Check the boiling point of ethanol and water at the current atmospheric pressure or at a given altitude. This lets you interpret the thermometer readings in the column head correctly.
Why does the boiling point change?
A liquid boils when its vapour pressure equals the ambient pressure. When pressure falls (low-pressure weather, high altitude), ethanol boils at a lower temperature. A change of 10 hPa shifts the boiling point by about 0.3°C.
Practical meaning for distillation
When running a column, watch temperature changes relative to the stabilised value rather than a fixed 78.3°C threshold. A precise electronic thermometer with 0.1°C resolution lets you catch the moment hearts turn into tails.
How we calculate
We use the Antoine equation: log₁₀P = A − B / (C + T), with parameters for ethanol (A = 8.20417, B = 1642.89, C = 230.3) and water (A = 8.07131, B = 1730.63, C = 233.426), P in mmHg. Pressure from altitude is calculated with the barometric formula.
Frequently asked questions
Why doesn't the thermometer show 78.3°C in the column?
78.3°C is the boiling point of pure ethanol at 1013 hPa. At lower pressure it is lower, and the azeotrope (96%) boils at 78.15°C. The weather alone can shift it by ±0.3°C.
How many thermometers does a still need?
At least one in the column head. A second one in the boiler or halfway up the column lets you spot changes earlier and run the process better.
At what temperature should hearts be collected?
Hearts are collected while the head temperature is stable and close to the azeotrope boiling point (approx. 78.1–78.3°C at 1013 hPa). A rise of a few tenths of a degree signals the tails.