Keil & Delitzsch, on Prov 20:9

9 Who can say I have made my heart clean, I am pure from my sins? It is the same thought that Solomon expresses in his prayer at the consecration of the temple, Kg1 8:46 : there is no man who sinneth not. To cleanse his heart (as Psa 73:13), is equivalent to to empty it, by self-examination and earnest effort after holiness, of all impure motives and inclinations; vid., regarding זכה, to be piercing, shining brightly, cloudlessly pure, Fleischer in Levy's Chald. Wrterbuch, i. 424. The consequence of זכּות is, becoming pure; and the consequence of זכּות לב, i.e., of the purifying of the heart, the being pure from sinful conduct: I have become pure from my sins, i.e., from such as I might fall into by not resisting temptations; the suffix is not understood as actual, but as potential, like Psa 18:24. No one can boast of this, for man's knowledge of himself and of his sins remains always limited (Jer 17:9.; Psa 19:13); and sin is so deeply rooted in his nature (Job 14:4; Job 15:14-16), that the remains of a sinful tendency always still conceal themselves in the folds of his heart, sinful thoughts still cross his soul, sinful inclinations still sometimes by their natural force overcome the moral resistance that opposes them, and stains of all kinds still defile even his best actions.