The calculative correction developed by PTB is based on a chemical analysis. As no reliable data about the behaviour of solved substances are known in the range of extremely small contamination concentrations, these data must all be obtained by experiment. The applied method determines the amount of contaminations with uncertainties of a few ppb (this corresponds to 10–9 mol/mol) and determines its influence on the phase transition temperature of the metal.
To avoid use of the large fixed point cells the employed so far, special minimized cells have – on the basis of investigations of the heat transport in a fixed point cell – been developed which are selectively contaminated to observe the change of the melting temperature. The data obtained in this way on the fixed points of indium, tin and zinc are – as far as the uncertainty and amount at low costs are concerned – unequalled in the international comparison. The result of the investigations was surprising: For the first time it could be demonstrated that not all contaminations affect the fixed point as expected. A detailed analysis as well as calculations showed that elementary contaminations sometimes form insoluble compounds which do not have to be taken into account for the correction.
