Kodak F-24

Two things make this bath worth a page. The first is what it is: the same 240 grams of hypo per litre that Kodak’s general-purpose hardening bath F-5 carries, with the alum, the boric acid and the acetic acid taken out and a single acid salt put in their place. The second is the sentence Kodak attaches to it, which is the only instruction attached to any fixing-bath formula on that page that reaches outside the fixing tray: the developer, the rinse bath and the wash water must not be warmer than 20 °C. A fixer that does not harden makes a demand on every other bath in the sequence, and Kodak states it in the same breath as the formula.

Ingredient	Quantity	Form the source specifies
Water	500 mL	about 50 °C (125 °F)
Sodium thiosulfate	240 g	pentahydrated
Sodium sulfite	10 g	anhydrous
Sodium bisulfite	25 g	anhydrous
Cold water	to make 1000 mL

Purpose

To dissolve the silver halide the developer did not reduce, and to arrest development while doing it, without tanning the gelatin. Kodak’s own scope is one sentence: this bath can be used for films, plates or papers when no hardening is desired.

The alum in a hardening fixer is not there to help the fixing. It is there to make the wet emulsion survive being handled, squeegeed, dried and — in the trade conditions Kodak was writing for — ferrotyped and belt-dried. Take it out and the fixing chemistry is unchanged, but the emulsion that comes out of the tray is the swollen, soft layer that came out of the developer. Everything downstream of the fixer has to be gentler, and everything downstream of the fixer also becomes easier: a toner, a bleach or a wash reaches an unhardened layer faster and more evenly than a tanned one.

Kodak Limited’s London handbook states the same case positively for its own non-hardening bath F-52, whose header reads “for use when hardening is not desired, or must be avoided” and names two period examples, Transferotype and Bromoil papers, both of which need a gelatin layer that can still swell and release. F-24 is the American answer to that question, built from different salts.

Recommended uses

Films, plates or papers, at Kodak’s word. That breadth is unusual — most of the fixing baths around it on the page are headed for one class of material — and it is a permission rather than an instruction to share a tray. The same publication says plainly, in its general account of fixing baths, that one fixing bath should be used for films and plates and another for papers. Paper carries far more developer into the fixer per unit of area than film does, and mixing the two shortens the life of both.

Anything that will be toned, bleached, or redeveloped. Every such process works by getting a reagent into the gelatin and out again, and hardening slows and unevens each stage of that. Kodak’s own toning section makes the related point about what must not be left in the layer — residual silver salts and hypo retained in the paper may cause uneven toning or staining — and the fastest way to get thiosulfate out of a print is not to have hardened the print first.

Anything whose washing matters more than its handling. An unhardened layer gives up thiosulfate faster, which is the whole argument for the archival end of the process; see the sulfite washing aid and HE-1 for what happens after the fixer.

Only where the whole chain can be held at or below 20 °C. This is the condition Kodak attaches and it is not decoration. The 1928 primer states the underlying rule for warm-weather work in one line: the secret lies in preventing abnormal swelling of the gelatin, for once it is swollen it is almost impossible to reduce it and to handle the film. An unhardened emulsion has no defence against a warm wash except the temperature of the wash.

Kodak publishes no fixing time for F-24. The entry gives quantities, a scope and a temperature ceiling, and no duration at all — unlike F-5, which the London handbook times at ten minutes. The course does not supply a number Kodak did not print. Use the clearing-time rule under Behaviour, which is the measurement any published time would have been derived from.

When another formula is preferable

• Where the wet emulsion has to be handled, dried warm, or ferrotyped, F-5, which is the bath F-24 is a subtraction from and which Kodak recommends for general use.
• Where the sulfur dioxide over the tray is the problem, F-5 or its F-6 modification. Kodak’s own note on the same page is that in warm weather and in inadequately ventilated darkrooms the odour of sulfur dioxide given off by F-5 may be objectionable, and that F-6 — F-5 with the boric acid replaced by twice its weight of KODALK — eliminates it almost entirely. F-24 carries more sulfur(IV) per litre than either of them, as the arithmetic below shows, so it is the wrong direction to move in.
• Where fixing speed is the constraint, Kodak’s rapid baths F-7 and F-9, which fix much faster and have a considerably greater capacity — but they harden, and they carry Kodak’s own caution against prolonged fixing, since the image may tend to bleach with fine-grain emulsions and with any paper print.
• For a printing-out process — salted paper, albumen, Van Dyke — the alkaline plain-hypo bath. Acid of any kind attacks the finely divided silver of a printed-out image, and the acidity of this bath is not gentle.
• If what is on the shelf is potassium metabisulfite rather than sodium bisulfite, F-52 is Kodak Limited’s published two-ingredient bath rather than a substitution into this one. The substitution is discussed under Variants; the point is that F-52 is a formula somebody published and a swap is not.
• To understand fixing with nothing else in the tray, plain hypo.

Mixing

Kodak’s order is the printed order, and Kodak says so twice. J-1 states that most Kodak formulas are arranged so that the ingredients are named in the order in which they should be dissolved, unless the directions state an exception; the 1928 primer puts it more firmly still — the order of chemicals in a formula is established carefully and should always be followed. For F-24 that order is hypo, sulfite, bisulfite.

1. 500 mL of water at about 50 °C. Not a convenience. Wall’s 1912 dictionary gives hypo’s solubility as one part in two of cold water, so 500 mL of cold water will only just hold the 240 g this formula calls for — and it gets worse as you stir, because dissolving hypo takes heat out of the solution. The 1928 primer gives exactly that as the reason for warm water: dissolve the hypo in warm water because the temperature drops considerably while the hypo is dissolving.
2. Hypo, 240 g, dissolved completely before anything else goes in. The primer’s warning about adding an acid to hypo that has not fully dissolved is that a precipitate of sulfur is likely to form.
3. Sodium sulfite, 10 g. The defence before the attack, which is the order every acid fixing bath in this formulary uses: the sulfite is in solution before anything acid arrives to decompose the hypo.
4. Sodium bisulfite, 25 g — the acid, and the step the callout below is about, because Kodak’s two documents do not agree on how warm the solution may be when it goes in.
5. Cold water to make 1.0 litre. The 1928 primer states what this line is for: “Cold water to make” is always given at the end of a formula, and it insures dilution to a definite volume and so a known concentration each time the formula is mixed. It also states the design intent of the warm-water line at the head — a volume at about 52 °C sufficient to dissolve all the chemicals, so that when the solution is finally diluted to volume with cold water it is at a working temperature of 18 to 21 °C.

!Kodak's own two instructions do not quite agree, and this page follows the cautious one

Step 4 is where the two Kodak documents this page rests on pull in opposite directions, and the disagreement is worth seeing rather than smoothing over.

What J-1 prints is a formula whose warm water is stated to be sufficient to dissolve all the chemicals, in an order that puts the bisulfite in before the cold water.

What the 1928 primer directs, for the whole class of acid fixing baths built on bisulfite or metabisulfite, is the opposite: do not add the bisulphite or acid sulphite solutions to the warm hypo solution or the hypo will turn milky. The solutions should be quite cold when mixed.

The two are reconcilable in practice and the course says which reading it follows and why. Dissolving 240 g of hypo pentahydrate in 500 mL is strongly endothermic — that is the primer’s own stated reason for the warm water — so a solution that started at 50 °C will have fallen a long way by the time the hypo is in. The course’s practice is to let the hypo solution reach room temperature, or to add most of the cold water, before the bisulfite goes in. That changes no quantity and no final volume; it changes the temperature at one step, on Kodak’s own warning about that step. What is not done here is quietly reprinting the formula in a different order: Kodak’s order is hypo, sulfite, bisulfite, cold water, and that is what the data on this page records.

🧮What the litre actually contains

240 g ÷ 248.19 g/mol = 0.967 mol/L, a 24 % w/v bath

Thiosulfate

10 g ÷ 126.05 g/mol = 0.079 mol/L

Sulfite

25 g ÷ 104.06 g/mol = 0.240 mol/L of hydrogensulfite

Bisulfite

Three numbers worth carrying away.

The hypo is F-5’s hypo. 240 g per litre, the identical figure on the identical page. Whatever else changed between the two formulas, the fixing agent did not, and the two baths are therefore directly comparable: Kodak gives both a useful capacity of 26 sheets of 8 × 10 inches per litre.

The bath is 24 per cent hypo, which is deliberately below the fastest strength. The 1928 primer puts the optimum for speed of fixation at 30 to 40 per cent and Wall sets 40 to 45 per cent as the strongest bath that should be used. F-24 is not the fastest bath Kodak could have written; more hypo would also mean more thiosulfate to wash out of an unhardened layer that is going to be washed hard.

The sulfur(IV) load is the largest of any fixer in this formulary. Sulfite plus bisulfite is 0.079 + 0.240 = 0.319 mol/L of sulfur in the +4 oxidation state. F-5 carries 0.119 mol/L, all of it sulfite; F-52 carries 0.225 mol/L, all of it hydrogensulfite from metabisulfite. That figure is this page’s Safety section in one number, and it is also its Waste section.

One comparison the course makes and Kodak does not. The primer’s general-purpose proportion for acidifying a plain hypo stock is 45 mL of a 50 per cent sodium bisulfite solution — 22.5 g — per litre of 35 per cent hypo, with the warning that any considerable excess turns the hypo milky, especially in warm weather. F-24 puts 25 g into a 24 per cent bath. Per gram of hypo that is about 10.4 g of bisulfite per 100 g against the primer’s 6.4, or roughly sixty per cent more acid salt per unit of hypo than the primer’s general rule. This is the course’s arithmetic on Kodak’s own published figures, not a statement Kodak makes, and it is offered as an explanation to test rather than a fact to repeat: it is consistent with the short tank life Kodak’s own table gives F-24, and nothing in the corpus confirms the connection.

Behaviour

Time it by clearing, because Kodak publishes nothing else for this bath. The rule is the 1928 primer’s and it is the one the craft still uses: the time for fixation is twice the time for the milkiness or opalescence of the unreduced silver salts to disappear. The primer then lists what the 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 of the solution, and the degree of exhaustion. Clear a scrap of undeveloped film, or an unexposed corner of the paper, and double it.

Capacity: 26 sheets of 8 × 10 inches per litre, 100 per US gallon — and the footnote on that column matters more here than anywhere else in the table. Capacity figures apply only if a stop bath is used. An acid fixer’s life is spent neutralising the alkali that arrives on each wet sheet, so the number assumes a bath that has already taken most of that alkali away. Without one, expect less, and measure rather than guess.

For roll film, Kodak’s own equivalence table on the facing page converts the figure: one roll of 135 (36 exposures) and one roll of 120 or 620 each count as one 8 × 10 sheet, 116 or 616 as one and a quarter, and 150 feet of 35 mm as thirty. So 26 sheets per litre is about 26 rolls of 135-36 per litre, which is a plausible number to hold in the head and a startling one to compare against the volume of fixer most darkrooms actually throw away.

The two-bath method is Kodak’s own answer to the capacity limit, and the second footnote on the same block points at it. J-1 describes it: treat the material in the first bath until the emulsion has cleared, then transfer it to a second bath for an equal time; the first bath does most of the work and the second removes the last traces of silver compounds. When the first approaches exhaustion, discard it, promote the second to first, and mix a fresh second. After five such changes, discard both and mix fresh solutions. For paper prints in particular this is not a refinement, it is the way the last of the silver leaves the sheet.

It keeps two months in a bottle and one week in a tank, and the tank figure is the anomaly of the table. Every other fixing bath Kodak lists — the packaged Fixer, EKTAFLO, KODAFIX, Rapid Fixer, Photo-Fix, F-5, F-6, F-7 — is given a month in a gallon tank. F-24 is given a week. Kodak states the figures and does not explain them, and the course will not invent a mechanism it cannot source. What can honestly be said is that two things in this formula point the same way: the 1928 primer’s observation that on keeping, an acid hypo solution gradually becomes milky, which is why its own recommendation is to keep the acid salt as a separate stock and add it to plain hypo as required; and the fact that this bath’s acid reserve is a bisulfite whose excess the same primer warns turns hypo milky in warm weather. Both are consistent with a short standing life. Neither is Kodak’s stated reason, and the table’s own preamble calls every figure in it an estimate based on experience, intended for use only as a guide.

Exhaustion has four published signs, all from the primer: the bath froths at the surface; it goes milky or sludges throughout; it fixes so slowly that there is a danger of removing the material before it is done; and, as the operational rule, when the clearing time for a slow-fixing film exceeds 12 to 15 minutes, discard it. Wall gives the reason to act on that rather than merely notice it — 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.

The only precipitate this bath can make is sulfur. With no alum in it, F-24 cannot produce the white aluminium sulfite sludge that ends an F-5. Pale yellow, slow to settle, and it means the acid has got ahead of the sulfite — through warmth, through a mixing error, or through contamination.

Temperature is a constraint at both ends and the constraint is tighter here. The primer recommends 18 °C for fixing and calls it dangerous practice to let a bath rise above 21 °C, because the solution is apt to precipitate sulfur; it also notes that a film needing 95 seconds to clear at 18 °C would clear in about 60 at 29 °C. Kodak’s ceiling for F-24 is 20 °C and it applies to the developer and the wash as well as to the fixer, because the gelatin, not the chemistry, is what the ceiling protects.

Image characteristics

A fixer used inside its life leaves no visible signature, and every visible signature it does leave is a fault. That holds for F-24 as for any other.

What it changes is invisible on the day and decisive afterwards. The negative or print that comes out of this bath has an unhardened emulsion, and that governs how fast it washes, how evenly it tones, how a bleach-and-redevelop sequence behaves, and how easily it scratches while wet. Fix two identical prints in F-24 and F-5 and tone both, and the difference shows up in the toned print as evenness — even though neither bath has left a mark of its own.

The faults it can produce are the standard acid-fixer faults. A bath that has gone alkaline through carried-over developer stains prints brown, which the primer states directly; a bath used past the clearing rule leaves the insoluble silver complex Wall warns about, invisible at the time and disfiguring years later. To those F-24 adds one of its own: an emulsion physically damaged because the sequence was run warm. Softened surfaces, frilling at the edges and prints sticking to each other are what the 20 °C instruction exists to prevent, and they are not chemistry, they are gelatin.

The mechanism

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the bis(thiosulfato)argentate complex

The fixing half of this bath is the fixing half of every bath in the section: thiosulfate holds silver more tightly than the halide lattice does, and holds it in solution, two thiosulfate ions to each silver. What is particular to F-24 is the acid system and the absence of a hardener.

Why an acid fixer at all. The primer gives the reason without reference to hardening: a plain hypo bath gradually becomes alkaline from the accumulation of developer carried over, the gelatin softens, and the image continues to develop in the fixing bath, so that if two prints stick together more development takes place at the point of contact, causing uneven development. An acid bath neutralises that alkali on contact. That argument stands entirely on its own for a non-hardening formula, which is why F-24 is acid at all when it has no alum to keep in solution.

Why the acid has to be a sulfite salt. Acid and thiosulfate are enemies:

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Acid attacking thiosulfate: the sulfur precipitate

Look at the right-hand side. Hydrogensulfite is the decomposition’s own product, so a bath already full of hydrogensulfite pushes that reaction backwards. This is why the primer can state flatly that all acid fixing baths contain either sodium bisulfite, potassium metabisulfite, or a mixture of sodium sulfite and a weak acid. A fixer acidified with hydrochloric acid would turn to sulfur while you watched.

What bisulfite is, in Kodak’s own terms. The primer defines it as sulfite plus sulfurous acid:

Na₂SO₃ + H₂SO₃ → 2 NaHSO₃

Kodak's own definition of sodium bisulfite

and metabisulfite, the salt that is actually in most tubs sold as bisulfite, as sulfite plus sulfur dioxide:

Na₂SO₃ + SO₂ → Na₂S₂O₅

Kodak's own definition of sodium metabisulfite

which is the same fact told twice: a bisulfite is a sulfite carrying an acid, and that is exactly what a fixing bath needs. The primer says so directly — bisulfite is used very largely as a preservative for fixing baths, supplying both the sulphite and the acid necessary.

Why, then, does F-24 also carry 10 g of separate sulfite? Kodak does not say, here or anywhere the course has read. What the primer does say is that a neutral solution can be produced by adding a small quantity of bisulfite to sulfite — which is a statement that the two are a conjugate pair whose ratio sets the acidity. F-24 runs that pair the other way round, with three moles of the acid member to one of the base member, and the smaller member is the reserve that has to be consumed before the pH begins to move. That reading is the course’s, not Kodak’s, and it is the kind of thing a pH meter and a sequence of ten prints can settle in an afternoon; there is an experiment for it below.

⚗Three parts acid to one part base, and no alum to referee

An acid hardening fixer is a three-body problem: the acid attacks the thiosulfate, the sulfite defends it, and the alum has to be kept in solution by the acid while not meeting the sulfite. Take the alum out and one whole side of the problem disappears.

What is gone. No aluminium means no aluminium sulfite sludge, which is the failure that ends an F-5 bath; no requirement to hold the pH low enough for alum to tan gelatin, which is why F-5 needs both acetic acid and a boric acid reserve; and no boric acid, which is the ingredient that makes F-5 a Level B page.

What is left is a buffer and a fixing agent. 0.240 mol/L of hydrogensulfite and 0.079 mol/L of sulfite, in the same solution as 0.967 mol/L of thiosulfate. The acid member arrests development and suppresses the thiosulfate’s decomposition; the base member is the reserve; and both are consumed by the alkali arriving on every wet sheet.

And what is lost with the alum is not chemical but mechanical. The emulsion leaves this bath as soft as it entered it. Kodak’s 20 °C ceiling on the whole sequence is the price of that, and it is a price paid in workflow rather than in the tray.

Function of every ingredient

Sodium thiosulfate, 240 g of the pentahydrated crystals. The fixing agent, and the only ingredient doing the job the bath is named for. It complexes silver out of the undeveloped halide, two thiosulfate ions to each silver ion, and the resulting bis(thiosulfato)argentate is soluble and washes away. At 0.967 mol/L this is a 24 per cent bath. More fixes faster up to the 30-to-40-per-cent optimum the 1928 primer states, above which there is no further gain and Wall’s 40-to-45-per-cent ceiling arrives; more also puts more thiosulfate into the emulsion for the wash to remove, which on an unhardened print is a cost you have already decided to care about. Less fixes more slowly, exhausts sooner, and moves the bath towards the region where the insoluble silver–thiosulfate complex forms instead of the soluble one — the invisible failure that ruins prints years later. The form matters as much as the number. Kodak specifies the pentahydrate at 248.19 g/mol, which is about two-fifths water by mass; 240 g of the anhydrous salt would make a bath a third stronger than Kodak wrote. Its solubility is what forces the warm water: Wall’s 1912 dictionary gives one part in two of cold water, so 240 g is at the limit of what 500 mL of cold water will take, and the solution chills itself as it goes.

Sodium sulfite, 10 g anhydrous. The preservative, in the role the 1928 primer assigns sulfite generally in a fixing bath: it prevents decomposition of the hypo. It is also a reducing agent, which is why an acid fixer containing it does not brown as quickly as the oxidised developer carried in on each sheet accumulates. In this formula it is additionally the base half of a sulfite–bisulfite pair, and at 0.079 mol/L it is a modest reserve — a third the size of the acid member. More would raise the pH and lengthen the bath’s resistance to carried-over alkali, at the cost of a bath less able to stop development on contact. Less, or an old tub that has oxidised to the sulfate, and the bath goes milky with sulfur sooner and browns sooner.

Be clear about what is and is not sourced here. That sulfite preserves hypo is Kodak’s statement. That Kodak chose this quantity to sit against this quantity of bisulfite, and why, is not: no source read for this course explains why F-24 carries both salts, or why in this ratio. The formula that most resembles it, Kodak’s own 1928 bisulphite fixing bath, pairs almost the same sulfite — 10.5 g — with only 5.3 g of bisulfite, so the pairing is clearly Kodak’s habit and the proportion is clearly not fixed. Treat the preservative role as established and the ratio as a design choice whose reasoning is unrecorded.

Sodium bisulfite, 25 g anhydrous. The acid, the second preservative, and the ingredient that gives this formula its character. In water it supplies hydrogensulfite: 0.240 mol/L of a weakly acidic, reducing ion that does three things at once. It makes the bath acid enough to neutralise developer alkali and arrest development on contact. It suppresses the acid decomposition of the thiosulfate, by being that reaction’s own product. And it reduces the oxidised developer carried in, so the bath does not brown — the primer’s own account of bisulfite in a fixing bath is that it prevents the formation of sulfur by the action of the acid on the hypo and prevents the developer carried into the bath from oxidising. More is not a stronger bath: the primer’s warning about excess over its general-purpose proportion is that the hypo rapidly turns milky, especially in warm weather, owing to the liberation of sulphur. Less gives a bath that goes alkaline early, lets development continue between sheets in contact, and stains prints brown.

What is actually in the tub, and what that does to the arithmetic. The 1928 primer is explicit that ordinary commercial bisulfite has been shown by analysis to consist chiefly of metabisulfite, which is converted into bisulfite when dissolved in water, and that Kodak’s own Tested Chemical of that name may be used with confidence for formulas calling for either. So “sodium bisulfite (anhydrous)” on a label is usually sodium metabisulfite, Na₂S₂O₅, and the two are not equivalent gram for gram. The course’s arithmetic on the published molar masses:

🧮What 25 g delivers, depending on what the tub really holds

25 g ÷ 104.06 g/mol = 0.240 mol/L of HSO₃⁻

As true sodium bisulfite

25 g ÷ 190.11 g/mol × 2 = 0.263 mol/L of HSO₃⁻

As sodium metabisulfite, two ions per formula unit

25 g ÷ 222.33 g/mol × 2 = 0.225 mol/L of HSO₃⁻

As potassium metabisulfite, the customary substitute

A spread of about sixteen per cent across three salts sold for the same purpose, all at the same weight. The primer calls the substitution of sodium bisulfite for potassium metabisulfite weight for weight customary and says the choice comes down to cost, which for a fixing bath’s acid reserve is a defensible position and is not the same as saying the three are identical. Note where the sodium metabisulfite figure lands: 0.263 against Kodak’s nominal 0.240, so a tub of metabisulfite weighed out as bisulfite gives a bath about ten per cent more acid than the formula specifies — in the direction the primer warns about.

Water: 500 mL at about 50 °C at the head, cold water to make 1000 mL at the foot. Two different statements doing two different jobs, and the 1928 primer explains both. The warm water is sufficient to dissolve all the chemicals and is warm because hypo takes heat out of solution as it dissolves. The cold water to make insures dilution to a definite volume, thus yielding a known concentration of chemicals each time the formula is mixed, and lands the finished bath at 18 to 21 °C — which is, conveniently, inside the 20 °C ceiling this formula demands of everything else.

Interactions

With the developer, which is what spends it. Every wet sheet brings alkali into the tray, and the bisulfite is consumed neutralising it. This is why Kodak’s capacity figure carries the condition capacity figures apply only if a stop bath is used: an acid rinse such as SB-1 spends its own acid instead, and the 1928 primer measures the gain for its hardening bath F-1 at a hundred 8 × 10 prints per gallon after a water rinse against a hundred and twenty-five after an acid rinse. The capacity of this bath is a property of your developer and your drain time as much as of the formula.

Sulfite against bisulfite, as a pair. The two salts are the conjugate base and acid of the same system, so they do not simply add — they buffer. Adding sulfite raises the pH and lengthens the bath’s resistance to carried-over alkali; adding bisulfite lowers it and shortens the bath’s keeping. It is one dial with two ends, and Kodak has set it at three to one on the acid side.

With warmth, badly, twice over. Warmth accelerates the milkiness on mixing, the milkiness on keeping, and the swelling of the unhardened gelatin. Kodak’s keeping figures are for storage at 18.5 to 21 °C and are stated to be proportionately less above that.

With the wash and with everything after it, favourably — which is the point. No alum means an open layer: thiosulfate leaves faster, a sulfite washing aid works on a layer that will let it in, and a toner reaches the silver evenly.

With a hardener stock, not as an upgrade. Adding F-53 or a proprietary liquid hardener to this bath does not improve it; it makes a different formula, and F-54 is what that formula is when Kodak does it deliberately, with the hypo strength and the hardener dose chosen together.

With an ammonium-thiosulfate rapid fixer, not at all. Different chemistry, different pH window, different waste behaviour; the ammonium thiosulfate page records why those baths must not be alkalised. Do not top one up with the other.

With the vessel. J-1 is specific and it applies here: glass, hard rubber, polyethylene, enamelled steel and stainless steel are all safe with any ordinary solution, but aluminium, zinc and galvanised iron should not be used with either developers or fixing baths, and tin, copper and their alloys cause trouble with developers. A bisulfite solution in contact with a reactive metal has a second consequence, recorded on its own page from CAMEO: flammable hydrogen.

Variants

Kodak’s own 1928 bisulphite fixing bath is the same construction at a fifth of the acid. Hypo 250 g, sodium sulfite 10.5 g, sodium bisulfite 5.3 g per litre, published without a formula number. Put the two side by side and only one number has really moved:

	1928 bisulphite bath	F-24
Hypo	250 g (1.007 mol/L)	240 g (0.967 mol/L)
Sodium sulfite, anhydrous	10.5 g (0.083 mol/L)	10 g (0.079 mol/L)
Sodium bisulfite, anhydrous	5.3 g (0.051 mol/L)	25 g (0.240 mol/L)

The hypo and the sulfite are the same formula to within five per cent. The acid is almost five times larger. These are not two printings of one bath, they are two different answers to the question of how much acid reserve a non-hardening fixer needs, published by the same company about half a century apart, and the course prints both rather than reconciling them. Which is better is an empirical question with a measurable answer, and the experiments below are how to get it.

F-52 is the London answer to the same question, and it reaches almost the same acid loading by a different route: 250 g of hypo and 25 g of potassium metabisulfite, no separate sulfite, 0.225 mol/L of hydrogensulfite against F-24’s 0.240. Two Kodak companies, two continents, two salts, and an acid reserve that agrees to within seven per cent. The difference that remains is F-24’s 10 g of free sulfite, which F-52 does without entirely.

F-5 and F-6 are the hardening siblings on the same J-1 page and are not variants of this formula in either direction. F-5 is Kodak’s general-purpose acid hardening bath; F-6 is F-5 with the boric acid omitted and twice its weight of KODALK Balanced Alkali substituted, which Kodak offers specifically to remove the sulfur dioxide odour in warm weather and inadequately ventilated darkrooms, and which it notes washes out of photographic papers more rapidly than baths with a greater hardening action. Neither is F-24 with something added.

No safer or simplified variant of F-24 is offered here. The obvious candidates — leaving out the 10 g of sulfite because bisulfite supplies sulfite anyway, or cutting the bisulfite towards the 1928 figure — would each be a different formula with a different life and a different pH, not a version of this one. Where a reader wants the two-ingredient bath, F-52 is one somebody published.

The bought non-hardening fixers are behaviour only. ILFORD’s RAPID FIXER and HYPAM are ammonium thiosulfate concentrates described by their maker as non-hardening, and neither composition is published; what can honestly be said about that class, including ILFORD’s own clearing test and silver limits, is set out under Variants on the F-52 page rather than repeated here.

Safety

Level A, and the criteria are worth setting out because this is a page where the level is decided by what is absent and the residual hazard is decided by what is present in quantity.

Nothing in this formula carries a corrosive or reproductive classification. Sodium thiosulfate has no aggregated GHS hazard statements at all. Sodium sulfite and sodium bisulfite are the working hazard, and the bisulfite page records its aggregated classification as a single statement — H302, harmful if swallowed, with the GHS07 pictogram and the signal word Warning — from an unusually uniform set of notifications. The controls are the standard darkroom ones: nitrile gloves, eye protection, an apron, dedicated utensils, ordinary extraction.

Compare what is not here. F-5 is Level B, and both reasons are missing from this tray. There is no boric acid, whose aggregated classification is H360, may damage fertility or the unborn child. There is no glacial acetic acid, which is corrosive and flammable and whose vapour is the reason F-5’s mixing stage needs ventilation. There is also no potassium alum and therefore no soluble-aluminium dust to weigh. If the reason for looking at a non-hardening fixer was the classification rather than the hardening, this page is an answer to that question too.

⚠The sulfur dioxide load is the largest in this formulary's fixers

This bath carries 0.319 mol/L of sulfur in the +4 oxidation state — 0.079 as sulfite and 0.240 as hydrogensulfite. That is 2.7 times F-5’s figure and about 1.4 times F-52’s. Every one of those moles is a sulfur dioxide molecule held as an ion in solution, and the whole protective mechanism of the bath consists of keeping it there.

Kodak’s own evidence that this is not theoretical sits on the same page as the formula: it prints the F-6 modification specifically because in warm weather and in inadequately ventilated darkrooms the odour of sulfur dioxide given off by F-5 may be objectionable. F-5 is the bath with less than half the sulfur(IV) of this one. That F-24 is therefore the more odorous bath is the course’s inference from Kodak’s two published figures, not a statement Kodak makes; it is a straightforward one, and it points at three practices.

Weigh the bisulfite without raising dust, and never in the room where sensitised material is handled. Keep the tray covered when it is not in use. And never let anything more strongly acid than the formula reach the bath or its waste bottle — a stop bath concentrate tipped in, or a spent fixer poured into a container that held acid, releases the gas directly. Sulfur dioxide’s workplace exposure limit is among the tightest of any substance in this course, and the bisulfite page carries the figures.

Eye protection is not optional at Level A. The bisulfite page records CAMEO’s description of the concentrated commercial solution as a strong irritant to skin and tissue, and a splash of a saturated solution is not something to establish by experiment.

Storage

Two months in a stoppered bottle; one week standing in a tray; one week in a covered gallon tank — Kodak’s own table, with the caveat the table itself prints: the figures are estimates based on experience, intended for use only as a guide, and the keeping figures are for solutions held at 18.5 to 21 °C and are proportionately less above that.

Read the tank figure rather than copying it. A bath that keeps eight times longer in a bottle than in a tank is telling you that its enemy is not simply time. The 1928 primer’s own recommendation for acid hypo solutions follows from that: keep the acid salt as a separate concentrated stock and add it to a plain hypo stock as required, rather than storing the mixed bath. If you mix F-24 in quantity, that is the way to do it, and it is Kodak’s advice rather than the course’s.

Keep the dry bisulfite dry, closed and cool. Its own page carries NIOSH’s note that it is slowly oxidised to the sulfate on exposure to air, so an old, faintly caked tub has lost strength as an acid and as a preservative at the same time — which on this formula means both of the jobs it does. The same applies to the sulfite.

Glass or plastic, never aluminium, zinc or galvanised iron. Label with the formula, the date and the running sheet count, per the labelling SOP; a fixer’s remaining life is a number of sheets, and a bottle with no tally on it has an unknown capacity left.

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 from the sulfite and the bisulfite directly and colloidal sulfur from the thiosulfate at the same time. This bath holds three sources of that gas rather than one; see incompatibilities.

Alkalis and carbonates, which neutralise the acid, end the bath’s ability to stop development, and let carried-over developer oxidise and brown it.

Oxidising agents — peroxide, persulfate, permanganate, hypochlorite — which oxidise both the sulfur(IV) and the thiosulfate. Note that HE-1, a hypo eliminator, is a peroxide solution: it belongs in the washing sequence, several trays and one thorough wash away, and never in or near this one.

Developer, in either 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.

Sulfide toners, whose waste meeting an acid fixer gives hydrogen sulfide. Different bottle, different stream.

Reactive metals. Aluminium, zinc and galvanised iron are ruled out for fixing baths by Kodak, and the bisulfite page records CAMEO’s finding that contact with metals can generate flammable hydrogen.

Waste

Silver-bearing, and chemically the simplest of the acid fixers in this formulary — thiosulfate, sulfite, bisulfite, sulfate, sodium and dissolved silver, with no aluminium and no borate in it. That simplicity is worth something at the waste stage and it does not make the stream dischargeable.

Three properties decide how it is handled. It carries silver as a soluble complex, which is the reason silver recovery exists and the reason a spent fixer is the most valuable waste a darkroom makes. It is strongly oxygen-demanding: thiosulfate and sulfite both consume oxygen in receiving water, and this bath carries more sulfur(IV) than any other fixer here. And it is a sulfur dioxide source if it meets an acid, which is why a fixer waste bottle never doubles as a stop bath bottle.

So: collect the spent bath in a labelled container that has never held an acid, keep the count on the label, send it for silver recovery where that route exists, and work through the silver-bearing waste SOP and the disposal ruling it rests on. What you may then lawfully do with it is decided by the authority where you live, and this course has no standing to tell you what that decision is.

Troubleshooting

The bath went milky as it was mixed. Sulfur, and it does not redissolve. The hypo was not fully dissolved before the bisulfite went in, or the solution was still warm — the primer’s two named causes. Remake it, and let the hypo solution cool first.

The bath goes milky after weeks in the bottle. The primer’s own observation about acid hypo solutions on keeping. Kodak’s two months is an outside figure and assumes 18.5 to 21 °C; store cool, and consider keeping the bisulfite as a separate stock.

A tank of it was dead in a week. That is Kodak’s published figure, not a fault. F-24 is the only fixing bath in Kodak’s table given one week rather than one month in a gallon tank.

Prints staining brown. The bath has gone alkaline through carried-over developer. Use a stop bath — Kodak’s capacity figure assumes one — and check the sheet count against the 26-per-litre figure.

Uneven density where two sheets touched. The same cause one stage earlier: the bath is spent enough that development is continuing in it, which is exactly the failure the primer describes for a plain hypo bath.

Fixing time creeping up. Clear a test strip and time it. The rule is to discard when a slow-fixing film’s clearing time passes 12 to 15 minutes.

Froth on the surface. An exhaustion sign in its own right, listed alongside milkiness and sludging.

A sharp smell over the tray. Sulfur dioxide. Either something acid has reached the bath or the ventilation has stopped. Do not diagnose this one by leaning closer.

The emulsion is soft, frilling at the edges, or prints are sticking to one another. The formula is working as designed and the sequence is too warm. Check the developer, the rinse and the wash against Kodak’s 20 °C ceiling; the wash is the one people forget, because it is the one that runs from a tap.

The negative marks easily when wet. Same answer. If that is unacceptable in your workflow, you wanted F-5 and the trade you made was the wrong way round.

Experiments

Measure the clearing time and build the exhaustion curve. Drop a scrap of undeveloped film into a beaker of fresh bath, time the disappearance of the milkiness, double it, and repeat every five sheets or rolls. Plot clearing time against count. Where the curve crosses the primer’s 12-to-15-minute rule is your capacity, in your water, after your developer, with or without a stop bath — and comparing it to Kodak’s 26 sheets per litre is a lesson in what a published capacity is and is not.

Price the stop bath, since Kodak’s figure depends on one. Run the same series twice, once after a plain water rinse and once after SB-1. The 1928 primer’s numbers for its hardening bath predict roughly a quarter more capacity with the acid rinse. Whether your figure matches matters less than being able to argue about it with data.

Settle the sulfite question with a pH meter. Mix a litre of F-24 as published and a litre with the 10 g of sodium sulfite left out, and exhaust both with the same developer and the same drain time, measuring pH after every sheet with the pH SOP. If the free sulfite is the reserve this page reads it as, the full formula’s pH curve will hold flat for longer before it turns. This is the one experiment on the page that addresses a question no source could answer, so record it as a laboratory report with a formula version code on it.

Test the fifty-year gap. F-24 against Kodak’s own 1928 bisulphite bath — nearly identical hypo and sulfite, five times the bisulfite — with clearing time and pH tracked in both to exhaustion. The prediction to test is that the 1928 bath goes alkaline sooner and that F-24 keeps worse.

Measure what the alum was for. Fix and wash two strips, one in F-24 and one in F-5, then use the primer’s own method: immerse each in water and heat it slowly until the gelatin flows away from its support. The primer puts baths of F-5’s class at 54 to 77 °C. The gap between the two strips is the whole content of the word “hardening”. Treat the hot water as the hazard and work at Level B for the heating step.

Race the wash. Fix matched prints in F-24 and F-5, wash both on the same schedule, and compare how long each goes on giving something up to a fresh tray of water — by conductivity if you have a meter, by Kodak’s own hypo test if you can source it. The claim under test is the central practical argument for a non-hardening fixer, and it should be visible as a difference in minutes.

Weigh the tub against the label. Mix one litre with 25 g from a tub sold as sodium bisulfite and one with the molar equivalent of true bisulfite worked out from sodium metabisulfite — 22.8 g of Na₂S₂O₅ delivers the same 0.240 mol/L of hydrogensulfite — and compare their pH. If the two differ measurably, the tub was metabisulfite and the arithmetic in the callout above is the reason.