|Easton's Bible Dictionary|
(Proverbs 25:20; R.V. marg., "soda"), properly "natron," a substance so called because, rising from the bottom of the Lake Natron in Egypt, it becomes dry and hard in the sun, and is the soda which effervesces when vinegar is poured on it. It is a carbonate of soda, not saltpetre, which the word generally denotes (Jeremiah 2:22; R.V. "lye").
Noah Webster's Dictionary
1. (n.) A white crystalline semitransparent salt; potassium nitrate; saltpeter. See Saltpeter.
2. (n.) Native sodium carbonate; natron.
3. (n.) See Niter.
Int. Standard Bible Encyclopedia
ni'-ter (nether; nitron): Nitre as used in the King James Version does not correspond to the present use of that term. Nitre or niter is now applied to sodium or potassium nitrate. The writer has in his collection a specimen of sodium carbonate, called in Arabic naTrun, which was taken from the extensive deposits in Lower Egypt where it is found as a deposit underneath a layer of common salt. Similar deposits are found in Syria and Asia Minor. This is probably the "nitre" of the Bible. the American Standard Revised Version has rendered niter "lye" in Jeremiah 2:22, and "soda" in Proverbs 25:20. Soda or lye has been used as a cleansing agent from earliest times. It effervesces energetically, when treated with an acid; hence, the comparison in Proverbs 25:20 of the heavy-hearted man roiled by the sound of singing to the sizzling of soda on which vinegar has been poured.
James A. Patch
Nitre (2 Occurrences)
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. (KJV JPS DBY WBS YLT)
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. (KJV JPS DBY YLT)