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 End Process in Chromatographic Column and Linear Relationship between Half Peak Width and Retention Time 色谱柱末端过程与峰宽规律性 短句来源 Studies on Internal Rotation of Methyl in Substituted Benzene by Using ~(13)C NMR Spin-lattice Relaxation Time 用~(13)C的NMR T_1研究苯环上甲基的内旋转 短句来源 Effect of Some Spectroscopic Buffers on Transite Time of Cu and Cr Atoms in DC Arc Discharge 直流电弧中若干缓冲剂对Cu及Cr原子停留时间的影响 短句来源 Determination of Dead Time in Reversed-Phase Ion Pair Chromatography ( I ) 反相离子对色谱中的死时间测定(Ⅰ) 短句来源 A Note on the Perterson Formula for the Determination of Dead Time in Gas Chromatography 关于Perterson公式的讨论 短句来源 更多

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 time
 The proof is based on a variant of Moser's method using time-dependent vector fields. We present two-sided singular value estimates for a class of convolution-product operators related to time-frequency localization. The Balian-Low theorem (BLT) is a key result in time-frequency analysis, originally stated by Balian and, independently, by Low, as: If a Gabor system $\{e^{2\pi imbt} \, g(t-na)\}_{m,n \in \mbox{\bf Z}}$ Gabor Time-Frequency Lattices and the Wexler-Raz Identity Gabor time-frequency lattices are sets of functions of the form $g_{m \alpha , n \beta} (t) =e^{-2 \pi i \alpha m t}g(t-n \beta)$ generated from a given function $g(t)$ by discrete translations in time and frequency. 更多
 An electrophotometric method has been worked out for the determination of magnesium in cast iron using titan yellow as indicator.Briefly,the method consists of dissolving the sample in dilute sulphuric acid,separating iron and other metals such as copper and nickel by electrolysis with mercury cathode and precipitating manganese from ammoniacal solution with ammonium persulfate.Ammonimn salt,silica and traces of aluminaare thenremoved. Finally,the prepared titan yellow reagent is added to the solutionand the... An electrophotometric method has been worked out for the determination of magnesium in cast iron using titan yellow as indicator.Briefly,the method consists of dissolving the sample in dilute sulphuric acid,separating iron and other metals such as copper and nickel by electrolysis with mercury cathode and precipitating manganese from ammoniacal solution with ammonium persulfate.Ammonimn salt,silica and traces of aluminaare thenremoved. Finally,the prepared titan yellow reagent is added to the solutionand the magnesium content determined from the electrophotometric reading.The time required for each determination is approximately 5 hours. The effect of interfering elements which are likely present in cast iron such as manganese, silicon,phosphorus, aluminum, copper, nickel and vanadium as well as ammonium ion and alkali and alkaline earth metals has been studied and discussed.It has been shown that manganese, silicon, aluminum,copper,nickel,iron and ammonium ions can be removed down to such low values as to cause no significant interference.Phosphorus under 0.4% has no appreciable effect.Vanadium is not separated in the present method,but fortunately it is usually present in insignificant amounts in ordinary cast iron.The effect of calcium is compensated by the introduction of saturated calcium sulphate solution before the titan yellow addition. Potassium and sodium have no appreciable effect when present in small amounts. The results obtained by the present method for the determination of magnesium in 11 known samples are good,the average difference being 0.002% in a range of 0.000% to 0.140% Mg.The data obtained by the present method are also in good agreement with those obtained by gravimetric methods On seven nodular iron samples made by the Metallurgical Division of our Institute.A detailed procedure for the determination of magnesium in cast iron has been suggested and included in this paper.In view of its rapidity and fair accuracy,the new photometric method might prove useful for the purpose of control in a nodular iron foundry. 本文报告将地丹黄比色法应用於测定铸铁中之镁的试验结果.如将铁、锰、铝、矽等干扰物质预先除去,则镁的测定并无困难.依照本文所擬的方法分析,所需时间约为五小时,较重量法迅速得多. The effects of copper and zinc ionsupon the precipitation of barium sul-phate have been noted for a long time.However,the results published wereeither not systematic(3,4b,5,6)or obtained under conditions apart fromusual analytical practice(7,8).Thus,it seems necessary to investigate thissubject in more detail by the usual methods of determination.The presentcommunication reports(a)the effect of various amounts of cupric and zincions and(b)the influence of different acidities during precipitation in thepresence... The effects of copper and zinc ionsupon the precipitation of barium sul-phate have been noted for a long time.However,the results published wereeither not systematic(3,4b,5,6)or obtained under conditions apart fromusual analytical practice(7,8).Thus,it seems necessary to investigate thissubject in more detail by the usual methods of determination.The presentcommunication reports(a)the effect of various amounts of cupric and zincions and(b)the influence of different acidities during precipitation in thepresence and absence of zinc ions.It is found that the interference of copper in the determination of sulphuras barium sulphate up to a ratio of 20:1 for Cu:S is negligible.The deviationsof the results obtained from theoretical values are all within ±2‰(see Table1),and do not show any regularity with the amount of copper added.The presence of zinc ions alone gives satisfactory results(slightly lowerthan theoretical values).As the amount of zinc ions in solution increases,thedeviations become somewhat wider(Table 2).The low results are presumedto be due to(1)the formation of soluble zinc-sulphate complex,which holdsa part of sulphate in solution,and/or(2)diverse ion effect.In the presenceof zinc ions,introducing hydrochloric acid up to 0.1 N before precipitationcauses essentially the same errors as if hydrochloric acid were absent(cf.Tables2 and 3),while at and beyond 0.2 N positive errors are obtained.A spectrographic examination of the precipitates reveals that zinc doescoprecipitate with barium sulphate,though only to a slight extent.The amountof zinc coprecipitated decreases with the increase of the acidity of the solutionfrom which the precipitate is thrown down. 本文报告铜及锌离子对于硫酸钡定硫的影响,找出了铜的影响几近于零,锌的影响亦微。锌离子能与硫酸钡共同沉淀。 When ceric iodate is precipitated from acid solution by the addition of asoluble iodate,a gelatinous precipitate is obtained,which is difficult to filterand wash.The precipitate must be redissolved and reprecipitated as oxalatefor final ignition to ceric oxide.A method has been deviced to improve the form of the ceric iodateprecipitate by slow precipitation in homogeneous solution.Advantage is takenof the fact that cerium exists in two distinct valence states,the cerous and theceric.Cerous iodate is soluble... When ceric iodate is precipitated from acid solution by the addition of asoluble iodate,a gelatinous precipitate is obtained,which is difficult to filterand wash.The precipitate must be redissolved and reprecipitated as oxalatefor final ignition to ceric oxide.A method has been deviced to improve the form of the ceric iodateprecipitate by slow precipitation in homogeneous solution.Advantage is takenof the fact that cerium exists in two distinct valence states,the cerous and theceric.Cerous iodate is soluble in dilute acid solution,while ceric iodate isinsoluble.Cerium is first reduced to the cerous state by hydrogen peroxide innitric acid solution.Ammonium iodate is added and no precipitation occurs atthis moment.The precipitation is brought about by the slow formation ofceric ion in the iodate solution by ammonium persulfate.Thus a denseprecipitate is formed slowly.This compact volume of the precipitate facilitatesfiltration and washing.The precipitate is ignited directly to ceric oxide,part of which is always stained with a brown color.This is remedied by evapora-tion of the ignited precipitate to dryness with a few drops of concentratedsulfuric acid and ignition to ceric oxide for final weighing.For samples containing 10-50 mg of ceric oxide,a total volume of 300-400 ml gives best results.The optimum concentration of the nitric acid presentis 0.4-0.5 N.A starting temperature of 40-60℃ favors the denseness of theprecipitate.Precipitation is continued at an elevated temperature(70-80℃ )on a hot plate.Stirring not only improves the crystallinity of the precipitateand may decrease the occlusion of impurities,but also shortens the time forcomplete precipitation.Dilute iodic acid solution(1-2%)is used as washsolution.This method is good for samples containing 2.5-250 mg of cericoxide.The composition of the precipitate is not reproducible.The precipitate is abasic iodate and the ratio of IO_3~-found/IO_3~- theoretical for Ce(IO_3)_4 increaseswith the concentration of the nitric acid present(up to 0.5 N)to a maximumvalue of about 0.92.The ratio never reaches unity.An analysis of the pre-cipitate dried at 40-45℃ for 2-3 days shows a fairly constant ratio of IO_3~- found/IO_3 theoretical for Ce(IO_3)_4 From the ratio of Ce:IO_3,an approximate formulaCe_2(IO_3)_7(OH)·XH_2O has been proposed for the precipitate obtained under theconditions described.The number of water molecules in the formula is in-definite,varying from 3 to 5. 1.在均匀溶液中沉淀的碘酸铈,性质紧密,便利于过滤和洗涤。烧灼成为氧化铈后,适合于作为铈的定量分析。2.利用碘酸亚铈能溶解于稀酸溶液而碘酸铈则不溶的区别,先用过氧化氢把全部铈离子还原,再加入碘酸铵,然后进行氧化,使碘酸铈在均匀溶液中沉淀出来。3.本方法适用于含氧化铈2.5—250毫克的样品。4.所得沉淀的分子式经初步测定为 C_(e2)(IO_3)_7(OH)·x H_2O,其中 x 的数值不定,涨落在3与5之间。 << 更多相关文摘
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