The following question and answer by Sheikh Yusuf Talal DeLorenzo attendees of the Dow Jones University Courses on Islamic Investement.
Question:: How exactly is investment income 'purified' and how is the amount that comes from interest calculated?
Answer: Please review Lesson Six: Zakah and Purification, particularly the section entitled: "Purification as a Fiscal Responsibility". You may also want to try the exercise that is given there.