Development and accreditation to ISO/IEC 17025 calibration status of a melting point measurement facility for the UK

A melting point measurement facility for the UK has been developed and accredited to ISO/IEC 17025 Calibration status for the determination of the liquefaction temperature of pure substances from 35 to 250°C. The facility is based upon a commercial instrument, i.e. an oil bath fitted with an aluminium block (Isotech, model 798 EHT), a precision multimeter (Isotech TTI-7), a thermocouple directly inserted in the sample under investigation and a platinum resistance thermometer (PRT) tracking the block temperature. The homogeneity of temperature bath/block was investigated and the PRT used for the traceability of the measurements was calibrated by NPL and traceable to ITS-90. The process was validated using four current LGC Certified Reference Materials (CRMs): Phenyl salicylate; material number: LGC2411, batch number: 001; liquefaction point: 41.85±0.05°C 4-Nitrotoluene; material number: LGC2401, batch number: 007; liquefaction point: 51.71±0.21°C Benzoic acid; material number: LGC2405, batch number: 005; liquefaction point: 122.37±0.21°C Carbazole; material number: LGC2409, batch number: 007; liquefaction point: 245.58±0.07°C Different approaches were used to identify reproducible features of the melting point (time-temperature) curves of these four CRMs. Excellent correlation was observed between the certified values for the liquefaction point of the four CRMs and the temperature at the end of their respective melting point curve plateau, determined using a temperature differential approach. An uncertainty budget was derived and the expanded uncertainty at the 95% confidence interval ( k =2) was found to be Phenyl salicylate: ±0.20°C; 4-Nitrotoluene: ±0.17°C; Benzoic acid: ±0.24°C; Carbazole: ±0.27°C