The triple-point of water is a standard fixed point in modem thermometry. Why ? What is wrong in taking the melting point of ice and the boiling point of water as standard fixed points (as was originally done in the Celsius scale) ?

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The triple-point of water serves as a crucial standard fixed point in modern thermometry due to its unique properties. At this point, water coexists in equilibrium in its three phases: solid, liquid, and gas. This equilibrium ensures a precise and consistent temperature measurement, regardless of external conditions such as pressure.

Now, let's address why using the melting point of ice and the boiling point of water as standard fixed points, as originally done in the Celsius scale, may not be ideal. While these points are convenient and readily accessible, they are dependent on atmospheric pressure, which can vary significantly. This variability introduces uncertainty into temperature measurements, compromising their accuracy and reliability.

In contrast, the triple-point of water remains constant at a pressure of 611.657 pascals, or 0.0060373 standard atmospheres. By utilizing this fixed point, thermometers can be calibrated with precision, ensuring consistent and accurate temperature readings across different instruments and locations.

In summary, while the melting point of ice and the boiling point of water were suitable as standard fixed points in the past, the triple-point of water offers superior accuracy and consistency in modern thermometry, making it the preferred choice for calibrating temperature measurements.