EFFECTS OF ACIDITY AND HYDROXYLAMINE ON THE DETERMINATION OF ALUMINUM WITH FERRON

Ferron (8-hydroxy-7-iodo-5-quinoline sulfonic acid) is a dibasic acid (H2A) which dissociates to HA- and A2- with increased pH. The results show that only HA-, which is the predominant species around pH 5, is reactive with Al. The Al-ferron complex yields an absorption maximum at 366 nm, with a molar absorptivity of 7,940. H2A and A2- are not reactive with Al, while they both yield a strong absorption background near 366 nm. The variation in the distribution of various ferron species in solution accounts for the acidity effect on Al determination. Based on the above information, the color-developing reagent was prepared by mixing ferron, Na acetate, and acetic acid together. This recommended reagent should have pH 5.2. With the addition of 5 mmol HCl to a sample aliquot in Al determination, the final color solution (50 mL) should have pH 4.75 when the solution is still dominated by HA-. The solution also has a high buffer capacity to tolerate the variation in acidity. The addition of hydroxylamine gradually alters ferron to a different chemical species. This hydroxylamine-modified species also reacts with Al and yields an absorption maximum at 362 nm, with a molar absorptivity of 10,500. Because of the changes in absorption maximum and molar absorptivity with time, it is necessary to age the reagent for at least 5 days before using. By deleting hydroxylamine in preparation, the ferron reagent can be used immediately after preparation.