[pure-silver] Re: Alkali Fix - How to Mix
----- Original Message ----- From: "BOB KISS" <bobkiss@xxxxxxxxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, August 27, 2007 8:06 AM
Subject: [pure-silver] Re: Alkali Fix - How to Mix



DEAR RICHARD,
Do you know if I might be able to make alkali fixer (ammonium thiosulfate) by mixing Ammonium Chloride or just Ammonia and Sodium Thiosulfate? If so, in what quantities do I mix to create either a stock 
solution or a working solution?
CHEERS!
BOB
First of all Ammonium thiosulfate fixer is not necessarily alkaline. Ammonium thiosulfate is used to make so called _rapid_ fixer. For some reason the Ammonium salt fixes faster than the sodium salt. In addition, it is less sensitive to Iodide ions leached out of fast film emulsions, which tend to retard the fixing rate of Sodium fixer. There are some older formulas for making up rapid fixer using Sodium Thiosulfate and an Ammonium salt. They were suggested because Ammonium thiosulfate, in powder form, is not too stable. The action of fixer using either salt is not dependant on its pH, as is, for instance, the action of a developer. Most fixing baths are acid for two reasons: one is that most contain some hardener, usually White Alum (Potassium Aluminum Sulfate). These hardeners require the bath to be acid within some range in order to work. The other reason is to prevent carried over developer from continuing to develop. Such continued development can cause dichroic fog because it can reduce some of the silver complexes dissolved in the fixing bath into metallic silver which is deposited on the film or paper. Of course, continued development in the fixing bath can also make control of development more difficult. Because an acid stop bath can not be used with a neutral or alkaline fixing bath it is more difficult to prevent carried over developer from being active. A water stop can be used but must be quite thorough, really a short but vigorous rinse. There are evidently alkaline agents which will stop development but I've only seen them mentioned by Ryuji Suzuki, but not specified. I have never seen a formula for one or seen the theory of operation described. Neutral fixing baths are common in some types of color processing because acid can convert the dyes into a colorless form. Neutral baths are also recommended by some for preserving the stain of imagewise staining or tanning developers, typically those using Pyro or one of its relatives. I have never experimented with these so I don't know how much reduction of stain is caused by an acid processing chain. Note that the imagewise stain is not a dye but rather a pigment related to Humic acid. Tanning is cased by other development reaction products produced by the same developers which produce the pigment. It may well be that normal acid stop and fixing bath will not damage the stain image. It is important to realize that some of the "tricks" for increasing the stain actually only produce an overall stain, which is not desirable since it is the equivalent of fog. I suggest developing a negative in the staining developer, cutting it in half and treating half in a normal acid stop and fixer and the other in a water stop and neutral or alkaline fixer. Note that for color processing the fixing bath does not have to be alkaline, only neutral. I see no advantage at all in making the bath alkaline. As far as washing goes, the use of a sulfite wash aid after normal acid processing results in a wash rate faster than that gotten by using either a plain alkali bath or an alkaline fixer. I've posted a long treatice about this to this list recently (unless it went to one of the Rolleiflex lists). In short, acid stop and fixer has become the standard method of processing for B&W for many decades simply because it works better than the non-acid system. Also note, Ammonium thiosulfate fixer, when acid, is a mild bleach for metallic silver. This is more important for prints than for film. It is the reason that a paper strength (more diluted) version of rapid fixers is given. The solution is simply not to over fix. When neutral the fixer does not have the bleaching property so when fixing time can not be closely regulated there is some justification for the use of neutral fixer. Also note that the addition of Citric acid to a rapid fixer makes a quite effective silver bleach, in fact Kodak recommends it for removing dichroic fog. Because of this the use of a Citric acid stop bath, like Ilford stop, should be avoided when an Ammonium thiosulfate fixer is used. 


Kodak F-7 Rapid Fixing Bath
Water (at 125F or 52C)                    600.0 ml
Sodium Thiosulfate (crystalline)           360.0 grams
Ammonium Chloride                          50.0 grams
Sodium Sulfite (desiccated)                15.0 grams
Acetic acid, 28%                           47.0 ml
Boric Acid (crystalline)                    7.5 ml
Potassium Alum                             15.0 grams
Cold water to make                          1.0 liter
Dissolve the hypo first then add the Ammonium chloride. Do not add the chloride to the completed fixer or a sludge may form.
This may be made into a neutral bath by leaving out the Acetic acid, Boric acid, and Alum. Where no acid is added the amount of Sulfite can be reduced to about 5.0 grams per liter but more sulfite will do no harm and will tend to eliminate overall staining due to carried over developer.
Kodak had an alternative formula where this one caused too much corrosion to stainless steel processing tanks. I can't find it right now but will look.
Note that any standard Sodium Thiosufate fixer can be converted to a rapid fixer by substuting Ammonium thiosulfate for the Sodium Thiosulfate in the amount of about half where the Sodium thiosulfate amount is given for the crystalline form as is usual in older formulas. However, again, powdered Ammonium thiosulfate is not particularly stable in storage. 

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx

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