Question.

Older colleagues tell of picric acid exploding with great violence, but always in other labs. Is there really a risk of explosion?

Answer.

From the late 19th Century until the First World War, picric acid was used as a high explosive in military shells. Its melting point (122C) is quite well separated from its exploding temperature (above 300C). Picric acid can be ignited by a nearby spark at temperatures above its flash point of 150C. More sensitive explosives can be formed by chemical reaction of picric acid with other substances. An example is ammonium picrate (which has been used in histology to fix vital stainings with methylene blue).

In 1915 a French freighter, the Mont Blanc, full of expired explosives, caught fire in the harbour of Halifax, Nova Scotia. The largest man-made, non-nuclear explosion followed, and it's customary to blame it on picric acid, which probably accounted for much of the cargo.

When you buy a bottle of picric acid for the lab, the yellow powder is mixed with 10% to 40% of its weight of water (varies with the supplier), so it is impossible for the temperature to go above 100C, let alone the 300C required for an explosion. If a jar of picric acid were to dry out, as a result of neglect, it's conceivable that a high temperature might develop from friction when unscrewing a tight bottle cap, but 300C seems highly improbable. Nevertheless, it's usual to loosen a tight cap by standing the jar upside down in water for a few minutes before applying force to it. Percussion can cause a locally high temperature, so you shouldn't hit dry picric acid with a hammer. One of its uses is in matches. Stories of picric acid explosions in labs are like sitings of ghosts: always second-or third-hand.

Various toxic effects are described, especially skin reactions. Oral LD50 values range from 60 to 250 mg/kg depending on the animal. (This puts it in the same league as ferrous sulphate.)

Sources: Various chemistry textbooks; Merck Index; Lange's Handbook of Chemistry; MSDS sheet.

John A. Kiernan,
Department of Anatomy & Cell Biology,
The University of Western Ontario,
LONDON,  Canada  N6A 5C1
(kiernan[AT]uwo.ca)