Chemistry of the Iron-based Processes

An Outline for Non-Chemists

When iron (symbol Fe, from the Latin "Ferrum") is chemically combined with other elements, it usually ends up in one or the other of two possible states:

either "ferrous" iron, also called iron(II) [pronounced "iron-two"] or Fe2+ for short

or "ferric" iron, also called iron(III) [pronounced "iron-three"] or Fe3+.

These numbers 2+ and 3+ are the oxidation states of the iron, signifying the positive electric charge that the - originally neutral - iron atom has acquired from the reaction. Oxidation is making an atom or molecule more positive (or less negative) in the electrical sense. So when iron forms compounds, it is oxidized.

Reduction is the converse: making an atom or molecule less positive (or more negative). e.g. oxygen gas (O2) is reduced to form oxides, which contain O2-. Both processes involve the transfer of electrons, which are the fundamental particles of negative electricity and make up the outer sphere of all atoms. Chemistry is governed by the redistribution of electrons between atoms.

Oxidation is the removal of electrons, reduction is their addition. Iron is easily oxidized (for example, rusting to form iron(III) oxide, Fe2O3) and consequently it is said to be electropositive. In contrast, metals such as platinum, palladium and gold are very resistant to oxidation, don't rust or even tarnish, and are described as noble.

All the iron imaging systems have the same basis -

The key compound is ferric oxalate, formula Fe2(C2O4)3, which contains Fe3+ and oxalate (C2O4)2- chemically bound together. Under the influence of ultra-violet light, these two undergo a reduction-oxidation reaction, which can be summarised in words as:

"Under UV light, iron(III) reacts with oxalate to give iron(II) and carbon dioxide gas" so, we can now "read" this chemical reaction when it's written symbolically:

UV + 2Fe3+ + (C2O4)2- → 2Fe2+ + 2CO2↑

[N.B. The large + signs simply mean "...added to..." or "...mixed with.." - they have nothing to do with electric charges - and the horizontal arrow → means "...react chemically to give..."; the upwards arrow ↑ means "evolved as a gas".] Notice how two electrons (two negative charges) have been transferred from one oxalate to two iron(III), to keep the number of electrons balanced in this chemical equation, because opposite charges 'neutralise' or cancel one another. The chemical reaction has been 'dissected' in this way, so you can see the oxidation-reduction process in principle. The actual equation for the photochemical reaction in the solid is usually written:

UV + Fe2(C2O4)3 → 2FeC2O4 + 2CO2↑

This light-induced change occurs in dry solid ferric oxalate, to give ferrous oxalate; but there is only a slight color change (from pale yellow-green to pale yellow-brown), and the result is not permanent, because the oxygen of the air will re-oxidise the iron(II) back to iron(III). To make a satisfactory photographic image, the iron(II) oxalate which is formed by the UV light must be reacted promptly with something else. Now, iron(II) is a reducing agent, because it readily gives up an electron and reverts to iron(III); so it can be used to reduce the compounds of a noble metal to the metallic state, as we'll see next.

The best example - platinum printing -

The traditional platinotype sensitizer consists of a mixture of aqueous [i.e. water] solutions of ferric oxalate and potassium chloroplatinite, which is now more accurately called potassium tetrachloroplatinate(II), formula K2PtCl4 , and which contains platinum in the oxidation state +2. The iron(II) formed as described above by the exposure to UV light is capable of reducing the platinum(II) to its solid metallic state, which is oxidation state zero (the 0 is not usually written):

2Fe2+ + Pt2+ → 2Fe3+ + Pt↓

The downwards arrow ↓ means "precipitated as a solid". Note that it takes two iron(II) to make one atom of platinum, because two electrons are needed to balance the charges. These ratios are important in formulating the sensitizer solution correctly. This is again a 'dissected' version of the chemistry; the actual molecules involved are complex, as we'll see in a moment.

However, this second reaction does not take place in the dry sensitizer, because the ions (charged atoms or molecules) are immobile and cannot encounter one-another. Only when dissolved in water are they free to roam: they can then collide and react to form tiny particles of platinum metal which appear black and are trapped in the paper fibres to constitute the final image. Now the iron(II) oxalate produced by UV light is insoluble in plain water; so to dissolve it, the traditional "developer" uses a solution of the salt, potassium oxalate, K2C2O4 which supplies oxalate ions - the potassium ions, K+, don't come into it, so we can omit them in writing the balanced equation for the "development" reaction like this:

2FeC2O4 + 4(C2O4)2- + [PtCl4]2- → 2[Fe(C2O4)3]3- + Pt↓ + 4Cl-

This is the traditional two stage process of platinum printing, as described in the C19th literature. Finally, all the excess unreacted sensitizer chemicals and soluble reaction products are removed from the paper by baths of dilute (2%) hydrochloric acid, thus 'fixing' or, more accurately, 'clearing' the platinum image.

The closely-related element, palladium, Pd, reacts in an exactly similar way.

A modernized version of platinum-palladium printing -

An alternative "printing-out" process, which we have researched, can offer some advantages. With this method, the sensitizer is now composed of solutions of ammonium iron(III) oxalate, formula (NH4)3[Fe(C2O4)3], and ammonium tetrachloroplatinate(II), formula (NH4)2[PtCl4], or the corresponding palladium compound. The sensitized paper is not fully dried but allowed to acquire a controlled degree of humidity prior to exposure. At normal relative humidity - around 70% - paper contains about 8% by weight of water. Under these conditions the platinum or palladium image is formed during the exposure, and requires little or no development afterwards. The reaction goes like this:

The light-sensitive substance is now the ferrioxalate ion:

UV + 2[Fe(C2O4)3]3- → 2[Fe(C2O4)2]2- + (C2O4)2- + 2CO2↑

But the iron(II) reaction product on the right hand side of this equation is soluble in water (unlike the traditional case), so if the paper is slightly humid it can react immediately with the platinum(II) salt:

2[Fe(C2O4)2]2- + [PtCl4]2- → 2[Fe(C2O4)2]- + Pt↓ + 4Cl-

The overall concerted reaction is the sum of these:

UV + 2[Fe(C2O4)3]3- + [PtCl4]2- → 2[Fe(C2O4)2]- + (C2O4)2- + 2CO2↑ + Pt↓ + 4Cl-

The traditional clearing agent of dilute hydrochloric acid tends to dissolve palladium and weaken the cellulose structure of the paper. A better modern reagent is disodium Edta, short for ethylenediaminetetraacetate, which is effective in binding iron(III) strongly under mildly acid conditions, and removing it from the paper. Finally a water wash completes the processing, leaving an archivally permanent print.

There are several other iron-based processes -

As alternatives to platinum and palladium, as described above, other noble metals have been used historically, e.g. gold (Chrysotype and Aurotype) or silver (Kallitype and Argentotype) or even mercury (Amphitype and Celaenotype), with appropriate changes in the chemistry, although the principles are the same.

Inexpensive methods are provided instead by reacting the iron(II) with ferricyanide (Cyanotype or Blueprint process) or gallic acid (Ferrogallate 'Ink' process). In these methods, use is made of substances that form highly-coloured insoluble pigments with the iron. Ferric oxalate and ammonium ferric oxalate are not the only light-sensitive iron(III) salts that can be used in the sensitizer: other salts of organic acids, such as the citrate or tartrate, are also employed, (e.g. in the Van Dyke, Brownprint and Argyrotype processes); the chemistry is similar in principle, but rather more complicated with the citrate ion, having the formula C6H5O73-.