Albert Barnes Commentary Jeremiah 2:22

"For though thou wash thee with lye, and take thee much soap, yet thine iniquity is marked before me, saith the Lord Jehovah." — Jeremiah 2:22 (ASV)

Nitre – Or, natron, a mineral alkali, found in the Nile valley, where it effloresces upon the rocks and surfaces of the dykes, and in ancient times was carefully collected and used to make lye for washing .

Soap – A vegetable alkali, now called “potash,” because it is obtained from the ashes of plants. Its combination with oils, etc., to form soap was not known to the Hebrews until long after Jeremiah’s time, but they used the lye, formed by passing water through the ashes. Thus, though Israel uses both mineral and vegetable alkalies, the most powerful detergents known, yet she will be unable to wash away the stains of her apostasy.

Thine iniquity is marked – i.e., as a stain.