Arsenic Trioxide
Properties
| State | Solid (sublimable) |
| Color | White |
| Solubility | Slightly soluble in water (20 g/L at 20 °C); soluble in HCl and alkali |
| Melting Point | 312 °C |
| Boiling Point | 465 °C |
About Arsenic Trioxide
Arsenic trioxide has had a remarkable career arc — from the most notorious poison in European history to an FDA-approved anticancer drug, with the same molecule wearing both labels. The compound was the assassin's reagent of choice through the 18th and 19th centuries because it ticked every poisoner's box: tasteless, nearly odorless, only slightly soluble in neutral water (so it didn't precipitate in food or drinks), and producing symptoms (vomiting, cramping, diarrhea, eventual cardiac arrest) that closely mimicked natural gastrointestinal illness. The forensic problem persisted until James Marsh's 1836 colorimetric assay made trace arsenic in tissue reliably detectable, which dropped the discovery rate of arsenic poisonings dramatically and effectively retired the molecule's career as a covert toxin. The drug-development arc started in the 1990s when Chinese clinicians at Harbin Medical University noticed that traditional 'Ailing-1' preparations — which contained arsenic trioxide — produced remarkable remissions in patients with acute promyelocytic leukemia (APL), a previously near-uniformly fatal blood cancer. Mechanistic work showed that As(III) binds cysteine residues in the PML/RARα fusion protein that drives APL, triggering its degradation and forcing the malignant promyelocytes to differentiate and die. FDA approval came in 2000 as Trisenox; combined with all-trans retinoic acid (ATRA), the regimen now achieves cure rates above 90% in newly diagnosed APL. Solid As2O3 exists as two polymorphs: cubic arsenolite, built from discrete As4O6 cage molecules isostructural with white phosphorus's P4 cage architecture, and monoclinic claudetite, built from infinite sheet structures.
Where you'll encounter it
If you read 19th-century murder-mystery fiction, arsenic poisoning is a recurring plot device, and the molecule the authors meant was almost always As2O3 — a white powder that fit into a sugar bowl or wine glass without detection. The modern medical use is the more interesting story: APL patients now receive intravenous Trisenox at doses around 10 mg/day for several weeks, in combination with ATRA, and the response rate is among the most striking in clinical oncology. The drug works by a mechanism that's hard to imagine being approved if it weren't so effective — at the molecular level, the same arsenic-cysteine binding chemistry that produces general toxicity at high doses produces specific anti-leukemic differentiation at controlled lower doses. In a forensic chemistry lab, As2O3 still serves as the reference compound for calibrating the modern instrumental methods (ICP-MS, atomic absorption) that have replaced the Marsh test, and it remains a teaching example of how analytical chemistry advances change criminal-investigation outcomes.
Common Uses
- Trisenox chemotherapy for relapsed acute promyelocytic leukemia
- Glass-decolorizing agent for high-clarity optical glass production
- Precursor for arsenate pesticides and semiconductor dopants
- Forensic and analytical reference standard for arsenic determination
- Wood preservative component in legacy CCA formulations (now restricted)
Safety Information
IARC Group 1 human carcinogen with documented causation of skin, lung, and bladder cancer at chronic low-dose exposures. Acute oral LD50 in rats is around 14 mg/kg — among the most acutely toxic common laboratory chemicals. The Trisenox clinical formulation is administered as a controlled-dose IV infusion under continuous cardiac monitoring because As(III) prolongs the QT interval and can trigger torsades de pointes arrhythmia. OSHA PEL is 0.01 mg/m³ as arsenic. Handle only in a fume hood with cytotoxic-drug PPE; treat all spills as regulated hazardous waste. Listed as a controlled substance in many national pharmaceutical schedules. GHS H301, H331, H350, H410.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.