Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03C—PHOTOSENSITIVE MATERIALS FOR PHOTOGRAPHIC PURPOSES; PHOTOGRAPHIC PROCESSES, e.g. CINE, X-RAY, COLOUR, STEREO-PHOTOGRAPHIC PROCESSES; AUXILIARY PROCESSES IN PHOTOGRAPHY
  • G03C5/00—Photographic processes or agents therefor; Regeneration of such processing agents
  • G03C5/26—Processes using silver-salt-containing photosensitive materials or agents therefor
  • G03C5/40—Chemically transforming developed images
  • G03C5/44—Bleaching; Bleach-fixing

Definitions

• the present invention relates to single powder photographic bleach mixtures containing an alkali metal ferricyanide as the oxidizing agent, and more particularly to the stabilization of such bleach powders.
• aqueous solution an alkali metal fern'cyanide such as lithium, potassium or sodium terricyanide as the oxidizing agent which converts the metallic silver formed during development into a silver salt which is soluble in silver halide solvents such as hypo.
• a bleach solution contains, in addition to the ferricyanide, a halogenizing or converting agent preferably an alkali metal chloride or an alkali metal bromide which converts the silver ferricyanide initially formed in the bleaching reaction into the less soluble silver chloride or silver bromide and thus impels the bleach reaction to completion.
• the ferricyanide bleach solutions contain a bufiering agent which maintains the desired pH even though small amounts of acids or alkali may be carried into the bleach by the film.
• the pH of the bleach solutions falls between a range of from 3 to 9, a pH between 4' and 6 being generally preferred.
• alkali metal phosphates such as disodium and monosodium phosphate, sodium citrate, sodium bisulfate, citric acid or phthalic acid.
• Some of the ingredients of the butter mixture act also as corrosion inhibitors and retard the corrosive action of the ferricyanide bleach on metallic developing tanks, trays and the like.
• the alkali metal ferricyanide bleaches are usually packaged and sold as dry powder mixtures. Normally, the stability of dry ferricyanide bleach mixtures leaves much to'be desired. However, their stability is further drastically curtailed by the presence of other ingredients which are added to ensure thatthe bleach possesses certain desirable photographic characteristics.
• a metal ferrocyanide is included which gives a tempering action to a freshly mixed bleach solution in order to simulate the performance of a partially used bleach.
• the rate of bleaching depends on the 7 oxidation reduction potential which in turnis a function of pH and the relative"ferrocyanide-fenicyanide concentration. No such oxidation reduction potential can be established so long as ferricyanide only is in the fresh solution, and no ferrocyanide has been formed as a result of the bleaching action;
• the desired tempering action is achieved by adding to a fresh bleach solution specified amounts of ferrocyanide thus establishing an oxidation reduction potential which otherwise would be set up only after part of the ferricyanide had been reduced to ferrocyanide.
• ferricyanide has an even greater tendency to cause caking and discoloration. It is believed that this discoloration is caused by the de- Patented July 5, 1960 2 composition of ferricyanide yielding ferrocyanide and iron salts leading in all likelihood to the formation of Prussian Blue or Turnbulls Blue.
• a gas is formed which creates a pressure'build-up in hermetically sealed packages. This gas has been identified as hydrogen cyanide which is highly toxic. 'Thus, the decomposition of ferricyanide bleach powders creates a serious health hazard.
• ferricyanide bleaching powder ingredients which afiect adversely the stability of the ferricyanide bleaching powder are alkali dichromates which are added to bleach solutions to inhibit the corrosion of processing equipment. Ferricyanide bleaches containing dichromates also cake and discolor with the formation of hydrogen cyanide.
• boric anhydride is an etiective stabilizer for photographic bleaches particularly those which contain, in addition to the alkali metal ferricyanide, a small amount of alkali ferrocyanide or alkali dichromate.
• Photogaphic bleach mixtures containing alkali metal ferricyanide as the oxidizing agent and stabilized with boric anhydride constitute the purposes and objects of the present invention.
• the photographic bleach powders which are contentplated herein contain as their essential ingredients an alkali metal ferricyanide and boric anhydride, together with other bleach adjuvants including buir'er mixtures, alkali metal halides, particularly alkali metal bromides andalkali metal ferrocyanides or alkali metal dichromates; Photographic bleach powders which we have found stable during storage for considerable periods of time may be compounded in the following proportions, it being understood that all parts are by weight and'that the resulting dry powder is to be used to make a liter of bleach solution:
• boric anhydride In compounding the above bleach powder, one may proceed by mixing the boric anhydride to the alkali metal ferricyanide and adding the other ingredients later; However, if preferred, all the ingredients may be admixed simultaneously.
• One of the most important advantages of boric anhydride lies in the fact that its stabilizing action is not impaired by changes in alkalinity and acidlty.
• the stabilizing action is not based on a dehydrating action and is not influenced by the selection of bleach ingredients with ditferent amounts of water of crystallization. While it is desirable to use a grade of boric anhydride which is completely free of water, it must be kept in mind that small amounts of moisture are absorbed by the anhydride during storage and before the mixing of the bleach. However, these small amounts of water absorbed which usually do not exceed 2 percent, but which may go up as high as 5 percent based on the weight of boric anhydride, do not interfere with the stabilizing action on the bleach powder.
• Example IV This powder mixture did not deteriorate after storage for "6 months at an average room temperature of 75 F.
• V l h is powder mixture 'was kept for 6 months at room temperaturewithout deterioration. "A mixture without boric anhydride caked badly' after a few days and discolored to a deep blue 'color with the-formation of hydrogen eyaziide gas.
• Example V1 r w Grams Potasium ferricyanide 75.0 Potassium ferrocyamde 5 .0 Sodium bromide. '2;. 1020 weightofan"alkalizmetal rlichroniate, theflpartsfbeiag from 2 to 12 parts by weight of alkali metal ferrocyanide,-
• a stable single powder photographic bleach ture containing as its essential ingredient-stolthe preparation of onelliter of bleach solution: from 30 to 120 parts by weight of alkalimetal ferricyanitle; from 2 to 20. parts 'by weight of alkali metal bromide; from 5 to 30 parts by weight of a butter-mixture comprising a salt selected from the group-consisting :of alkali metal phosphates and alkali :metal 'citr'ates, and an acid :selected from the. group 'consistingzof citric and phthalic acid; g
• a stable'sin'gleipowder photographic bleach mixture comprising an alkali metal fenicyanidq an alkali m'etal ferrocyanide, ania'lkalianetal bromide and 'a-stabiliiing' amount of boric 'anhydride inaS quantity sufficient to prevent 'cakingfand disodlorationbf said bleach mixture;
• a stable photographic bleach mixture having the following composition:
• a stable photographic bleach mixture having the following composition:
• a stable photographic bleach mixture having the following composition:

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• 0
  • BE BE567236D patent/BE567236A/xx unknown
• 1957
  • 1957-05-15 US US659213A patent/US2943935A/en not_active Expired - Lifetime
• 1958
  • 1958-05-06 DE DEG24469A patent/DE1062114B/de active Pending
  • 1958-05-07 GB GB14652/58A patent/GB834972A/en not_active Expired

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