Dentin disks prepared from human extracted teeth were grouped as follows to prepare different situations of dentin hypersensitivity : ( 1 ) those sonicated for 20 minutes ; ( 2 ) those treated with 0.5 mol/L EDTA for 2 minutes ; ( 3 ) those treated with 6 % citric acid for 2 minutes ; ( 4 ) those treated with 50 % citric acid for 2 minutes .
