Mephibosheth1 is correct.
While dry ice is ok in a pinch or last resort, it's not as humane as using a Co2 tank that has a valve to regulate flow. As Mephibosheth said, there is no way to regulate the flow of Co2 using dry ice. If the sublimation is too quick, it causes distress and panic to the animals and they will suffocate before losing consciousness first.
If you do dry ice, make sure the container is large. Add very little amounts of water to keep the Co2 rate slow. That way it's slow enough for the feeders to loose consciousness first before suffocating. Once they're knocked out, you can go ahead and pour more water into the container for a rapid release of co2 to euthanize them.
Two humane methods of euthanasia is:
1) Using essentially the same method as the dry ice method. But instead of Dry ice, it's a Co2 canister(such as the kind used for paint balling) with a release valve. Do a slow gradual release to knock out the rats first, then a rapid release of gas to euthanize them. They tend to panic more if they're awake all the way to the point of death. Plus it's a painful process. Co2 gas stings the nose and eyes, so it's better if they are unconscious for the most part.
or
2) Cervical Dislocation: Breaking the neck.
Put your rodent onto a flat surface, such as a table or counter. You can use something like screwdriver or stick to put behind it's neck. Push the rodent into the table with the screwdriver behind it's neck with one hand and hold onto it's tail with the other hand. Pull on the base of the tail until you feel/hear the vertebrae pop several times and the rat is dead. It takes like 2 seconds.
And then there's always the good ol' fashion snake method. ;p