How isothermal is an isotherm
Nitrogen adsorption at the boiling point of liquid nitrogen, N2@77K, has become the established method for quality control. However, scientific surface and pore investigations are increasingly being performed with different adsorptives at higher temperatures, such as Ar@87K, CO2@195K or CO2@273K. One question for every measurement is the accuracy of the used measuring temperature. As example in technical articles, the specification of the experimental temperature with 77.35 K as the boiling temperature of liquid nitrogen suggests an unrealistic accuracy of two decimals if a standard liquid nitrogen dewar is applied. In scientific articles, however, the adsorption temperature of N2 measurements is often given as 77 K, 77.3 K, 77.4 K, 77.5 K or 78 K. Few users are aware that their reported temperature could very likely vary by as much as 0.5 K because of the dependence of the boiling temperature both from the purity of the liquid nitrogen, but mainly from the ambient pressure. Not only must the temperature dependence of the saturation vapor pressure be evaluated for very accurate results, but also the exact measuring temperature and its constancy must be known over the complete measuring time. So far, this is the state of the art for relating thermostats with temperature accuracies of 0.01 K close to room temperature and should be aimed at for other temperature ranges as well. The new developed cryoTune 77 option offers an easy-to-handle technical solution for such significant temperature stability improvement for accurate sorption studies.
