The pH in hatching eggs seems to be an important factor for embryo development, although not everything of it is understood yet.

When the egg is laid, the pH of both the albumen and the yolk is approximately 6.5-6.7, almost neutral and identical to the normal pH in the blood of the bird. Also the carbon dioxide level is identical to that of the blood of the bird. Once the egg is laid, the carbon dioxide is released from the albumen, and due to that, the pH of the albumen rises in a few days to approximately 9.3-9.5. This pH is too high for bacteria to penetrate, and it seems to function as an antibacterial defence system. The yolk remains at a pH of 6.5 which means that the difference between yolk and albumen is 3 pH units.

3 pH units represent a 1000 fold difference in concentration of H+ ions, as the pH is a log value. As the embryo is located between albumen and yolk and at the moment of laying only one or a few cells layers thick, it experiences an enormous difference in pH. No other organism is known to be able to handle such pH differences.

Once the embryo starts developing, it produces CO2 and as a result the pH of the albumen decreases. This is probably necessary for the embryo, as the early embryo has an internal pH of approximately 8.5. The difference in pH between the yolk and the embryo plays a role in the functioning of the ion pumps, that transport ions like K+ and Na+.

It can be speculated that a slow starting incubation process, combined with a poor albumen quality does not allow the pH around the embryo to increase fast enough to give it the correct pH condition, as the CO2 will be produced very slowly and will disappear rapidly through the albumen. This might explain the positive effect of fast heating of the eggs at the start of incubation, but also the negative effect of poor albumen quality on hatchability. It can also explain the positive effect of starting the incbuation process in a CO2-enriched environment, as it will help the embryo to create the optimum pH level.

The development of the pH could also be the explanation why eggs should not be set directly after lay, as the pH by that time has not reached the optimum level of 8.5 yet.