Ferricyanides bleaching

Fixer that has been partially exhausted will cause bleached prints to stain more easily. At the same time, ferricyanide bleach rapidly destroys fixer. Use fresh fixer and discard after the bleaching session after neutralizing it by dumping the remaining bleach solution into it. Plain Hypo is useful for this purpose as it uses only one ingredient and must be discarded after use anyway (Formulas Fixers Plain Hypo). 

Ana.lytica.1 Methods. Various analytical methods involve titration with oxidants, eg, hexacyanoferrate (ferricyanide), which oxidize dithionites to sulfite. lodimetric titration to sulfate in the presence of formaldehyde enables dithionite to be distinguished from sulfite because aldehyde adducts of sulfite are not oxidized by iodine. Reductive bleaching of dyes can be used to determine dithionite, the extent of reduction being deterrnined photometrically. Methods for determining mixtures of dithionite, sulfite, and thiosulfates have been reviewed (365). Analysis of dithionite particularly for thiosulfate, a frequent and undesirable impurity, can be done easily by Hquid chromatography (366). 

The most common bleaching agents are copper sulfate, potassium permanganate, potassium ferricyanide, and potassium or ammonium dichromate. Most B R toners require a darker print than usual as there is often a loss of density. However, this is not always the case as copper bleaches will intensify the print. 

There are several ways in which to apply the bleach. One is to dip a piece of cotton into the bleach and squeeze until it is almost dry. Beginning at the top, so the bleach will run down over the area to be reduced, lightly wipe the cotton over the area for 2 or 3 seconds. Do not use friction, which could abrade the wet print. Instead, let the chemical action do the work. If you have the correct dilution of bleach it should take several applications to achieve the results you want. However, if the print is exceptionally dense in the area being worked on you can strengthen the working solution by adding more ferricyanide. 

Fixer acts as a neutralizer, halting the bleaching action. How quickly it stops the bleach from working depends on how much ferricyanide is in the solution. However, the fixer also acts as a catalyst, even while it is neutralizing the bleach. This means that if the print looks perfect on the board it will probably be too light after it is placed in the fixer. This is another reason it is important to work up slowly to the lightening you want. 

Pour a small pile (approximately the size of a dime) of potassium ferricyanide crystals into a small white plastic container. Add about 90.0ml of water, then a splash of plain hypo. The splash will vary based on desired contrast of the mixture. The hypo acts as a catalyst, but ultimately neutralizes the bleach. A solution strong in ferricyanide and low in hypo is fairly contrasty. One lower in ferricyanide and higher in hypo is lower in contrast and slower acting. With experience, you will be able to judge bleach strength by its color saturation. 

NOTE 1 This solution should be stored in the dark as ferricyanide solutions are light sensitive. Should the solution turn blue, the bleach should be discarded. 

Local bleaching, use the same proportions of A and B as for overall reduction. Increasing the amount of ferricyanide solution will increase the rate of reduction. 

Alternately, dissolve approximately 14 teaspoon of ferricyanide in 200.0ml of water. If you are working on lighter tones you will want to start with less ferricyanide in solution as the bleach works rapidly on light areas you can always add more ferricyanide to speed things up. 

When potassium ferricyanide is warmed with a solution of bleaching powder to 70° C. a considerable evolution of gas takes place, and a reddish deposit of ferric oxide and calcium carbonate is formed. The filtered solution is concentrated and the potassium nitroprusside extracted with alcohol, and converted into the insoluble copper salt by addition of cupric chloride. This latter is decomposed with sodium hydroxide, yielding the sodium salt, which may be further purified by dissolving in a little water, addition of alcohol, and subsequent evaporation after filtering off any insoluble material.2 The constitution to be assigned to sodium nitroprusside in particular, and hence to nitro-prussides in general, has been a subject of debate, Browning s3 formula is —... 

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