Hey Yummybuds.....
Co2 is great stuff, asuming you are using HID lighting.
I've not heard of placing an open soda in a grow room for added co2.
I doubt it would do much more than go flat.
Co2 is only beneficial during the lights on cycle.
Here is a simple recipe that I came across, I believe it has it's merits:
Items Required:
* 10lb. white sugar
* 5 gallon clean bucket W/lid
* 4 1/2 gallons of water
* A piece of toast browned and hard
* A table spoon of dry active yeast (for baking bread)
First boil the water, (this will ensure clean water) remove from the heat and add the sugar. Stir until sugar dissolves. Let cool until water is room temperature (if you don't let it cool down it won't work).
After the sugar water has cooled, float the piece of toast on top of the water. Now, empty the tablespoon of yeast over the toast. After a few days, the yeast will take over the toast and start making bubbles (CO2) in the bucket. After a week, the amount of bubbling (CO2) will increase.
Keep the lid airtight on the bucket. CO2 travels up the dispersion tubing, and due to it being heavier than air, falls directly onto your plants. Timing your exhaust, is essential in maintaining an effective level of CO2.
I hope this helps you.
Peace