From Richard Knoppow over in USENET:(posted 8508 days ago)D-76 mixed according to the original formula will increase in activity over a period of weeks. This is full strength developer. The rise in activity is due to a complex interaction between the sulfite and hydroquinone. Mike Gudzinowicz once posted a quite complete explanation of the chemical process. It may be available in the data base currently on Deja.com. In any case its worth a name search there.
Packaged D-76 is a modified formula using a buffer. It does not suffer from the rise in pH and activity of the original formula. Diluted D-76 has a short life because there is not enough sulfite to prevent fairly rapid oxidation. At 1:1 or greater its strictly a one-shot developer. For those who mix their own the difference in the formulas is that the original D-76 contains 2.0 grams/liter of Borax. In the buffered variety this is replaced by 8.0 grams of borax/liter plus 8.0 grams/liter of boric acid. The initial activity of the two is the same but the buffered version maintains that activity for very long periods. The original version starts to increase activity immediately.
The original research paper describing the effect and cure shows that after several weeks the time required for a give contrast is nearly halved with the unbuffered form but remains the same for the buffered type. Mixed D-76 stock from the packaged powder should have a life of a few months in a sealed bottle. Kodak gives two months but that is quite pessimistic. D-76 should be nearly clear to very slightly straw colored. A darker brown indicates excessive oxidation and discolored solutions should be discarded.