Calculate the Vapor Pressure of Liquid Potassium at 100 C
Use a Clausius Clapeyron based model with editable thermodynamic inputs. Defaults are preloaded for potassium, so you can calculate instantly and compare pressures across units and temperature ranges.
Expert Guide: How to Calculate the Vapor Pressure of Liquid Potassium at 100 C
Vapor pressure is one of the most useful properties in high temperature materials work, vacuum system design, alkali metal heat transfer, and reaction engineering. If you are trying to calculate the vapor pressure of liquid potassium at 100 C, you are solving a thermodynamics problem that sits at the intersection of phase equilibrium and practical engineering safety. Potassium has a much higher boiling point than water and many organic liquids, so at 100 C its equilibrium vapor pressure is very low, but not zero. Knowing this value matters when you estimate evaporation losses, contamination in sealed volumes, and gas phase composition over molten or partially molten alkali systems.
In this calculator, the default approach uses the Clausius Clapeyron equation with a reference point at the normal boiling point. For potassium, a commonly used reference set is a boiling point near 759 C and an enthalpy of vaporization around 76.9 kJ per mol. With these values, the predicted equilibrium pressure at 100 C is on the order of hundredths of a pascal or below, depending on constants and assumptions. That is why potassium can still be treated as very low volatility at 100 C compared with many other liquids, even though it is chemically very reactive.
Why this specific calculation matters
- It supports inert atmosphere and vacuum process design.
- It helps estimate potassium carryover in heated loops and vessels.
- It informs pressure limits for storage and sealed test cells.
- It improves hazard analysis when heated potassium is present.
- It gives a quantitative basis for comparing alkali metals.
Core equation used by the calculator
The model applies the integrated Clausius Clapeyron form:
ln(P2/P1) = -(Delta Hvap / R) x (1/T2 – 1/T1)
Where P1 is a known reference pressure (often 1 atm at the normal boiling point), T1 is the corresponding reference temperature in Kelvin, T2 is your target temperature in Kelvin, Delta Hvap is enthalpy of vaporization, and R is the gas constant 8.314462618 J per mol per K. Rearranging gives:
P2 = P1 x exp [ -(Delta Hvap / R) x (1/T2 – 1/T1) ]
For potassium, if T2 is 373.15 K (100 C), T1 is 1032.15 K (759 C), Delta Hvap is 76900 J per mol, and P1 is 1 atm, you get a very small equilibrium pressure. This is expected because 100 C is far below the normal boiling point.
Step by step procedure
- Set target temperature to 100 C (or 373.15 K).
- Set boiling point to 759 C (or equivalent Kelvin).
- Set enthalpy of vaporization to 76.9 kJ per mol.
- Set reference pressure to 1 atm at the boiling point.
- Choose your output unit, such as Pa or mmHg.
- Click Calculate Vapor Pressure and review the formatted result.
If you are comparing literature sources, remember that vapor pressure correlations can differ due to chosen temperature range, fitting method, and whether Delta Hvap is treated as temperature dependent. The simple integrated equation assumes Delta Hvap is constant across the interval. Over a broad interval such as from 759 C down to 100 C, that introduces some error, but the estimate remains highly useful for first pass engineering work.
Reference data and comparison across alkali metals
The table below summarizes widely cited physical property values used in calculations and design references. Values are representative and can vary slightly by source.
| Metal | Melting Point (C) | Boiling Point (C) | Delta Hvap (kJ/mol) | Molar Mass (g/mol) |
|---|---|---|---|---|
| Lithium | 180.5 | 1342 | 147.1 | 6.94 |
| Sodium | 97.8 | 883 | 97.4 | 22.99 |
| Potassium | 63.5 | 759 | 76.9 | 39.10 |
| Rubidium | 39.3 | 688 | 72.2 | 85.47 |
| Cesium | 28.5 | 671 | 63.9 | 132.91 |
Because potassium has a lower boiling point than sodium and lithium, its vapor pressure at a fixed sub-boiling temperature tends to be higher than those two metals, but still very small at 100 C. A second comparison table, using the same Clausius Clapeyron framework and 1 atm at each normal boiling point, shows approximate order of magnitude differences.
| Metal | Estimated Vapor Pressure at 100 C (atm) | Estimated Vapor Pressure at 100 C (Pa) | Relative to Potassium |
|---|---|---|---|
| Lithium | ~3.3 x 10^-16 | ~3.3 x 10^-11 | Far lower |
| Sodium | ~5.9 x 10^-10 | ~6.0 x 10^-5 | Lower |
| Potassium | ~1.3 x 10^-7 | ~1.4 x 10^-2 | Baseline |
| Rubidium | ~6.6 x 10^-7 | ~6.7 x 10^-2 | Higher |
| Cesium | ~4.0 x 10^-6 | ~4.0 x 10^-1 | Much higher |
Interpreting the result correctly
A low predicted equilibrium vapor pressure does not mean no vapor exists. It means that at thermodynamic equilibrium, the gas phase partial pressure of potassium is low compared with atmospheric pressure. In real equipment, transient heating, splashing, aerosol generation, and surface reaction with oxygen or moisture can produce behavior that looks more aggressive than a simple equilibrium number suggests. Always combine thermodynamic calculation with transport analysis and chemical compatibility review.
The result is also sensitive to input assumptions. If you adjust Delta Hvap by only a few percent, the pressure can shift significantly because the model uses an exponential term. Temperature input uncertainty also matters. A difference of even 10 C can alter pressure by a noticeable factor. That is why careful unit handling and validated constants are crucial.
Good engineering practice for potassium vapor calculations
- Convert all temperatures to Kelvin before applying equations.
- Use consistent pressure units across reference and output values.
- Document the exact constants and source edition used.
- Check whether your application needs a temperature dependent Delta Hvap model.
- For safety systems, apply conservative margins and scenario analysis.
Where to get high quality reference data
For traceable constants and thermodynamic datasets, consult primary technical sources. The following links are authoritative and useful for this topic:
- NIST Chemistry WebBook (.gov): Potassium data and reference thermophysical information
- MIT OpenCourseWare (.edu): Thermodynamics foundations including phase equilibrium
- CDC NIOSH Pocket Guide (.gov): Potassium safety and handling context
Limitations of the simple model
The integrated Clausius Clapeyron approach is a first order model. It assumes ideal vapor behavior, negligible liquid volume in the phase relation, and constant enthalpy of vaporization over the temperature interval. More advanced work can use Antoine or Wagner style equations, fit coefficients over limited temperature windows, or tabulated vapor pressure data directly from evaluated databases. If your process is tightly controlled and near hazard limits, use the best available correlation for the exact range and compare multiple sources.
Another limitation is chemical purity. Trace impurities can alter effective activity in molten systems, and oxidation products can influence surface behavior. If potassium is part of a mixture, such as sodium potassium alloy, pure component vapor pressure must be corrected through solution thermodynamics. In that case, Raoult law or activity coefficient models may be needed depending on composition and temperature.
Practical takeaway at 100 C
For most engineering purposes, liquid potassium at 100 C has a very low equilibrium vapor pressure, often estimated around 0.01 to 0.02 Pa when using common reference constants. This is tiny compared with atmospheric pressure, but still meaningful in high vacuum or high sensitivity instrument environments. The calculator above gives you a fast, transparent way to compute and visualize this pressure and to test how assumptions change the output.
Safety reminder: Potassium reacts violently with water and can ignite in air under certain conditions. Always follow material specific handling protocols, inert gas practices, and institutional safety guidance.