What happens as developer nears exhaustion?

I'm cheap and don't like to pollute more than I have to, so I use
developer until I've processed what the maker says it can. I have 2
liters of D-76 in which I've developed 30 4x5 sheets so far. Kodak
sites 32 as the maximum (4 8x10 sheets per liter). I have 2 sheets
left, which is not worth my time since the tank holds 10. I also have 2
rolls of T-max 120. In what way does image quality degrade? Is there
some way to pre-treat developer before dumping it? Neutralize ph, put
it in the sun?

When increasing dev time to compensate for exhaustion, is the 15%
increase compounded? I would think so, eg 10 min, 11.5 min, 13.2, 15.2
(compounded) as opposed to 10, 11.5, 13, 14.5 (additive). It's not a
huge difference, and well below my error in dunk/pour time, but I'm
still curious. Why does Kodak say not to reuse diluted developer, even
in the same sitting?

What happens is not so much a using up of the developing agents but
a build up of the reaction products of development. These have the
effect of lowering the film speed. While extending the development can
partially compensate there will be a progressive loss of shadow detail.

If you want to extend the developer you can use the replenisher.
The replenisher for D-76 is a special formula, I think it is still put
up in packages. This replenisher is used by replacing some of the
developer based on the amount developed. For D-76R it is one ounce per
80 square inches, the area of one 120 roll, or a 36 exposure roll of
35mm film. There is some loss of shadow detail at first but the
replenished system will reach equilibrium after a time. Since there is
a build up of bromide replenishment by this method can not be carried
out forever, but it will very considerably extend the capacity of the
developer while maintaining reasonably constant quality. Kodak used to
publish full instructions for replenishment. Some newer developers can
be used as both developer and replenisher, for instance Xtol and T-Max
RS.
Generally, for small volume processing its more economical and
yields better uniformity to use D-76 diluted 1:1 and discard it after
one use.

--
Richard Knoppow
Los Angeles, CA, USA
dickburk DeleteThis @ix.netcom.com