What were the required operating conditions regarding temperature and pressure for the successful synthesis?

Answer

Pressures around 150 to 250 atmospheres and temperatures between $400^{\circ}\text{C}$ and $500^{\circ}\text{C}$

Haber determined the necessary conditions involved extremely high pressures, typically 150 to 250 atmospheres, combined with moderately high temperatures ranging from $400^{\circ}\text{C}$ to $500^{\circ}\text{C}$.

What were the required operating conditions regarding temperature and pressure for the successful synthesis?
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