Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Reexamination Certificate
1998-11-02
2001-10-23
Tung, T. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C205S789000, C205S789500
Reexamination Certificate
active
06306283
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for screening gold chemical sensitizers for their use in silver halide photographic elements. The method utilizes the electrochemical reduction potentials of the sensitizers as the screening criteria.
BACKGROUND OF THE INVENTION
It is well known in photographic science that the sensitometric performance of silver halide microcrystals can be altered by so-called “chemical sensitizers.” Operationally, the function of this special class of photographic addenda is to decrease the number of photons required to create a development center (i.e., increase photographic speed), independent of wavelength.
For most applications, chemical sensitizers are compounds which incorporate sulfur and/or gold into silver halides during emulsion making and/or finishing. Until relatively recently gold sensitization was frequently combined with sulfur sensitization through the use of formulations often based on aurous dithiosulfate [e.g., Na
3
Au(S
2
O
3
)
2
2 H
2
O]. However, during the past decade the discovery and synthesis of various “gold-only” Au(I) sensitizers, such as described in U.S. Pat. Nos. 5,049,485; 5,700,631; and 5,620,841, has led to increased flexibility of chemical sensitization and increased control of components introduced into photographic makes.
Promising gold-only sensitizers are Au(I) complexes with relatively labile ligands, but without active sensitizing sulfur. Historically, the syntheses of suitable gold-only sensitizers has been difficult for a variety of reasons. Frequently, the materials compounded are either too insoluble to make an aqueous “doctor” solution or their aqueous solutions are too unstable and degrade rapidly upon keeping. Other materials, for example, some Au(I) complexes, i.e., KAu(CN)
2
and some Au(I) phosphino complexes, have good solution stabilities, but are so stable that emulsions prepared with them do not exhibit speed enhancement.
Because of the complexity of photographic chemistry in general and the minute quantities of silver and/or gold necessary to create a development center, little is known of the incorporated gold species which is formed during chemical sensitization. One of the explanations proposed for photographic enhancement by gold sensitization is based on the observation that silver is plated onto a gold electrode surface at a less negative potential than is silver deposited onto a silver electrode. In the electrochemical literature this phenomenon is known as “underpotential deposition” (or UPD) and has been observed for a variety of electrode metals and metal-ion combinations (Bard and Faulkner, “Electrochemical Methods,” John Wiley and Sons, New York (1980), p. 308).
In the chemical sensitization mechanism, UPD of silver onto Au(0) which has been deposited onto the surface of the silver halide microcrystal would theoretically accelerate development and/or render otherwise subdevelopable latent images developable. Either route would produce an increase in photographic speed (Hillson and Adam, “On Latent Images of Gold and Silver, ”
J. Photogr. Sci,
23, 104 (1975)).
In the late 1970's, the chemical sensitization of photographic AgBr-polyvinyl alcohol (PVA) emulsions with Au(I) complexes of sulfur or phosphorous-containing ligands was investigated (Suss, Schroter, Reinhold, Zwanziger, and Hoyer, “Gold Sensitization of Silver Bromide Layers in Polyvinyl Alcohol on Glass,”
J. Signal AM
5 (1977) and Hartung, Schroter, Reinhold, Zwanziger, Dietzsch, and Hoyer, “Chemical Sensitization of Photographic Silver Bromide-Polyvinyl Alcohol Emulsions with Gold(I) Complexes of Sulfur or Phosphorus-Containing Ligands,”
J. Signal AM
58 (1980)). In the earlier of these two papers, literature values of standard reduction potentials were shown to correlate, within certain limits, with a given complex's ability to chemically sensitize AgBr in a PVA matrix. In the second paper, polarographic half-wave potentials obtained in acetonitrile were used to experimentally verify the relationship between the ease of electrochemical reduction of a complex and its tendency to chemically sensitize.
While concentrating on the mechanistic relationship between electrochemical reduction potentials and gold (I) chemical sensitization, neither of these aforementioned studies investigated water-soluble gold (I) complexes nor related the stability of the complex in aqueous solution to photographic usefulness. The inventors herein have discovered a method which permits the establishment of a “utility window” of electrochemical potentials which permits the complex to be screened for practical manufacturing suitability.
Such a method for easily and accurately screening gold chemical sensitizers is needed to avoid time consuming and costly experimentation in evaluation such compounds.
SUMMARY OF THE INVENTION
This invention provides a method of screening a water soluble gold complex for use as a sensitizer in a silver halide photographic element comprising experimentally measuring the electrochemical potential of the gold complex and then determining if the electrochemical potential falls within a predetermined utility window.
The screening method of this invention is a fast and accurate method to determine whether specific water soluble gold complexes will be effective as gold sensitizers for silver halide photographic elements. This invention further provides a proven predetermined utility window which may be utilized during the assessment of the gold complexes.
DETAILED DESCRIPTION OF THE INVENTION
In this invention a utility window has been defined in which water soluble gold complexes are effective as chemical sensitizers for silver halide emulsions. Specifically, the electrochemical potential at which Au(0) is formed from solutions of various Au(I) complexes was determined and correlated with each complex's solution stability and ability to sensitize AgX microcrystals within a gelatin matrix (emulsion) in commercial photographic formulations. Ultimately, a “utility window” of potentials was defined which can be used to screen potential Au(I) sensitizers. The effective utility window is about −380 mV to −850 mV, with −407±34 mV to −833±13 mV being preferred. (Potentials are reported versus a standard calomel (Hg/Hg
2
Cl
2
), or SCE, reference electrode.)
In order to practice the invention, the electrochemical potential of a water-soluble gold complex is experimentally determined. The electrochemical potential can then be compared to the predetermined utility window to determine whether the gold complex will be useful as a chemical sensitizer.
Several standard electrochemical methodologies may be used to determine characteristic redox potentials for the reduction of the Au(I) complexes. These include voltammetry at stationary and moving electrodes, potentiometry, chronoamperometry, as well as other less common eletrochemical methodologies. In general, the electrochemical apparatus includes an electrochemical cell composed of a working electrode, a reference electrode, and, in cases where current is passed through the cell, an auxiliary electrode. The instrumentation used is a standard potentiostat, such as the EG&G Princeton Applied Research Model 173/179.
Silver halides are electronic insulators, therefore, it is not possible to perform electrochemistry directly on bulk-like silver halide surfaces. Preferably the electrochemical potential is measured in an aqueous system using a platinum, carbon, mercury, gold or other electrochemically inert, but conductive electrode. To avoid possible complications associated with the plate out of dissimilar metals it is most preferable that the electrochemical potential of gold (I) chemical sensitizers be measured in an aqueous system with a gold electrode.
It is well known that the mechanism of deposition of metals onto an electrode is highly sensitive to both solution and surface conditions. While a variety of electrolytes may be used for the reduction of the gold complexes, for example unbuffered KCI, KCI
Brandt E. Steven
Cleary Brian P.
Lok Roger
White Weimar W.
Eastman Kodak Company
Meeks Roberts Sarah
Tung T.
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