I have a two gallon batch of cider going that I would like to have a medium carbonation. I was thinking of using apple juice to prime, would that work and how much should I add?
You can use juice, or juice concentrate for that matter. You can generally assume that each gravity point you add will produce 0.5 volumes of CO2, and fermentation typically leaves about 0.8-1.0 volumes of residual carbonation. So if you add enough juice to increase your gravity by 3 points, you should end up with about 2.3-2.5 volumes of CO2 in the bottle.
So if your juice has a gravity of 1.040, then you want to add 3/40 (0.075) gallons (9.6 ounces) of juice for every gallon of finished cider.
It’s 1.050, so around 7.5 ounces per gallon?
Yep, that sounds right to me.