COPRECIPITATION OF TRACE PHOSPHATE WITH MIXED HYDROXIDE OF IRON(III) AND BISMUTH(III) AS A MEANS OF PRECONCENTRATION FOR THE SPECTROPHOTOMETRIC DETERMINATION

A simple and rapid method for determining trace amounts of phosphate in aquatic environment was proposed. A 0.5 ml portion of 1 mol.dm-3 iron(III) nitrate solution and 0.3 ml of 1 mol.dm-3 bismuth(III) chloride solution were added to a 1 l of test solution, and the pH was adjusted with sodium hydroxide in the range from 7 to 8 to precipitate mixed hydroxide of iron(III) and bismuth(III) with which phosphate coprecipitated. The suspension was stirred for 30 min and filtered through a 1-mu-m membrane filter. The precipitate on the filter was dissolved in 5 ml of 5 mol.dm-3 sulfuric acid and then 6 ml of 10 mol.dm-3 sodium hydroxide was added to the solution to release the phosphate from the regenerated precipitate. The total volume of the suspension and washings was made up to 20 ml with distilled water and stirred for 10 min. After centrifugal separation, the pH of the solution was adjusted from 3.0 to 4.0 with sulfuric acid and ammonium molybdate and tin(II) chloride were added to develop a blue color. The absorbance was measured at 710 nm. Phosphate as low as 3 ppb could be determined. Up to 60 ppm of silicate did not interfere. The method was applied to the determination of phosphate in river water samples.