Please read the patent carefully. It explains why a
sequestering agent is included. Also, it it appears to be the
origin of the use of a selenium toner mixed in for image
stabilization. I don't know of Kodak was making KRST at the time
this patent was applied for, the toner used is mixed according to
the formula included in the patent. It also acts as an
anti-pluming agent. Pluming is the discoloration produced when
some papers are dried at high heat. I have forgotten the details
but believe that its caused by the production of extremely fine
silver by the heat. I used a dryer at one time and remember
seeing this very ugly discoloration.
Note that the patent mentions that a neutral pH is especially
useful. This is discussed in the technical paper describing the
wash aid. By making the gelatin nearly neutral the mordanting
action of the alum hardener is eliminated so it does not bind the
thiosulfate ions. Also, because photographic gelatin has an
isolectric point slightly on the acid side of neutral making it
alkaline breaks the electrical bonding. When neutral the
hardening action of the alum is retained. Very clever.
On 3/1/2021 9:14 PM, `Richard Knoppow wrote:
FWIW, this is the patent for Kodak Hypo Clearing Agent. The PDF is not too large so I thought I would post it instead of just the patent number.