doesn't matter the weight on curls matters about the form, if using dumbbells lock your elbows on your sides keep your back up against a wall, hold them by your side with your thumbs facing forward, slowly curl one side up slowly twisting your wrist trying to bump your fingernails into your chest, slower lower it down then do the other side, repeat.
Great info. I like to put back against the wall and do ezbar curls and go slow on the negatives. Also geting on your knees and doing curls that way minimizes cheating and makes the rep harder too. After working about about 2 years ive noticed biceps are my genetic weak point its kind of lame but whatever ,for alot of people its calves it seems.