Distillation Method Using Miniature Apparatus for Determination of Total Cyanide by FIA

Abstract

Acid distillation is commonly carried out before the determination of total cyanide in aqueous samples to decompose metal-cyano complexes into HCN. We designed the miniature distillation apparatus, which consists of a 20 mL distillation tube, a 30 mL outer tube as a receiver, and an inner tube for air cooling. This apparatus (16 cm in width, 40 cm in height, and 27 cm in depth) was equivalent to about one-tenth of a standard distillation system in size. Sample and reagent solutions used were reduced to one-fiftieth of those used in the standard method. The proposed procedure is as follows: 5 mL of a sample, 0.2 mL of phosphoric acid (85 %), and 0.2 mL of EDTA (100 g L⁻¹) were added to the distillation tube. Then, 2 mL of NaOH (20 g L⁻¹) was added to the outer tube, in which the inner tube was covered. A maximum of 8 sets were heated by a dry block bath at 150°C for 20 min. The CN⁻ in the distillate was determined by FIA. Under the distillation conditions, potassium hexacyanoferrate as well as KCN was converted into HCN, whereas thiocyanate was not. The CN⁻ was completely recovered from a KCN solution (0.05 mg L⁻¹ as CN) in the presence of Fe(III) (≤1000 mg L⁻¹), Mn(II) (≤100 mg L⁻¹), and Zn(II) (≤100 mg L⁻¹). Satisfactory recoveries of CN⁻ were found for a recovery test using the leaching solution from soils. The proposed distillation method is easier to operate, and reduces the distillation time, space, and consumption of chemicals and energy.