858
precipitates yellow insoluble iron(II) oxalate, evolving
carbon dioxide gas:
UV light + Fe 2 (C 2 O 4 ) 3 → 2FeC 2 O 4 + 2CO 2
iron(III) oxalate → iron(II) oxalate + carbon dioxide
This equation is the sum of two half-reactions, involv-
ing the transfer of electrons from the oxalate anion to
reduce the iron(III) cation:
C 2 O 4 2– → 2CO 2 + 2e–
Fe3+ + e– → Fe2+
Iron(II) oxalate is too feebly coloured to constitute a sat-
isfactory image, so a second reaction must be employed
to make a permanent print, either by reducing a noble
metal salt to the metal, such as silver, gold, palladium
or platinum, or by forming a pigment such as Prussian
blue, or ferrogallic ink.
In 1842, Sir John Herschel was the first to use
iron(III) salts photographically, as the commercially-
available ammonium iron(III) citrate or tartrate; the
photochemistry is more complex than the oxalate, but
the same principle of reduction of iron(III) to iron (II)
applies. As iron(III) carboxylates are sensitive only to
the ultraviolet and blue-green portions of the spectrum,
they had to be exposed to daylight or sunlight in the
nineteenth century.
Uranium
Light sensitivity in uranium salts was noted by Adolph
Gehlen in 1804, and fi rst used for photographic pro-
cesses by Charles Burnett in 1855. Salts of uranium(VI),
such as uranyl nitrate UO 2 (NO 3 ) 2 , (once called “uranic”
salts) can be photochemically reduced on paper to
uranium(IV) (once called “uranous”):
UV light + UO 2 2+ + 2e– + 4H+ → U4+ + 2H 2 O
Uranyl cation + electrons + hydrogen ions → uranium(IV)
cation + water
This lower oxidation state of uranium can then reduce
a noble metal salt to form the metal image of silver:
U4+ + 2Ag+ + 2H 2 O → UO 2 2+ + 2Ag + 4H+
palladium:
U4+ + PdCl 4 2– + 2H 2 O → UO 2 2+ + Pd + 4HCl
or gold:
3U4+ + 2AuCl 4 – + 6H 2 O → 3UO 2 2+ + 2Au + 4H+ +
8HCl
Alternatively, the uranium(IV) cation can be reacted
with potassium ferricyanide to form the stable red
pigment, uranyl ferrocyanide (UO 2 ) 2 [Fe(CN) 6 ], in the
Uranotype process, analogous to the Cyanotype. Owing
to its toxic and radiological hazards, uranium is no lon-
ger employed in photography, but it did enjoy a passing
signifi cance in the nineteenth century.
Chromium
Dichromates were discovered by Vauquelin in 1797
and used for tanning leather, before Mungo Ponton dis-
covered in 1839 that papers coated with them changed
colour on exposure to light, so launching this method of
photographic imaging. The yellow-orange dichromate
can oxidise many organic substances; the chromium(VI)
is itself reduced to the state of blue-green chromium(III)
ultimately (passing through brown intermediates of
uncertain identity, possibly chromium(IV) dioxide,
CrO 2 ). Acidic conditions are needed for the reduction
half-reaction:
Cr 2 O 7 2– + 14H+ + 6e– → 2Cr(H 2 O) 6 3+ + H 2 O
Dichromate + hydrogen ions + electrons →
hexaquachromium(III) cations + water
Light stimulates dichromate to oxidise organic matter
in a number of ways, for example, oxidising a primary
alcohol group (present in cellulose) to an aldehyde:
RCH 2 OH → RCHO + 2e– + 2H+
Alcohol → aldehyde + electrons + hydrogen ions
So a typical overall reaction would be:
UV light + Cr 2 O 7 2– + 3RCH 2 OH + 8H+ → 3RCHO +
2Cr(H 2 O) 6 3+ + H 2 O
Dichromate + alcohol + hydrogen ions → aldehyde +
hexaquachromium(III) + water
The product hexaquachromium(III) cation,
Cr(H 2 O) 6 3+, is capable of hardening (i.e. rendering insol-
uble) many macromolecular colloids that are normally
soluble in water: either proteins such as gelatin, casein,
and animal or fi sh glues, or carbohydrates such as starch
or plant gums. This hardening is believed to result from
the chromium(III) complex forming cross-links between
the long organic chains to create a net-like molecular
structure which is no longer soluble: carbohydrates bind
to chromium(III) via their hydroxyl (–OH) groups, and
proteins via their amino (–NH–) groups. The hardened
colloid then acts as a vehicle to bind a pigment image
in the Carbon and Gum bichromate processes, or as an
etching resist in the Photomechanical processes.Organic Substances
The “Heliographic” process of Nicéphore Niépce entails
the light-induced hardening of bitumen, which becomes
insoluble in lavender oil and petroleum. Bitumen has
a complex and varied structure of polycyclic aromatic
hydrocarbons (linked benzene rings), containing a
small proportion of nitrogen and sulphur; its hardening
is undoubtedly due to further cross-linking, as is the
hardening of tree resins (colophony, or abietic acid) by
light, fi rst observed by Jean Sénébier in 1782. The pho-
tochemistry of these processes, which have been studied
by Marignier in the 1990s, remains rather obscure.