[pure-silver] Re: Hypo Clearing: citric acid or sodium citrate?
----- Original Message ----- From: "Jordan Wosnick" <jwosnick@xxxxxxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, April 27, 2007 4:19 PM
Subject: [pure-silver] Re: Hypo Clearing: citric acid or sodium citrate? 




John,
More than a slight effect on pH. Solutions of citric acid are acidic, whereas solutions of sodium citrate are alkaline (salt of a weak acid). My guess is that if citric acid is added to hypo clear, it is done for pH control, in which case sodium citrate is not an acceptable substitute.
Bisulfite and sulfite form a buffer, but if the citric acid concentration is high, the buffer system can easily be overwhelmed. (Buffers have a fixed capacity.) 

Jordan
The MSDS for Kodak Hypo Clearing Agent specifically lists Citric Acid as an ingredient. The patent lists only the EDTA Tetrasodium salt but mentions Citric Acid among acceptable sequestering agents along with several others. The target pH is 7.0. The formula in the patent has this pH but, if the small amount of Citric Acid added to the commercial product is enough to affect the pH the proportions of Sodium Sulfite and Sodium Bisulfite are probably adjusted to compensate. Citric acid is a sequestering agent with a somewhat different effectiveness than EDTA Na4. I expect Kodak found that the stuff worked better with both. Citric Acid seems to be a sequestering agent for Aluminum, for instance, Citric acid can not be used to acidify fixing baths using White Alum hardener and should not be used as a stop bath with them. So, the purpose may be to hold carried over alum hardener which otherwise will cause deposits on the film or paper. The EDTA Na4 may also have this effect but is certainly present to sequester Calcium and Magnesium salts in the water, again to prevent deposits.
The patent lists EDTA Na4 as being present in the working solution in the quantity of 0.5 grams/liter. This is equivalent to 2.5grams/liter in the normal concentrate. The MSDS shows quantity only as a _range_. Both EDTA Na4 and Citric Acid are listed as being present in the quantity of 1 to 5 percent in the concentrate. So, its possible both are present in the same amount but that is a guess and may be wrong. Unless one is prepared to make a chemical analysis probably the pH is the key to gettting the correct amounts. The pH of KHCA is not very critical as long as it is neutral or slightly alkaline. If no hardener has been used the pH is even less critical. At pH 7.0 the hardening action of White Alum is not destroyed, as it is by a higher pH but its ability to mordant Thiosulfate and reaction complexes to the emulsion is broken. If one does not need to preserve hardening the pH can be higher although it should not be so high as cause excessive swelling. According to Ryuji Suzuki the measurement of the pH of photographic solutions is not trivial. I believe he has an article about this on his web site. 

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


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