Experiment: Small-Scale Metallic Replacement

Purpose

To run, in three jam jars on a tray, the industrial process the previous page describes — and to find out, by trying, how much a bench test can and cannot tell you about the liquid that comes out.

The hypothesis. Metallic iron will reduce silver out of its thiosulfate complex and take its place in solution, so a measured volume of silver-laden fixer standing over weighed steel wool will deposit a black solid, will lose enough dissolved silver to change how a polished copper strip responds to it, and will end up carrying dissolved iron instead.

The controls, and there are two, because one is not enough.

Control B — the same spent fixer, no steel wool, in an identical jar beside it for the same time at the same temperature. It answers “would this have happened anyway?” A spent fixer left standing can throw a precipitate of its own if it is over-acid or old, and that precipitate is pale sulfur rather than black silver; without B you would have no way to say which you were looking at.

Control C — fresh working-strength fixer, with steel wool, weighed the same way. It answers the question that decides whether you can weigh anything at the end: does the iron dissolve when there is no silver to recover? Kodak says it does — “the natural corrosivity to steel of many photographic processing solutions consumes steel wool even when silver is not being removed” — and C is where you watch that happen instead of taking it on trust.

The one variable that changes between jar A and control B is the presence of metallic iron. Between jar A and control C it is the presence of dissolved silver. Held constant across all three: the fixer brand and dilution, the volume, the jar, the temperature, the standing time, the swirl schedule, the copper strip’s dip time, and the person reading it.

=What each outcome means, decided before you start

A black or grey-black sludge in jar A and nothing in B is the hypothesis standing. The colour matters: silver from this reaction is a finely divided metal and looks black, not grey-white.

A pale, milky or yellowish precipitate in A and B is not silver. It is sulfur, thrown by an over-acid or long-standing thiosulfate bath, and it means the spent fixer you brought was outside the working range before the experiment began. Record the pH you measured and stop treating the run as a recovery experiment; it has become a stop-bath-contamination experiment instead.

Control C’s wool visibly attacked, with little or no black deposit, is the expected and the important result. It is the reason the mass of the wool is not a measurement of recovered silver.

Copper strips that look identical before and after mean one of three things and the analysis section separates them: the contact time was too short, the fixer was not as loaded as you assumed, or — most likely — both readings sat on the same side of the strip’s detection floor.

No visible change in A at all after an hour points at pH first. Below about pH 4 the iron is being consumed by acid faster than by silver; above pH 7 the reaction is too slow to see.

Learning objectives

Design an experiment where one control is not enough; predict a reaction’s direction from the course’s own electrochemical ladder rather than from a rule of thumb; compute a stoichiometric requirement and explain why the practical quantity is fifteen times it; read a qualitative test against its published detection floor and state a result as a bound rather than a value; and recognise a measurement the course declines to ask you to make because a manufacturer warns against the step that would make it.

Prerequisites

Silver recovery and the responsible darkroom, which sets out the four methods, the concentrations and the economics this session tests at bench scale, and clearing time and fixer capacity, which is where the silver-laden fixer comes from and which recorded the work that went through it.

From Part III, oxidation and reduction, which owns the electrochemical ladder this page reads off, and complex formation, which numbers the stepwise thiosulfate complexes. From Part IX, experimental design for the darkroom. From Part II, chemical waste and silver waste.

Safety classification

Level A on the course rubric, and it stays there because of what the session deliberately does not do.

What applied. Working-strength fixer, already diluted, handled by the hundred millilitres in open jars. No concentrate opened, no powder weighed, no solution heated, no pH adjusted, no acid added to anything, no mains equipment beyond a timer. The rubric’s Level A waste criterion is met exactly: what leaves the bench is silver-bearing fixer collected for recovery, and nothing that needs specialist neutralisation.

The one thing that had to be designed out. Kodak’s refining publication carries a warning that looks like housekeeping and is not: a rinsed cartridge is not to be emptied, because “wet steel wool exposed to air will rust, creating heat which may cause combustion”. Finely divided iron with a large surface area, wet, in air, is an oxidation reaction with a heat balance that can run away in a heap. This page therefore never asks you to remove the wool from the liquid and dry it — which also removes the only route to weighing the deposit, and the Analysis section treats that as a result rather than a disappointment.

What is not a hazard here, and why. No acid is added, so the incompatibility that dominates every other page in this part — thiosulfate meeting acid and giving sulfur dioxide — is not created; the fixer’s own mild acidity was already in the bottle when it left the tank. Nothing is heated, so there is no vapour to extract. Nothing is weighed as a powder, so there is no dust. Steel wool and copper wire are not classified substances, and the hazard they carry is mechanical — fine wire splinters that go into a fingertip and stay there — which gloves and tongs answer rather than a data sheet. And the silver is dissolved in a bath at the concentration it left the fixing tank at, which is a disposal problem and an aquatic hazard rather than a hazard to the person holding the jar.

What no source in this course’s corpus establishes. Whether prolonged contact between an ammonium thiosulfate fixer and iron releases hydrogen sulfide. Kodak documents that gas from overdosing a sulfide precipitation, ILFORD documents it from vigorous electrolysis, and neither documents it here — and an absence of documentation is not an absence of risk. So the controls are stated anyway and are cheap: an open jar on a ventilated bench, a bounded contact time, no heat, no added acid, no pH raised toward the range where an ammonium bath gives off ammonia, and the plain instruction below about a rotten-egg smell.

Hazards

• Spent fixer, at the concentration it left the tank. Ammonium or sodium thiosulfate with sulfite and a weak acid; an eye and skin irritant on contact, and carrying dissolved silver at grams per litre, which is the aquatic hazard that makes every drop of it waste rather than rinse.
• Acid incompatibility, carried into the room rather than created here. A thiosulfate bath meeting an acid gives sulfur dioxide at once; the incompatibility matrix has the equation. Nothing acid comes onto this bench.
• Fine steel wool, mechanically. Splinters, and a large reactive surface area: it is kept wet in its jar from the moment it goes in until it leaves in a sealed container.
• The black deposit, which is finely divided metal in a wet sludge. It stains, it travels on a glove, and it is the most concentrated silver-bearing solid the course produces.
• Hydrogen sulfide, unquantified. No source in the corpus establishes that this reaction produces it. The rule is behavioural rather than numerical: if a rotten-egg smell appears, cap nothing, open the window wider, leave the room, and come back to a bench that has aired. Note that the sense of smell is unreliable as a warning at higher concentrations, which is why the response is to leave rather than to sniff again.

Required PPE

Nitrile gloves, single-use, the 0.2 mm grade the HSE’s sheet for manual film development specifies, changed the moment the black sludge gets on them and never worn a second time afterwards; the glove table governs the choice. Safety glasses whenever a jar is poured or swirled — the failure mode here is a splash of silver-laden fixer, not a burn. Tweezers or tongs dedicated to the session, so wool and copper are never handled with fingers: one control for the splinters and the stain at once.

Ventilation

The control is a through draught — the general ventilation of better than five air changes an hour that the HSE’s COSHH essentials sheet for manual film development sets for a darkroom, which in a domestic room means an openable window and a second opening or a fan. Nothing here produces a vapour by design; the ventilation is present because the corpus does not rule out a gas from this particular contact, and because a bench where nothing is expected is exactly where an unexpected smell needs somewhere to go.

Materials

Item	Quantity	Notes
Plain steel wool, fine grade	about 4 g	Plain only. Not a soap-filled pad, not an oiled one, not stainless: the soap and the oil coat the surface the reaction needs, and stainless steel is alloyed precisely to resist being dissolved.
Bare copper wire or sheet	three 60 mm strips	The 1.5 mm² solid earth core from a length of household cable is bare copper and is the cheapest source. Cleaned to bright metal with abrasive paper immediately before use.
Fine abrasive paper	one sheet	To polish the copper. A strip is only a comparator if all three start identical.
Absorbent paper	a few sheets	For blotting strips. It becomes silver-bearing solid waste.
Labels and a marker	—	Three jars, two of which look identical and contain different things.

Chemicals

Chemical	Quantity	Form
Ammonium thiosulfate or sodium thiosulfate	200 mL spent, 100 mL fresh	Working-strength fixer, already diluted. Whichever your darkroom uses; do not mix the two.
Sodium sulfite	as present	The fixer’s preservative, not added here.
Acetic acid	as present	The weak acid already in a rapid fixer, which is what puts the bath in the pH band the reaction needs. Not added.
Silver	about 0.5 to 1 g dissolved	Held as the second stepwise thiosulfate complex. It is the reagent this session is chasing, and it arrived in the fixer rather than out of a bottle.
Silver sulfide	trace, possible	The EPA’s guide notes that photographic streams carry small amounts of it alongside the complex; it would report as part of the sludge.
Iron(II)	produced, not added	The encyclopaedia’s iron(II) sulfate entry is its record of this oxidation state and its exposure limit. The course states no formula for the iron(II) species this reaction leaves in a thiosulfate solution, because no source in the corpus gives one.

Two things on this bench have no encyclopaedia entry at all: metallic iron and metallic copper. Rather than invent a row for the table, they are inventoried under Materials, where a piece of wire belongs.

Equipment

Three identical jars of about 250 mL with lids that are not used during the run. A 100 mL measuring cylinder. A balance reading to 0.01 g. Narrow-range pH strips. A timer. Dedicated tweezers. A containment tray. The labelled silver-bearing waste container, standing in the tray before anything is poured. A sealable pot for the wet solids at the end.

Estimated cost

Band £, and the session is the cheapest practical page in Part XII because the reagent is a waste product.

The planner carries the containment tray, the storage bottles, the balance and the silver-waste container as items in their own right. It does not price steel wool, copper wire or abrasive paper: all three are hardware-shop consumables bought in quantities that will outlast the course many times over, and the planner records what it could not price rather than guessing. The fixer is stock you already hold, and the 200 mL of spent bath was on its way to the waste container either way.

Estimated consumables cost

Almost nothing, and that is the point of the page: the reagent is a waste product you were going to pay to dispose of either way.

Consumed	This session	Sourced price	Cost this session
Spent fixer	200 mL	Already bought, already spent, already destined for the waste container	—
Steel wool, plain and oil-free	one pad	None. The planner records it as a hardware-shop consumable it could not price	—
Copper wire	a short length	None, for the same reason	—
Abrasive paper	one sheet	None, for the same reason	—

All three unpriced items are bought in quantities that outlast the course many times over, and the planner records what it could not price rather than guessing.

Waste streams

Two, and one of them is unusual in being more concentrated leaving than arriving.

• Silver-bearing liquid: all three jars’ contents, the copper strips’ rinse, and any spillage on the tray. Into the labelled container under the silver-bearing waste procedure. Note that control C is fresh fixer and still goes there: it now carries dissolved iron, and it has been in contact with the session.
• Silver-bearing solids: the wool with its sludge, the copper strips, the blotting paper, the gloves. Bagged wet, sealed, labelled and kept with the liquid container. Nothing is dried, for the reason given under Safety classification.

Every one of these is subject to the disposal caveat, and the standing sentence applies: the chemistry above is general, what may lawfully be done with the waste depends on where you are, it differs between authorities inside one country, and it changes. Check your local regulations.

Preparation

Before the session, and not on the day.

1. Source the spent fixer and write down its history. Which fixer, which dilution, how many rolls or 8 × 10 equivalents went through it, and when it was retired under the Part XI criterion. That history is what turns the volume into an expected silver mass later; without it this is a colour-change demonstration.
2. Measure its pH with a strip and record it. ILFORD gives its rapid fixer at 1+4 as pH 5.0 to 5.5, which sits inside Kodak’s usable band of 4 to 6.5 and just below its ideal 5.5 to 6.5. Record the value and do not adjust it. Adjusting would add a second variable to a one-variable experiment, and a reading outside the band is a result about your bath.
3. Label three jars A, B and C with their contents and the date, following the labelling procedure, and stand them on the tray with the waste container.
4. Weigh two portions of steel wool to 0.01 g, about 2 g each, and record both masses. They will not be used to compute a recovery; they are recorded so that the reason they cannot be is a measurement rather than an assertion.
5. Polish three copper strips to bright metal and set them aside dry. One is the reference and is never dipped.

🧮How much iron does 0.5 g of silver actually need?

The stoichiometry comes straight off the equation: one iron atom hands two electrons to two silver atoms.

55.845 ÷ (2 × 107.87) = 0.259 g of iron per gram of silver

Iron required per gram of silver recovered

So 100 mL of a spent film fixer carrying 5 g/L — half a gram of silver — needs 0.13 g of iron, about a sixteenth of the 2 g of wool this page specifies.

Why use fifteen times what the equation asks for? Three reasons, all of them Kodak’s. The reaction happens at a surface, so the mass that matters is the mass that is wetted, not the mass in the jar. The steel channels as it is consumed, and a channel lets solution past untreated. And the bath attacks the iron whether or not there is silver in it, which is what control C exists to show. An excess is not sloppiness; it is the design.

Turn the same figure round for the sludge: 1 g of iron can take out 3.86 g of silver. Now remember that Kodak’s fully exhausted cartridges assay at only 20 to 40 per cent silver by dry weight, and the gap between those two numbers is the whole story of why this is the cheapest equipment and the most expensive material to refine.

Procedure

Three jars, one variable at a time

1. 1Jar A — 100 mL spent fixer + 2 g weighed steel wool — the treatment; black sludge collects below the wool
2. 2Jar B — 100 mL spent fixer, no wool — control for anything that would have happened on standing
3. 3Jar C — 100 mL fresh fixer + 2 g weighed steel wool — control for iron dissolving with no silver to recover
4. 4Three copper strips: reference, A-before, A-after — polished together, dipped for the same time, read side by side
5. 5Silver-bearing waste container, on the tray from the start — every liquid and every solid ends here, wet

The two controls are what make this an experiment rather than a demonstration: B says the change needed the iron, C says the iron would have been consumed anyway.

1. Put on gloves and glasses. Stand all three jars, the waste container and the solids bag on the tray.
2. Measure 100 mL of spent fixer into jar A and 100 mL into jar B. Measure 100 mL of fresh working-strength fixer into jar C. Use the same cylinder, rinsing between the spent and the fresh, and put the rinse into the waste container.
3. Dip the “before” copper strip into jar B — not A, because A is about to change and B is its twin — for exactly 60 seconds, held with tongs, then blot it and lay it on the card beside the untouched reference. Sixty seconds is this course’s own convention, not a published figure: Kodak names the polished copper strip as an on-site technique and gives its sensitivity, but no dip time, so the number is held constant across every reading rather than treated as meaningful in itself.
4. Wet both portions of wool with a little water and let them stand a minute before use. Kodak pre-conditions a new cartridge the same way — with a mildly acidic solution, or at the very least water — to etch the steel and to keep the solution from finding a channel through a dry mass.
5. Drop one weighed portion of wool into jar A and the other into jar C. Note the time. Do not press it down; let it sit loosely on the bottom.
6. Swirl each jar once, gently, every ten minutes, and observe at 5, 10, 20, 40 and 60 minutes. Record what you see in each jar at each time, including “no change”. A still jar has no flow, so contact here is diffusion plus your swirl — which is the bench-scale version of the dwell time Kodak calls the single biggest factor in a cartridge’s performance.
7. At 60 minutes, stop. Let jar A stand undisturbed for 10 minutes so the sludge settles.
8. Dip the “after” copper strip into jar A’s clear upper liquid for exactly 60 seconds, blot it, and lay it on the card between the reference and the before strip. Read all three side by side, in the same light, at once.
9. Measure and record the pH of all three jars.
10. Do not filter, decant, dry or weigh anything. Cap the jars, and see Clean-up.

Expected observations

Jar A, in the first minute or two. The wool darkens where the liquid touches it and fine dark particles begin to detach and drift down — fast, because the outer surface is clean and wetted, then slower, because the surface is the reagent and it is now coated.

Jar A, over the hour. A black to grey-black layer builds under and around the wool, the liquid above may look faintly cloudy before the particles settle, and the wool loses its springy brightness and looks eaten. Anything standing above the liquid line goes orange-brown, which is ordinary rust rather than the reaction you came for.

Jar B. Nothing. If anything appears it is pale rather than black, and see the callout under Purpose.

Jar C. The wool is attacked — the surface dulls, fine bubbles may cling to it — but the bottom of the jar stays clean of any black deposit. This is the observation the whole session turns on, and it is also why iron sits below hydrogen on Part III’s ladder: an acid bath will take electrons from iron with nothing else on offer.

The copper strips. The before strip, dipped in a bath at grams of silver per litre, should darken to a dull brown-grey along the wetted length within the minute; the after strip should be visibly less marked and may look untouched. The reference strip stays bright, so that “untouched” is a comparison.

Smell. The ordinary fixer smell; anything resembling rotten eggs is the stop condition in Hazards.

What is happening chemically

One reaction, read two ways.

2 [Ag(S₂O₃)₂]³⁻ + Fe → 2 Ag + Fe²⁺ + 4 S₂O₃²⁻

Metallic replacement in a fixer: iron reduces the complexed silver and passes into solution

That is Kodak’s own overview reaction, written for the species actually present. The silver bromide entry and Part II’s silver nitrate lesson carry the same chemistry written for free silver ion —

Fe + 2 Ag⁺ → Fe²⁺ + 2 Ag

The same displacement as bookkeeping for free silver ions, the case in a silver nitrate solution

— and Part II said explicitly that a spent fixer’s balance sheet is not that one and that Part XII would do it properly. This is that page. The difference is not cosmetic: in the fixer the silver is held in the second of Part III’s stepwise thiosulfate complexes, and four thiosulfate ions are released for every iron atom that dissolves.

Why it goes at all, and only just. Part III’s ladder gives the free silver couple at +0.7996 V and the same silver locked in the bis(thiosulfato) complex at +0.017 V — barely above the hydrogen reference. Binding silver in the complex is what makes fixing possible, and it is the same thing that makes recovering it slow: iron is still below the complexed couple, so the reaction runs, but with almost none of the driving force the free ion would give. That is the chemistry behind Kodak’s insistence on dwell time. The course prints no E° for the iron-metal couple, because it has not read one in the source its ladder comes from; the direction is established by the ladder and by the observation in jar C, not by a number this page would be quoting from memory.

What else the iron is doing. In parallel, the bath’s own acidity attacks it:

Fe + 2 H⁺ → Fe²⁺ + H₂

The general-chemistry account of Kodak's sentence that photographic solutions consume steel wool with or without silver present

That reaction is written as the arithmetic of Kodak’s words rather than as a quotation of them, and it is why the pH band matters at both ends. Below about pH 4 to 5 it dominates and the wool is spent before the silver is out; above pH 7 the wool is barely activated and the silver goes past. Most fixers, as Kodak notes, sit in the usable band without being adjusted — which is a lucky fact rather than a designed one.

The sludge. Kodak describes an exhausted cartridge’s contents as “a combination of degraded metallic iron and collected silver”, assaying at 20 to 40 per cent silver by dry weight, and the EPA guide notes small amounts of silver sulfide travelling with the complex in these streams. So the black solid in your jar is not silver; it is silver mixed with what used to be the wool.

And why the treated fixer cannot be reused. Kodak is explicit: solutions passed through a cartridge are not returned to a processor, because the dissolved iron and the other reaction by-products contaminate the tank. You have traded a silver problem for an iron one — and iron has a mean regulated sewer limit of its own, 17 mg/L.

Data to record

In the lab notebook, and the first three lines are the ones that make the rest mean anything.

Record	Why it is on the list
Fixer brand, dilution, and the work that went through it	Converts a volume into an expected silver mass. Without it the session has no quantitative half.
pH of the spent fixer, before and after	Places the run in or outside Kodak’s band, and the change says how much acid the iron consumed.
Mass of each wool portion, to 0.01 g	Recorded so that the reason it cannot be used for a recovery figure is demonstrated rather than asserted.
Volume in each jar, and the temperature of the room	The two things most likely to differ between your run and somebody else’s.
Observation of all three jars at 5, 10, 20, 40 and 60 min	Including “no change”, which is data in jars B and C.
Copper strip dip time, and the three strips read together	Photograph or tape them into the notebook; a described colour is not a comparison.
The expected silver mass, computed	From the work log and Part XI’s per-film and per-print figures.
The stoichiometric iron requirement, computed	And the ratio between it and what you actually used.
Anything smelt	With the time.

Analysis

First, the qualitative result, stated as a comparison. Jar A changed, jar B did not: the change required the iron. Jar C’s wool was attacked without producing a deposit: the iron would have been consumed anyway. Those two sentences together are the hypothesis confirmed and its main practical caveat, and neither needs a number.

Second, the expected mass. Multiply the work through the bath by Part XI’s figures — 0.33 to 0.42 g per 135-36 roll by the upper of its two routes, 50 mg per 8 × 10 fibre print — and divide by the bath’s volume to get grams per litre; multiply by 0.1 L for what was in jar A. Compare it with Kodak’s published 3 to 7 g/L for a worked black-and-white fixer. If your figure is an order of magnitude below, the bath was retired early, which is a legitimate result and should be said.

Third, and this is the section’s real work: what you may and may not conclude from the strips.

!Reading a qualitative test against its own floor

Kodak states the detection limit of this class of test plainly: a strip “can be used to estimate silver concentrations greater than 1 gram/litre”, and soaking it longer to see below that is “extremely qualitative and not reliable to quantify low levels”. ILFORD says the same thing from the other end — silver estimator papers “are usually not sensitive enough to test the very low silver levels suitable for optimum permanence”.

Now put that beside what a working cartridge achieves: below 5 mg/L, in Kodak’s own figures.

The floor of the test is two hundred times higher than the result the method is capable of. So a blank after-strip does not say the liquid is desilvered. It says one sentence, and this is the sentence to write in the notebook:

After treatment, the dissolved silver was below about 1 g/L, which is the detection floor of the test used. No statement about the actual concentration is possible from this session.

That is not a failure of the experiment. It is the experiment’s most useful output, because it is exactly why on-site methods “cannot typically be used to demonstrate regulatory compliance” — Kodak’s words — and why the treated liquid in jar A goes into the silver container along with everything else. A test that cannot see the difference between compliance and a hundredfold breach is not a permit.

Fourth, the recovery mass you cannot measure, and the three independent reasons. (a) The wool cannot be dried, because Kodak warns that wet steel wool in air rusts with enough heat to risk combustion, so the mass difference is unavailable by design. (b) Even dried, the solid is 20 to 40 per cent silver, so its mass is not the silver’s mass. (c) Control C shows iron leaving the wool with no silver arriving, so the mass lost is not the mass displaced either. What would measure it is a laboratory analysis of the liquid for total silver before and after — the analysis Kodak says on-site methods cannot replace — and if you ever have one done, the course would like the number.

Troubleshooting

Nothing happens in jar A. Check the pH you recorded. Below 4 or above 7 explains it, in opposite directions. If the pH is in band, suspect the wool: a soap-filled or oiled pad has a coated surface, and stainless steel is alloyed not to do this.

A pale precipitate in both A and B. Sulfur, from an over-acid or long-standing bath, not silver. The run is not a recovery experiment; record it as a result about the fixer’s condition.

The black deposit will not settle. Give it longer, undisturbed. Do not filter: Kodak notes that silver precipitates plug filter media, and a loaded filter is a disposal problem you did not need.

All three copper strips look the same. Either the before strip met a bath that was not as loaded as you assumed — check the work log — or the strips were not polished to the same finish. A comparator only compares if it starts identical.

The after strip is as dark as the before strip. Contact was too short, or the wool channelled and the liquid passed the mass without meeting much of it. Kodak’s advice for a cartridge applies to a jar: when in doubt, longer.

The wool has broken into floating fragments. Normal, and they go with everything else.

Clean-up

1. Pour the liquid from jars B and C into the silver-bearing waste container.
2. Pour jar A’s liquid off the sludge into the same container, keeping the solid in the jar.
3. Add a little water to jar A so the wool and the sludge stay covered, then transfer the whole wet contents into the sealable pot or bag. Nothing is dried, nothing is spread out, nothing is left in an open dish overnight.
4. Put the copper strips, the blotting paper and the gloves in with the solids.
5. Rinse the jars and the cylinder into the waste container, not the sink.
6. Wipe the tray. Label the solids pot with its contents, the date and the process it came from, and stand it with the liquid container.

Storage

Nothing from this session is stock and nothing is kept for a future run: the fresh fixer that became control C now carries iron and is waste. The wet solids pot is sealed and stored with the silver-bearing liquid container in its tray until both go by the route you checked — stored wet, deliberately, for the reason the safety classification gives.

Disposal considerations

The chemistry is the interesting part and it does not change with jurisdiction. This session takes a liquid whose silver was dissolved and converts most of it into a solid whose silver is metallic, mixed with degraded iron, at 20 to 40 per cent silver by dry weight if it ran well. That solid is, industrially, what goes to a refiner, and it is the most concentrated silver-bearing object a home darkroom will hold.

The liquid that comes off is not clean. It is a fixer whose silver has been reduced by an unknown amount, below a detection floor two hundred times above the target, and which now carries dissolved iron — the by-product that stops the industrial process being used on rinse water and that appears in its own right at a mean sewer limit of 17 mg/L. Kodak lists “discharges iron, limited by some sewer codes” among metallic replacement’s standing disadvantages. Nothing about having run this experiment makes the liquid more disposable than it was; if anything it has one more regulated metal in it.

Both streams go, together, by the route in the silver-bearing waste procedure. And the disposal caveat governs, as it does everywhere: the chemistry above is general, what may lawfully be done with this waste depends on where you are, it differs between authorities within one country, and it changes. Check your local regulations.

Questions

1. Iron reduces silver in jar A. Given that the complexed silver couple sits at +0.017 V and the free silver ion at +0.7996 V, what does binding silver in thiosulfate do to the driving force for this reaction — and what practical consequence does Kodak draw from it?
2. Control C contains no silver, and its wool is attacked anyway. Write the sentence that explains why this makes the mass of the wool useless as a measure of recovery.
3. Your after-strip is blank. State, in one sentence that would survive somebody reading your notebook in ten years, exactly what that entitles you to conclude.
4. A bath at pH 3.5 and a bath at pH 8 both fail to recover silver efficiently. Say why, in each case, and say which failure wastes the wool and which wastes the silver.
5. The sludge is 20 to 40 per cent silver and electrolytic flake is above 90 per cent. Both hold silver a refiner can smelt. Why does that difference change the answer to “is recovery worth it at my volume?” more than the difference in recovery efficiency does?

Further experiments

Contact time as the variable. Four jars of the same fixer and the same weighed wool, stopped at 15, 30, 60 and 120 minutes and read with a strip each. Not a concentration, but possibly an ordering, and the point where the strips stop changing is your bench’s version of the dwell-time curve Kodak calls the single biggest factor.

Surface area as the variable. Same mass of iron, different forms: fine wool, coarse wool, a plain steel nail. Kodak lists wool, filings, screen wire and iron on a rigid support as the industrial forms and says wool is chosen “for its surface area” — and a nail in the same fixer for the same hour makes that argument better than a sentence does.

A second pass. Stand jar A’s treated liquid over fresh weighed wool for another hour. Industry runs two cartridges in series for exactly this reason. Whether you can see any difference at all is itself an answer to a question about where the strips’ floor lies.

Check your understanding

Question 1. You are treating 250 mL of spent fixer that your capacity log says carries about 6 g of silver per litre. How much iron does the reaction stoichiometrically require, and what mass of plain steel wool would you actually use following this page’s design?

• About 0.39 g of iron; about 6 g of wool
• About 1.5 g of iron; about 2 g of wool
• About 0.39 g of iron; about 0.4 g of wool
• About 5.8 g of iron; about 6 g of wool

Show the answer and why

Answer: About 0.39 g of iron; about 6 g of wool

250 mL at 6 g/L is 1.5 g of silver. One iron atom serves two silver atoms, so the requirement is 1.5 × 55.845 ÷ (2 × 107.87) = 1.5 × 0.259 = 0.39 g of iron. The page uses roughly a fifteenfold excess for three sourced reasons — the reaction is a surface reaction so only wetted iron counts, the mass channels as it is consumed, and the bath dissolves iron whether or not silver is present — which puts the practical charge near 6 g. Working the ratio the other way is the more memorable form: a gram of iron can take out 3.86 g of silver, and yet a fully exhausted cartridge assays at only 20 to 40 per cent silver, because the iron that did the work is in the sludge too.

Question 2. After an hour, your after-strip is blank while the before-strip is dark. What may you write in the notebook?

• That the fixer has been desilvered and may now be treated as a non-silver waste
• That the dissolved silver is now below about 1 g/L, which is the test’s detection floor, and that no statement about the actual concentration is possible
• That the silver concentration is below 5 mg/L, since that is what metallic replacement achieves
• That recovery was better than 95 per cent, which is Kodak’s stated efficiency for the method

Show the answer and why

Answer: That the dissolved silver is now below about 1 g/L, which is the test’s detection floor, and that no statement about the actual concentration is possible

The strip’s published floor is about 1 gram per litre and Kodak says reading below it is extremely qualitative and unreliable. The concentration a working cartridge reaches is below 5 milligrams per litre — two hundred times lower than the floor. So the test cannot distinguish a fully desilvered bath from one still carrying nearly a gram of silver in every litre, which is itself hundreds of times a mean regulated sewer limit. The third and fourth options both import a figure from Kodak’s equipment and attach it to your jar, which is precisely the move the page exists to prevent; the first adds a disposal conclusion on top of it.

Question 3. Why does control C — fresh fixer with weighed steel wool and no silver — earn its place in a two-hour session?

• It shows whether the fixer itself darkens on standing
• It demonstrates that the bath consumes iron with no silver present, which is why the wool’s mass loss is not a measure of silver recovered
• It provides a blank for the pH measurement
• It confirms that fresh fixer contains no silver, which the manufacturer already states

Show the answer and why

Answer: It demonstrates that the bath consumes iron with no silver present, which is why the wool’s mass loss is not a measure of silver recovered

Kodak states that the natural corrosivity of photographic solutions consumes steel wool even when silver is not being removed, and control C is where that stops being a sentence you believed and becomes something you watched. It matters because the obvious way to quantify the experiment — weigh the wool before and after — is invalid for three separate reasons, and this is the one you can demonstrate at the bench. The other two are that the wool cannot safely be dried to be weighed at all, and that the deposit is only 20 to 40 per cent silver. Control B, not C, is what answers the first option.

Question 4. Two darkrooms run this experiment. One uses a fixer at pH 3.5, the other at pH 8. Both recover very little silver. Which statement describes the two failures correctly?

• Both baths dissolve the wool too fast; the difference is only in rate
• At pH 3.5 acid consumes the iron before it can displace silver, wasting the wool; at pH 8 the iron is barely activated, so the silver passes through and is wasted
• At pH 3.5 the silver precipitates as sulfide; at pH 8 the complex dissociates
• At pH 3.5 the reaction is unaffected; at pH 8 ammonia is released and stops it

Show the answer and why

Answer: At pH 3.5 acid consumes the iron before it can displace silver, wasting the wool; at pH 8 the iron is barely activated, so the silver passes through and is wasted

Kodak gives an ideal band of 5.5 to 6.5 and names both failure modes: below pH 5 the steel is catalysed to dissolve too rapidly and the cartridge’s capacity is significantly reduced, and above pH 7 the dissolution reaction is slow so optimum silver removal may not take place. The parasitic reaction at low pH is acid attack on iron, which produces the same iron(II) but recovers no silver — so one bath wastes the reagent you bought and the other wastes the silver you were trying to keep. The fourth option contains a true fact in the wrong place: ammonia release from an ammonium bath is a reason not to raise the pH above about 8 during electrolysis, not the reason replacement slows at pH 8.

Question 5. The treated liquid poured off jar A is going into the silver-bearing waste container rather than anywhere else. Which reason is the one this page adds that the earlier pages did not?

• It still contains thiosulfate, which is an oxygen-demanding waste
• It now carries dissolved iron, a by-product with a regulated limit of its own, so the treatment has substituted one regulated metal for part of another
• Silver compounds carry H400 and H410
• The disposal caveat says local regulation governs

Show the answer and why

Answer: It now carries dissolved iron, a by-product with a regulated limit of its own, so the treatment has substituted one regulated metal for part of another

The other three are all true and all established elsewhere — Part II owns the oxygen demand and the aquatic classification, and the disposal caveat is the standing sentence on every waste section in the course. What this session adds is specific to the method: metallic replacement works by putting iron into solution in place of silver, Kodak lists "discharges iron, limited by some sewer codes" among its standing disadvantages, and the mean regulated sewer limit for iron is 17 mg/L. It is also why the industry does not use metallic replacement on rinse water it wants to reuse.

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