Verses with the word nitre in the Old Testament (2 verses):

Proverbs 25:20
As he that taketh away a garment in cold weather, and as vinegar upon nitre, so is he that singeth songs to an heavy heart.

Jeremiah 2:22
For though thou wash thee with nitre, and take thee much soap, yet thine iniquity is marked before me, saith the Lord GOD.