Continuing our study on aluminum, tin, and silver points, a study on the realization of the zinc point was conducted. Zinc-point cells were newly fabricated using 6N-nominal grade zinc samples, impurity elements of which were analyzed extensively based on glow-discharge mass spectrometry (GDMS). The present paper reports the temperature measurements done using the newly fabricated cells during the zinc freezing process, under which the zinc fixed point is defined, and the analysis of the freezing curve obtained. Comparisons of zinc-point temperatures realized by the newly fabricated cells (cell-to-cell comparisons) were also conducted. Zinc-point depression due to impurity elements was calculated based on the sum of individual estimates and the impurity element analysis. One of the cells evaluated was drawn out from its crucible and analyzed by GDMS at four points, namely, at around the center of the top, of the middle, of the bottom, and around the outer part of the middle area. The purpose of this cell disassembly is to see whether or not there has been some difference before and after cell fabrication, as well as difference in impurity element distribution within the ingot. From the aforementioned studies, some findings were obtained. First finding is that the homogeneity of the zinc ingot was within 30 %, except for Pb, which was more concentrated in the center part. Second finding is that the cell-to-cell temperature difference changes along with the progressing solidification process. As a consequence, for an accurate cell-to-cell comparison, the locus in the freezing plateau where the comparison is done should be determined. Third finding is that the slope analysis estimates accurately the cell-to-cell comparison, and is consistent with the impurity analysis. This shows that the slope analysis gives extensive information about the effect of impurity to the zinc-point realization, especially after the cell fabrication.

The twenty-one participating laboratories in the international key comparison of water-triple-point cells (CCT-K7) can be classified into three groups: two laboratories that corrected the effect of the isotopic composition of water, four laboratories that had information on the isotopic composition but did not correct the effect, and the remaining laboratories that had no information. There were significant differences in the realized national standard for the triple point of water (TPW) between those laboratories that applied the isotopic correction and those that did not. The isotopic correction is now considered essential for the triple point of water. Since the National Metrology Institute of Japan (NMIJ) did not apply the isotopic correction and estimated large uncertainties at the time of the CCT-K7 comparison, we subsequently developed new cells for the TPW to improve the reliability and to reduce the uncertainty of the realization as a national reference. The isotopic compositions of seven cells were analyzed, and a chemical impurity analysis of one cell was performed. The good consistency among seven cells was shown in the results obtained when the isotopic correction was applied to the realized temperatures measured experimentally. The expanded uncertainty of the new national reference of NMIJ is estimated to be 49 µK (k = 2), and as a result of this improvement, the expanded uncertainty for calibrating a water-triple-point cell is 80 µK. The previous reference of NMIJ, reported in CCT-K7 to have an expanded uncertainty of 302 µK, is 42 µK lower than the new one. The new reference value is within the uncertainty of the previous national reference, and the new uncertainty is completely covered by the previous uncertainty. Furthermore, the new reference of NMIJ shows good agreement with the national references of the six laboratories able to apply isotopic corrects to their results for CCT-K7. These facts confirm the validity and the linkage to the CCT-K7 of both the previous and the new national references of NMIJ.