Kodak F-5

Five ingredients, and four of them are there to protect the fifth from the third. That sentence is the whole design of an acid hardening fixer, and it is why this formula is worth a page rather than a line: the thiosulfate does the work, the acid is needed to stop development and to make the alum harden, the acid attacks the thiosulfate, the sulfite defends it, and the boric acid keeps the acid from running out before the bath does. Take any one away and the bath fails in a different, diagnosable way.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate	240 g	crystalline hypo (or 150 g anhydrous)
Sodium sulfite	15 g	anhydrous (the handbook’s main column gives 30 g crystalline)
Acetic acid	17 mL	glacial
Boric acid	7.5 g	crystals
Potassium alum	15 g
Water	to make 1000 mL

Purpose

To dissolve the silver halide the developer did not reduce, to stop development while doing it, and to leave the gelatin hard enough to handle warm and wet. Kodak Limited’s header is “tropical acid hardening fixing bath for films and plates”, and the course prints that header rather than the general-purpose description this formula usually receives, because it is what the source says.

The 1928 primer states the general case that F-5 is a member of: a plain hypo solution is seldom used as a fixing bath, and is used instead with a weakly acid salt or, more commonly, with an acid hardening solution containing a preservative, sodium sulfite, which prevents decomposition of the hypo; an acid, usually acetic acid, to neutralise any alkali carried over in the film from the developer and thereby arrest development; and a hardening agent, either potassium alum or chromium alum. F-5 is that description with one addition, the boric acid, and the addition is the interesting part.

Recommended uses

Films and plates, ten minutes in a fresh bath. That is the handbook’s whole instruction, and it comes with a warning attached to the same sentence: prolonged immersion at high temperatures is harmful. Ten minutes is not a minimum to be exceeded for safety.

Where the emulsion has to survive warm processing. The header says tropical, and the alum is why. An unhardened wet emulsion at 25 °C is soft enough to take a fingerprint and, at the temperatures the 1928 primer is discussing, soft enough to flow off the base. The primer gives the property in numbers: baths of this class are compounded to give a hardening of 54 to 77 °C, measured by fixing and washing a strip, immersing it in water and heating the water slowly until the gelatin flows away from its support. That temperature, not an adjective, is what the alum buys.

Where a bath has to last. 120 sheets of 8 × 10 inches per 160 fluid ounces, and three months in a full stoppered bottle. The boric acid is a large part of why.

When another formula is preferable

• When the film will be toned, or when hardening is unwanted, F-52 — Kodak Limited’s non-hardening acid fixing bath, which the handbook offers for exactly the cases where hardening “is not desired, or must be avoided”. A hardened emulsion resists every bath that follows it.
• For paper, F-54, which is the same chemistry at a lower hypo concentration and is built by adding the F-53 hardener stock to a hypo solution rather than dissolving five things in one vessel.
• For a printing-out process — salted paper, albumen, Van Dyke — an alkaline plain-hypo bath. An acid fixer attacks the finely divided silver of a printing-out image and bleaches the highlights; this page’s acid is a defect there rather than a feature.
• When you want to understand fixing rather than buy hardening, plain hypo is the whole mechanism with nothing else in the tray.
• When the vapour of glacial acetic acid is the problem, F-52, whose acid arrives as a solid salt and whose page also records the citric bath the course could not publish.
• When the reproductive-hazard classification of boric acid decides it. F-54 does the same job with no boric acid in it. See Safety.

Mixing

The order is not a convenience, and Kodak says so twice. The 1949 handbook prints “dissolve chemicals in the order given” under the formula and explains it in the front matter: when an acid hardening fixing bath is made up it is essential that the ingredients be dissolved in the proper order if decomposition of the hypo and precipitation of the alum are to be avoided — hypo first, then the sulfite, then the acid, and finally the alum.

1. Hypo, and dissolve it completely. The 1928 primer: if the hypo is not thoroughly dissolved before the hardener is added, a precipitate of sulfur is likely to form.
2. Sulfite, so that the defence is in place before the attack arrives.
3. Acetic acid, slowly, into a cool solution.
4. Boric acid. Buy the crystals rather than the powder; Kodak’s own footnote elsewhere is that powdered boric acid does not dissolve easily.
5. Potassium alum, last. The primer explains the alternative order and why it is worse: adding alum to a sulfite solution before the acid gives a white sludge of aluminium sulfite that redissolves only with difficulty.

Both solutions cold, if you mix a hardener separately. Where the hardener is made as a stock — which is what F-53 is for — the handbook’s rule is that it goes into the hypo solution slowly, with vigorous stirring, and that both solutions must be cold. The 1928 primer adds the temperature: an acid fixing bath mixed or held above about 29 °C will not remain clear.

🧮What the litre actually contains

240 g ÷ 248.19 g/mol = 0.967 mol/L

Thiosulfate, from the crystalline salt

150 g ÷ 158.11 g/mol = 0.949 mol/L

Thiosulfate, from the anhydrous salt

Kodak’s two forms differ by under two per cent in moles, which is the test of whether an “or anhydrous” line was computed or guessed. The bath is about 24 per cent w/v as the crystalline salt, which sits just under Wall’s ceiling — he gives 40 to 45 per cent hypo as the strongest bath that should be used and the most rapid in action — and inside the 1928 primer’s stated optimum of 30 to 40 per cent for speed of fixation. F-5 is therefore not the fastest bath Kodak could have written; it is a bath whose strength was chosen alongside four other things.

17 mL × 1.049 g/mL × 0.995 = 17.7 g, or 0.295 mol/L

Acetic acid

7.5 g ÷ 61.84 g/mol = 0.121 mol/L

Boric acid

15 g ÷ 474.4 g/mol = 0.0316 mol/L, and so 0.0316 mol of aluminium

Potassium alum

The acetic acid figure is worth holding on to: a litre of F-5 carries the same 0.295 mol of acetic acid as a litre of SB-1, the stop bath that precedes it. The fixer is, among other things, a stop bath that keeps working.

Behaviour

Fixing time is defined, not chosen. The 1928 primer gives the rule the whole craft still uses: the time for fixation is twice the time for the milkiness or opalescence of the unreduced silver salts to disappear. It then lists what that clearing time depends on — the strength of the hypo, the material (portrait films 3 to 5 minutes, lantern slides 30 seconds to a minute), the temperature, and the degree of exhaustion. Kodak’s ten minutes is the answer for a fresh bath and a film of its period. Your film in your bath has its own clearing time, and doubling it is the measurement that replaces the number.

It gets slower as it works, and that is the exhaustion signal. The primer: discard the bath when the clearing time for a slow-fixing film exceeds 12 to 15 minutes. Wall gives the reason to care rather than merely to notice — the more a bath is used, the greater its saturation with silver salts, and the greater the chance of forming insoluble, transparent silver salts which are difficult to wash out. An under-fixed negative does not look under-fixed.

Three other signs of a finished bath, all from the primer: it froths at the surface, it goes milky or sludges throughout, or it simply fixes so slowly that there is a danger of taking the film out before it is done. A milky bath is diagnosable by the colour and settling rate of its precipitate, which is under Troubleshooting.

Hardening rises before it falls. The primer’s account of an alum bath in use is not a straight decline: during the first stages of use the hardening properties increase slightly, after which they fall off rapidly, because the developer carried in is neutralising the acid. So a fixer that has done a little work hardens better than a fresh one, and one that has done a lot hardens worse — and the sludge of aluminium sulfite that ends its life is the same neutralisation carried to its conclusion.

Temperature is a constraint at both ends. The primer: a film needing 95 seconds to clear at 18 °C would take about 60 seconds at 29 °C, but it is dangerous practice to let the bath rise above 21 °C because the solution is apt to precipitate sulfur. That the same handbook calls F-5 a tropical bath and that its own keeping table cuts the figures above 24 °C are two halves of the same fact: heat helps the fixing and destroys the bath.

Image characteristics

A fixer used inside its life should leave no visible signature at all, and every visible signature it does leave is a fault. That is worth saying in a section that on a developer page would be about grain and tone.

What it must not do: bleach the image, stain it, or leave it. The 1928 primer’s own warning is that prints fixed in a bath that has gone alkaline are likely to become stained brown, and that is the commonest way a fixer signs its work. On a printing-out paper the acid itself attacks the image silver, which is why Reilly’s alkaline bath exists.

What it does change, invisibly, is the emulsion’s mechanical and chemical future. A hardened layer takes toner, selenium and dye more slowly and less evenly, washes more slowly, and resists reticulation and scratching more strongly. Hardening is a decision about everything that happens after the fixer, and the reason Kodak published F-52 alongside this bath is that the decision sometimes has to go the other way.

The mechanism

Why an insoluble salt dissolves. Silver bromide is insoluble in water because the lattice holds the silver ion more tightly than water can. Thiosulfate holds it more tightly still, and does it in solution:

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the bis(thiosulfato)argentate complex

Two thiosulfate ions per silver, which is why an excess of thiosulfate is not a luxury. The 1924 edition of Kodak’s primer states the consequence the course cares about most: two such compound thiosulfates exist, one almost insoluble in water and the other very soluble, and as long as the bath has any appreciable fixing power only the soluble one is formed. A bath worked past that point makes the insoluble one, which is invisible and does not wash out.

Why the acid is needed, and why it is dangerous. The acid neutralises the alkali arriving on the film, which arrests development in the fixer itself; and the alum will not harden unless the bath is acid. But acid and thiosulfate are enemies:

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Acid attacking thiosulfate: the sulfur sludge

The 1928 primer describes the same decomposition in words — a few drops of a strong acid in a weak hypo solution turn it milky as sulfur precipitates — and its list of causes for a pale yellow, slowly settling precipitate is exactly this reaction’s list: too much acid in the hardener, too little sulfite or an impure sulfite, or too high a temperature.

Why the sulfite is the defence. Sulfite is the reaction’s own product, so its presence pushes the equilibrium back; and it is a reducing agent that mops up the oxidised developer carried in on the film, which is the primer’s second reason for it — bisulfite and sulfite prevent the developer carried into the fixing bath from oxidising and turning brown.

Why the alum hardens, and how far the course can go. Alum dissolved in water is weakly acid, because the hydrated aluminium ion sheds a proton; the potassium alum page gives that hydrolysis and the sludge reaction that ends the bath. What that page also records is that the molecular account of the bond between aluminium and gelatin could not be verified from a source meeting this course’s standard. The effect is measured, and Wall’s table of Alums as Hardening Agents puts numbers on the comparison — 6 parts of potash alum to render 100 parts of dry gelatine insoluble in hot water, against 2 parts of chrome alum — but the period sources call it “tanning” and stop. This page does not go further than they did.

Why boric acid, and the honest limit of the answer. Boric acid is about three thousand times weaker than acetic acid, with a pK_a of 9.27, so 7.5 g of it barely moves the pH while adding a large reservoir of undissociated acid. That reservoir is spent slowly as carried-over developer neutralises the bath, and it is the obvious explanation for F-5’s long life. The boric acid page states the limit plainly and this page keeps to it: no source read for this course states why Kodak chose boric acid rather than more acetic acid, or by what mechanism it lengthens the life of an alum bath. Treat the buffering role as well supported and the detailed mechanism as open.

⚗The window this formula lives in

An alum fixer has a wall on each side and the whole formula is an argument about where to sit between them.

Too acid and the thiosulfate decomposes to colloidal sulfur, which mists the bath and the film. Too alkaline and the aluminium and the sulfite find each other as a white gelatinous sludge, hardening stops, and prints fixed in the bath stain brown.

The primer adds a complication that makes the choice a genuine compromise rather than a target: with a fixed quantity of alum, hardening rises as acid is added, peaks, and then falls away until the bath does not harden at all — while the minimum acid needed for a long life before aluminium sulfite precipitates is usually more than the quantity that hardens best. A long-lived alum fixer is therefore a slightly worse hardener than a short-lived one. F-5 is Kodak’s chosen point on that curve.

Function of every ingredient

Sodium thiosulfate, 240 g of the crystals or 150 g of the anhydrous salt. The fixing agent, and the only ingredient doing the job the bath is named for. It complexes silver ions out of the undeveloped halide, two thiosulfates to each silver. More fixes faster up to about 30 to 40 per cent, above which the primer records no further gain and Wall sets 40 to 45 per cent as a ceiling; more also means more thiosulfate to wash out of the emulsion afterwards, which is a permanence cost paid at the other end of the process. Less fixes more slowly, exhausts sooner, and crosses into the region where the insoluble silver–thiosulfate complex forms. The two forms are not interchangeable gram for gram: 240 g of the pentahydrate and 150 g of the anhydrous salt are the same number of moles because the crystals are two-fifths water by mass, and using 240 g of the anhydrous salt would give a bath a third stronger than Kodak wrote.

Sodium sulfite, 15 g anhydrous, or 30 g of the crystalline salt. The preservative, and it does two separate jobs. It suppresses the acid decomposition of the thiosulfate by being that reaction’s own product, and it reduces the oxidised developer carried in on each film so the bath does not brown. Less, or an old tub that has oxidised to the sulfate, and the bath goes milky with sulfur — the primer names insufficient or impure sulfite as one of three causes. More is not free: it is more sulfite to meet the alum when the acid finally runs out, and aluminium sulfite is what ends the bath. Note which column you are working from. The handbook prints the crystalline salt first, and 30 g of the crystals is 15 g of the anhydrous; take 30 g of the anhydrous and the bath carries twice the sulfite Kodak specified.

Acetic acid, 17 mL of the glacial acid. Two jobs again. It neutralises the alkali arriving from the developer, which arrests development inside the fixing bath rather than leaving it to run on; and it holds the bath acid enough for the alum to harden at all. It is a weak acid on purpose, for the reason Part X works through: neutralising capacity is a count of total hydrogen, dissociated or not, so a large quantity of a weak acid gives a long life at a mild pH. More acid hardens better up to a point, then worse, and increases the risk of sulfur; less gives a bath that goes alkaline early, sludges, and stains prints brown. It is also the ingredient that carries most of this page’s hazard.

Boric acid, 7.5 g. The acid reserve, and the ingredient that distinguishes F-5 from the older acid hardening baths. With a pK_a of 9.27 it is a very weak acid indeed, which means almost all of its hydrogen is still on the molecule and available to replace hydrogen ions as carried-over developer consumes them. That is capacity without strength, and it is consistent with F-5’s long dish and bottle life. More would not make the bath more acid to any useful degree; less, and the bath’s pH falls out of the window sooner. Note again that the mechanism by which it specifically lengthens an alum bath’s life is not established in this course’s corpus. It is also the reason this page is Level B.

Potassium alum, 15 g. The hardener, present as 0.032 mol of aluminium per litre. It tans the gelatin so that a wet emulsion can be handled and dried warm without softening, melting or scratching, and the property it delivers is measurable as the temperature at which the layer melts off its base — 54 to 77 °C for baths of this class. More alum overshoots into brittleness, which the primer names explicitly; less fails to reach the minimum quantity that hardens at all. Potash alum rather than chrome alum is a deliberate choice: Wall’s table makes chrome alum three times as efficient by weight, and the primer’s reason for preferring potash alum anyway is life — a chrome alum bath loses its hardening properties rapidly whether it is used or not.

Water, to make 1000 mL. The handbook gives no mixing temperature here, but its general rule governs: the hypo must be fully dissolved and the solution cold before the acid and the alum go in. Warm water speeds the hypo up; a warm bath at the moment the acid arrives is the primer’s third cause of a sulfur sludge.

Interactions

Acid against thiosulfate, restrained by sulfite. The central three-body problem of the formula, set out under The mechanism. Everything that goes visibly wrong with an acid fixer is this triangle out of balance.

Alum against sulfite, refereed by acid. The second triangle, and the one that decides the bath’s life. As long as there is free acid the aluminium stays in solution; when the acid is gone, aluminium sulfite comes out as a white sludge and hardening stops.

Alum against the developer’s alkali, through the film. Every film brings alkali in. The primer’s account is a sequence rather than a slope: acid neutralised, hardening briefly improved, then hardening lost and the bath sludged. A stop bath between the two trays is the intervention, and the primer measures the gain — its acid hardening fixer F-1 fixes a hundred 8 × 10 prints per gallon after a thorough water rinse, and a hundred and twenty-five after an acid rinse.

Boric acid against the sulfite, in the waste bottle rather than the tray. The boric acid page records that boric acid belongs to a weak-acid reactive group whose reactions with thiosulfates, sulfites and nitrites generate gas, and that an acid fixing bath contains all three together only because it is buffered well above the pH at which thiosulfate breaks down. Adding a stronger acid to spent fixer removes that protection at a stroke.

With the wash, and with everything after it. A hardened emulsion washes more slowly than an unhardened one and holds thiosulfate longer. That is a cost this bath imposes on the washing stage, and it is the reason a sulfite washing aid matters more after a hardening fixer than after a plain one.

Variants

F-54 and its films-and-plates twin F-54a, in the same handbook, are not variants of F-5 but a different construction of the same chemistry: a hypo solution to which the separately made F-53 hardener stock is added. They carry no boric acid. Kodak numbered them separately and published them for different materials, so they have their own entries rather than appearing here.

The 1928 primer’s F-1 is the older design this formula descends from: 480 g of hypo made to two litres, with a hardener of 30 g of sodium sulfite, 96 mL of 28 per cent acetic acid and 30 g of powdered potassium alum in 160 mL of water at about 52 °C, added separately. It has no boric acid, and its acid is specified as a 28 per cent solution rather than the glacial acid, so it is not this formula at a different strength but a differently constructed bath. It is fully sourced and publishable, and the course has simply not written its entry yet; nothing about its provenance stands in the way. F-1’s capacity and behaviour data are cited throughout this page, because that is where the 1928 measurements were made.

No safer variant of this formula is offered here. The obvious one would be F-5 without the boric acid, and it would be a different formula with a different life, not a safer version of this one. Where the boric acid is the objection, the honest answer is F-54, which Kodak published, rather than a subtraction the course invented.

Chrome alum instead of potash alum is a real historical variant — Kodak Limited’s own F-16 is the chrome alum bath — and the course does not publish it. The chromium ruling permits chromium(III) only where a lesson genuinely requires an in-emulsion hardener, and a fixing bath is not that case when potassium alum, which carries no classification at all, does the same job in a bath that also lasts far longer.

Safety

Level B, and the level is set by two ingredients rather than by the fixer.

Boric acid is the reason this page cannot be Level A. Its aggregated classification is H360, may damage fertility or the unborn child, with reproductive toxicity as the only hazard class in it; the route that matters is ingestion, and the control is dust discipline while weighing and strict hygiene afterwards. That is a serious classification on a substance sold as a household product, and it deserves to be read on its own page rather than summarised here.

Glacial acetic acid is the other. H314 and H226, a flash point of 39 °C, and a vapour that can reach a harmful concentration in a small room by evaporation at 20 °C. The course’s standing practice, set in Part X, is to buy acetic acid already diluted to a stated strength and to compute the volume; the SB-1 page carries that arithmetic and it applies unchanged here.

The two substances everyone worries about are the two that carry no classification. Sodium thiosulfate and potassium alum both have aggregated GHS entries with no hazard statements at all — on thin evidence in the first case and on a small number of unanimous notifiers in the second, which is an absence of an agreed classification rather than a finding of safety. The alum page adds the control the GHS codes do not mention: soluble aluminium salts carry a workplace exposure limit for dust.

The bath’s own hazard is a gas it can be made to release. Thiosulfate plus a strong acid gives sulfur dioxide; sulfite plus a strong acid gives it directly. Both live in this tray. Nothing acidic beyond the formula goes into it, and nothing from this tray goes into a bottle that has held acid.

Ventilation is the ordinary darkroom provision — an extract that changes the room’s air, a covered tray, and a door that opens — rather than a special control for this formula. What makes it necessary here is the glacial acid at the mixing stage, not the finished bath.

Storage

Three months in a full stoppered bottle, two weeks half full, one month in a covered tank and one week standing in a dish, from Kodak’s own table — with a dagger against two of those cells whose footnote warns of much shorter life above 24 °C, and which the course does not assign because the two footnote marks cannot be told apart in the scan it read.

The pattern is worth reading rather than just copying. A fixer is not destroyed by air the way a developer is, so the full-to-half-full penalty is milder than a developer’s; but it is destroyed by warmth and by standing, so a dish left out overnight in a warm room has lost a week’s life in a night.

Glass or plastic, never metal. Label with the formula, the strength and the date, per the labelling SOP, and add the count: a fixer’s remaining life is a number of sheets, and a bottle with no tally on it has an unknown capacity left.

Store the boric acid and the glacial acid as the hazards they are — the acid cool, away from ignition sources, bases and oxidisers; the boric acid dry, closed, separated from strong bases, and in a container that has never held food.

Incompatibilities

Any acid stronger than its own. Sulfuric, hydrochloric or a concentrated stop bath tipped into this tray or its waste bottle liberates sulfur dioxide and colloidal sulfur. The incompatibilities page treats this as the most likely serious accident in a home darkroom, and it is the reason stop bath and fixer never share a waste container.

Developer, in the other direction. One pair of tongs per tray. A splash of fixer in a paper developer dissolves halide out of the paper and fogs or bleaches it.

Alkalis. Raising the pH of a thiosulfate bath is what ends it; on an ammonium-based rapid fixer it also releases ammonia, which is why the ammonium thiosulfate page records Kodak’s ceiling on pH for those baths.

Oxidising agents. Thiosulfate is a reducing agent, and its oxidation product, tetrathionate, attacks image silver. Persulfate and permanganate reducers, hypochlorite bleaches and hydrogen peroxide all belong in different bottles.

Sulfide toners. Acid meeting a sulfide gives hydrogen sulfide. Sulfide toner waste is kept away from this bath and from every acid stream.

Chrome alum baths, mixed by accident. Kodak’s SB-3 and its relatives are chromium(III) baths and a different waste stream; keeping them separate is what keeps the chromium ruling’s containers honest.

Waste

This is the silver stream, and it is the most valuable waste a darkroom makes. Spent F-5 carries dissolved silver as the thiosulfate complex, together with sulfite, acetate, borate and aluminium.

Recovery first, disposal second. The 1928 primer already treated an exhausted bath as a resource and listed the period’s methods — sulfide precipitation for large volumes, zinc dust for smaller ones, electrolytic units — while noting that the sulfide route evolves hydrogen sulfide and the zinc route does not. Wall gives the brass-sheet method and the tests for completeness. For a home darkroom the practical route is collection rather than recovery.

Borate is persistent and boric acid is a registered pesticide, so spent fixer does not go on the garden — a habit that survives from the days when fixer was thought of as merely salty.

Collect it, label it, keep it away from acid wastes, and follow the silver-bearing waste SOP. The disposal ruling governs and the jurisdictional caveat is not a formality: local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

A pale yellow precipitate that settles slowly. Sulfur. The primer gives three causes: too much acid in the hardener, too little sulfite or an impure sulfite, or too high a temperature. Check the mixing order and the age of the sulfite tub before blaming the formula.

A white sludge, and a bath that no longer hardens. Aluminium sulfite. The acid has been used up by carried-over developer. The bath is finished; a stop bath before it is the fix for next time.

Prints or negatives staining brown. The bath has gone alkaline. The primer names this outcome directly.

Fixing taking much longer than it did. Clear a test strip and time it. When a slow film’s clearing time passes 12 to 15 minutes, the primer’s rule is to discard the bath.

Froth on the surface. An exhaustion sign in its own right.

A milky bath immediately after mixing. The hypo was not fully dissolved, or the solution was warm, or the hardener went in too fast. Remix; a sulfur precipitate does not redissolve.

A brittle, curling negative. Over-hardening — too much alum, or a long soak in a bath whose hardening had briefly risen. The primer names brittleness as the consequence of excess alum.

A softened emulsion in warm weather despite the hardener. Check the bath’s age against the keeping table rather than the alum: hardening falls away rapidly once the acid is spent.

Experiments

Measure your own clearing time, then stop using Kodak’s ten minutes. Drop a scrap of undeveloped film into a beaker of fresh bath and time the moment the milkiness goes. Double it. Repeat every ten films and plot the clearing time against the count. You will have drawn the exhaustion curve for your film in your bath, and the point where the curve crosses the primer’s 12-to-15-minute rule is your capacity — measured, not quoted.

Test the hardening rather than assuming it. The primer’s own method needs a beaker, a thermometer and a hotplate: fix and wash two strips, one in F-5 and one in plain hypo, immerse each in water and heat slowly until the gelatin flows off the base. The difference between the two temperatures is what the 15 g of alum bought. Do this at Level B with eye protection, and treat the hot water as the hazard.

Watch hardening rise and then fall. Take the strip test above and run it on a fresh bath, on a bath that has fixed ten films, and on one near exhaustion. The primer predicts the middle bath will harden best. If it does, you have measured a non-monotonic behaviour that almost no modern text mentions.

Price the stop bath. Fix one series after a water rinse and one after SB-1, tracking clearing time against sheet count in both. The 1928 primer’s figures predict about a quarter more capacity with the acid rinse. Whether your figure matches matters less than the fact that you can now argue about it with numbers.

Isolate the boric acid. Mix F-5 as published and a second litre with the boric acid left out, and exhaust both with the same developer and the same drain time, testing pH at every step. This is the one experiment on this page that addresses a question the course could not answer from any source, and the result belongs in a laboratory report with a formula version code on it.