垣内 正久

The hydrogen isotopic fractionation factors between the crystal water (CW) in crystalline hydrates and the saturated aqueous solution of cobalt dichloride, alpha CW-st.sol , were experimentally determined in the temperature range from 10 to 55 degrees C under equilibrium condition: alpha(CW-st.sol) in cobalt dichloride hexahydrate, CoCl2 center dot 6H(2)O, from 10 to 43 degrees C, and in cobalt dichloride dihydrate, CoCl2 center dot 2H(2)O, at 50 and 55 degrees C. The empirical relationship between ln alpha C.W.-st.sol and (1/ T-2) on the CW of CoCl2 center dot 6H(2)O in the temperature range from 10 to 43 degrees C was obtained as: 10(3) ln alpha(CW-st.sol) =-2.46(10 6 /T-2 )+17.6. The CW of dihydrate of cobalt dichloride exhibits larger depletion of deuterium than that of hexahydrate. The values of alpha(CW-st.sol) in CoCl2 center dot 2H(2)O are similar to that of CuCl2 center dot 2H(2)O and BaCl2 center dot 2H(2)O at 25 degrees C. This fact indicates that the hydrogen fractionation factor between the CW and the saturated aqueous solution of cobalt dichloride strongly depends on the structure of crystalline hydrate rather than the chemical species of cation and/or the crystallized temperature.

The distribution of isotopic water molecules, H2O, HDO, and D2O, in vapor and liquid phases in pure water and aqueous solution systems is related to the D/H fractionation factor between liquid and vapor phases, alpha(L-V), and the vapor pressure ratio between pure H2O and D2O, P-H2O/P-D2O. As pure HDO does not exist, it is complicated to compare the experimental data of the vapor pressure ratio of pure H2O and D2O with those of a D/H fractionation factor between liquid and vapor phases directly. n pure water and aqueous solution systems, the gamma value, defined as gamma = In(P-H2O/P-D2O)/ln(P-H2O/P-HDO) = ln(P-H2O/P-D2O)/ln alpha(L-V), is calculated with the assumption that the experimental data of the D/H fractionation factor alpha(L-V) is equal to the vapor pressure ratio between pure H2O and hypothetical pure HDO, P-H2O/P-HDO. The gamma(0) values in a pure water system decreased from 1.92 to 1.88 in the temperature range from 0 to 100 degrees C, and the gamma(m) values in aqueous solutions for alkali halide salts and urea at 25 degrees C were similarly evaluated using the value of gamma(0) = 1.92. Using the gamma values, the equilibrium constant of the liquid phase for the reaction, H2O + D2O = 2HDO, in pure water and aqueous sodium chloride solution systems, can be estimated from the equilibrium constant for the vapor phase. Copyright (C) 2000 Elsevier Science Ltd.

環境水におけるトリチウム濃度の測定精度の向上のために, すでに著者らは電解前後における重水素濃度を測定してトリチウムの濃縮率を推定する方法を提案した。近年, 操作が簡便で安全性の高い固体高分子電解質を用いたトリチウム電解濃縮装置が開発された。本装置における電解時の試料水の重水素とトリチウムの濃縮率の相関を調べた。電解時における各濃縮率の対数比k (=α (β-1) /β (α-1) ) は, 実験により1.173±0.007と求められ, 一定になることが示された。ここで, αとβは重水素とトリチウムの分離係数である。本電解装置で得られたkの値はこれまで一般によく用いられてきた重水素とトリチウムの分離係数の相関を示すβ/αおよびlogβ/logαの値の変動と比較して, より変動幅が小さいことが確かめられた。この装置で電解濃縮を行う際このkの値を用いれば, 電解前の試料水のトリチウム濃度 (T/H) ₀は, (T/H) ₀= (T/H) _f [ (D/H) _f/ (D/H) ₀] ^-k式より, 電解前後の試料水の量を測定することなしに, 電解前後の試料水の重水素濃度 ( (D/H) ₀と (D/H) _f) と, 電解後のトリチウム濃度 (T/H) _fを測定することで正確かっ精密に求められる。この際, 電解前後の水の重水素濃度を質量分析計で測定すると, 重水素の濃縮率がよりいっそう精密に求められ, トリチウムの濃縮率の誤差が小さくなり, 試料水のトリチウム濃度がより正確に求められることがわかった。