How Fixer Works

A working fixer will hold about forty grams of silver bromide in every litre. Pure water will hold about a tenth of a milligram. That is a factor of three hundred thousand, it is obtained by adding one cheap salt, and the whole of this page is an attempt to make the size of it feel inevitable rather than lucky — and then to explain why a bath that capable still needs three minutes.

The problem, in three numbers

Part IV’s halide page owns the solubility products and this page uses them without re-deriving anything. What matters here is that they run in one direction and that the direction is the same one that made the halides useful in the first place.

Salt	Ksp at 25 °C	Free silver ion, from the Ksp	Dissolves to
Silver chloride, AgCl	1.6 × 10⁻¹⁰	1.3 × 10⁻⁵ mol/L	1.93 mg/L
Silver bromide, AgBr	5.0 × 10⁻¹³	7.1 × 10⁻⁷ mol/L	0.135 mg/L
Silver iodide, AgI	1.5 × 10⁻¹⁶	1.2 × 10⁻⁸ mol/L	0.0028 mg/L

The middle column is computed here from the solubility product; the last is Part IV’s, taken from the manufacturers’ and Merck Index figures on PubChem, and the two agree to within a few per cent as they should.

Nothing on that line about washing is a rounding error. A 135-36 roll of film has to shed something like a third of a gram of silver, which is a little over half a gram of silver bromide. Divide by 0.135 mg/L and the water required is of the order of four thousand litres, run to equilibrium, per roll. LibreTexts, working the same case, says “tens of thousands of litres”; it does not state the mass it assumed, so the two figures differ by the amount the assumption differs. Either way the conclusion is the same one and it is not a close call: water is not a fixer, and no amount of washing will ever be one.

🔬Why the trend runs that way, and why it matters twice

Chloride to bromide to iodide, the solubility product falls by three orders of magnitude each step, because the lattice gets more covalent and the bigger, softer halide ion holds the silver more tightly. Part IV used that trend to explain sensitivity: the harder a halide holds its silver, the longer it holds a photoelectron, and the more useful the latent image.

The same trend now reappears with the sign reversed. What made silver iodide the best material for a latent image makes it the worst material to dissolve. That is not a coincidence and it is not a nuisance to be worked around; it is one property being read twice, and a reader who has felt that will predict the rest of this page rather than memorise it.

The reagent, and the ladder it climbs

Part III’s complex-formation page established the formation constants and, importantly for everything below, numbered the steps. This part uses that numbering and adds nothing to it.

Ag⁺ + S₂O₃²⁻ ⇌ [Ag(S₂O₃)]⁻

Step one: the mono complex, one thiosulfate per silver

[Ag(S₂O₃)]⁻ + S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻

Step two: the bis complex, which is what a working fixer makes

[Ag(S₂O₃)₂]³⁻ + S₂O₃²⁻ ⇌ [Ag(S₂O₃)₃]⁵⁻

Step three: the tris complex, where thiosulfate is in large excess

Part III was explicit that the course has found no stepwise constants for this system in a source it has read, and gives none. What it does give, sourced, is the overall constant for step two: K_f = 4.7 × 10¹³. Part XI inherits both the number and the gap.

The gap matters less than it looks, because the photographic literature had already reached the same distinction from the other end — by watching what happens rather than by measuring a constant.

Which rung the ladder stops on is the single most useful idea in this part, so it is worth being precise about what decides it. Every step is an equilibrium, and every one of them is pushed to the right by free thiosulfate. Where thiosulfate is abundant the silver goes straight past rung one, because rung one’s product is immediately consumed by the next addition; where thiosulfate is scarce, rung one is where a silver ion stops. Nothing about that requires the stepwise constants the course does not have. It requires only Le Chatelier’s principle, which Part III owns, and the observation that a fixer’s free thiosulfate is not uniform: it is high in the tray, lower inside the gelatin, and lowest of all at the surface of a grain that is actively dissolving.

The third rung is the mirror image of the first and it is why the concentration section at the foot of this page has an optimum in it rather than a maximum. In a bath of very large thiosulfate excess the tris complex, [Ag(S₂O₃)₃]⁵⁻, becomes available, and a silver ion that ends up there has taken three thiosulfate ions out of circulation instead of two. That costs capacity by simple bookkeeping — the same reagent buys two thirds as much silver — and it is one of the standing hypotheses for why a very strong hypo bath fixes more slowly rather than faster. The course states it as a hypothesis, because no source it holds attributes the effect to that species.

⚗Kodak, 1924: two compounds, and only one of them is any use

Kodak’s teaching primer puts the whole of this part’s permanence argument into three sentences, sixteen years before anybody published a stability constant for the system:

In the process of fixation the silver bromide is dissolved in the hypo by combining with it to form a compound sodium silver thiosulphate. Two of these compound thiosulphates exist, one of them being almost insoluble in water, while the other is very soluble. As long as the fixing bath has any appreciable fixing power the soluble compound only is formed.

The course reads those two compounds onto Part III’s ladder, and says plainly that the reading is its own. Step one has one ligand and one negative charge; step two has two ligands and three. More ligands means more charge, more hydration and more solubility, so the sparingly soluble member of Kodak’s pair is the mono complex and the very soluble one is the bis complex. No document this course holds makes that identification, and if it is wrong the argument still works, because what the argument actually needs is only that a low-thiosulfate silver species exists, is insoluble, and forms when thiosulfate runs short. Kodak establishes all three.

Doing the maths: adding the two equilibria

This is the calculation the rest of the part rests on, and it takes two lines. Silver halide dissolving and thiosulfate complexing share the silver ion, so the equations add and the constants multiply.

AgBr + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing silver bromide: K = Ksp x Kf

K = K_sp × K_f

The combined constant

K is the equilibrium constant for the whole business of fixing, K_sp is the solubility product of the halide being dissolved and K_f is the formation constant of the bis complex. Do it for all three halides with the same K_f and the answer is the shape of the entire photographic process.

Halide	Ksp	K = Ksp × 4.7 × 10¹³	What that means at the tray
AgCl	1.6 × 10⁻¹⁰	7.5 × 10³	goes essentially to completion; chloride papers clear in seconds
AgBr	5.0 × 10⁻¹³	24	a perfectly ordinary constant, comfortably greater than one; film clears in minutes
AgI	1.5 × 10⁻¹⁶	7.1 × 10⁻³	less than one. Thiosulfate does not dissolve silver iodide

🧮How far a fixer can actually go

Take a plain bath at 0.5 mol/L of thiosulfate and ask how much silver bromide it will dissolve at equilibrium. Let s be the molar solubility. Each dissolved silver takes two thiosulfate ions with it, so the free thiosulfate left is 0.5 − 2s, and the bromide and the complex are both s.

K = s² ÷ (0.5 − 2s)² = 24

Substituting into K

Taking the square root of both sides turns a quadratic into a line: s = √24 × (0.5 − 2s) = 4.90 × (0.5 − 2s), so 10.80s = 2.449 and s = 0.227 mol/L.

As mass, using silver bromide’s 187.77, that is 42.6 g of silver bromide per litre, carrying 24.5 g of silver. Against 0.135 mg/L in water it is a factor of 3.2 × 10⁵.

Two things are worth noticing. First, the stoichiometric ceiling — one silver per two thiosulfate — is 0.25 mol/L, and the equilibrium answer is 0.227, or 91 per cent of it. The constant of 24 is large enough that the reaction runs nearly to the point where the reagent is physically used up, which is why Part III could treat capacity as almost pure stoichiometry. Second, no manufacturer works anywhere near it: ILFORD’s ceiling for a film bath is 8 to 10 g of silver per litre. The gap between 24.5 and 10 is not thermodynamics. It is everything else on this page.

What the combined constant looks like across the three halides

• log₁₀ K for AgX + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + X⁻

Show the numbers behind this plot

Three points plotted against a horizontal axis carrying silver chloride at position one, silver bromide at position two and silver iodide at position three, with the vertical axis showing the base-ten logarithm of the combined equilibrium constant for dissolution in thiosulfate. Silver chloride sits at plus 3.88, silver bromide at plus 1.37 and silver iodide at minus 2.15, so the line falls steeply and almost straight across the three. A horizontal guide is drawn at zero, where the constant equals one and the reaction has no thermodynamic preference in either direction. Silver chloride and silver bromide sit above that line, by four orders of magnitude and by one and a half; silver iodide sits two orders of magnitude below it. The teaching point marked on the plot is that the line crosses zero between bromide and iodide, so a fixer that dissolves the first two does not dissolve the third, and every practical consequence on the page follows from where that crossing falls.

Series	Halide, in order of decreasing solubility product	log₁₀ of the combined constant K
log₁₀ K for AgX + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + X⁻	1.00	3.88
log₁₀ K for AgX + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + X⁻	2.00	1.37
log₁₀ K for AgX + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻ + X⁻	3.00	-2.15

Computed, not measured: each point is the product of OpenStax Appendix J's solubility product and the formation constant Part III adopted. The positions on the horizontal axis are labels, not a quantity. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Iodide, which is why cyanide existed

A constant of 0.0071 says that in a solution of pure silver iodide and thiosulfate, the equilibrium sits on the side of the solid. Kodak’s 1928 primer records the practical consequence without any of this arithmetic: potassium cyanide was used for fixing wet collodion plates, “which, being made from silver iodide, are not easily fixed in hypo”. Run the same multiplication with cyanide’s formation constant of 10²¹ and the combined constant for silver iodide comes out near 1.5 × 10⁵ — five orders of magnitude the other side of one. The wet-plate workers were not being reckless for the sake of it. They had a salt that hypo could not touch, and one reagent that could.

Modern films are not silver iodide, and that is the whole difference. A camera film is an iodobromide: silver bromide carrying a few mole per cent of iodide in solid solution, not a separate iodide phase. The iodide does not have its own equilibrium to satisfy; it comes out of the lattice with the bromide. What it then does is sit in the bath as free iodide, where — on the course’s own solubility table — it is the ion best placed to take silver back out of solution. That last step is the course’s inference from its own constants and not a statement any source it holds makes, and it is the only mechanism the course can offer for iodide shortening a bath’s life.

⁇Do tabular-grain films fix badly? Two manufacturers, two answers

Kodak says yes, in print, on every T-Max sheet. F-4043 and the processing chart both carry the same standing note: “your fixer will be exhausted more rapidly with this film than with other films”, with a magenta stain after fixing given as the sign of a bath near exhaustion or a time too short, and refixing in fresh fixer prescribed where the stain is pronounced and irregular.

ILFORD publishes no such difference. The technical sheets for Delta 100 and Delta 400, its tabular-grain films, give exactly the figures the sheets for HP5 Plus and FP4 Plus give: RAPID or HYPAM at 1+4, 2 to 5 minutes at 20 °C, 24 films of 135-36 per litre unreplenished. Four films, two grain technologies, one set of numbers.

What the course uses. Both statements, as statements about products rather than about grain shapes. The honest reading is that fixing behaviour is a property of a particular emulsion — its halide composition, its grain surface area, its coating weight — and not of the word “tabular”; a tabular grain has more surface per unit volume, which should help, and a high-iodide emulsion has more iodide, which should hurt, and the two are independent choices a manufacturer makes. Neither company publishes the iodide content of any of these films, so the course cannot attribute Kodak’s warning to iodide, and does not. What it can say is: follow the sheet for the film in the tank, and if your fixer is being asked to work on T-Max, test it more often.

Two stages, and why one of them is invisible

Here is the puzzle the arithmetic leaves behind. A fixer is thermodynamically capable of forty grams of silver bromide a litre. Diffusion across the emulsion is fast. So why three minutes?

🧮The transport arithmetic, which does not explain the answer

Part III’s diffusion page gives the rule of thumb.

t ≈ x² ÷ 2D

Diffusion time over a distance

t is the time, x the distance and D the diffusion coefficient. Put an emulsion’s order of ten micrometres and a small ion’s free-solution value of 1 × 10⁻⁹ m² s⁻¹ into it and thiosulfate reaches the base of the layer in about 0.05 seconds.

Now count how many times the layer has to be flushed. A 135-36 film is about 0.055 m² of emulsion; at ten micrometres that is 0.55 mL of gelatin. Shedding a third of a gram of silver takes 3.1 mmol of silver and therefore 6.2 mmol of thiosulfate, while 0.55 mL of a 1 mol/L bath holds only 0.55 mmol. The layer must therefore be recharged about a dozen times over, and at 0.05 seconds a charge that is still under a second.

The measured time is two to five minutes. The arithmetic says under a second. The course cannot close that factor of three hundred from any source it holds, and it will not pretend to: it must sit in the rate at which a grain surface actually reacts, and in a diffusion coefficient for a triply charged complex threading gelatin loaded with silver halide, which is a very different quantity from a small ion’s in free solution. The research plan for this page asked for that coefficient from Mees and James; the course holds those texts as cite-only with no readable text, and has no figure to give.

What the arithmetic does establish is negative and still useful: fixing time is not set by how long it takes a molecule to cross the layer. Anything you do that only speeds up transport across ten micrometres will change nothing.

So the useful model is not one stage but two, and it is Kodak’s 1924 primer rather than any modern sheet that states it in a form you can act on.

Stage one — the halide goes. Grains dissolve, the milky opalescence of undeveloped silver halide clears, and the film becomes transparent where it is not black. This is the stage you can see. It ends at the clearing time.

Stage two — the complexes go. At the instant of clearing, the gelatin is a gel loaded with silver thiosulfate complexes and locally depleted of free thiosulfate. Where thiosulfate is short, the ladder does not climb past its first rung, and Kodak’s warning is exact: the first compound “is invisible”, so a negative transferred to the wash the moment it looks clear takes some of it with it, and it stays in the dried film. Stage two is the extra time in which fresh thiosulfate arrives, converts the mono complex to the bis complex, and the bis complex diffuses out.

What happens at one grain, and what has to happen afterwards

1. 1Early: thiosulfate is plentiful — silver leaves as the bis complex, step two, which is very soluble
2. 2At clearing: thiosulfate is locally short — silver leaves as the mono complex, step one, which is not
3. 3After clearing: the bath recharges the layer — mono becomes bis and the bis diffuses out — this is the second half of the fixing time

Drawn to show sequence, not scale or population. The identification of Kodak's insoluble compound with Part III's step-one complex is the course's own reading and is argued for above.

Clearing time, and the rule three manufacturers agree on

The rule is the oldest piece of quantitative practice in the darkroom and it has not moved.

• Kodak, 1928. “The time for fixation is usually taken as twice the time for the milkiness or opalescence of the unreduced silver salts to disappear.”
• Kodak, 2023, in the current processing chart: “Fix for twice as long as it takes the film to clear (lose its milky appearance).”
• ILFORD, 2010. “In order to avoid the risk of insufficient fixing, film should remain in the fixer for twice the time it takes the emulsion to clear.”

Ninety-five years, two companies, one number. It is the only place in this course where that happens, and the reason is the two-stage argument above: stage one is visible and stage two is not, so the sanest available estimate of the invisible half is the visible half.

ILFORD also gives the discard rule in the same paragraph, and it is a different rule doing a different job: throw the bath away when the clearing time in used fixer exceeds twice the clearing time in fresh. Kodak’s 1928 version is an absolute rather than a ratio — discard when a slow-fixing film takes more than 12 to 15 minutes to clear — which is the same judgement made with fewer measurements.

ILFORD's clearing-time method, which can be done in room light

1. 1Take a scrap of unprocessed filmA leader, the end of a roll, or outdated stock. It has never been developed, so all of its halide is still there and it is milky.
2. 2Put one drop of working-strength fixer on the emulsion sideLeave it until the emulsion under the drop is a clear spot. ILFORD says around 30 to 60 seconds. This gives you a reference patch on the same piece of film.
3. 3Immerse the whole piece and start the clockJudge clearing by comparing the surrounding film with the clear central spot, which removes the argument about what "clear" looks like.
4. 4The time for the rest to clear is the clearing timeFix for twice it.
5. 5Repeat as the bath agesWhen the clearing time passes twice the figure you got in fresh fixer, the bath is finished.

Method as ILFORD's RAPID FIXER sheet gives it. The drop-first step is the part people leave out and it is the part that makes the reading objective.

Sodium against ammonium

The rate difference is not subtle. Kodak Limited’s 1949 handbook expects its F-5 sodium bath to fix films and plates “properly in 10 minutes”. ILFORD’s ammonium thiosulfate fixer, at 1+4, does general-purpose film in 2 to 5. Kodak’s own current chart puts the same contrast inside one sentence: 2 to 4 minutes with a liquid-concentrate fixer, 5 to 10 with a powder one.

What is sourced is the effect, and the course’s encyclopaedia entry is candid that the mechanism is not. The strongest evidence in the corpus is Wall’s, from 1924, and it is better than a modern assertion because it is a controlled comparison: adding 2.5 to 5 per cent of ammonium chloride to a 20 per cent hypo bath speeds fixing, 10 per cent ammonia water does the same, and — the detail that identifies the culprit — the addition has no effect at all on a 40 per cent hypo bath. So the acceleration belongs to the ammonium ion, and it appears only where thiosulfate is not already in large excess. Whether that is because ammonium changes which complex forms, or how fast it forms, or only how fast it moves, is not established by any open source this course holds.

One thing that is not the explanation is worth saying out loud, because it is the obvious guess. It is not simply that the ion is different. The thiosulfate ion is the same ion; the equilibrium written above is the same equilibrium; the constant is the same constant. Whatever ammonium does, it does to the rate and not to the position.

Temperature, agitation and concentration

Three knobs, and one of them turns the wrong way.

Temperature helps, and is capped by the bath rather than by the film. Kodak’s 1928 primer gives a measured pair: a film needing 95 seconds to clear at 18 °C takes about 60 seconds at 29 °C. Eleven degrees for a 1.6-fold speed-up is the ordinary temperature dependence Part III’s kinetics page sets out, and it is worth noticing that it is a good deal gentler than development’s. The primer then closes the door: it is “dangerous practice” to let the bath rise above 21 °C, because an acid fixing bath at that temperature is apt to precipitate sulfur. The limit is a property of the acid bath, not of the fixing reaction, and it is one of the arguments for a neutral or alkaline fixer.

Agitation helps, and it is the one that is free. Everything in the two-stage model is transport across the boundary layer at the film surface: fresh thiosulfate in, complexes out. Kodak’s T-Max sheets ask for “vigorous agitation” and frequent agitation during fixing, and the reason is stage two rather than stage one.

Concentration has an optimum, and going past it makes things worse. Kodak’s 1928 primer states that 30 to 40 per cent hypo “fixes most rapidly”; Wall’s 1924 formulary puts the useful ceiling at 40 to 45 per cent and calls that the strongest worth using. Above it the rate falls again. No source this course holds gives the reason, and the plausible candidates — the tris complex of step three forming at the expense of the bis, the solution’s viscosity rising, the activity of the ion falling as the solution becomes crowded — are hypotheses and are labelled as such here.

Going the other way is the error with real consequences, and Part III already worked it: a bath mixed too weak does not merely fix more slowly, it fixes worse and less, because dilution cuts the driving concentration and the stoichiometric capacity at the same time. ILFORD says the practical version in one line — if the concentration “is too high or too low efficiency is reduced and poor fixing can be experienced” — and gives the specific gravity of a correctly mixed bath so that you can check.

Film, resin-coated paper and fibre base are three different transport problems

The same fixer, on the same day, is given four different times by ILFORD’s own sheet: general-purpose film 2 to 5 minutes at 1+4, resin-coated paper 30 seconds at 1+4, fibre-base paper 1 minute at 1+4, and 2 minutes at 1+9. Stage one explains part of that — a paper emulsion is thinner and carries less silver than a camera film — but the interesting comparison is the one between the two papers, whose emulsions are much the same.

The difference is entirely underneath. Resin-coated paper is sealed with polyethylene on both faces, so the fixer meets a thin emulsion on an impermeable floor and never enters the base at all. Fibre base is bare paper and behaves like blotting paper: fixer goes in, and — this is the part that matters for the rest of the part — fixer that goes in has to come out again in the wash. Kodak’s toning manual draws exactly that conclusion and turns it into a limit in the other direction: do not fix prints for longer than the recommended time, approximately ten minutes for fibre base and two for resin-coated, because prolonged fixing “expands the paper and allows the solution to penetrate the base”, where it is difficult to remove and turns a selenium- or sulfide-toned print yellow. Prolonged fixing, the same page adds, can also reduce the silver image.

So a print has an upper bound on fixing time as well as a lower one, and film effectively does not. That asymmetry is worth carrying forward: for film, the risk of under-fixing dominates and generous time is nearly free; for fibre-base paper, both errors are real and the working answer is short times in fresh fixer rather than long times in tired fixer. ILFORD’s own archival print sequence takes exactly that position — one minute at 1+4 — and the capacity lesson works out why.

Recap

Silver halides are insoluble to a degree that makes washing hopeless, and progressively more so from chloride to bromide to iodide. Thiosulfate fixes that by coupling a hopeless equilibrium to an overwhelming one, and multiplying the two constants gives 7,500 for chloride, 24 for bromide and 0.0071 for iodide — which is why chloride papers clear in seconds, film in minutes, and pure silver iodide not at all. A bath at 0.5 mol/L will hold 42.6 g of silver bromide per litre at equilibrium, and manufacturers stop at less than half the silver that implies, for reasons that are kinetic and archival rather than thermodynamic.

Fixing runs in two stages. The first is visible and ends at the clearing time. The second is invisible, is the conversion of a sparingly soluble low-thiosulfate complex into a soluble one and its removal, and is the reason every manufacturer for a century has said to fix for twice the clearing time. Temperature and agitation speed both stages; concentration has an optimum near 30 to 40 per cent and a hard floor below which capacity itself is lost; ammonium in place of sodium roughly halves the time, by a mechanism the course could not source.

Check your understanding

Question 1. Using the course's constants — Ksp for silver chloride 1.6 × 10⁻¹⁰ and Kf for the bis(thiosulfato)argentate ion 4.7 × 10¹³ — work out the combined constant for fixing silver chloride, and say what it predicts about a chloride contact-printing paper.

• About 3 × 10⁻³; the paper will not fix in thiosulfate at all
• About 24; the same as film, so a chloride paper needs the same fixing time
• About 7.5 × 10³; the reaction runs essentially to completion, so a chloride emulsion clears far faster than a bromide film
• The constants cannot be multiplied, because one is a solubility and the other a formation constant

Show the answer and why

Answer: About 7.5 × 10³; the reaction runs essentially to completion, so a chloride emulsion clears far faster than a bromide film

1.6 × 10⁻¹⁰ × 4.7 × 10¹³ = 7.5 × 10³. The equations for dissolution and for complex formation share the silver ion, so adding the equations multiplies the constants — that is the whole trick, and it is why a number as small as a solubility product can produce a reaction that goes. Three hundred times the constant for bromide is why Kodak's 1928 primer could report lantern slides clearing in 30 seconds to a minute where portrait film took 3 to 5 minutes. Note what this does not say: a fast reaction is not a complete one, and a chloride print still needs the second, invisible stage.

Question 2. A plain bath is mixed at 0.5 mol/L of thiosulfate and worked until it stops. Roughly what mass of silver will it be holding, and how does that compare with the manufacturer ceiling for a film bath?

• About 24 g/L, which is between two and three times ILFORD's 8 to 10 g/L ceiling for a film bath
• About 8 g/L, which is exactly ILFORD's ceiling, because the ceiling is a thermodynamic limit
• About 100 g/L, because the constant of 24 means 24 times the stoichiometric limit
• About 0.5 g/L, because only the free thiosulfate can hold silver

Show the answer and why

Answer: About 24 g/L, which is between two and three times ILFORD's 8 to 10 g/L ceiling for a film bath

Solving s² ÷ (0.5 − 2s)² = 24 gives s = 0.227 mol/L, which at 107.87 g/mol is 24.5 g of silver. ILFORD allows 8 to 10 g/L in a film bath, so the published working limit is well under half the thermodynamic one. That gap is the useful part of the answer: a fixer is retired long before it is chemically spent, because the rate collapses as free thiosulfate runs down and because the sparingly soluble step-one complex starts to form and be retained. Capacity in practice is set by what the bath leaves behind, not by what it can hold.

Question 3. Why does every manufacturer say to fix for twice the clearing time rather than, say, clearing time plus twenty per cent?

• Because the reaction is second order and doubles the time needed
• Because clearing marks the end of only the visible stage; the second stage — converting a sparingly soluble low-thiosulfate complex into a soluble one and diffusing it out — is invisible, and doubling is the sanest available estimate for it
• Because a safety factor of two is conventional in all darkroom timings
• Because the emulsion must be cleared from both sides

Show the answer and why

Answer: Because clearing marks the end of only the visible stage; the second stage — converting a sparingly soluble low-thiosulfate complex into a soluble one and diffusing it out — is invisible, and doubling is the sanest available estimate for it

Kodak's 1924 primer names the thing the rule is protecting against: two compound sodium silver thiosulfates exist, one almost insoluble and one very soluble, and "this first insoluble compound is invisible", so a negative moved to the wash the moment it looks clear can keep some of it. There is no measurement in the corpus that fixes the ratio at two rather than 1.7 or 2.5; it is a working rule that three independent documents ninety-five years apart all state, which is a different and weaker kind of evidence than a measurement, and the page says so.

Question 4. A wet-collodion worker fixes silver iodide plates in potassium cyanide rather than hypo. On the constants this page uses, what justified that, and what does it imply about a modern iodobromide film?

• Cyanide was simply cheaper; the chemistry is the same either way
• For silver iodide the thiosulfate combined constant is 0.0071 — below one, so the reaction does not go — while cyanide's much larger formation constant puts it near 10⁵; a modern film is bromide with a few mole per cent of iodide in solid solution, so it fixes in thiosulfate, but the released iodide is the ion best able to take silver back out of the bath
• Silver iodide is more soluble than silver bromide, so it needed a weaker reagent
• Cyanide fixes faster but the equilibrium constants are identical

Show the answer and why

Answer: For silver iodide the thiosulfate combined constant is 0.0071 — below one, so the reaction does not go — while cyanide's much larger formation constant puts it near 10⁵; a modern film is bromide with a few mole per cent of iodide in solid solution, so it fixes in thiosulfate, but the released iodide is the ion best able to take silver back out of the bath

Multiply 1.5 × 10⁻¹⁶ by 4.7 × 10¹³ and the answer is 7.1 × 10⁻³: a fixer facing a separate silver iodide phase is pushing an equilibrium uphill. With cyanide's formation constant near 10²¹ the product is about 1.5 × 10⁵ and the reaction runs. The modern case is different in kind and not only in degree, because iodide in a camera film is dissolved in the bromide lattice rather than present as its own solid, and comes out with it. What survives is the second-order effect: free iodide accumulating in the bath. That mechanism is the course's inference from its own solubility table, not a statement any source it holds makes.

Question 5. ILFORD publishes identical fixing times and capacities for Delta 100 and HP5 Plus, while Kodak warns on every T-Max sheet that fixer exhausts more rapidly with those films. What should a reader conclude?

• ILFORD is wrong, because tabular grains are known to fix badly
• Kodak is wrong, because a tabular grain has more surface area and should fix faster
• That fixing behaviour belongs to a particular emulsion rather than to the word "tabular", and since neither company publishes iodide contents the difference cannot be attributed to grain shape at all — so follow the sheet for the film in the tank and test the bath more often when it is working on T-Max
• That the two companies measure clearing time by different methods, so the figures are not comparable

Show the answer and why

Answer: That fixing behaviour belongs to a particular emulsion rather than to the word "tabular", and since neither company publishes iodide contents the difference cannot be attributed to grain shape at all — so follow the sheet for the film in the tank and test the bath more often when it is working on T-Max

Both statements are manufacturer statements about their own products and there is no reason to reject either. What cannot be done is to promote them into a general claim about tabular grains, because the two variables that would decide it — halide composition and coating weight — are not published for any of the four films. A tabular grain has more surface per unit volume, which should help fixing; a higher iodide content should hurt it; and those are independent choices. Holding two sourced statements side by side without inventing a mechanism to reconcile them is the correct outcome, not a failure to reach one.

Question 6. A printer decides that if a fixer at 1+4 is good, one at 1+9 will be gentler and can simply be given twice the time. What is wrong with that, and which of ILFORD's published checks would catch it?

• Nothing is wrong; halving the concentration and doubling the time is equivalent
• Dilution halves the driving concentration and the stoichiometric capacity together, so the bath fixes both more slowly and less completely and retires sooner; the specific-gravity check catches a bath that is too dilute
• The bath will fix faster, because a dilute solution diffuses more quickly
• Only the pH check would catch it, since dilution changes pH but not capacity

Show the answer and why

Answer: Dilution halves the driving concentration and the stoichiometric capacity together, so the bath fixes both more slowly and less completely and retires sooner; the specific-gravity check catches a bath that is too dilute

Capacity is very nearly stoichiometric — the worked example above lands at 91 per cent of the one-silver-per-two-thiosulfate ceiling — so halving the thiosulfate halves the number of films, exactly, and no extra time recovers it. ILFORD's own sentence is blunt: if the concentration is too high or too low, efficiency is reduced and poor fixing can be experienced. The sheet publishes specific gravity at both dilutions, 1.070 to 1.080 at 1+4 and 1.030 to 1.040 at 1+9, precisely so that a hydrometer can tell you which bath you actually have. ILFORD does publish a 1+9 dilution — for paper, with its own longer time and its own capacity figure — which is a different thing from improvising one.

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