Zinc sulfate is an adulterant that intereferes with urine drug testing and which may interfere with detection of alcohol, methamphetamine, cocaine and marijuana. This effects occurs when the zinc sulfate concentration exceeds 10-15 mg per mL.
Two spot tests can identify zinc sulfate at a concentration >= 10 mg/mL:
(1) sodium hydroxide (NaOH)
(2) sodium chromate
Sodium hydroxide spot test:
(1) 1N sodium hydroxide solution as reagent
(2) addition of 3 or 4 drops to 1 mL urine results in a white precipitate
(3) addition of excess NaOH causes the precipitate to become soluble
Sodium chromate spot test:
(1) 1% sodium chromate solution and 1N sodium hydroxide solution as reagents
(2) addition of 3-4 drops of sodium chromate to 1 mL urine followed by 4-5 drops of 1N NaOH results in formation of zinc chromate as a yellow precipitate
False positive results can occur with excessive lead in the urine.
The presence of zinc can be confirmed by either atomic absorption spectrophotometry or inductively coupled plasma mass spectrometry (ICP-MS).
