The equilibrium 2 HI (g) 
H2(g) + I2(g)
was the first gas phase equilibrium to be thoroughly studied. It was investigated by
Max Bodenstein (1871-1942) in the late 1890's in Germany. He found that the reaction
could be studied between about 380 and 500 °C, but that at higher temperatures the
reaction was shifted too far to the product side.
To study the reaction, Bodenstein sealed various amounts of HI in glass containers. These were then heated to the desired temperature for various intervals of time. The containers were then removed and cooled rapidly, and the contents analyzed for all three components. Bodenstein was able to determine the equilibrium constant for the reaction from the quantities of hydrogen gas, iodine vapor that had formed, and hydrogen iodide that remained.