Jps pt/Pd paper

5 DISCUSSION

In this section it is suggested that the image quality depends quite critically upon the aquation of the platinum metal complex in the sensitizer. Several aspects of the process are discussed on this basis, including the effect of various metal salt additives that have been recommended in the past.

5.1 Differences between the Platinum and Palladium Sensitizers.

Previous accounts of these processes have implied that there is little difference between them, apart from observing that palladium yields a 'warmer' colour. However, the present work brings to light some clear differences in the behaviour of the two metals, that demand explanation. When all the parameters of the process are kept the same, it is found that the palladium sensitizer is:

(i) 'faster' than platinum by a factor of about 2.5, on most papers

(ii) less contrasty, with a logH value of ca. 2, compared with 1.5 for platinum,

(iii) capable of greater colour variation, from brown to black, with different R.H.,

(iv) capable of a finer image having higher resolution, whereas platinum images tend to take on the fibrous appearance of the paper base.

Unlike conventional silver halide photography, the iron-based processes do not involve latent image formation and development. These four characteristics -higher speed, lower contrast, browner colour and higher resolution- suggest a smaller particle size in the sensitized layer. This implies that the palladium sensitizer is more finely dispersed within the paper structure than is its platinum counterpart. In seeking an explanation for this it should be remembered that the molecular structure of cellulose has an extensive capability for hydrogen-bonding, both internally and with adsorbed species. The complex anion [Fe(C₂O₄)₃]^3- is likely to be strongly adsorbed (it is known to be extensively hydrogen-bonded in the hydrated crystalline state), but the precious-metal anions [PtCl₄]^2- and [PdCl₄]^2- will have little propensity for binding to the cellulose hydroxyl functions or to the interstitial water molecules. However, these anions are not the only species present in the sensitizer, because both tetrachloro-anions are known to undergo an aquation reaction to the aquotrichloro-anion, but to differing extents. The reaction equilibrium:

[MCl₄]^2- + H₂O = [MCl₃(H₂O)]^- + Cl^- reaction <3>

has its equilibrium constant, K_aq, defined by

K_aq = [MCl₃(H₂O)^-][Cl^-]/[MCl₄^2-]

and K_aq has been found to take the value 0.17 for palladium²³, but only 0.015 for platinum²⁴ , at 20 °C. These data apply at high values of the ionic strength typical of a sensitizer solution. It follows from a simple mass-action calculation that, at the concentrations used in the sensitizers, at least 50% of the total palladium is present as [PdCl₃(H₂O)]^-, but less than 20% of the platinum as [PtCl₃(H₂O)]^-, the rest of the metal being mostly in the form of [MCl₄]^2-. The tendency for the palladium to be more finely dispersed within the cellulose is therefore understandable on the grounds that it is present in a form likely to hydrogen-bond strongly with this substrate.

On this hypothesis, several other experimental observations become comprehensible:

(i) it is important to allow freshly-made solutions of [PtCl₄]^2- to stand for a least a day before use;

(ii) the addition of chloride ions (as ammonium chloride or hydrochloric acid) to the sensitizer tends to suppress the 'printing-out' process because it diminishes the concentration of [MCl₃(H₂O)]^-;

(iii) the residual yellow 'stain' of unexposed sensitizer is harder to wash out of palladium rather than platinum papers, because the former is more extensively bound to the cellulose;

(iv) the addition of salts of mercury(II) or lead(II) to the platinum sensitizer can improve the image quality, making it more like that of palladium.

The observations (i) and (iv) are discussed more fully in the following sections.

5.2 Maturation of the Platinum Solution

The kinetics of the aquation reaction <3>, have been investigated by Martin et al., who found that the half-time for the approach towards equilibrium is ca. 2.4 hours at 25 °C. It is customary to allow ten half-times to elapse for the effective completion of a reaction. Bearing in mind also that the rate will be slower at the more usual ambient temperature of 20 °C, it seems appropriate that at least twenty-four hours should be allowed for completion of the aquation of freshly-dissolved [PtCl₄]^2-. This accords with the observation that the image quality suffers unless this 'maturation time' is allowed before first use. A similar maturation time does not appear to be necessary for the palladium solution owing to the much higher aquation rate of the more labile [PdCl₄]^2- complex.

5.3 Use of Additives containing Mercury(II) or Lead(II)

The addition of mercury(II) salts, such as the nitrate or citrate, to the platinum sensitizer has long been recommended. It results in an image more resembling that of palladium, i.e. having a lower contrast, warmer colour and smoother texture. This observation is partly explicable on the grounds that mercury(II) has a strong affinity for chloride ions by forming the undissociated species HgCl⁺ and HgCl₂²⁵ for which the equilibrium constants are:

Hg²⁺ + Cl^- = HgCl⁺ log K₁ = 6.74

HgCl⁺ + Cl^- = HgCl₂ log K₂ = 6.48

The presence of mercury(II) will therefore promote the aquation reaction of the tetrachloroplatinate(II) thus:

Hg²⁺ + [PtCl₄]^2- + H₂O  HgCl⁺ + [PtCl₃(H₂O)]^-

and the higher concentration of the aquotrichloroplatinate(II) ion will lead to better dispersion of the platinum within the cellulose fibres. D/logH curves for a platinum/mercury sensitizer in which the molar ratios were Pt:Hg = 10:1 are shown in Fig.5, and their parameters are summarised in Table 4.

The effect of mercury(II) is not confined to scavenging chloride ions, however. Because of its redox potential:

E^o(Hg²⁺/Hg) = 0.854 V

mercury(II) is itself susceptible to reduction by the iron(II) photoproduct to give elemental metallic mercury which will co-precipitate with the platinum, possibly forming a finely-divided amalgam.

It is of some interest to determine the amount of mercury in finished images obtained by this means. Accordingly, a platinum sensitizer solution was prepared containing mercury(II) nitrate at a final concentration of 0.34 mol/dm³ (molar ratio Pt:Hg = 1:1), coated and exposed for a range of times in the usual way. The amounts of mercury and platinum in the processed metallic images were measured by X-ray fluorescence spectrometry, and compared with the amounts measured in the unexposed sensitized paper. Fig. 7 shows the variation with exposure time of the fractions of total metals deposited in the paper. It can be seen that mercury is precipitated more readily than platinum, consequently the proportion of mercury in the final image is higher than in the sensitizing solution. The archival properties of platinum images containing mercury are not known with certainty, but it is thought that many of the sepia platinotypes made at the turn of the century, and now surviving apparently undegraded, may contain mercury.

In contrast to the behaviour of platinum, we have found little change and no benefit in adding mercury(II) salts to the palladium sensitizer.

The aquation reaction <3> can also explain Willis's original preference for adding lead(II) salts to his sensitizers. In this case, the low solubility product of lead(II) chloride will 'scavenge' chloride ions:

K_sp = [Pb²⁺][Cl^-]² = 1.6 x 10^-5 at 25 °C

but the possibility of forming a precipitate of PbCl₂ within the sensitizer seems rather undesirable. Some contemporary recipes recommend the use of lead oxalate, but it is hard to see how this can have any useful effect because it is such an insoluble material:

K_sp = [Pb²⁺][C₂O₄^2-] = 2.74 x 10^-11 at 18 °C

5.4 Use of Gold(III) Salts as Additives

It has been recommended that salts of gold(III) such as the chloride (actually tetrachloroauric acid, H[AuCl₄].3H₂O) may be added to a platinum sensitizer to 'tone' the image and improve its quality. In view of the redox potentials:

E^o([AuCl₄]^-/Au,4Cl^-) = +1.00 V

E^o([PtCl₆]^2-/[PtCl₄]^2-,2Cl^-) = +0.68 V

it is evident that tetrachloroauric acid should oxidise the platinum(II) in the sensitizer to platinum(IV) and be itself reduced to metallic gold:

2[AuCl₄]^- + 3[PtCl₄]^2-  3[PtCl₆]^2- + 2Cl^- + 2Au

An investigation²⁶ of the reaction between platinum(II) and gold(III) has shown this reaction to be rapid at low concentrations, with the formation of an intermediate gold(I) complex, presumably [AuCl₂]^-; but the latter is not stable at higher concentrations²⁷ and will disproportionate to gold(III) and gold metal. Tests on the platinum sensitizer solution, to which tetrachloroauric acid or ammonium tetrachloroaurate was added, showed the precipitation of metallic gold to be quite rapid; i.e. any more than a trace of gold(III) in a platinum sensitizer will be decomposed before it can even be coated or exposed, and will simply impart a fog of colloidal gold to the paper, which is often coloured lilac or purple. There seems little to be gained from its use.

The same is not true of palladium, however. Thanks to its higher redox potential

([PdCl₆]^2-/[PdCl₄]^2-,2Cl^-) = +1.288 V

a mixed gold(III)/palladium(II) solution is stable with respect to oxidation-reduction, and may be used as a sensitizer. Even so, precautions must be taken with this sensitizer, because gold(III) will quite rapidly oxidise any free oxalate ions arising from the partial dissociation of trisoxalatoferrate(III):

2[AuCl₄]^- + 3C₂O₄^2-  2Au + 8Cl^- + 6CO₂

Provided that the ambient temperature is not too high and that the coating and drying operations are carried out rapidly, it is possible to make mixed images in gold-palladium. These can display a wide range of colours, depending on the exact chemistry; this printing system is currently under investigation and further details will be published in due course.

5.5 Mixed Platinum-Palladium Prints

The platinum and palladium sensitizer solutions described in §3.2 may be mixed in any proportion, provided that their total volume approximately equals that of the iron(III) solution. The resulting image will consist of a mixture of the two metals but not in the same proportion as the sensitizing solution, because palladium 'prints out' about 2.5 times faster than platinum (in the middle tones). The consequence of this was checked analytically by X-ray fluorescence spectrometry of coated papers both before and after exposure and processing. A mixed sensitizer solution, having a molar ratio of Pt:Pd = 1:1 was used. Before exposure, the X-ray fluorescence analysis of the coating agreed with the proportion in the bulk sensitizer; after exposure and processing the ratio of each metal to the total was found to vary with the exposure time in the manner shown in Fig. 8, where the proportion of palladium in the image is always seen to be higher than platinum. It may be concluded from these results that, if an image containing approximately equal amounts of platinum and palladium is required, then the paper should be coated with a sensitizing solution in which the molar ratio of Pt:Pd = 2:1 or more. It should also be remembered that palladium is not as resistant to chemical attack as platinum, and may therefore be less permanent archivally.

5.6 Control of Contrast

The benefit of using a mixed platinum-palladium sensitizer is that it provides a means of controlling contrast and colour, within certain limits. The traditional method of contrast control for platinum printing (which is ineffective with palladium) is to include an oxidising agent such as potassium chlorate in the sensitizer. This has the effect of truncating the high print values and so gives the impression of a shorter printing range, although the change in tonal gradation is not uniform across the scale. Users of this method agree that it has another disadvantage: the presence of potassium chlorate tends to cause granularity in the image, which degrades the print quality. It seems a better philosophy to omit the potassium chlorate entirely and ensure that one's negatives have their density range correctly matched to the intended printing process in the first place. Any 'fine-tuning' of the contrast that may then be necessary can be provided by the humidity control, or by the use of an aqueous prewash before EDTA treatment, or by mixing the two metals.

6 CONCLUSIONS

There is no novelty in claiming that high quality archival images may be contact-printed in platinum and palladium - after all, the process has been used successfully for over a hundred years. Although it may no longer be viable on a commercial scale, the process should not be dismissed as "obsolete" since it can offer image characteristics that are unobtainable with conventional silver-gelatine papers, and it need cost no more than a similar-sized colour print. In order to optimise the reproducibility of the results and minimise the labour of hand-coating the paper, it is hoped that the chemical principles outlined above may prove helpful. The print colour may be controlled over a range of brown and black tones by simple means, and the printer can exercise a wide choice of colour and texture in the paper base. The low sensitivity of the material naturally imposes the limitation of printing by contact only, but the wet-processing procedure is simple and uncritical, and darkroom facilities are not required at any stage. While the procedures described here have been developed with the aim of simplifying the variables in this process, their number remains so large that it is hoped further improvements may yet be found by others. Ars longa, vita brevis.

7 ACKNOWLEDGEMENTS

I wish to express my gratitude to Messrs. Kodak Ltd., for the award of a Photographic Bursary in 1984 to support this work.

I am indebted to Pradip Malde for many stimulating ideas and for the pleasure of seeing in his work a most expressive use of the platinum-palladium printing medium.

My thanks also go to Jane Routh and John Malcolm for their enthusiasm, encouragement and perceptive observations.

Dr. John Roberts of the Department of Paper Science at the University of Manchester Institute of Science and Technology kindly supplied advice on paper structure, and Dr. John Esson of the Department of Geology at the University of Manchester generously provided the expertise and instrumentation for the analyses by X-ray fluorescence spectrometry.

8 APPENDIX

The Preparation of Ammonium trisoxalatoferrate(III)

34 g of iron(III) nitrate nonahydrate, Fe(NO₃)₃.9H₂O, is gently heated to 50 °C in a water bath, until the pale purplish-brown crystals have dissolved entirely in their own water of crystallization to give a deep red-brown solution. To this is added 35 g of finely powdered ammonium oxalate, (COONH₄)₂.H₂O, with stirring at 50 °C until all is dissolved to yield a clear green syrupy solution.

This solution can be used directly as a sensitizer without further purification: it must be diluted by adding ca. 14 cm³ of water, which should then give a total of 60 cm³ of a 1.4 molar solution of ammonium trisoxalatoferrate(III) (this solution contains excess ammonium nitrate: its effect is to make the sensitized paper more hygroscopic, and the resulting image therefore more neutral in tone.)

If it is required to isolate the pure solid (NH₄)₃[Fe(C₂O₄)₃].3H₂O, the undiluted green syrup should be set aside in the dark to cool and crystallise. The fine grass-green crystals of ammonium trisoxalatoferrate(III) trihydrate are filtered off and washed with methanol, in which ammonium nitrate is quite soluble. The product may be recrystallised from a water-ethanol mixture, (recrystallization from water-methanol tends to give a product with methanol incorporated in the lattice). The solid should be dried and stored in the dark, (drying over anhydrous calcium chloride or silica gel will cause efflorescence and the loss of water of crystallization). The salt is very soluble, a saturated solution at 20 °C having a concentration of ca. 1.4 mol/dm³.

The Preparation of Ammonium Tetrachloropalladate(II) Solution.

3 g of ammonium chloride, NH₄Cl, is dissolved in 35 cm³ of hot (70-80 °C) distilled water. 5 g of finely powdered palladium(II) chloride, PdCl₂, is added with stirring until dissolved (about one hour) and the solution made up to a final volume of 40 cm³, which has a concentration of 0.70 mol/dm³ of tetrachloropalladate(II).

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