To prepare the 20% hydroxylamine hydrochloride solution, we dissolved 20 g of hydroxylamine hydrochloride in sufficient DI
water to make ~60 mL. Five drops of 0.1% thymol blue in the ethanol solution were added, and the pH was adjusted with ammonium
hydroxide until a yellow color appeared. We added and dissolved 0.4 g sodium diethyldithiocarbamate, and allowed it to stand
for 5 min. The solution was extracted five times with successive 10 mL of chloroform. Drop by drop, we added 3 N hydrochloric
acid until the solution became pink, and then diluted it to 100 mL with DI water.
Figure 1: The complex of lead and dithizone formed in weak basic solution. Lead to ligand ratio is 1:2.
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All reagent solutions were kept in a refrigerator and were stable for at least four weeks.
Analytical procedure. In a weak alkaline solution, dithizone can form a stable complex with lead in a 2:1 ratio (see Figure 1). This complex has
a maximum absorbance at 520 nm (see Figure 2). To obtain reliable results, the key step was to quantitatively dissolve and
then isolate lead from the sample matrix. The optimized and validated analytical procedure follows.
Figure 2: Spectrophotometric scans of a blank dithizone solution (blue), 1 μg/5 mL (red) and 4 μg/5 mL (purple) of lead-dithizone
solutions.
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Step 1.
Into a 250-mL beaker, weigh an appropriate amount of samples containing ≤25 μg of lead, such as 1–2 g of inorganic sample,
5 g of a mixture sample of inorganic and organic, or 10 g of organic sample. Add 10 mL of 1:1 H2SO4 (1:1 of H2SO4–H2O, v/v) and a few glass beads into the beaker. Cover the beaker with a piece of watching glass and heat the beaker on a hot
plate until the sample becomes brown or dark brown.
Step 2.
Drop by drop carefully add H2O2 (35%), until the organic materials are completely destroyed, which is indicated by the solution becoming clear. For mineral-containing
samples, some white residue will remain.
Step 3.
For some samples of mixed organic and inorganic materials, H2SO4–H2O2 may not be able to destroy the organic components completely. In this case, alternatively adding H2O2 and HNO3 could speed the digestion. Wait for evolution of SO3 smoke before switching solvents.
Step 4.
For alkaline earth-salt samples such as dicalcium phosphate, H2SO4 should be avoided for digestion because it can form heavy precipitation with the metal ions and make the lead separation
impossible. For these types of samples, neat nitric acid is sufficient.
Step 5.
After sample digestion, let the solution cool to room temperature. Wash the beaker with approximately 10 mL of DI water.
Heat the solution again until SO3 smoke is evolved. Repeat the washing and heating one more time to remove any residual H2O2. If H2O2 is not completely removed, it will fail the lead extraction separation.
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