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Clarke和Glew研究了0～373.15 K温度范围内NaCl纯溶液的渗透系数. Holmes和Mesmer以氯化钠为参考物质，研究了383.5～474.2 K高温范围内NaCl-KCl-H2O体系的渗透系数、活度系数和吉布斯自由能等热力学性质. Robinson和 Bower采用等压法研究了298.15 K常温下NaCl-CaCl2-H2O体系的活度系数等热力学性质. 然而，308.15 K下NaCl-CaCl2-H2O体系的等压研究迄今未见报道. 本文采用改进的自组装等压设备测定了NaCl-CaCl2-H2O及其纯盐体系在308.15 K下的等压平衡浓度、水活度，计算了体系的渗透系数和饱和蒸汽压. 根据Pitzer渗透系数方程，获得了该体系NaCl和CaCl2的Pitzer纯盐参数和混合离子作用参数，为含钠钙卤水体系溶液模型建立提供了重要的基础数据. 1 试剂和溶液 实验所用水皆为去离子水经二次蒸馏纯化所得，电导率为1.0×10-4 S•m-1. NaCl（分析纯，天津科密欧化学试剂厂）经三次重结晶，在873.15K下灼烧恒重后配实验储备液，采用AgCl重量分析法和称重法两种方法相对照确定其浓度，相对偏差小于0.05%；CaCl2(分析纯，天津科密欧化学试剂厂)的纯化，首先根据CaCl2+H2O的相图确定出CaCl2•nH2O的结晶区域，大致在CaCl2质量百分含量为50%~75%，在(-233.15~293.15) K的条件下可析出CaCl2•nH2O晶体；浓度过高则易形成CaCl2过饱和溶液，控温不当则易形成一种粘稠状玻璃体，无法过滤分离结晶体. 本文对实验条件进行了多次摸索，不断调整CaCl2浓度及结晶温度，最终选择60%~70%浓度范围、在263.15 K下结晶并放置过夜，产生棒状晶体，经四次重结晶后获得易存放的CaCl2•2H2O，用以配制实验储备液. 储备液经原子吸收光谱对提纯前后的CaCl2进行杂质分析，镁离子杂质含量降低了30倍. CaCl2储备液浓度由AgCl重量法标定，三个平行样分析结果相对偏差小于0.05%. 混合储备液的配制，首先参考我们已报导的溶解度数据确定液相区的实验点分布，再由NaCl和CaCl2等水活度对应的纯盐浓度值和设定的不同CaCl2质量摩尔分数YB(0.0，0.1，0.3，0.5，0.7，0.9，0.97，1.0)来确定待测溶液的浓度. 2 仪器设备与实验方法 实验使用改进的等压装置及方法在以往文献中[9]已有较详细描述. 简言之，实验装置主要由等压箱(用镀有防腐层的铝材制成，带有螺旋加盖装置，箱内放置有14个带螺纹凹槽的镀铬紫铜传热板，可在同一组等压实验中放置14个镍铬合金等压杯)、恒温系统、自动传动装置、抽真空和通洁净干燥空气系统五部分组成. 在等压实验中，等压箱放入精确控温为(308.15 ± 0.01)K的恒温水浴中. 等压箱内共有14个等压杯，其中两个等压杯装NaCl参考溶液(在低浓度区域以NaCl(aq)和CaCl2(aq)做双参考(较高浓度区域时，NaCl易于结晶析出，采用CaCl2(aq)溶液做参考)，另10个等压杯分别装不同YB值的待测溶液. 根据预测的等水活度线上的点选择每一组实验中需要做的YB，每一组等压实验中配有一个YB值的平行样，与参考溶液的平行样做比较，当平行样品平衡浓度偏差小于0.5%时，作为等压平衡的标志. 实验表明，NaCl-CaCl2-H2O体系在0.6524~16.6631 mol kg-1范围内的等压平衡时间一般在10 d以内，平行样间等压平衡浓度的平均绝对误差为0.001 mol kg-1，最大为0.003 mol kg-1，平均相对偏差为0.1%. 3 等压平衡浓度与水活度 在308.15K下NaCl+CaCl2+H2O体系热力学性质的研究，该体系的溶解度数据取自文献,首先根据该体系的溶解度曲线确定液相区域，然后按照预测图中各实验点的分布，使等压测定的实验点均匀的分布在该体系的液相区内.高浓度点接近溶解度饱和线.在308.15 K下，实验测得的待测溶液的等压平衡浓度列于表1. 根据等压法的实验原理，待测体系溶液的水活度，在同一组实验达到等压平衡后，待测溶液与参考溶液具有相同的水活度。在NaCl-CaCl2-H2O与参考溶液NaC1(aq)或CaC12(aq)达到等压平衡时，体系的水活度按(1)式计算： 其中，*表示等压参考NaC1(高浓度时等压参考为CaCl2)，m*是等压参考溶液平衡浓度，Mw为分的水子量，v*为参考溶液分子完全离解的离子数(NaC1作参考时为2、CaCl2作参考时为3)，Ф*表示参考溶液渗透系数. NaCl-CaCl2-H2O体系的水活度列于表1，该体系不同YB值的等水活度线绘于图1，共绘制出14条等水活度线，高浓度点接近溶解度饱和线. 图1上方的曲线为NaCl-CaCl2-H2O体系的饱和溶解度曲线，所有实验点较均匀地分布于溶液度曲线下方. NaCl-H2O和CaCl2-H2O纯盐体系分别在0.5666~5.9265 mol kg-1、0.3943~5.5573 mol kg-1范围的等压平衡浓度及水活度见表1和图2. 由图2可见，CaCl2和NaCl溶液的水活度都随质量摩尔浓度增加而减小，在相同质量摩尔浓度下，NaCl溶液的水活度高于CaCl2溶液的水活度，说明CaCl2与水结合力远强于NaCl与水结合力。 4 溶液渗透系数与离子强的关系 当待测溶液NaCl-CaCl2-H2O与参考溶液NaCl(aq)或CaCl2(aq)达到等压平衡时，渗透系数Ф计算如式： 其中，*表示等压参考NaC1或CaCl2 (高浓度时等压参考为CaCl2)，m*是等压参考溶液平衡浓度，Mw为分的水子量，v*为参考溶液分子完全离解的离子数(NaC1作参考时为2、CaCl2作参考时为3)，Ф*表示参考溶液渗透系数，mi代表等压平衡时溶液中各物种的浓度，Ф表示NaCl-CaCl2-H2O体系的渗透系数. 在308.15 K下，NaC1作参考时的渗透系数Ф*数据取由文献[10]报导的浓度与相应的渗透系数的实验数据，经拟合获得Ф*与m*的关系为(3)式，拟合偏差为1.73×10-3. 当CaCl2作参考时的渗透系数Ф*也源自文献[10]，采用最小二乘法拟合渗透系数与浓度关系为(4)式，拟合偏差为SD = 1.57×10-3. 按(2)式计算的NaCl-CaCl2-H2O体系的实验渗透系数与参考溶液NaC1和参考溶液CaCl2的渗透系数列于表1，NaCl+CaCl2+H2O及其纯盐体系渗透系数与离子强度的关系见图3. 由图3可见，NaCl和CaCl2纯盐体系中(图3左)，纯盐溶液的渗透系数均随质量摩尔浓度的增加而增大，CaCl2溶液的增幅较大， NaCl参考溶液增幅较小. 在混合体系中(图3右)，YB值相同条件下，渗透系数随着离子强度的增加而增大，且随YB值增大这种变化趋势越明显；同一离子强度时，随着YB值增大，渗透系数随之变大，且混合盐渗透系数的变化介于NaCl和CaCl2纯盐变化趋势线之间。 5 平衡气相饱和蒸汽压的计算 根据实验计算的NaCl和CaCl2水活度，通过牛顿迭代法解式(5)、(6)方程组，求得308.15 K下NaCl-CaCl2-H2O体系的饱和蒸汽压. 其中，T为绝对温度(K)，aw为溶液的水活度，fs和fs0分别为308.15K下体系和纯水的平衡气相中水的逸度(Pa)；Ps和Ps0分别为体系和纯水在308.15 K的平衡气相蒸汽压(Pa)，R为普适气体常数；B2(T)是维里系数(m3 mol-1). NaCl-CaCl2-H2O体系及其纯盐体系的饱和蒸气压计算结果分别见表1. 图4是NaCl+CaCl2+H2O及其纯盐体系平衡气相饱和蒸汽压与离子强度的关系图. 由图4可见，NaCl和CaCl2纯盐体系中(图4左)，NaCl和CaCl2纯盐溶液的饱和蒸汽压随质量摩尔浓度的增大而减小，且CaCl2溶液饱和蒸汽压随质量摩尔浓度的变化较为显著. 在混合体系中(图4右)，YB相同条件下，饱和蒸汽压随离子强度的增加而减小，YB值越小这种变化趋势越显著；同一离子强度下，饱和蒸汽压随YB增大而增大，不同YB混合溶液的趋势线皆在CaCl2纯盐饱和蒸汽压曲线下方. 6 Pitzer离子相互作用模型的应用 Pitzer离子相互作用模型广泛地应用于强电解质水盐体系热力学性质研究[9,11]. 本实验根据Pitzer渗透系数方程(文献9和011有较详细介绍)，采用多元线性回归法分别拟合了对实验测定的6组NaCl-H2O纯盐体系离子强度在0.5666~5.9265 mol kg-1范围内的渗透系数数据和13组CaCl2-H2O纯盐溶液离子强度在0.3943~5.5573 mol kg-1范围内的渗透系数数据，求得NaCl、CaCl2的Pitzer纯盐参数β(0)、β(1)、Cφ，其结果列于表2，标准偏差分别为0.0004、0.012. 利用表1中NaCl+CaCl2+H2O体系在0.65～16.66mol kg-1的离子强度范围内对混合溶液等压平衡浓度进行多元线性回归，得到二离子θNa,Ca和三离子ψNa,Ca,Cl混合离子作用参数(表3)，标准偏差分别为0.0062和0.0064. 利用Pitzer模型计算了混合体系渗透系数，计算值与渗透系数实验值的偏差绘于图5，绝大部渗透系数实验值与模型计算值的偏差在0.01～-0.01之间，表明Pitzer方程可较好地表述该体系的热力学性质. 7 结论 本文采用等压法首次报导了308.15K下NaCl-CaCl2-H2O及其2个纯盐体系的等压平衡浓度和水活度，计算了渗透系数及饱和蒸汽压，获得了该体系渗透系数和饱和蒸汽压随离子强度的变化规律. 应用Pitzer离子相互作用模型，多元线性回归拟合获取308.15 K下NaCl和CaCl2的Pitzer单盐参数β(0)、β(1)、Cφ分别为0.3181、1.5346、-0.0014；0.07972、0.3067、0.00075和；Pitzer混合离子作用参数θNa,Ca、ψNa,Ca,Cl分别为0.06821、-0.0076. 应用Pitzer模型计算了混合体系渗透系数，计算值与渗透系数实验值吻合较好，表明Pitzer电解质溶液模型可成功地描述了NaCl-CaCl2 -H2O体系的热力学性质，为含钠钙卤水体系溶液模型建立提供了重要的基础数据. 表1 在308.15 K下NaCl-CaCl2-H2O体系的等压平衡浓度、渗透系数、水活度和蒸汽压 图3 308.15K下NaCl和CaCl2纯盐体系(左)和NaCl+CaCl2+H2O体系(右)渗透系数与离子强度的关系 图4 308.15K下NaCl和CaCl2纯盐体系(左)和NaCl-CaCl2-H2O体系(右)水蒸汽压与离子强度的关系 图5 308.15 K下NaCl-CaCl2-H2O体系渗透系数计算值与实验值的偏差 |
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The basic features, the applications, and the recent advances of the isopiestic method have been well de- scribed by Rard and Platford [24,25] in previous publi- cations. An improved isopiestic chamber employed in this work was similar with the one used in the literature [5]. The chamber was designed for some precise meas- urements from very low to higher molalities or at ele- vated and lower temperatures. The schematic diagram of the apparatus is given in figure 1. The chamber was made from stainless steel. Eight tantalum cups and tef- lon caps tight fitting to the cups with O-rubber rings were used in this work. A internal capping device was mounted in the top of the chamber for lowering and pressing down the caps onto the simple cups to seal the cups under equilibrium conditions before they were removed from the chamber to avoid effectively the experimental error caused from evaporation losses of solution simples or condensation of vapor. The experiments were performed at T = 298.15 K in a water-filled constant temperature bath which was con- trolled to ±0.01 K under the room temperature (296 ± 0.5) K kept by an air conditioner. Two stirrers were placed in the bath to minimize temperature gradi- ents. The temperature was measured by means of a mer- cury thermometer that was calibrated by a standard platinum resistance thermometer. A thick pure copper block with eight cylindrical holes whose surface plated by gold was used for holding the isopiestic cups and for tight contacting the bases of the cups with the block, obtained rapid heat transfer. The chamber with 8 mm thick stainless steel wall, the thick copper block, and the small area of contact between them formed effective thermal buffer. These were able to damp out the fluctu- ations in bath temperature and to eliminate internal temperature gradients between the cups. A rocking device was installed in the bath, powered by a low speed electric motor, isopiestic chamber was rocked back and forth for 30 min per hour with a fre- quency of 50 cycles per minute to mix well the sample solutions and the vapor in the chamber. A Sartorius analytical electronic balance to accuracy of ±0.0001 g was used for all weighings in this experiment. After themass of each empty cup covered with cap had been determined, a total amount of about 2 g of the sam- ple solution was weighed into every sample cup. Aqueous NaCl solution was used as isopiestic reference standard. After closed, the isopiestic chamber was then slowly evac- uated and the solutions were carefully degassed to being near free of air. Equilibrations were performed generally within 7–12 days for intermediate and supersaturated solutions and about 15 days for very lowmolalities.When equilibriumwas attained the sample cupswere closedwith the caps fixed previously on the capping device inside iso- piestic chamber, and then the chamber was removed from the thermostat bath, clean dry air was admitted to the chamber, all of the cups sealed with the caps were placed into a desiccator and stayed for 30 min, were then weighed. Fromthe vacuum-correctedmasses of solutions and the molalities of the stock solutions the isopiestic equilibrium molalities of the solutions were determined. Duplicate or triplicate samples were used for the all salts samples in the all-experimental runs. The molalities of several replicate samples whose original molalities dif- fered by several percent came to be near equal,which indi- cated that equilibrium was attained. The water purified by deionization followed by distil- lation twice (once from K2MnO4) with conductance of 1Æ104 SÆm1 was used for all sample purifications, preparations, and dilutions in the experiment. Li2B4O7 commercial reagent (made in Beijing Xinhua Reagent Factory, A.R. grade) was recrystallized twice. The stock solution of Li2B4O7 was prepared from purified Li2B4O7 and water in which CO2 was removed, and analyzed by mass titration in the presence of mannitol using NaOH standard solution as titrant and phenolphthalein as indi- cator, triplicate samples agreed to 0.1% or better. The NaOH(aq) from which carbonate was removed previ- ously had been standardized with primary standard Na2B4O7 solution which was prepared from borax Na2- B4O7 Æ10H2O (Shanghai Reagent, Standard grade) keep- ing and equilibrating over a saturated solution of sucrose and sodium chloride in a desiccator by precisely weighing the borax and the water, the deviations were less than 0.05% for five replicate samples of NaOHIsopiestic equilibrium molalities of aqueous Li2B4O7 lutions in stoichiometric form of tetraborate and Na- (aq) reference solutions are given in column 2 and 5 of ble 1, respectively. The equilibrium molalities were determined to better an ±0.001 mol Ækg1 in nearly all cases (see table 1). he average values of the molalities at isopiestic equilib- um from replicate samples were taken as equilibrium olalities, and the absolute difference smaller than 0003 mol Ækg1 for molalities lower than 0.1052 ol Ækg1 was obtained by using the improved isopiestic amber and extending the equilibration time, which re comparable to the accuracy usually observed and quired for isopiestic measurements. When aqueous NaCl reference solutions and aqueous 2B4O7 solutions were at thermodynamic equilibrium, ey will have equal solvent activities. The water activi- s of the aqueous NaCl reference standards were calcu- ed by the following equation: aw ¼m m Mw/ =1000; ð1Þ here quantities with asterisks are denoted for isopiestic erence standards, m* = 2 denotes the number of ions rmed by the complete dissociation of one molecule NaCl, m* is isopiestic equilibrium molality, Mw is olar mass of H2O, /* is osmotic coefficient of NaCl erence standard estimated by using a least square uation fitted to the smoothed experimental data given Hamer and Wu (see [26]), with a standard deviation 0.0003, then the water activities of aqueous Li2B4O7 lutions at the equilibrium molalities with that of Na- (aq) were obtained. All of our experimental water tivities were plotted as a function of molalities in |
10楼2010-07-03 19:05:33
