Key Instruction Points
Review hazards of chemical combinations prior to start.
Use proper PPE for the task.
Lower hood sash to the lowest possible height.
Description of Incident
A teaching assistant was preparing a 30L solution 0.04 M KMnO4 in 0.5 M H2SO4 for use in chemistry instructional laboratories. She was working by herself. This is a laboratory where all solutions for use in the general chemistry laboratories are prepared. Her supervisor was working in his office, which is adjacent to the preparation lab and connected by a doorway. A chemistry class was underway in a lab across the hallway. This teaching assistant had a BS and MS in chemistry and working at the university for two years. Her responsibilities included preparing solutions for the chemistry program since the start of her employment.
She indicated she was following a procedure she has followed without incident in previous semesters (estimated 10-12 previous preparations). She indicated the procedure was contained in a notebook containing standard procedures for preparing all of the reagents used in the two classes. On the day of solution preparation, was not referring directly to the written procedure since she had used it sufficiently in the past that she did not need to refer to it. The quantity prepared is typically 10-20 liters ; this was the first instance she could recall in which 30L was to be prepared.
The notebook containing the written procedure has not been located.
To make 30 L of the solution, 833 ml of 18 M sulfuric acid was needed. This was measured out, transferred to a one-liter beaker which was placed on a stir plate in the hood. The volume of acid was obtained from two sources: the last 200-300 mL of sulfuric acid remaining in a bottle that had already been opened and the balance from a new unopened bottle. Both sulfuric acid bottles were 2.5 liters in size. A 40-liter Nalgene tank was filled with approximately 22 liters of deionized water and placed in the hood. Solid reagent grade potassium permanganate (189.648 grams) from a bottle dated 9/10/96 was weighed out into a clean beaker on a top loading balance on the lab desk. This was slowly added to the acid on the stir plate (estimated over a period of 30 seconds). The solution heated quickly and began to boil in one to two minutes. Because it was spattering, the solution was picked up with gloved hands so that it could be transferred into the DI water and diluted before any more was lost. Before any of the solution could be transferred to the DI container, the beaker broke, spraying acid and permanganate everywhere.
The teaching assistant sustained chemical burns to the upper body, any uncovered areas. She was wearing safety glasses, not goggles, gloves, and a short sleeved shirt.
The supervisor later indicated that the calculated amounts described above for preparation of the 30 L solution were correct. If the material had not broken, the contents would have been added to the 22-liter DI container over the course of a few minutes, the container would have been filled to 30 liters with DI, stirred, and then dispensed into three two-liter containers. The remaining solution would be placed into a 20-liter carboy for subsequent refills of the two-liter containers for lab work later in the week.
Probable Cause of the Incident
Several pertinent references in the literature regarding the mixtures of potassium permanganate and sulfuric acid indicate caution. Bretherick's Handbook of Reactive Chemical Hazards states:
"Addition of concentrated sulfuric acid to the slightly damp permanganate caused an explosion. This was attributed to formation of permanganic acid, dehydration to dimanganese heptoxide and explosion of the latter, caused by heat liberated from interaction of sulfuric acid and moisture. A similar incident was reported previously, when a solution of potassium permanganate in sulfuric acid, prepared as a cleaning agent, exploded violently.
Manganese heptoxide is formed as a dense green-brown oil by reaction between potassium permanganate and concentrated sulfuric acid. Kleinberg, Argersinger and Griswold, Inorganic Chemistry, p. 534 states that the reaction between potassium permanganate and sulfuric acid is:
2 KMnO4 + H2SO4 = Mn2O7 + K2SO4 + H2O
A manganese heptoxide begins to lose oxygen at 0B and decomposes with explosive violence when warmed.
Durrant and Durrant, Introduction to Advanced Inorganic Chemistry pg 1014 states, "A Manganese heptoxide, exists as dark green, explosive crystals. It is made by adding powdered potassium permanganate to cooled concentrated sulfuric acid. A dark green solution is formed which is explosive. Manganese heptoxide is stable at -5B, but it begins to give off oxygen at 0B, and at about 10B it explodes yielding manganese dioxide."
The most probable cause of this incident was the explosion of manganese heptoxide formed by the reaction of potassium permanganate and concentrated sulfuric acid.
An alternate procedure for preparing the solution will be developed. Procedures for preparation of all instructional lab reagents will be reviewed.
Use of protective equipment will be re-emphasized. The teaching assistant wore glasses and gloves, but no lab coat and face shield, sustaining burns to the face, arms, and upper torso.
Re-emphasize the availability and requirement to review health and safety information resources provided to supervisors and employees. This information (ACS booklet) indicates caution in mixing of sulfuric acid and potassium permanganate.