amber glass or the supplier’s own container, closed, in the dark. The supplier’s sheet lists the condition to avoid as, simply, light.
• The full label, by the course’s scheme: substance, concentration, date mixed, date first opened, hazard wording, your initials. An unlabelled silver solution cannot be identified by looking at it, because it looks like water.
• Nothing organic on the same shelf. No paper, no packaging, no cloth, no wooden shelf liner.
• The silver waste container beside it, so that the collection habit does not depend on remembering where the container went.

Write the whole thing out once, as a page in the notebook, and you have your silver nitrate standard operating procedure; the course’s own version is handling silver nitrate, and yours should sit beside it with your own bench in it. It becomes a live procedure in Part IV, where silver halides are made and the first sensitised paper is coated, and again in Part V for emulsions. This page is what those pages will assume you already have.

!What this course could not verify about buying it

The research plan for this page asked whether silver nitrate is restricted for domestic purchase in the United Kingdom under the Poisons Act 1972 and the regulations made under it, and what the outline position is in the European Union and the United States. This course could not verify any of that from a primary source in its corpus, and therefore states no position. It does not tell you that silver nitrate is freely purchasable, and it does not tell you that it is restricted. Read your supplier’s terms and your own national regulator before you order, and record what you find in your notebook with the date you checked, exactly as you record a safety data sheet version.

Recap

• Silver nitrate, AgNO₃, CAS 7761-88-8, relative molecular mass 169.87, very soluble in water, carries the harmonised classification H272 (may intensify fire), H314 (severe skin burns and eye damage), H400 and H410 (very toxic to aquatic life, with long lasting effects), signal word Danger.
• Four properties, four habits: corrosive means goggles, gloves and an eyewash before the cap; oxidising means nothing organic near it and no loaded rag left to dry; reducible means amber glass, dated solutions and stains that arrive late; aquatic toxicity means everything silver-bearing is collected.
• The stain is metallic silver bound to skin protein. Rinse at once to limit absorption, then let it wear off with the skin; the course does not recommend chemical stain removers because it found no source it would accept that evaluates them.
• Argyria is the cumulative, permanent deposition of silver in the tissues. NIOSH names blue-grey skin, eyes, nasal septum and throat as symptoms, and Reilly records that more serious consequences have also been reported. No source in this course’s corpus gives a dose. The exposure limit for silver, as Ag, is 0.01 mg/m³.
• Ammonia and silver nitrate produce a solution that can deposit silver nitride, a contact explosive, on standing or drying. No ammoniacal silver solution is stored, and none is left to dry.
• Distilled or deionised water only: chloride precipitates silver chloride, and a clouded solution is a failed batch of unknown concentration.
• Metals more reactive than silver plate it out of solution. A steel spatula is an uninvited silver-recovery cell.
• Solid spills are lifted dry before anything wet touches them; solution spills are absorbed from the edge inwards; everything loaded goes into a sealed bag and everything liquid into the silver waste container.

Check your understanding

Question 1. You spill about 5 ml of 10 per cent silver nitrate solution on a wooden bench. Which sequence is right, and which two actions must not appear in it?

• Flood the area with water to dilute it, wipe with kitchen roll, bin the roll
• Goggles and heavier gloves on; absorb from the edge inwards; bag the loaded absorbent; wash the surface with a small volume of water and collect that rinse; record it
• Wipe it up quickly with a dry cloth before it stains, then rinse the cloth in the sink
• Leave it to dry so the silver is easier to scrape off later, then scrape and bin the scrapings

Show the answer and why

Answer: Goggles and heavier gloves on; absorb from the edge inwards; bag the loaded absorbent; wash the surface with a small volume of water and collect that rinse; record it

Two actions are excluded on this page and three of the options contain one or both. Flooding does not remove the silver, it multiplies the volume of silver-bearing solution you must then store, and it carries the spill past the edge of the tray. Rinsing anything silver-bearing to the drain conflicts with the H400 and H410 classification and with the sheet's own instruction to prevent spillage entering drains. A dry cloth spreads a solution rather than lifting it, and a loaded cloth left to dry is an oxidiser sitting in cellulose, which is why the kit instructions say never to put solid silver nitrate in a wastepaper basket.

Question 2. A student dissolves silver nitrate in tap water and the solution turns cloudy white. What has formed, and can the solution still be used at its stated concentration?

• Silver oxide; yes, it will redissolve on standing
• Silver chloride; no, because silver has left the solution and you cannot know how much
• Silver metal; yes, once it is filtered out
• Nothing chemical - the cloudiness is dissolved air and will clear

Show the answer and why

Answer: Silver chloride; no, because silver has left the solution and you cannot know how much

Chloride ions in the supply meet Ag+ and precipitate silver chloride: AgNO3 + NaCl gives AgCl and NaNO3. Silver chloride is essentially insoluble - PubChem records 1.93 mg per litre at 25 degrees C - so it does not redissolve, and it is itself light-sensitive, so it will grey. The concentration you calculated from the mass you weighed is no longer the concentration in the bottle, and there is no way to correct it because the amount lost is unknown. Distilled or deionised water is specified for silver work precisely so this cannot happen.

Question 3. Why does this course insist that no ammoniacal silver solution is stored or allowed to dry out, rather than simply keeping ammonia off the silver shelf?

• Because ammonia fumes would spoil the silver nitrate crystals in the bottle next to it
• Because diammine silver solutions can deposit silver nitride, a contact explosive, on standing or drying, so the hazard is created by time and evaporation rather than by proximity
• Because ammonia dissolves silver nitrate and would waste it
• Because the mixture gives off a toxic gas as soon as it is made

Show the answer and why

Answer: Because diammine silver solutions can deposit silver nitride, a contact explosive, on standing or drying, so the hazard is created by time and evaporation rather than by proximity

The shelf rule and the handling rule address different mechanisms. Segregation on the shelf protects against a container failing and two substances meeting by accident. The handling rule exists because a deliberately made ammoniacal silver solution is stable at the moment it is made and becomes dangerous later: Ware records that these solutions can with time deposit a black precipitate of silver nitride, historically called fulminating silver, which detonates at a touch and sometimes even when wet. Time and drying are the variables, so the control is to make it immediately before use and quench it immediately after.

Question 4. A supplier sheet for silver nitrate names a chronic aquatic hazard of H413 while the harmonised European classification gives H410. What should you do?

• Use the supplier sheet, because it describes the product you actually bought
• Use whichever is less strict, because the classification is only advisory
• Take the more severe classification, note both, record the sheet's version and revision date, and cross-check against ECHA or PubChem
• Ignore both; aquatic classifications do not apply to a home darkroom

Show the answer and why

Answer: Take the more severe classification, note both, record the sheet's version and revision date, and cross-check against ECHA or PubChem

A supplier sheet is about a product as supplied and is the authority for first aid, handling and the supplier's own emergency contacts. A harmonised entry under Regulation (EC) No 1272/2008 is a legally fixed classification for the substance. Where they disagree the honest response is to record both, take the stricter, and note when the sheet was revised - the one quoted on this page is dated 2012 and also mis-names its own component. The disagreement between H410 and H413 is two categories of severity for the same behaviour, which is exactly the kind of divergence the safety data sheet page teaches you to expect and to date.

Question 5. Spent black-and-white fixer is quoted by Kodak at 3,000 to 7,000 mg of silver per litre; Kodak also gives the mean municipal sewer limit for silver as 1.2 mg/L. What does the ratio between those numbers tell you about diluting silver-bearing waste before discharge?

• That about a thousandfold dilution would meet the limit, so dilution is a reasonable method
• That the limit is a concentration limit on the discharge, so dilution changes the number without changing the mass of silver released, and the silver is recoverable in any case
• That the figures must be wrong, because no waste is that concentrated
• That silver limits only apply to commercial laboratories, so the comparison is irrelevant at home

Show the answer and why

Answer: That the limit is a concentration limit on the discharge, so dilution changes the number without changing the mass of silver released, and the silver is recoverable in any case

The arithmetic is right - roughly a thousandfold - and that is exactly why the answer is not dilution. The same mass of silver reaches the same treatment works and the same watercourse; only the number on a meter changes. The waste page develops the general form of this argument, that dilution is not a disposal method. The second half of the answer is the one that makes the habit stick: at those concentrations the silver is a recoverable resource worth several grams a litre, and Part XII shows how it is recovered.

Question 6. NIOSH lists tartaric acid among the incompatibilities for silver, yet a published Van Dyke brown kit instructs you to combine a tartaric acid solution with a silver nitrate solution. How should a reader resolve this?

• The kit is wrong and should not be followed
• NIOSH is wrong, because photographers have used the combination for a century
• They answer different questions: an incompatibility list governs bulk quantities in storage, while a specific published procedure governs a designed reaction at a stated dilution and order
• The two substances are only incompatible when dry

Show the answer and why

Answer: They answer different questions: an incompatibility list governs bulk quantities in storage, while a specific published procedure governs a designed reaction at a stated dilution and order

Tartaric acid and oxalic acid are both reducing agents for silver, which is why they appear on the card. An incompatibility list answers the question "what if these two containers meet by accident", and it is written for the quantities and concentrations of storage. A published process answers "what happens when I add 1.5 g of one to 3.8 g of the other in 66 ml of water, in this order" - and that kit tells you a precipitate may form and to disregard it. The rule this course draws is asymmetric on purpose: follow a specific published procedure where one exists, and let the incompatibility list govern everywhere else, because improvising a mixture a list names is where the evidence runs out.

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