Can I use citric acid instead of lemon juice when canning? I am making wine jelly and have run out of lemons. I have some citric acid in the pantry. Can I use the citric acid instead? The jelly calls for half a cup of lemon juice. If I can use it, how much citric acid would I use?
Yes, you can.  In fact, many canning and jarring recipes specifically call for citric acid.

Presumably you are using citric acid in its dried, crystalline form.  In that case, a solution of around 4% citric acid (e.g. 4gm in 100ml of water) should be around the same strength as lemon juice.