Smith’s Bible Dictionary
Nitre: Mention of this substance is made in (Proverbs 25:20) --"and as vinegar upon nitre"--and in (Jeremiah 2:26) The article denoted is not that which we now understand by the term nitre i.e. nitrate of Potassa--"saltpetre" --but the nitrum of the Latins and the natron or native carbonate of soda of modern chemistry. Natron was and still is used by the Egyptians for washing linen. The value of soda in this respect is well known. This explains the passage in Jeremiah. Natron is found In great abundance in the well-known soda lakes of Egypt.