The Research And Prepared Of Os Gravimrtric Standard Material

In the field of earth science, the radioactive isotope system is the important tool for dating geologic body/geological events, and tracing the material sources. At present commonly used radioactive isotope systems include U-Pb, Rb-Sr, Sm-Nd, K-Ar, Lu-Hf, Re-Os, etc. Unlike other lithophile isotope systems, both Re and Os are highly-siderophile elements, therefore can be used to dating metal sulfides directly. Furthermore, the geochemical behaviour difference between these two elements make Re-Os system became an ideal tracer for material sources. By virture of these, Re-Os isotopic system has been a key research method in the study of deposit, the mantle geochemistry, marine chemistry and cosmochemistry.Although has a lot of advantages, Re-Os isotopic system also suffers from a few drawbacks. One of the greatest shortcoming is the relative low precision comparering with others. For example, the uncertainty of U-Pb method is less than 0.1%, and the best is 0.05%; the uncertainty of Ar-Ar method is 0.1%, and the worst is 0.3%; that of Re-Os isotopic method can be up to 1.4%. The greatest error contribution to the Re-Os method coms from the uncertainty of calibrating spike（about 0.2%）. The precision of the spike calibration depends on the availability of high-purity gravimetric standard material with known chemical composition. Re metal is in simple value state, high pure, and easy to dissolve, so it is a mature method that use Re metal as gravimetric standard material to prepare standard solutuion and calibrate Re spikes. The greatest challeng is looking for appropriate Os gravimetric standard materials. Os metal is difficult to be dissolved under atmospheric pressure, even dissolved under high-temperature and high-pressure, the product Os O4 is a volatile compound. The alternative is to select a high-purity Os salt commodity, accurately measure its composition, and then calculate the content of Os. This Os salt with determined chemical composition can be used Os gravimetric standard material.In this thesis, three brands of high purity（NH₄）₂OsCl₆ are selected as candidates of Os gravimetric standard material. In order to determine their chemical composition, purity and content of Os, we carried out a series of experiments including thermogravimetric analysis, drying condistion test, ignition and reduction condition experiment, and determinations of impurity and Os isotope composition. Through these work, following conclusions are drawn :（1） Thermogravimetric analysis showed that with temperature increasing, the weight of three candidates decreased to varying degrees, which proved that they all cotain some water. The weight variance of（NH₄）₂OsCl₆（A）is largest, and its weight change rate varied with time and temperature. Therefore, this candidate contains not only water, but also some other material influencing its thermostability;（2） Some researchers have dried（NH₄）₂OsCl₆ under 70℃, but our experiment proved that it is not high enough to decompose NH4 Cl may exsiting in the Os salt. Drying under 120℃, water will evaporate more fast, and some material with poor thermalstability, such as NH4 Cl, will breakdown. Increasing drying temperature can improve improve the results of the subsequent ignition and reduction experiments;（3） The shape and property of commercially available（NH₄）₂OsCl₆ is different due to different production process.（NH₄）₂OsCl₆ in powder form can be dried more thoroughly under 120℃, and react with the reducing gas more fully. The granular（NH₄）₂OsCl₆, which is more compact and with smooth surface smooth, cannot release all the water and NH4 Cl contained by the drying, and the core of（NH₄）₂OsCl₆ grain cannot contact and react with reductive gas in the reduction process. After the reduction, because the product still contains some unreacted（NH₄）₂OsCl₆, its weight cannot be used to calculate Os content.（4） According to results of reducetion experiments, impurity dertermination and Os isotopic composition dertermination, the purity and Os content of（NH₄）₂OsCl₆ are calculated. Os content of A is 43.24%,and the theoretical value of Os content in the samples is 43.33%. The calculated value is lower than the theoretical value（43.33%）. The purity of（NH₄）₂OsCl₆ is 99.87%, far lower than the theoretical value of 99.99%. Os content of（NH₄）₂OsCl₆（C） is 43.32%, slightly lower than the theoretical value（43.33%）, and the purity is 99.97%.Because of good thermal stability, low volatile matter content, high purity and Os content close to the theoretical value,（NH₄）₂OsCl₆（C） is finally selected as Os gravimetric standard material. After drying under 120℃, two Os gravimetric standard solution in different concentration are prepared. Their Os content are 0.99712 ±0.00024 mg/g and 0.48526±0.00016 mg/g, respectively.