Transition Temperatures of the Hydrates of Na2SO4, Na2HPO4, and KF as Fixed Points in Biomedical Thermometry

The hydrate transition temperatures of Na 2 SO 4 ·10H 2 O to Na 2 SO 4 , KF·2H 2 O to KF, and Na 2 HPO 4 ·7H 2 O to Na 2 HPO 4 ·2H 2 O were established using ACS grade salts as 32.374 °C, 41.422 °C, and 48.222 °C, respectively. A simple and reliable procedure involving inexpensive materials was used to realize these transitions as temperature fixed points. Each transition temperature was attained within 30 minutes of hydrate initiation and remained constant to within ±0.002 °C for more than 10 hours if the mixture was stirred. The established transition temperatures were sensitive at the 0.001 °C level to the amount of impurities, so the materials used should be of the highest quality available. These systems fill a gap in the existing spectrum of temperature standards and should be useful in biomedical laboratories for calibrating thermometers.