Nitre does not mean the substance which now bears that name, but "natron," a mineral alkali, deposited on the shores and on the bed of certain lakes in Egypt, especially those in the Wady Nat-run (the ancient Nitria, whence came so large a store of precious Syriac manuscripts).
In ancient times, this natron was collected to make lye from for washing purposes (comp. Proverbs 25:20). Sope; rather, potash; the corresponding vegetable alkali (comp. Isaiah 1:25). Thine iniquity is marked.
So Kimchi and Gesenius (through a doubtful etymology); but the Aramaic use of the word favors the rendering stained, i.e. filthy. The word is in the participle, to indicate the permanence of the state (comp.
"Will all great Neptune's ocean wash this blood," etc.? 'Macbeth').