Prussian blue

Every cyanotype is a picture made of this pigment, and it was already a hundred and thirty years old when Herschel made the first one. It is the oldest modern synthetic pigment, the reason blueprints were blue, and one of the few image substances in photography whose colour can be explained down to the electron.

In photography

It is the image, and nothing else in the print is. A cyanotype has no silver and no dye. Light reduces an iron(III) salt of an organic acid to iron(II); the iron(II) meets hexacyanoferrate; the pigment precipitates in the paper fibres and everything unused washes away. Both routes are on the potassium ferricyanide and potassium ferrocyanide pages. What belongs here is the substance they leave.

⚗Why it is blue

The glossary defines intervalence charge transfer and this is the compound it was defined for. Ware gives the spectroscopy. Absorption is a strong, broad band centred near 700 nm, in the red, and what is left of white light is blue. The transition is the movement of an electron from the low-spin iron(II) centre to the high-spin iron(III) centre — between two atoms of the same element in different oxidation states, bridged by cyanide — and it is allowed, because the electron goes from a t₂g orbital on one centre to a t₂g orbital on the other with no change in net spin.

A second, weaker band sits at 400 nm; it is the t₂g-to-e_g version of the same transfer, which local orthogonality forbids. The gap between the two, 10,500 cm⁻¹, is the ligand-field splitting for an FeN₆ centre, which is the evidence that the assignment is right. The maximum differs between the varieties — 690 nm for the “soluble” form, 730 nm for the “insoluble” — and Ware gives 1.55 × 10⁴ dm³ mol⁻¹ cm⁻¹ as a working mean for the extinction coefficient. An intense colour from very little substance is exactly what a printing process needs.

The composition is a family, not a formula. Ware’s Table II.1 lists soluble Prussian blue KFe[Fe(CN)₆], insoluble Prussian blue Fe₄[Fe(CN)₆]₃, Turnbull’s blue Fe₃[Fe(CN)₆]₂·X, Prussian white K₂Fe[Fe(CN)₆], Berlin green and Prussian yellow. They share one face-centred cubic lattice and differ in how many hexacyanoferrate sites are vacant and what has moved into the holes — cations, anions, water. “Soluble” is a misnomer, for the reason the solubility entry above gives.

⁇Turnbull's blue is Prussian blue, and the name is kept anyway

Two routes reach a blue precipitate. Iron(III) with hexacyanoferrate(II) gives what has always been called Prussian blue; iron(II) with hexacyanoferrate(III) gives what has always been called Turnbull’s blue. The classical assumption was that these were different compounds — iron(III) hexacyanoferrate(II) and iron(II) hexacyanoferrate(III).

They are not. Ware states the modern position without hedging: the precipitate from iron(II) and hexacyanoferrate(III) is definitely not iron(II) hexacyanoferrate(III); an irreversible electron transfer between the two iron centres immediately gives iron(III) hexacyanoferrate(II) — essentially Prussian blue. He keeps the historical name only to record which way the preparation went.

Fe²⁺ + [Fe(CN)₆]³⁻ → Fe[Fe(CN)₆]⁻

The cyanotype route, before the internal electron transfer that makes the product identical to Prussian blue

“Essentially” is doing some work, and Ware says so. Reguera’s Mössbauer spectroscopy finds the two iron centres of Turnbull’s blue in a 3:2 ratio, against 1:1 for the soluble form and 4:3 for the insoluble one, which needs an extra anion in the lattice for charge neutrality — hence Fe₃[Fe(CN)₆]₂·X, a defect structure with more vacancies still. Its X-ray powder pattern matches the insoluble form’s closely. The same compound in every way a photographer can detect, then, and not quite the same compound to a spectroscopist.

Three reactions undo it. Reduction gives Prussian white, colourless because the mixed valence is gone and there is no intervalence transition left to make; that is reversible, by air over hours or by Ware’s half-minute bath of 0.3 per cent w/v hydrogen peroxide at once. Alkali is not reversible — it hydrolyses the lattice and the blue is gone. Concentrated oxalate is a third and distinct route: Ware points out that it is neither hydrolysis nor reduction but the small oxalate ion penetrating the lattice and complexing the iron(III) out of it.

Fe[Fe(CN)₆]⁻ + 3 C₂O₄²⁻ → [Fe(C₂O₄)₃]³⁻ + [Fe(CN)₆]⁴⁻

Ware's equation for oxalate destruction: both products are soluble

Properties

Fe₄[Fe(CN)₆]₃ for the insoluble form, relative molecular mass 859.2, CAS 14038-43-8. Six cyanide ligands surround each iron(II) and every cyanide points its nitrogen end at an iron(III); the colour, the insolubility and the fragility to alkali all follow from that one arrangement.

The solubility entry above is a genuine disagreement between good sources and is left standing: HSDB’s 6 mg/mL against Ware’s insistence that every form is highly insoluble and that what looks like a solution is a peptized colloid. A printer meets the argument as a practical fact — blue in the wash water, and blue that keeps coming.

Handling

The cyanide question, answered carefully. Prussian blue carries eighteen cyanide groups per formula unit and is not a cyanide poison. The Photographers’ Formulary kit sheet gives the reason for the related salts: the cyanide groups are chemically bound to the iron atom and not free to act as a poison. The Environment Agency’s WM3 says it in regulatory language, assigning the hazard statement for liberating a very toxic gas on contact with acid to cyanide salts with the explicit exception of complex cyanides such as ferrocyanides and ferricyanides. Ware supplies the most striking evidence: Prussian blue is used as an ion exchanger and a detoxifying agent, taken internally, because its lattice traps other cations while giving up nothing itself.

None of that makes a hexacyanoferrate a substance to be casual with, and this course does not say so anywhere. Strong acid can release hydrogen cyanide from these compounds, which is why the reagent pages classify potassium ferricyanide at Level B and keep acid, heat and stop bath away from it. The distinction the course holds to is between free cyanide, among the most acutely toxic substances in the historical darkroom, and complexed cyanide, which is what both the pigment and its parent salts are.

⚠Why Level A, and the one operation that is not

Against the rubric, the pigment as this course meets it is Level A. It is met as a product: precipitated in paper, insoluble, with a classification that 86 per cent of notifiers decline to make at all and that the remaining 10 per cent put no higher than harmful by skin contact or inhalation. Gloves, eye protection and an open window are the controls, and they are the controls the sensitiser needs anyway.

The one operation that takes it out of Level A is bringing strong acid to it. Not the dilute citric, nitric or sulfamic acids of cyanotype clearing, which are the process as published, but a concentrated acid in a bottle of scrap or a bath left to evaporate. The rule is the same one the potassium ferricyanide page sets, and the course takes the stricter reading because the cost of being wrong is asymmetric.

Waste

A used cyanotype bath is iron, hexacyanoferrate and a fine dispersion of the pigment. It is not classified as a cyanide waste in Great Britain, for the reason given above, which is a statement about classification and not a licence to pour: bottle it, label it, keep acid away from it, and take it where your authority directs. Check your local regulations.

History

Ware’s account of the discovery is worth having because so much of it is accident. Prussian blue has never been found in nature and was unknown before the eighteenth century. It was first made, probably in 1706, by Johann Jacob Diesbach, a Swiss colour-maker in Berlin who was trying to make a crimson lake from cochineal, alum and iron(II) sulfate and, lacking potash, bought some from an alchemist working in the same building. That potash was contaminated with a distillate of animal residues, which supplied the one element the reaction needed and nobody had yet identified: nitrogen. Manufacture began in 1708 and Diesbach kept the method secret until somebody else published it in 1724.

The order of events is unusual for this course. Normally the image substance is discovered with the process; here the pigment was a hundred and thirty years old and on every painter’s shelf when Herschel found a way to make light put it where he wanted it.