Jamieson, Fausset and Brown on Isaiah 6:7

7. mouth ... lips--(See on Is 6:5). The mouth was touched because it was the part to be used by the prophet when inaugurated. So "tongues of fire" rested on the disciples (Ac 2:3, 4) when they were being set apart to speak in various languages of Jesus.

iniquity--conscious unworthiness of acting as God's messenger.

purged--literally, "covered," that is, expiated, not by any physical effect of fire to cleanse from sin, but in relation to the altar sacrifices, of which Messiah, who here commissions Isaiah, was in His death to be the antitype: it is implied hereby that it is only by sacrifice sin can be pardoned.