The Influence of Impurities on the Zinc Fixed Point

Abstract

Impurities are considered to be the most significant source of uncertainty for the realization of the International Temperature Scale of 1990 by means of metal fixed points. The determination and further reduction in this uncertainty require a traceable chemical analysis of dissolved impurities in the fixed-point metal and accurate knowledge of the specific temperature change caused by impurities (slope of the liquidus line). We determined the slope of the liquidus line for three binary systems and present results and conclusions from the chemical analysis of zinc with a nominal purity of 7N. For the Fe–Zn system, we determined a liquidus slope of ($-0.91\pm 0.14$) mK / ($\upmu \hbox {g}{\cdot }\hbox { g}^{-1}$) from the evaluation of freezing plateaus and ($-0.76~\pm 0.20$) mK / ($\upmu \hbox {g}{\cdot }\hbox { g}^{-1}$) from the evaluation of melting plateaus; for the Pb–Zn system, the corresponding results are ($-0.27~\pm 0.05$) mK / ($\upmu \hbox {g}{\cdot }\hbox { g}^{-1}$) and ($-0.26~\pm 0.05$) mK / ($\upmu \hbox {g}{\cdot }\hbox { g}^{-1}$). Although for the Sb–Zn system, we determined a liquidus slope of about $-0.8$ mK / ($\upmu \hbox {g}{\cdot }\hbox { g}^{-1}$), our investigations showed that a correction of the influence of antimony is highly questionable because antimony can be found in zinc in a fully dissolved state or precipitated as an insoluble compound. Iron is the only impurity where a correction of the fixed-point temperature was possible. For the realization of the zinc fixed point at PTB, this correction is between 2 $\upmu$K and 16 $\upmu$K depending on the batch of zinc used. The influence of the sum of all impurities was estimated by means of the OME method. The resulting uncertainty contribution is between 12 ${\upmu }\hbox {K}$ and 48 ${\upmu }\hbox {K}$.