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Remedy for These Problems - Bleach Accelerator, Clear Holder for Physical Developer, The Bunte Equilibrium

silver mercaptane thiosulfate complexes

To correct the silver dye-bleach processing problems discussed above, the following tailor-made components have been used.

Bleach Accelerator

Silver ligands forming strong silver complexes at low pH (0-1) are useful as bleach accelerators. Thiourea and a number of other organic sulfur compounds have been investigated. Some are active as accelerators but all have clear disadvantages: they have a bad smell, form insoluble reaction products, are corrosion stimulating, or are suspected to be carcinogenic.

Aliphatic phosphines are excellent silver ligands, but they are quite basic (pK 8-9). Aliphatic phosphines are not useful in strongly acidic solutions where they are protonated; moreover they are very sensitive to oxidation. Aromatic phosphines, or tris-cyanoethyl-phosphine, have low pK values but have low water solubility.

An ideal ligand for our purpose is an aliphatic phosphine with very low basicity. This has been invented and patented by Ilford.

3BZ (NCCH 2 CH 2 )2PCH 2 CH 2 CH 2 SO 3 K

Biscyanoethyl-sulfopropyl-phosphine pK ~ 1

The stability constants for Ag(3BZ) and Ag(3BZ) 2 are 10 9.5 and 10 16.1 .

The substitution of one cyanoethyl group by a sulfopropyl group increases the water solubility remarkably. 3BZ forms very stable silver complexes even in strongly acidic solutions and is not extremely sensitive to oxidation. Nevertheless, 3BZ protects all components of the bleach bath from oxidation including iodide. No further antioxidant has to be added to a bleach bath containing 3BZ. On oxidation, 3BZ is transformed to the corresponding phosphine-oxide.

Silver dye-bleach processing can be simplified to a two-bath sequence if iodide as silver ligand in the bleach bath is completely substituted by 3BZ. Under these conditions all silver is bleached and dissoluted (fixed out) in the bleach bath. Two-bath processes are not very stable due to the continuously changing silver potential over the exhaustion of the bleach bath. Two-bath processes are only of interest for special applications.

Clear Holder for Physical Developer

It was known that physical developers containing thiosulfate can be kept clear for a limited time by the addition of a small amount of mercaptanes. Mercaptanes block the surface of any silver nuclei and suppress the precipitation of silver sludge, but higher concentrations lead to fogging of the silver halide emulsions because silver is developed everywhere. Mercaptanes are oxidized to disulfides by the oxygen in the air. Ilford has found that solutions containing only the oxidation product disulfide keep clear. A detailed investigation of disulfide-containing developers has shown the following fact. Sulfite, which is a component of every black and white developer, and disulfide in alkaline solution in equilibrium form the corresponding mercaptane and a substituted organic thiosulfate, a so-called Bunte-salt (Hans Bunte, Karlsruhe, 1848–1925). Kolthoff did research work on equilibria of this kind long ago, but the kinetics of these reactions had not been investigated.

The clear holder used in silver dye-bleach processing for Ilford professional materials is the disulfide (MSSM) of 3-phenyl-3-mercapto-propionic acid (MSH).


The Bunte Equilibrium

MSSM + SO 3 2- ? MS – + RSSO 3 – (R = C 6 H 5 C + HCH 2 COOH) produces from disulfide and sulfite a certain quantity of mercaptane, which is effectively the clear holding substance. Under practical concentration conditions (MSSM ~ 0.003 M, Na 2 SO 3 ~ 0.2 M, pH ~ 9.6) about 20 percent of the disulfide is transformed to the mercaptane. The equilibrium is established within about an hour.

Keeping the developer clear is related to the mercaptane; the disulfide has no reactivity toward the silver surface.

Two intrinsic properties of the clear holder system are fundamental for the practical effect:

  1. The concentration of the active mercaptane is buffered.
  2. The establishment of the equilibrium is relatively slow.

The mercaptide ion (M) is an excellent silver ligand; the stability of the silver mercaptide complex is considerably higher than that of the silver thiosulfate complexes.

Stability constants for silver thiosulfate and silver mercaptide:

The Bunte Equilibrium
pK (MSH) 10.2

T = thiosulfate; M = mercaptide

The silver mercapto complexes are much more stable under the conditions of practical developer (pH 9-10) than the silver thiosulfate complexes. The mercaptane is an efficient inhibitor of the nucleation reaction of silver metal. In the absence of the clear holder, nucleation and, consequently, the formation of silver sludge in the developer starts during development. With clear holder in the developer the sludge problem is eliminated.

The Behavior of Clear Holder Under Practical Conditions

There seems to be an interesting contradiction. On one side the mercaptane ion efficiently suppresses the silver sludge formation in the developer; on the other hand, it does not inhibit the masking process. Remember that the masking process is based on diffusion transfer of silver thiosulfate complex into the neighboring layer and the deposition of silver there. Why is the growth of silver nuclei in the developer inhibited and in the masking layer absolutely possible?

When exposed, silver dye-bleach material is soaked in the developer; thiosulfate and a small concentration of mercaptane penetrates into the top layer which contains pure AgBr. Both ligands react here under quantitative formation of the corresponding silver complexes. The developer, now free of mercaptane, arrives in the masking layer. The silver thiosulfate complex forms a silver image by diffusion transfer, which is controlled by iodide ions emanated from the lower magenta and cyan layers.

The silver mercapto complex is very stable and therefore inactive. The reformation of mercaptane by the Bunte equilibrium is sufficiently slow enough to be neglected during the few minutes needed for the practical development process.

Basic Requirements for this Very Efficient, Buffered, and Robust Clear Holding System

A pure silver bromide photographic layer has to be coated on top of the masking layer that contains colloidal silver. No silver iodide is allowed in the top emulsion, because iodide inhibits physical development. The AgBr layer transforms the mercaptane in the penetrating developer quantitatively to silver mercapto complexes.

The silver mercapto complexes have to be more stable than the silver thiosulfate complexes. The equilibrium concentration of mercaptane must be small compared to the thiosulfate concentration, giving more silver thiosulfate complexes for physical development in the masking layer and only a few of the inactive silver mercapto complexes.

The establishment of the Bunte equilibrium producing mercaptane from the initially added disulfide has to be slow enough to avoid any significant reformation of mercaptane in the masking layer during development. At the same time it must be fast enough to guarantee a continuously sufficient concentration of mercaptane in the bulk of the developer to keep the solution sludge free.

The disulfide of 3-phenyl-3-mercapto-propionic acid is the best clear holder known today. It is also used in X-ray developers containing high concentrations of sulfite, which can dissolve a considerable amount of silver. This kind of developer is also susceptible to silver sludge formation.

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