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On the triple point with standard thermometer as a fixed point.

Qmechanic
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Taukir Sheikh
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3 Answers3

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For a pure component, the triple point involves a unique and very specific combination of temperature and pressure, meaning that only a very specific combination of temperature and pressure will demonstrate this point. Whether or not this point is "easy to reproduce", the fact that you can demonstrate the triple point of a pure substance means that you know that your substance is at that exact combination of temperature and pressure, with high precision.

David White
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Suppose we use the freezing point of water as the bottom marker for our scale. The trouble is that the freezing point of water depends on the pressure. Have a look at Freezing point of water with respect to pressure. for more details on this.

So our reference point can't just be the freezing point of water, it would have to include pressure as well and be the freezing point of water at a pressure of 101325Pa.

However the temperature of the triple point is not dependent on pressure. It is always 0.01°C, so it is simple to use as a reference.

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John Rennie
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Triple point of a substance is the point(coordinate) in the pressure-temperature graph where three states of it coexist(i.e. solid, liquid, and gas). As a matter of fact, substances have different values of boiling and freezing points at different pressure. Thus, simply boiling and freezing points can't be taken as the fixed point in standard thermometers, instead, triple point is used which has a fixed value for a substance.
Eg:triple point for water is (p,t)=(0.00603659 atm,0.01°C)
triple point for CO2 is (p,t)=(5.10 atm,−56.60 °C)

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