If you can you're better off with a headstock tuner. They're cheap, pretty easy to use, and are pretty accurate. But if you're just looking for an app, I've had good luck with the Guitar Tuna app. Drop C was the lowest I used with it before I bought a headstock tuner though. It would occasionally have a hard time picking up the low C unless I set my phone pretty close to my guitar. I believe the free one only shows E Stnd but if you pay for the full app you can set the tuning you want and it'll give you the notes you need to tune to as well as recognizing fairly well which string you're playing. It also has some chord charts that can be interesting to go through.