The triple point of water is a standard fixed point in modern thermometry. Why ? Why melting point of ice or boiling point of water not used as standard fixed points.(as was originally done in celsius scale)

The triple point of water is a standard fixed point in modern thermome

1.	The triple point of water is a standard fixed point in modern thermometry. Why ? Why melting point of ice or boiling point of water not used as standard fixed points.(as was originally done in celsius scale)
Answer» Solution :The melting point of ice as well as the boiling point of water changes with CHANGE in pressure. The presence of IMPURITIES ALSO changes the melting and boiling POINTS. However the triple point of water has a unique temperature and is independent of external factors. It is that TEMPERATURES at which water, ice & water vapour co-exist that is 273. 16 K and pressure 0.46 cm of Hg.