读者园地

读者园地问:为什么有些专著中把钪和钇也称为稀土元素?浙江读者———张虹答:钪(Scandium,元素符号Sc,原子序数21)和钇(Yttrium,元素符号 Y,原子序数 39)分别是第 4周期和第5周期的第一个外过渡元素,同属ⅢA族。它们的最外层电子的排列方式与镧系元素(即稀土元素)的最外层电子排列相似,导致它们的许多化学性质也与稀土元素相似。而且在一些矿物中它们也常与稀土元素共生。因此,在化学中常把它们与稀土元素放在一起讨论,有些书刊中甚至把它们也称为稀土元素。元素钪的最外层电子构型为3d14s2,它的性质与稀土元素中的镧(Lanthanum,符号La,…     

为什么有些专著中把钪和钇也称为稀土元素？

钪(Scandium，元素符号Sc，原子序数21)和钇(Yttrium，元素符号Y，原子序数39)分别是第4周期和第5周期的第一个外过渡元素，同属ⅢA族。它们的最外层电子的排列方式与镧系元素(即稀土元素)的最外层电子排列相似，导致它们的许多化学性质也与稀土元素相似。而且在一些矿物中它们也常与稀土元素共生。因此，在化学中常把它们与稀土元素放在一起讨论，有些书刊中甚至把它们也称为稀土元素。     

金属材料分析方法的选择和施行第六讲金属材料中稀土元素的测定(续)

钇的离子半径介于镝与钬之间,因此它的性质更接近于重稀元素。当元素的价态发生变化时,离子半径也发生变化。例如三价铕离子的半径为1.13,而二价铕的半径就增加到1.2~1.3,这就与锶的离子半径(1.27)接近了。因此,二价铕离子的性质与碱土金属离子比较相似,因而,就有可能用硫酸锶来共沉淀铕,以富集微量的铕。表6-3中还列出了稀土元素三价离子的颜色,可作为鉴别的参考。1.4镧系元素的化学性质在1.2及1.3节中简要地提到了镧系元素的电子结构和价态的关系以及镧系收缩和原子半径,离子半径的递变规律。这些元素的上述性质和现象对它们的化学性质也…     

金属材料分析方法的选择和施行——第六讲 金属材料中稀土元素的测定（续）

2.5钪及其螯合滴定2.5.1钪的性质21号元素钪位于周期系ⅢB族,常作为稀土元素之一,其3d14s2的结构使它的某些性质与镧系元素相似,特别是其电子构型与镧(5d16s2)相似。但钪的离子半径(81 pm)和共价半径(144 pm)比镧系元素小得多。在水溶液中,钪(Ⅲ)具有很大极化力,容易形成络合物。钪(Ⅲ)的有些性质较类似于重稀土离子,如能生成硫酸复盐沉淀,草酸盐沉淀和氟化物沉淀等,而另一些性质又与钍、锆等离子相似,如能生成硫代硫酸盐沉淀等,总的说来,钪不是一个非常典型的稀土元素。钪含量较高的矿物只有钪钇石[(ScY)2Si2O7,含Sc2O3(w%)约35%~42%]和…     

今日稀土

稀土元素是包括原子序数从57到71的15个镧系元素,加上周期表中同属Ⅲ副族的钪和钇,共有17个元素。由于决定它们的化学性质的外层电子结构基本相同,要分离出纯的单一稀土化合物比较困难,又由于它们的化学性质活泼,不易还原为金属,所以比其它常见元素发现得较晚。从1794年发现钇,直到1947年从铀裂变产物中分离得到钷为止,17个元素全部分离出来,整整用了150年之久。     

名词小议(续十一)

(七九)镧系元素共十五种,原子序数从57(镧)到71(镥)。另加21号钪和39号钇,统称为希土元素。因此‘希土和钪’(见分析测试通报1992,11(2)44)这种提法是不恰当的。希土已包括钪,何必单列出来。如要突出钪,可称为镧系元素和钪。英文题目也要相应改     

金属材料分析方法的选择和施行第六讲金属材料中稀土元素的测定(续)

第三次沉淀是草酸沉淀，经过前两次分离，干扰稀土（Ⅲ）最终以草酸盐沉淀的共存元素已经除去。这次沉淀是以草酸盐的形式得到稀土的沉淀并最终得到符合化学计量要求的稀土氧化物。应予说明，试样如含有钇及钪，在整个分离过程中均与稀土元素在一起，故最终也包括在总稀土的含量之中。     

希土金属及其在工业上应用的展望

近年来,由于新技术的需要,钍、钛、铌和钽的生产显著地增长。因此在矿物原料中与这些元素共生的希土金属的利用就成为重要的国民經济任务。在門捷列夫周期系中,镧及其以后从铈(58)到(?)(71)的14个元素称为希土金属或者镧系元素。钪(21)和钇(39)与希土元素的性貭很接近。希土金属占目前所有已知元素的六分之一,約为地壳总重的0.01％。它在自然界中的儲量較锌、銅、鉛、錫和貴金属要大几十倍甚至几百倍。由于希土金属原子的外层电子結构相同,因此,它們的物理-化学性貭也很相近。它們都具有三个外层价电子。但它們在4f层上的电子数是各不相同的(4f层能容納14个电子)。铈     

PMBP萃取-偶氮胂Ⅲ分光光度法测定茶叶中稀土元素总量

正稀土元素是镧系元素(15种)及与其性质相似的钪、钇两元素共17种元素的总称。稀土元素不仅可以促进植物根系的发育、促进幼苗生长,还能提高作物的产量、改善品质、增强作物的抗逆性和抗病性,因此在农业上得到了广泛的应用[1]。由于茶叶主要种植在高山上,容易受到干旱的影响,于是越来越多的茶农喷洒稀土药剂,以达到抗旱增产的目的,因此茶叶中会残留一定量的稀土元素,但国家标准GB 2762-2005《食品中污染物限量》中已规定茶叶     

金属材料分析方法的选择和施行 第六讲 金属材料中稀土元素的测定（续）

1.5镧系元素的物理性质1.5.1镧系元素的晶体构型镧、铈、镨三元素是具有六方密堆积(h.c.p.)和立方密堆积(c.c.p.)晶型的双晶形金属,元素钪是面心立方(f.c.c.)和六方密堆积晶型的双晶形金属,钐是斜方六面体晶形,铕是体心立方晶形,镱是面心立方晶形,其余稀土金属都是六方密堆积晶形。稀土金属大都具有同素异晶的构型,由于晶型的转变,使其与其它金属的作用复杂化。钕的熔点为1 016℃,但以前曾把它由六方晶系向体心六立晶系转变的温度(840℃)误作为该金属的熔点。1.5.2镧系元素的密度钪的密度为2.99 g·cm-3和钇的密度为4.47 g·cm-3。其余元素…     

Generation and detection of levuglandins and isolevuglandins in vitro and in vivo

Levuglandins (LGs) and isolevuglandins (isoLGs), formed by rearrangement of endoperoxide intermediates generated through the cyclooxygenase and free radical induced oxidation of polyunsaturated fatty acids (PUFAs), are extraordinarily reactive, forming covalent adducts incorporating protein lysyl ε-amino groups. Because they accumulate, these adducts provide a dosimeter of oxidative injury. This review provides an updated and comprehensive overview of the generation of LG/isoLG in vitro and in vivo and the detection methods for the adducts of LG/isoLG and biological molecules in vivo.     

Enthalpies of solution of purine and adenine in water and in dimethylsulfoxide

Journal of Solution Chemistry - Enthalpies of solution of purine and adenine in water and in demethylsulfoxide were measured calorimetrically in the temperature range 25–40°C. ΔH s...     

Determination of diazepam and nordazepam in milk and plasma in the presence of oxazepam and temazepam

For studies on the excretion of drugs into milk a sensitive high-performance liquid chromatographic assay was developed to quantitate diazepam and nordazepam in the milk and plasma of humans and rabbits in the presence of their major metabolites, oxazepam and temazepam. Flurazepam was used as an internal standard. The assay involves extractions with diethyl ether and an additional acid clean-up step. Chromatographic separation was achieved by a LiChrospher 60 RP-select B (5 microns) column and KH2PO4- acetonitrile (69:31, v/v) adjusted to pH 2.80 as a mobile phase. The same extraction and chromatographic conditions were suited to both types of samples, milk and plasma. The limits of determination using ultraviolet detection at 241 nm was for diazepam 20 ng/ml and for nordazepam 15 ng/ml. The absolute recoveries of diazepam, nordazepam and flurazepam in human milk were 84, 86 and 92% and in human plasma 97, 89 and 94%, respectively. The within- and between-day accuracy and precision for diazepam and nordazepam in milk and plasma at all concentrations tested (20-1500 ng/ml) were better than 8%. The high fat content which occurs in rabbit milk presented no limitation for the extraction of lipophilic diazepam: the method was successfully used to monitor milk and plasma concentrations of diazepam and nordazepam in lactating New Zealand White rabbits during 26-h infusions of diazepam (1.4 mg/h).     

Coassembly of Nanorods and Nanospheres in Suspensions and in Stratified Films

The entropically driven coassembly of nanorods (cellulose nanocrystals, CNCs) and nanospheres (dye‐labeled spherical latex nanoparticles, NPs) was studied in aqueous suspensions and in solid films. In mixed CNC‐latex suspensions, phase separation into an isotropic latex‐NP‐rich and a chiral nematic CNC‐rich phase took place; the latter contained a significant amount of latex NPs. Drying the mixed suspension resulted in CNC‐latex films with planar disordered layers of latex NPs, which alternated with chiral nematic CNC‐rich regions. In addition, fluorescent latex NPs were embedded in the chiral nematic domains. The stratified morphology of the films, together with a random distribution of latex NPs in the anisotropic phase, led to the films having close‐to‐uniform fluorescence, birefringence, and circular dichroism properties.     

Comparison and correlation of in vitro, in vivo and in silico evaluations of alpha, beta and gamma cyclodextrin complexes of curcumin

In the present study investigated the effect of curcumin (CUR) alpha (α), beta (β) and gamma (γ) cyclodextrin (CD) complexes on its solubility and bioavailability. CUR the active principle of turmeric is a natural antioxidant agent with potent anti-inflammatory activity along with chemotherapeutic and chemopreventive properties. Poor solubility and poor oral bioavailability are the main reasons which preclude CUR use in therapy. Extent of complexation was β-CD complex (82 %) > γ-CD (71 %) > α-CD (65 %). Pulverization method resulted in significant enhancement of CUR (0.002 mg/ml) solubility with CUR α-CD complex (0.364 mg/ml) > CUR β-CD complex (0.186 mg/ml) > CUR γ-CD complex (0.068 mg/ml). Gibbs-free energy and in silico molecular docking studies favour formation of α-CD complex > β-CD complex > γ-CD complex. With reference to CUR, relative bioavailability of CUR α-CD, CUR β-CD and CUR γ-CD complexes were 460, 365 and 99 % respectively. CUR–CD complexes exhibited increased bioavailability with an increase in t_½, tmax, Cmax, AUC, Ka, and MRT; and a decrease in Ke, clearance and Vd values. AUC increase was CUR α-CD complex > CUR β-CD complex > CUR γ-CD complex. Significant difference (p < 0.05) was observed between CUR α-CD complex and CUR γ-CD complex by one-way ANOVA and Dunnett’s post hoc test for multiple comparison analysis. Correlation observed between in vitro, in vivo and in silico methods indicates potential of in silico and in vitro methods in CD selection.     

Homo- and hetero-complexes of exchangeable apolipoproteins in solution and in lipid-bound form

The self-association state of human plasma apolipoprotein E (apoE) in solution and in complexes with dimyristoylphosphatidylcholine (DMPC) varying in stoichiometry was studied in sub-micromolar concentration range by gel filtration, fluorescence anisotropy, fluorescence quenching and energy transfer measurements with apolipoprotein labeled with lysine-specific fluorescent dyes. Together, these results confirm the equilibrium scheme for various apoE structures in solution: oligomer (in aged preparations) <==> 'closed' tetramer <==> 'open' tetramer ('molten globule' state) <==> native or partially denatured monomer <==> fully denatured monomer. Within DMPC:apoE discoidal complex (125:1) the apolipoprotein association state seems to be intermediate between that in solution and in larger vesicular complex (1000:1); for both complexes, the degree of exposure of fluorescein chromophores into water phase decreased. Hetero-associates of apoA-I and apoC-III-1 in solution and in the complexes with DMPC appear to behave similarly to apoE. When extrapolated to native HDL particles, 'molten globule' state seems to be a structure responsible for the interaction of exchangeable apolipoproteins with phospholipid. For a first time, the location of various apolipoprotein molecules on disc periphery was confirmed. The lysine residue(s) seems to locate closely to reacting residue(s) within apolipoprotein molecules in associates, however, with different package constraints for discoidal versus vesicular complexes with phospholipid.     

Electrochemical Fluorination of Benzamide and Acetanilide in Anhydrous HF and in Acetonitrile

Electrochemical fluorination of benzamide in anhydrous hydrogen fluoride does not involve the amide group but occurs exclusively at the aromatic ring, yielding isomeric fluoro- and difluorobenzamides and 3,3,6,6-tetrafluoro-1,4-cyclohexadienecarboxamide. Electrochemical fluorination of benzamide in acetonitrile as solvent gives the same products, as well as benzonitrile and its fluorinated derivatives and products of hydrolysis and fluorination of acetonitrile. Electrochemical fluorination of acetanilide in anhydrous HF leads to complete tarring of the reaction mixture, while its fluorination in acetonitrile results in selective formation of m-fluoroacetanilide.