Spectrophotometric detection of iodide and chromic (III) in urine after oxidation to iodine and chromate (VI)

J Anal Toxicol. 2005 Oct;29(7):658-63. doi: 10.1093/jat/29.7.658.


Tests for oxidizing adulterants in urine are a continuing challenge to the drug-testing program. Iodine was found to destroy morphine and 6-acetylmorphine almost immediately. The effects were less evident on 11 -nor-delta9-tetrahydrocannabinol-9-carboxylic acid (THC-acid). When the urine solution was tested for iodine by a chromogenic substrate, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), no iodine was detected. Masking drug and adulterant simultaneously made iodine a preferred oxidizing adulterant for drug abusers. In this study, the reduced iodide was oxidized by sodium nitrite to iodine. The excess nitrite was decomposed by sulfamic acid and the iodine was detected by ABTS. Linearity was 12.7 to 635 mg/L (0.1 to 5 mmol/L, y = 0.9966x + 0.0016, R2 = 1.0000). Precisions (coefficient of variation) were within +/- 4.1% and quantitative accuracies were within 97% of expected values (n=5). Chromate, iodate, periodate, and persulfate interfered with the method. To alleviate the problem, the positive specimens were tested again by an iodine-specific method. After oxidation, the samples were treated with sodium azide and ammonium thiocyanate. In presence of thiocyanate, the azide reduced iodine to iodide almost immediately, and the solutions showed negative response to ABTS. The results were compared with that of a control group tested without thiocyanate. When iodine was present, the ratios of thiocyanate to control were less than 6%. Chromate was also found to destroy THC-acid in urine, and during storage most of the chromate changed to chromic (III). In this study, chromic was oxidized to chromate by hydrogen peroxide and sodium hydroxide and detected by 1,5-diphenylcarbazide. Linearity was 5.2 to 156 mg/L (0.1 to 3.0 mmol/L, y = 1.0285x - 0.0034, R2 = 0.9998). Precisions were within +/- 8.5% and quantitative accuracies were within 92% of expected values (n=5). The test was not interfered by other oxidizing agents. Both iodide and chromic oxidation methods showed urine backgrounds less than 1.27 and 0.52 mg/L, respectively (< 0.01 mmol/L). It indicated that a response more than 10 times of the background could be considered as oxidant contamination or adulteration of urine specimens.

MeSH terms

  • Chromium / chemistry
  • Chromium / urine*
  • Drug Contamination*
  • False Positive Reactions
  • Humans
  • Iodides / chemistry
  • Iodides / urine*
  • Oxidation-Reduction
  • Reproducibility of Results
  • Spectrophotometry
  • Substance Abuse Detection / methods*


  • Iodides
  • Chromium
  • chromium hexavalent ion