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Research paper−187 Synthesiology - English edition Vol.1 No.3 (2009) several international organizations such as the International Union of Pure and Applied Chemistry (IUPAC) issued a recommendation to re-measure the absolute value of the density of water with known isotropic compositions at the relative combined standard uncertainty less than 1×10-6 in density. In response, the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO)[6] and the National Metrology Institute of Japan (NMIJ, formerly the National Research Laboratory of Metrology)[7] conducted independent absolute measurements of the density of chemically pure water with isotropic composition equivalent to the standard mean ocean water (SMOW)[8] in 1990s. The independent absolute measurements were thus obtained by Australia and Japan. Since there was a relative density difference of 2.1×10-6 that is greater than the uncertainty between the two values, the two data were analyzed by the Working Group on Density (WGD) of Comite Consultatif pour la Masse et les Grandeurs Apparentèes (CCM), Comité International des Poids et Mesures (CIPM), and recommended values for the density of water having isotopic compositions equal to SMOW have been issued in a range 0 to 40 °C with the absolute value of 999.9749(8) kg/m3 at 4 °C and 101.325 kPa[9]. The number in parenthesis expresses the expanded uncertainty (k=2) of the last digit. This value is widely used as the internationally recommended value. However, the density of water changes due to effects of dissolved gases and isotropic compositions, and several corrections are necessary depending on the actual condition of the water sample.Other than water, mercury has been used as a standard for high density. Absolute measurement for the density of mercury was conducted by the National Physical Laboratory (NPL) of UK in 1957 and 1961 to establish the standard for pressure[10][11]. When the average values of 1 IntroductionWater has been used as a density standard since ancient time. The International Prototype of the Kilogram (IPK) currently used as the standard of mass was originally defined as the mass of one litter of water measured using the Prototype Meter created at the end of the 18th century[1]. In the International System of Units (SI, from Le Système International d'Unités)[2], the unit of density (kg/m3) is the SI derived units composed from the SI base units, kilogram (kg) and meter (m). To measure the density according to the definition of SI, only mass and length standards are needed, and it seems there is no necessity to establish a new standard for density. However, to start from absolute measurements of mass and length to measure the physical quantity of density, large-scale measurement facility is required and measurements are extremely difficult, and it is much easier to make a relative density measurement of unknown material with respect to the density of material whose absolute value is already known accurately. Therefore, general method used is to measure the absolute value of density of a widely available material with high reproducibility, and then to make relative measurement of unknown material using this substance as a reference standard. The material that serves as the standard for density is called density reference material or density standard material[3][4].Water is a density standard material that was used first, and has been used widely to calculate the density and volume of other materials. The absolute measurement of the density of water was done by the Bureau International des Poids et Mesures (BIPM) around 1890 to 1910[5]. Since this measurement was conducted before the discovery of isotopes, the issue of uncertainty caused by the ambiguity of isotropic composition of water remained. Therefore, - Using silicon single-crystals as the top of traceability in density measurement -Kenichi FujiiNational Metrology Institute of Japan, AIST Tsukuba Central 3, Umezono 1-1-1, Tsukuba 305-8563, Japan E-mail: Water is conventionally and widely used as a reference standard to measure physical quantities such as the density, volume, internal volume, and concentration. Water is also used as a standard for other physical properties such as the specific heat and surface tension. However, water density is influenced depending on its isotopic composition and content of dissolved gases. Researches on density measurement using solid material with stable density such as silicon single-crystals were therefore started in the 1970s. Demands have been raised from industry for densimetric technology to improve reliability of product and to maintain traceability of measurement. With this background, AIST established a new density standard using silicon single-crystals. Shifting the density standard from liquid to solid will not only improve the accuracy but also promotes development of new material evaluation technology for thin films as well as metrological standard technology for next generation.A new density standard replaced from waterKeywords : Density, standard, water, silicon single-crystals, traceability, Avogadro’s constant[Translation from Synthesiology, Vol.1, No.3, p.201-211 (2008)](33)−

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