The psalmist prays for forgiveness: Purge me with hyssop. The verb is the intensive form of the verb “to sin,” and means (as Anderson picturesquely puts it) “to de-sin” or “un-sin.” Hyssop was a small bush plant used for sprinkling water or blood on things and people in ceremonies of atonement and purification (see Exo 12.22; Lev 14.1-7; Num 19.1-6, 16-19). Good News Translation has chosen to abandon the literal language and give the meaning, since it is not ritual purification the psalmist prays for but inner forgiveness. So Bible en français courant “Cause my fault to disappear, and I will be pure.”
The verb wash is the one used in verse 2a.
In the parallel lines of verse 7, line b‘s metaphor whiter than snow represents going beyond line a‘s I shall be clean. The intensification may be reflected by rendering line b, for example, “More than that, wash me and I will be whiter than snow.”
The expression wash me, and I shall be whiter than snow presents the translator with several problems. In some areas where snow is unknown or where, in spite of the presence of snow, it is not used as a proverbial likeness of something that is very white, it is necessary to avoid the word snow. In some languages it may be satisfactory to use some other article for comparison. However, in this case it is not the suitability of comparison of whiteness so much as it is the meaningful use of the comparison that is important. Consequently the translator must select for his language the solution that is the most natural and meaningful for the readers. One may say, for example, “and I will be white as…,” where the dots represent some local object known for its whiteness, or one may say “and I will be very white.” It is also appropriate to translate “and I will be very clean,” since it is the purification by the removal of sin that is in focus. If it is necessary the translator can also employ a simile to indicate the figurative aspect of the cleanliness.
Quoted with permission from Bratcher, Robert G. and Reyburn, William D. A Handbook on the Book of Psalms. (UBS Helps for Translators). New York: UBS, 1991. For this and other handbooks for translators see here .