Plain hypo fixing bath

One ingredient and one solvent, and every other fixer in this formulary is this bath with defences bolted on. Reading it first is the cheapest way to understand the rest: once you know what a plain hypo bath does well and what goes wrong with it, F-5 stops being a list of five chemicals and becomes an argument about four specific failures.

Ingredient	Quantity	Form the source specifies
Sodium thiosulfate	400 g	crystals
Water	to make 1000 mL	hot

Purpose

To dissolve the silver halide that development did not reduce, and to do nothing else whatever. No acid, so development does not stop when the film enters. No sulfite, so nothing protects the thiosulfate or the carried-over developer. No alum, so the emulsion leaves as soft as it arrived.

Wall assigns it, with the alkaline bath, to printing-out papers, and the acid and acid-alum baths to plates, film and developed papers. Kodak’s 1928 primer puts the same division from the other side: a plain solution of hypo is seldom used as a fixing bath, and is used instead with a weakly acid salt or an acid hardening solution.

Recommended uses

Printing-out papers, where an acid bath is a liability rather than an asset because it attacks the finely divided silver of the image. For albumen and salted paper the course publishes Reilly’s alkaline version of this bath rather than this one: the 2 g of carbonate he adds is a small change with a specific job, and his procedure carries times, temperatures and a capacity that Wall’s line does not.

As the laboratory bath. A beaker of plain hypo and a scrap of undeveloped film is the whole demonstration of fixing, uncontaminated by hardening or by pH. It is also how a clearing time is measured, and clearing time is the number that governs every fixing decision you will make afterwards.

Where you have nothing else. It fixes. Wall’s own instruction that the temperature be held about normal, 18 °C, and that a bath not be used too long, is enough to work by.

When another formula is preferable

• For anything developed-out — film, plates, enlarging papers — almost always. Kodak’s reason is specific and is not about hardening: in a plain bath development continues in the fixing bath, so if two prints stick together more development takes place at the point of contact and the result is uneven. An acid bath kills the alkali carried over and prevents it.
• For albumen, salted paper and other printing-out work, the alkaline bath, which is this formula with a deliberate 2 g of carbonate and a published procedure.
• Where the emulsion must survive warm handling, F-5, whose alum raises the temperature at which the gelatin melts off its base.
• Where you want acid without hardening, F-52.

Mixing

Hot water first, then the crystals. Hypo dissolves with a marked cooling — the crystals are two-fifths water by mass and their dissolution is endothermic — so a litre made in cold water ends up noticeably below room temperature and takes a long time to get there.

Wall’s own trick, which is worth having. Tie the crystals in a cloth or a piece of Canton flannel and suspend the bag in a vessel of hot water. The dense solution sinks as it forms and the undissolved lump stays out of the way, and, in Wall’s words, this obviates any necessity for filtering the solution. Period hypo carried grit; a bag also keeps the last insoluble tenth of a cheap batch out of the bath.

There is no order to get wrong, and that is the one advantage this formula has over every other fixer here. Nothing precipitates, nothing decomposes, nothing has to be cold before something else goes in.

🧮What a 40 per cent bath is, in moles

400 g in 1000 mL = 40 % w/v

As a mass concentration

400 g ÷ 248.19 g/mol = 1.61 mol/L of thiosulfate

As moles

Wall gives 40 to 45 per cent as the strongest bath that should be used and the most rapid in action; Kodak’s 1928 primer puts the optimum for speed of fixation at 30 to 40 per cent. Those two statements are compatible and together they bracket the answer: this formula sits at the top of the useful range and at the bottom of Wall’s ceiling.

Two thiosulfate ions are consumed per silver ion, so 1.61 mol/L is in principle enough thiosulfate to complex 0.8 mol of silver — about 87 g. No bath is ever worked near that, and the reason is the whole subject of Behaviour below: long before the thiosulfate runs out, the bath stops making the soluble complex.

If you have the anhydrous salt, 400 g of the crystals is 255 g of it, because 400 × (158.11 ÷ 248.19) = 255. Weighing 400 g of the anhydrous salt instead gives 2.53 mol/L — the equivalent of a 63 per cent crystalline bath, and half as strong again as Wall’s ceiling.

Behaviour

It fixes quickly, because it is strong and because nothing in it is fighting anything else. Clearing time is the measurement: the time for the milky opalescence of the undeveloped halide to disappear, with the fixing time taken as twice that.

It goes on developing. This is the behaviour that disqualifies a plain bath for developing-out materials, and it is easy to see: put an unrinsed print into plain hypo and watch the shadows deepen for several seconds. Where two sheets touch, the alkali cannot escape and the local development runs on longer, which is Kodak’s account of the uneven patches a plain bath produces.

It exhausts in a way that is invisible. Wall’s warning is the important sentence on this page: the more a bath is used, the greater its saturation with silver salts, so the longer it takes to fix properly and the greater the chance of forming insoluble, transparent silver salts which are difficult to wash out. Kodak’s 1924 primer gives the chemistry underneath it — two compound thiosulfates exist, one almost insoluble in water and the other very soluble, and only the soluble one is formed while the bath retains appreciable fixing power. Reilly puts the same finding in modern terms: thiosulfate must be present in excess or insoluble complexes form that cannot be washed out of the image layer.

Nothing about that failure is visible. The negative clears, the print looks finished, and the compound that will stain it in ten years is colourless.

Which is why the answer is two baths, not one long one. Kodak’s 1924 edition draws the conclusion explicitly, and Reilly makes it procedure: the first bath does the bulk of the complexing, the second ensures the complexes that end up in the print are the soluble kind. When the first bath is spent it is discarded, the second is promoted, and a fresh second bath is made.

Longer is not safer. Reilly: prolonged fixation is much more injurious to prints than is generally believed, because thiosulfate that penetrates the paper fibres becomes almost impossible to remove. The instinct to leave a print in the fixer while you deal with something else is the wrong instinct.

No capacity and no shelf life are published here, and the course does not supply them. Wall gives neither for the plain bath. What he gives instead is the rule that governs both — the bath slows as it saturates — and the clearing-time test turns that rule into a number for your own materials.

Image characteristics

On a developed-out negative or print, none. A fixer used inside its life leaves the image it was given. The visible signatures of a plain bath are all faults: uneven density where prints stuck together, and, much later, the staining and fading that follow an insoluble complex left in the paper.

On a printing-out paper, a large and expected change. Reilly describes it: during fixation the original brilliant purple or brown becomes a much yellower, duller brown, with a loss of density. That is not the fixer staining the print. The silver chloride is being removed and the metallic silver particles are packing into aggregates, so the covering power falls and the colour shifts — a physical change in the image layer, which is why the print also reddens when it first meets wash water and neutralises again as it dries. A student meeting this for the first time reasonably thinks they have ruined the print.

The mechanism

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

The soluble complex: bis(thiosulfato)argentate

Silver bromide is insoluble in water because the lattice holds Ag⁺ more tightly than water molecules can. Herschel put the same observation in 1819 as a property of a new class of salts, that their solutions dissolve the muriate of silver and hold it in permanent solution, which is where Part I picks the story up. Thiosulfate binds it more tightly still, through sulfur, and the resulting anion is freely soluble. The halide leaves as bromide or chloride ion, which is why a fixer accumulates halide as well as silver.

Why excess matters, in one sentence. The complex above is not the only one that forms. With too little thiosulfate relative to silver, complexes with fewer thiosulfate ligands per silver form instead, and those are the “almost insoluble” compounds Kodak’s 1924 primer describes and Reilly’s chapter blames for the failure of under-fixed prints. Excess thiosulfate is not a safety margin on the fixing time; it is a condition for making the right product.

⚗What this bath cannot do, and what that teaches

Three absences, each of which is another formula’s reason to exist.

No acid. So development continues; and so the bath will not harden with alum even if you add some. It also means the bath is defenceless in the other direction: a splash of stop bath in it liberates sulfur.

No sulfite. So there is nothing to reduce the oxidised developer arriving on each sheet, and a plain bath that has taken a few unrinsed prints goes brown. Sulfite is also what makes an acid fixer possible at all.

No alum. So the emulsion leaves soft. Do not fix that by adding alum: alum in water is weakly acid, and Kodak’s primer records what happens when it meets plain hypo with no sulfite present — the bath turns turbid and precipitates sulfur. That reaction is not a spoiled fixer by accident in only one context; it is the hypo-alum toner working on purpose in the other.

Function of every ingredient

Sodium thiosulfate, 400 g of the crystals. The fixing agent and the entire formula. It converts insoluble silver halide into a soluble anionic complex by supplying a ligand that binds silver more strongly than the halide lattice does, two thiosulfate ions to each silver. More — above Wall’s 40 to 45 per cent — gains no speed, costs money, and loads the emulsion with more thiosulfate to be washed out afterwards, which is a permanence cost. Less fixes more slowly and, more seriously, brings the bath closer to the region where the insoluble complex is the product. It has no aggregated GHS hazard statements, on a thin evidence base rather than on a body of evidence, and the chemical page says so carefully.

The form matters and the label lies about it. Photographic “hypo” is the pentahydrate, Na₂S₂O₃·5H₂O, at 248.19 g/mol; the anhydrous salt is 158.11. A shop that sells “sodium thiosulfate” without saying which will sell you either. Weigh 400 g of the wrong one and the bath is 57 per cent rather than 40.

Water, to make 1000 mL, hot. Not an inert ingredient here in one respect: dissolving 400 g of a hydrated salt absorbs a substantial amount of heat, so the finished bath is colder than the water it was made from and needs to come back to Wall’s 18 °C before it is used. Hot water also dissolves it far faster, which matters when the quantity is this large.

Interactions

With the developer, badly. Alkali carried over is not neutralised, so development continues; oxidised developer is not reduced, so the bath browns. Both are arguments for a rinse or a stop bath even where the fixer itself contains no acid — the rinse is doing work the bath cannot do for itself.

With alum, dramatically. See the callout above: alum plus plain hypo with no sulfite present is a sulfur precipitate. Adding hardener to a plain bath is not an upgrade; it is a different formula, and F-5 is what that formula looks like when it is done properly.

With acid, dangerously. Thiosulfate plus acid gives sulfur dioxide and colloidal sulfur, with nothing in the bath to buffer or restrain it.

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Acid attacking an unprotected thiosulfate bath

With the wash, favourably. This is the one place a plain bath wins outright. There is no alum hardening the layer, so thiosulfate diffuses out of the emulsion faster than it does from a hardened one. A sulfite washing aid still shortens the wash, and Reilly’s sequence uses one, but the starting position is better.

With toners and after-treatments, favourably again. An unhardened emulsion takes selenium, sulfide and gold evenly. That is why Kodak recommended a non-hardening fixer for prints that were to be toned.

Variants

The alkaline bath is this formula at 150 g/L with 2 g of sodium carbonate added, from Reilly’s conservation monograph, and it is what the course actually uses for printing-out papers. It is a separate published formula rather than a variant of this one.

Wall’s own acid bath, in the same chapter, is 150 g of hypo with 25 g of potassium metabisulfite per litre for papers, and he says plainly that for negative work the hypo should be increased to 400 g — this formula’s strength with acid added. The course publishes Kodak’s F-52 as its non-hardening acid fixer, because the 1949 handbook carries capacities and keeping figures for it that Wall’s line does not, and F-52 is that same construction at 250 g/L.

Wall’s alkaline bath could not be published. It appears on the same page — hypo 125 g, dry sodium carbonate 7 g, salt, and water to 1000 ccm, with the note that dry sodium sulfite may be substituted for the carbonate — but the quantity against “salt” is illegible in the copy the course holds, and neither column of the table can supply it without inference. A formula with an unreadable quantity is not a formula, so it is not printed here even in outline.

A 25 per cent bath is what Kodak’s 1928 primer assumes when it describes adding a hardener stock to “a 25 per cent cool hypo solution”. That is a plain hypo bath too, at a different strength, and it is worth knowing that the number in a formula’s hypo line is not a constant of nature.

Safety

Level A, and this is the only fixer in the formulary that is. The bath contains one substance whose aggregated GHS classification carries no hazard statements at all, in hot water.

That absence is thin evidence rather than a clean bill of health, and the chemical page is careful about it: 277 of 281 reports state that the substance does not meet GHS criteria, but only about 1.4 per cent of companies supplied any information. Gloves and eye protection for the solution, splash goggles when making a concentrated one, and an apron, because dried fixer leaves a white bloom that will mark a print it later touches.

The hazard belongs to what you might add. Every serious hazard in fixing chemistry — sulfur dioxide from acid, hydrogen sulfide from a sulfide toner, ammonia from an alkalised ammonium bath — arrives with something that was not in this formula. A plain bath is the safest fixer to mix and the easiest to ruin.

Hot water is the ingredient with the injury record. Four hundred grams is a large quantity to dissolve and it wants a vessel big enough to stir.

Storage

Kodak publishes no keeping figures for a plain bath and neither does Wall, so the course prints none. What is established is the direction of travel: the chemical page records that thiosulfate on heating or on long standing in solution can decompose with the release of sulfur dioxide, and Reilly records that unused fixer solutions break down. A plain bath is therefore made when it is wanted rather than kept, and Reilly’s own instruction for printing-out work is that both baths be freshly made up the day on which they will be used.

Dry crystals keep well if kept dry. They deliquesce slightly in moist air and cake into a block that is still weighable but no longer pours.

Label the bottle with the formula, the strength, the date and — because this is a fixer — the count of sheets it has taken, per the labelling SOP.

Incompatibilities

Acids, above everything. No sulfite is present to restrain the reaction, so this bath is the most acid-sensitive fixer in the formulary. Stop bath, spent stop bath, descaler, vinegar: none of them near it or its waste bottle. See incompatibilities.

Alum and alum-containing hardeners, which turn a plain bath turbid and precipitate sulfur.

Oxidising agents. Thiosulfate is a reducing agent; oxidation gives tetrathionate, which sulfides silver and attacks an image. Hydrogen peroxide, persulfate, permanganate and hypochlorite all belong elsewhere — which is also the argument against hypo eliminators as a class.

Silver nitrate, which precipitates and then complexes, and which is why fixer and sensitiser never share a work surface. The silver nitrate SOP has the sequence.

Heat and long standing, which decompose it in the bottle.

Waste

Silver-bearing, and it is the concentrated end of the silver stream — a plain bath used for printing-out papers carries more silver per litre than almost anything else a darkroom produces, because printing-out papers are coated with far more silver than developing-out ones.

There is no alum, no borate and no acid in it, which makes it the simplest fixer waste to describe and does not make it dischargeable. It is oxygen-demanding, it carries silver as a soluble complex, and the first wash after it carries more.

Collect it, label it, keep it away from acid wastes, and follow the silver-bearing waste SOP, which cites the disposal ruling. Local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Uneven density, especially in patches the shape of another sheet. Development continued in the fixer where two sheets were in contact. Separate the prints and keep them moving, and use a rinse or a stop bath before the fixer.

The bath has gone brown. Oxidised developer, unreduced because there is no sulfite. Rinse before fixing, or use a bath that contains sulfite.

Fixing is taking much longer than it did. The bath is saturating. Time a clearing test; when the time has roughly doubled against fresh, the bath is at the point past which insoluble complexes become likely.

Milkiness, a smell of burnt matches, a pale precipitate. Something acid has got in. Ventilate, stop work, and read incompatibilities. The bath is finished.

A print that has fixed and cleared but yellows over months. The classic under-fixing failure: an insoluble silver–thiosulfate complex left in the paper. It cannot be diagnosed at the time and cannot be cured afterwards, which is why two baths and a clearing test are procedure rather than perfectionism.

Crystals that will not dissolve, or a gritty bath. Wall’s cloth bag. Or a caked, damp tub whose contents are no longer the mass you weighed.

Experiments

Measure a clearing time, and then measure it again. A scrap of undeveloped film in a beaker, a stopwatch, and the moment the milkiness goes. Repeat on the same bath after every few films. The curve you plot is the only capacity figure that will ever be true for your materials, and no source publishes one for this formula.

Watch development continue. Develop two prints, rinse one in water for thirty seconds and put the other straight into plain hypo, and compare the shadows. This is the 1928 primer’s argument made visible in one tray, and it takes five minutes.

Prove the two-bath rule with a residual-silver test. Fix one print in a single bath worked hard, and one in two baths under Reilly’s four-minutes-each schedule, and compare them after a year in the same envelope. It is a slow experiment and it is the one that matters most, because the failure it is testing for is invisible on the day.

Find the strength where speed stops improving. Mix 20, 30, 40 and 50 per cent baths and clear a strip in each at 18 °C. Kodak’s primer predicts the curve flattens between 30 and 40 per cent. A result that disagrees is telling you something about your film’s halide rather than about the primer.

Weigh the temperature drop. Take the water’s temperature, dissolve the 400 g, and take it again. The fall is the enthalpy of solution doing something you can feel, and it is the reason Wall wrote “hot”.