Materials and instruments.
The following materials were used: lead stock standard solution, 1000 ppm (analytical grade); chloroform (HPLC grade); dithizone
(diphenylthiocarbazone, p.a. grade); diethyldithiocarbamic acid sodium salt (p.a. grade); hydroxylamine hydrochloride (p.a.
grade); ammonium hydroxide (ACS reagent grade); nitric acid (ACS reagent grade); hydrogen peroxide 35% (p.a. grade); and sulfuric
acid (p.a. grade), which were all purchased from Acros (Morris Plains, NJ). Citric acid (reagent grade) and potassium cyanide
(reagent grade) were from Sigma-Aldrich Chemical, Co. (St. Louis, MO). Deionized (DI) water was from a reverse osmosis system at Culligan (Missoula, MT). All other chemicals,
including hydrochloric acid, ethanol, phenol red, thymol blue, and so forth, either reagent or better grades, were obtained
from Fisher Scientific (Denver, CO). 1% HNO3, 1% HCl, and 3 N HCl were prepared by mixing appropriate amount of DI water and the concentrate HNO3 and HCl, respectively. All samples were either obtained from the Department of Quality Assurance of Nutritional Laboratories International Inc. (Missoula, MT) or purchased locally. The instrument was a Cary 50 Bio UV–vis spectrophotometer from Varian (Walnut Creek,
First, we diluted the 1000 ppm of lead stock solution with 1% HNO3 to 1.0 ppm. Then a 0.003% dithizone extraction solution was made as follows: 30 mg dithizone were dissolved in 1000 mL of
chloroform, and 5 mL of ethanol were added as a stabilizer. The solution was stored in a refrigerator. Each time before use,
we washed a suitable volume of the dithizone solution with half of its volume of 1% HCl. After washing, the aqueous phase
was discarded. We prepared a 0.001% standard dithizone solution by dissolving 10 mg of dithizone in 1000 mL of chloroform
and storing it in a refrigerator.
To prepare the 30% ammonium citrate solution, we dissolved 60 g of citric acid in approximately 100 mL of DI water. Two drops
of 0.1% phenol red in ethanol were added, and the pH was adjusted with ammonium hydroxide until the indicator turned from
yellow to pink. Any lead present in the citric acid was removed by extracting the solution with 10 mL of 0.003% dithizone
extraction solution. The extraction was repeated until the dithizone solution did not change color. The citrate solution was
then diulted to 200 mL.
To prepare the 2% ammonium cyanide solution, we dissolved 2 g of potassium cyanide in 15 mL of ammonium hydroxide, then diluted
it with DI water to 100 mL. For the 5% potassium cyanide solution, we dissolved 25 g of potassium cyanide in sufficient water
to make 100 mL. We removed lead from this solution by repeated extracting with 10 mL of 0.003% dithizone extraction solution
until the dithizone solution did not change color. We removed any remaining dithizone from the aqueous solution by extracting
with 10 mL of chloroform and repeated this process three times. We then diluted the cyanide solution with DI water to 500