For though thou wash thee with nitre - It should be rendered natar or natron, a substance totally different from our nitre. It comes from the root נתר nathar, to dissolve, loosen, because a solution of it in water is abstersive, taking out spots, etc., from clothes. It is still used in the east for the purpose of washing. If vinegar be poured on it, Dr. Shaw says, a strong effervescence is the immediate consequence, which illustrates Proverbs 25:20; : "The singing of songs to a heavy heart is like vinegar upon natron;" that is, there is no affinity between them; opposition and strife are occasioned by any attempt to unite them.
Thine iniquity is marked before me - No washing will take out thy spots; the marks of thy idolatry and corruption are too deeply rooted to be extracted by any human means.
Nitre - Or, natron, a mineral alkali, found in the Nile valley, where it effloresces upon the rocks and surfaces of the dykes, and in old time was carefully collected, and used to make lye for washing (see Proverbs 25:20).
Sope - A vegetable alkali, now called “potash,” because 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 then, though Israel use both mineral and vegetable alkalies, the most powerful detergents known, yet will she be unable to wash away the stains of her apostasy.
Thine iniquity is marked - i. e., as a stain.