Rehalogenating ferricyanide-bromide bleach

Two salts, one litre, and a photograph turned back into an undeveloped one. This is the bath that stands at the front of every bleach-and-redevelop process in the course, and the reason it has an entry of its own is that it is not part of any single toner. Kodak Limited prints it twice in one handbook; Kodak’s American primer uses it on a negative to intensify it; every sulfide, selenium and thiourea toner that works indirectly begins here.

Ingredient	Quantity	Form the source specifies
Potassium ferricyanide	50 g
Potassium bromide	50 g
Water	to make 1000 mL	A 5 per cent solution of each salt. Kodak Limited gives no temperature; both salts dissolve readily in cold water, and a ferricyanide solution should not be warmed.

Purpose

To convert the metallic silver of a finished image back into silver bromide, without removing any of it. Kodak’s 2006 toner sheet states the job in one line about its packaged sepia toner: the bleach bath converts metallic silver in the print to light-sensitive silver bromide. What was a photograph becomes, chemically, an unexposed and undeveloped one — in exactly the pattern the photograph had.

That is the whole of it, and everything else on this page follows from the difference between this bath and Farmer’s reducer. Both start by oxidising silver with ferricyanide. Farmer’s has hypo in it, which dissolves the oxidised silver and carries it out of the film; this one has bromide in it, which precipitates the oxidised silver where it stands. One bath takes the picture away; the other holds it in place for a second treatment. The two differ by one ingredient and they are opposite operations.

Recommended uses

Step one of a sulfide sepia toner. Kodak Limited’s T-52 and T-56 both open with this solution, and Kodak’s American T-7a opens with its close relative. The bleached, washed print then goes into a sulfide bath which converts the silver bromide directly into silver sulfide.

Step one of a redevelopment intensifier. The 1928 primer is explicit that perhaps the simplest method of intensification for negatives consists of bleaching in the ferricyanide-and-bromide formula used for the sepia toning of prints, and then blackening with sodium sulfide exactly as in print toning. The negative gains density because the redeveloped image is a different, more opaque substance.

Step one of any indirect toner. Moersch’s toning guide makes the general case from the modern bench: a thiourea toner cannot tone remaining metallic silver, only the areas where it finds a silver salt. Whatever the second bath is, the bleach decides how much of the image it can reach.

Partial bleaching, as a control. Because the redeveloper can only work where the bleach has been, stopping the bleach early leaves the shadows as metallic silver and converts only the highlights and mid-tones. Moersch records both ends of that in seconds rather than minutes with a much weaker bath; see Variants.

When another formula is preferable

• When the silver is to be removed rather than converted, Farmer’s reducer. A reducer and a rehalogenating bleach look almost the same on the shelf and do opposite things in the tray.
• When blue spots have been a problem, the T-7a bleach, which carries potassium oxalate for exactly that reason and acetic acid against blistering. Its quantities are under Variants.
• When the second bath is a thiourea or polysulfide toner sold as a system, use the bleach that maker publishes with it. Moersch’s own bleach is a quarter of this one’s strength and works in seconds; a bath designed around a 30-second bleach behaves differently in a 5 g/L bath than in a 50 g/L one.
• When the print is to be toned directly, no bleach at all: the hypo-alum route (T-1a) and the selenium and gold toners work on the silver image itself.

Mixing

One bottle, cold water, two salts, in this order.

1. Potassium ferricyanide, 50 g, dissolved in about 700 mL of cold water. It goes in first because it is the larger dissolving job and because there is no reason to warm a ferricyanide — the international chemical safety card records that it decomposes on heating and that heating produces toxic gases.
2. Potassium bromide, 50 g. It dissolves readily in the ferricyanide solution.
3. Water to make 1000 mL.

Store it in a dark or yellow bottle. The 1928 primer records that a potassium ferricyanide solution turns blue on prolonged exposure to light through the formation of Prussian blue, and blue pigment in a bleach bath is blue pigment on the next print.

🧮What five per cent of each actually buys

The bath is 50 g/L of potassium ferricyanide and 50 g/L of potassium bromide. In moles, at 329.24 g/mol and 119.00 g/mol:

50 g/L ÷ 329.24 g/mol = 0.152 mol/L

Potassium ferricyanide

50 g/L ÷ 119.00 g/mol = 0.420 mol/L

Potassium bromide

Bromide is in nearly threefold molar excess over ferricyanide, although the two weights are the same. That is not a coincidence of round numbers: the reaction consumes one bromide for every silver atom and one ferricyanide for every silver atom, so an equimolar bath would exhaust its bromide and its ferricyanide together, and a bath short of bromide would start dissolving silver rather than precipitating it. Kodak’s equal weights leave the halide in comfortable surplus, which is the safer side to err on.

Wall’s 1924 handbook makes the opposite point about the same figure and it is worth holding both: with the halide at the quantities he prints there is no loss in the intensity of the image, which will be the case if they are used stronger. Too much bromide begins to act as a solvent for silver bromide in its own right.

Behaviour

It announces its progress by colour and its exhaustion by inaction. The black of the shadows goes first to brown and then to a pale buff, and the whole sheet takes on the yellow of the bath. Kodak Limited’s instruction after bleaching is to wash until the yellow stain is removed; that stain is bath, not image.

There is no published time and that is honest rather than careless. Kodak Limited gives an end point — bleach in Solution A — and a wash, and no minutes. Kodak’s American sheets, for their own stronger and weaker relatives, give 5 to 8 minutes for the packaged sepia bleach and about one minute for the 1928 T-7a bleach. The two differ by a factor of five or more, which is the best possible argument for judging this bath by eye.

The bleached image is light-sensitive, and barely. Kodak’s 2006 sheet says both halves plainly: you may want to bleach under safelight illumination to minimise the effect of light on the image, however, the effect is extremely small, and may not be noticeable. Print in room light if you like; know that you are holding an undeveloped print.

It is not the same bath after it has worked. Every silver atom oxidised leaves a ferrocyanide behind, and ferrocyanide is the ingredient of Prussian blue. A bath that has bleached several prints carries a ferrocyanide load, and the iron that arrives from a tray or a clip has more to react with than it did at the start.

Image characteristics

Nothing is lost and nothing is gained. The distinguishing property of a rehalogenating bleach is that the silver stays put. Density is not reduced, it is made temporarily invisible; contrast is not changed; grain is not changed, because the halide is formed in place from the grain that was there.

What follows depends entirely on the second bath, and this bath’s only contribution to the final colour is how much of the image it converted. A complete bleach hands over the whole picture. A partial bleach hands over the highlights and mid-tones and keeps the shadows as metallic silver, which is the classical route to a print with warm mid-tones and neutral blacks — and, as Moersch notes, the only way to get one from a toner that cannot touch metallic silver.

One image characteristic is a defect and it is blue. See Interactions.

The mechanism

Two things happen at once and the second is what makes this a bleach rather than a reducer.

⚗Oxidise, then catch

Hexacyanoferrate(III) takes one electron from metallic silver, exactly as it does in Farmer’s reducer, and becomes hexacyanoferrate(II).

Ag + [Fe(CN)₆]³⁻ + Br⁻ → AgBr + [Fe(CN)₆]⁴⁻

The whole of the bleach, in one step

The silver ion never gets anywhere. In a bath holding 0.42 mol/L of bromide it meets a bromide ion long before it meets anything else, and silver bromide is insoluble, so it comes down where it was made — in the gelatin, on the site of the grain it came from.

Put hypo in the bath instead of bromide and the same oxidation gives a soluble thiosulfate complex that walks out of the film. That single substitution is the difference between this page and R-4a.

Why the bromide has to be in excess of the ferricyanide rather than equal to it. The oxidation and the precipitation are a race. Where bromide runs short, the silver ion has time to find a ferrocyanide ion, and what precipitates then is silver ferrocyanide — a pale salt that no sulfide bath will convert cleanly and that hypo, if it arrives later, will dissolve.

Why the resulting print is light-sensitive. Because it is silver bromide in gelatin, which is what photographic paper is. The reason the effect is nonetheless small is that a bleached print carries no developer and gets none: the printed-out silver a few seconds of room light can produce is a tiny fraction of the image, and the redeveloper is about to convert everything anyway.

Function of every ingredient

Potassium ferricyanide, 50 g to the litre. The oxidiser, and the ingredient that decides how fast the image goes. It is a one-electron oxidant: each hexacyanoferrate(III) ion takes one electron from one silver atom and becomes hexacyanoferrate(II), so the bath’s capacity for silver is set by how many moles of it are present — 0.152 mol/L here. More bleaches faster and exhausts the bromide sooner; less bleaches more slowly and more evenly, which is what Moersch’s much weaker modern bleach exploits to get a controllable 30 seconds instead of an uncontrollable 8. Note the form: there is no hydrate, so 50 g is 50 g, and the ruby crystals give a yellow solution. Its own page carries the hazard that governs this bath, and it is not the bleaching.

Potassium bromide, 50 g to the litre. The halide, and the ingredient that makes this a rehalogenating bleach rather than a reducer. It supplies the bromide ion that precipitates the oxidised silver as silver bromide in place. More than Kodak’s quantity is not a free improvement: Wall’s 1924 rule is that at the printed quantities there is no loss of image intensity, and that there will be a loss if the halide salts are used stronger, because bromide in quantity is itself a solvent for silver bromide. Less risks silver ferrocyanide instead of silver bromide, and a print that will not redevelop evenly. Wall also records that the halide need not be a bromide at all: a chloride or an iodide bath works, each at its own quantity, and the halide chosen changes the colour of the redeveloped image because it changes the grain that redevelops.

Water, to make one litre. The dilution is the speed control, and it is the only control this bath has. Kodak Limited publishes no dilution and no time; halving the strength roughly doubles the working time and makes an eight-second bleach into something a person can watch, which is precisely the design of the modern bleaches under Variants.

Interactions

With iron, and this is the classical failure. The 1928 primer names it: if a trace of iron is present in the ferricyanide-bromide bleach — from a defective enamelled tray, in its own example — blue spots composed of ferric ferrocyanide are liable to form. Iron(III) from the tray meets the ferrocyanide the bleach has just made.

4 Fe³⁺ + 3 [Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃

The blue spot, which is Prussian blue

The primer also gives the cure, and it is a formulation rather than a procedure: potassium oxalate in the bleach reduces the tendency to spot, because the blue iron salt is soluble in the oxalate. That is why T-7a carries 195 g of it. This bath does not, so this bath needs plastic or glass trays and no steel tongs.

With acid. Never. See Safety. The hazard is the ferricyanide’s, not the bromide’s, and it does not depend on the bath having worked.

With hypo, in either direction. Hypo carried into this bath on a print, or a bleach carried into a fixing bath, gives the ferricyanide something to dissolve silver with, and a rehalogenating bleach that dissolves silver is a reducer nobody asked for. Wash the print before bleaching it and after.

With the sulfide bath that follows, through the wash between them. Kodak Limited’s instruction — wash until the yellow stain is removed — is the single most important line in the sequence, and the reason is on the sodium sulfide page rather than this one: acid carried into a sulfide bath is how hydrogen sulfide gets made. This bleach contains no acid at all, which is one real advantage it has over the American T-7a bleach, but a print that has been through a stop bath or an acid fixer carries acid whatever the bleach is.

With light. A ferricyanide solution photolyses to Prussian blue, per the 1928 primer. Dark bottle.

Variants

Kodak T-7a’s bleach stock, the American formula. Potassium ferricyanide 75 g, potassium bromide 75 g, potassium oxalate 195 g and 28 per cent acetic acid 40 mL, in water at 20 °C to 2 litres, used diluted 1 + 1. The working bath is therefore 18.75 g/L of each of the two salts — about a third of this bath’s strength — with the oxalate against blue spots and, per the 1928 primer, acetic acid to prevent possible formation of blisters. Its quantities and its consequences are on the T-7a page. It is the better bleach on both counts and it puts acid next to a sulfide bath, which is the trade.

Wall’s halide series, 1924. The same ferricyanide bath with ammonium bromide, with common salt, or with potassium iodide, each at its own weight, giving a colder or warmer sepia after redevelopment. Wall’s quantities are printed in a scan whose figures this course cannot read with confidence, so they are described here and not reproduced as a formula.

Moersch’s modern bleach. 5 g/L of ferricyanide and 20 g/L of bromide — a tenth of the ferricyanide here, with the bromide ratio pushed much further in the halide’s favour. His published timings show why: 30 seconds to bleach into the deep shades, eight seconds for a light bleach. A bath that fast is a bath that can be stopped where you want it.

No course variant is offered. There is nothing here to make safer by changing the formula — the hazard is the ferricyanide meeting an acid, and no reformulation removes it — and the two published alternatives above already bracket this one on strength.

Safety

Level B, and the reason is not the bleaching.

Never bring an acid near a ferricyanide bath, its bottles or its waste. The international chemical safety card for potassium ferricyanide states that it reacts with acids and that this generates a toxic hazard. Princeton University’s guidance records that potassium ferricyanide will release hydrogen cyanide gas if heated, if hot acid is added, or if exposed to strong ultraviolet, and that cases of cyanide poisoning have occurred in darkrooms through treating a ferricyanide bath with acid. Read the potassium ferricyanide page before mixing this, and the incompatibilities table before setting out the bench.

In practice: no stop-bath tray in the same run, no acid fixer into this bath’s waste, no acid used to clean a tray that has held ferricyanide, and no warming of the solution.

Gloves, eye protection and ventilation as for any bench chemistry, and keep the dust down when weighing; Kodak’s 1928 mixing instructions single out ferricyanide among the salts whose dust should not be raised. Potassium bromide is unremarkable on its own and its own page says so with the classification behind it.

Where this bleach is the first bath of a sulfide toner, the sequence carries a hazard the bleach does not, and it is set out on the sodium sulfide page and on the toner entries themselves. The bleach’s contribution to that hazard is any acid it brings, and this formula brings none.

Storage

A dark or yellow bottle, stoppered, labelled and dated. The ferricyanide photolyses in light and the solution is worth protecting from it.

Kodak Limited publishes no keeping figure for this solution and none is invented here. What can be said from this page’s own sources is negative and useful: a ferricyanide solution that has gone green or blue has made pigment and should not go near a print, and a bath that has bleached prints carries ferrocyanide and is more vulnerable to iron than a fresh one.

Label it against Farmer’s reducer, in words rather than by colour. Two pale yellow solutions on the same shelf, one that takes silver out and one that keeps it in, are indistinguishable at arm’s length. Write the formula, the strength and the date.

Incompatibilities

Acids — stop baths, acid fixers, acetic and citric acid, and above all hot or concentrated acid. Hydrogen cyanide.

Heat. The safety card states that the solid decomposes on heating, producing toxic gases including hydrogen cyanide. This bath is never warmed.

Strong ultraviolet, including a carbon arc.

Iron and steel — trays, tongs, clips, chipped enamel. Blue spots.

Hypo, which turns a rehalogenating bleach into a cutting reducer.

Sulfides, in the sense that the two baths belong in separate bottles with separate tongs, and their wastes in separate containers. The sodium sulfide page carries that rule and the reasoning behind it.

Developer, in either direction, as for every bath in the darkroom.

Waste

Its own labelled container, and no acid stream ever goes into it. The spent bleach carries ferrocyanide, unreacted ferricyanide and bromide, and — where prints have been bleached in it — very little silver, because the point of the bath is that the silver stays in the paper.

It is not a silver-bearing stream in the way a fixer is, which is worth knowing when the containers are labelled: the silver from a bleached print is still in the print until a fixer takes it. Collect it as photographic chemistry all the same.

Never neutralise it, and never combine it with a sulfide toner’s waste. Both rules exist for the same reason as the tray rules above.

Follow the silver-bearing waste SOP and the disposal ruling, and note the standing caveat: local regulation decides, and this course cannot tell you what it says where you are.

Troubleshooting

Blue spots. Iron, from a tray, a clip or a pair of tongs. The 1928 primer names the mechanism and the formulation cure; without oxalate in the bath the answer is to remove the metal.

The print bleaches unevenly, or in blotches. Too fast to be even. Dilute the bath and agitate from the first second; a bath that finishes in under a minute is being watched rather than controlled.

The bleached image looks grey rather than buff, and redevelops thin. Bromide short, or a bath that has already bleached several prints. The silver has come down as ferrocyanide rather than bromide. Mix fresh.

Yellow stain that will not wash out of the whites. Not enough washing between bleach and toner — Kodak Limited’s instruction is to wash until the yellow stain is removed, which is a criterion rather than a time.

The shadows never bleach. They are the densest silver and they take the longest. If they stop changing, the bath is exhausted rather than slow.

The redeveloped print is weaker than the original. Something in the sequence dissolved silver rather than converting it: hypo carried in on the print, hypo in the tray, or an old bath. See Interactions.

Experiments

Prove that nothing left. Bleach a print completely, wash it, and redevelop it in a sulfide bath; read the maximum density before and after with the same instrument. A rehalogenating bleach should hand back everything it took, in a different substance. Then do the same with Farmer’s reducer in place of the bleach and watch the difference between converting and removing.

Find your own bleach time. Identical prints, this bath at full strength, at 1 + 1 and at 1 + 3, timed to the same visible end point. Plot time against dilution. You are looking for the dilution at which the end point stops arriving faster than you can act.

Bleach half way on purpose. Strips pulled at a quarter, a half, three-quarters and the full end point, all redeveloped together in one bath. This is the whole of split toning in one tray, and it makes visible the fact that the second bath can only reach where the first one went.

Test the halide. Wall says a chloride or an iodide bath gives a different sepia. Make a bromide bath and a chloride bath at the same molar halide concentration, bleach twin prints, redevelop both in one sulfide bath and compare the colour. Nothing in the corpus shows this comparison; the arithmetic to match the molarities is on this page.

Make the blue spot deliberately. A drop of an iron salt in a spent bleach, on a scrap print. Once you have seen Prussian blue arrive in the highlights you will never again put a steel clip in a toning line.