# Triple point of water

1. May 9, 2014

### gunparashar

why triple point of water is used to define SI base unit of thermodynamic temperature even though for triple point we need to define both temperature (. 01) and pressure(partial pressure = 611.73 pa). this thing we can do with ice point and boiling point which is same when temperature and pressure is defined.

2. May 9, 2014

### ModusPwnd

The ice point and boiling point change depending on your pressure. The triple point does not.

3. May 9, 2014

### dauto

The boiling point and freezing point of water depend on pressure. At higher pressure the boiling point moves up (higher temperatures) while the freezing point moves down. So boiling point and freezing point are not uniquely defined while the triplr point temperature is unique.

4. May 9, 2014

### 256bits

i think it is the other way around, isn't it. The triple point defines the temperature and pressure.

Reproducibility is the key.
Take a container of water and evacuate all the air so there is just liquid water and gas. Cool the container until some ice forms. You now have the triple point at the solid, liquid, gas interface and any temperature probe can be calibrated for 273.16 K.

With you method, one has to know the pressure beforehand for the ice point, as said above that can change with pressure. If the pressure is off, so is the temperature.

Luckily, at normal atmospheric temperature there is little difference. I believe it is 273.15K.