Is there anybody who has worked on the thermal behavior of NaCl by TGA/DSC. On my TGA/DSC profiles I have an endothermal transition without loss of weight before the melting (803°C), phase transition ?)
see the attached document
Many thanks,
Richard
The examined sodium chloride salt seemingly may contain both NaBr and NaI as impurities.
Hi Carlos,
traces of NaI or/and NaBr can not explain the elevated enthaly value of this endotherm, 1194 J.g-1 !!!!!!!
Dear Richard:
The calculation of the enthalpy for the broad endotherm was carried between about 375 ºC to the very start of the NaCl fusion peak. Because the peak is so broad, there is some confusion between the peak and the baseline, so that the integrated area largely depends on how the baseline is traced. That also contributes with some (related) uncertainty for the peak limits. As the baselines were drawn for both peaks, the baseline slope would be negative for the first and positive for the second, with a step increase by about 775 ºC, at the (signalled as immediate) transition from the first to the second. That bears no ground on what can be reasonably expected for a sodium chloride salt sample, seemingly nearly anhydrous based on TGA trace. I would suggest to review the baseline criterium, if reliable quantitative information is to be extracted for the first peak. Baseline criteria have been discussed in the literature. For the second peak ― NaCl fusion ― the measured enthalpy seems somewhat overestimated compared to published data; by about 8 %, after the reference below (*), while not accounting for the sample humidity. Therefore, you may possibly consider to revise also the enthalpy calibration protocol. Please note that the TGA-DSC instrument should be calibrated for the experimental conditions used with the actual sample ― for appropriate baseline drift compensation. It could be helpful to check also the cooling plot, yet for a sample taken to a lower maximum temperature ― to reduce halide evaporation. The exact nature of the examined sample was not disclosed at the enunciate, what limits further discussion.
(*) O. Kubaschewski, C.B. Alcock, P.J. Spencer, "Materials Thermochemistry", 6th ed., Pergamon Press, 1993.
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