...both Flaubert's Madam Bovary (1856) and Nicolai Leskov's Lady Macbeth of the Mtsensk District (1865) featured arsenic.
My thinking at this point is that Tolstoy didn't know the science or the history of arsenic and just cribbed something that was in the air, mistakenly thinking there was an antidote in 1811. But I'd like more certainty.