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左边木虫 (正式写手)
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[交流]
每天跟晶体打交道,还是没搞很懂
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近来三个月做的项目是每一步的产品都要重结晶的,刚好遇到了很多晶体结晶方面的很多问题,实践中学习,积累了后才了解结晶是门很复杂的学问了.希望能多结交些跟晶体打交道的虫友. 现在遇到了问题是产品脱干溶剂后得到油状物质,我先前用石油醚热提取方法,能得到90%纯度的,但是还是油状物. 又试过冷冻析晶,和保温析晶. 现在做到保温搅拌析晶,分析结果还没出来,不知道纯度如何. 以前结晶做得不多,现在长见识了. 在此拜会各位大虫了. [search]结晶[/search] [ Last edited by daiqiguang on 2007-8-25 at 19:55 ] |
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cjkong
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treat about the oil just few ways ,followed :
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treat about the oil just few ways ,followed : The separation of a second liquid phase, commonly known as an" oil," instead of the expected crystalline solid, sometimes occurs during recrystallisation. The oil often solidifies on standing, although at times a considerable period may elapse before crystallisation occurs. The resulting crystals will probably occlude some of the mother liquor, and the purity will therefore not be high. If the substance has a low melting point, it must not be assumed that this apparently abnormal behaviour is due to the presence of impurities ; it is probably an example of a system in which the solid phases consist of the pure components and the components are only partially miscible in the liquid state (see Section 1,18). The separation of the oil may be avoided by diluting the solution considerably, but this will lead to large losses. It is probably best to re-heat the mixture until a clear solution is obtained, and allow it to cool spontaneously ; immediately the oil commences to separate, the mixture is vigorously stirred so that the oil is well dispersed in the solution. Eventually, crystals will separate and these will grow in the bulk of the solution and not in a pool of oil, so that occlusion of the mother liquor is considerably reduced. When all the oil has disappeared, stirring may be stopped and the crystals allowed to accumulate. Sometimes the addition of a minute quantity of the crude compound in order to " see'd " the solution may facilitate the initial crystallisation. Occasionally substances form supersaturated solutions from which the first crystals separate with difficulty ; this is sometimes caused by the presence of a little tar or viscous substance acting as a protective colloid. The following methods should be tried in order to induce crystallisation:— 1. By scratching the inside of the vessel with a glass rod. The effect is attributed to the breaking off of small particles of glass which may act as crystal nuclei, or to the roughening of the surface, which facilitates more rapid orientation of the crystals on the surface. 2. By inoculating ("seeding "  the solution with some of the solid material or with isomorphous crystals, crystallisation frequently commences and continues until equilibrium is reached. The " seed crystals " may be obtained by cooling a very thin film of liquid to a low temperature. Several drops of the solution are placed in a test-tube or beaker and spread into a thin film by rotating the container ; the latter is then cooled in a mixture of ice and salt or in some other suitable freezing mixture. A better procedure, which avoids the necessity of subsequently scrapingthe surface to remove the " seed crystals " and the attendant melting if the compound is impure or of low melting point, is to moisten a small glass bead with the supersaturated solution, place it in a test-tube, cool the latter in a freezing mixture and thus form crystals on the surface of the bead. The glass bead can then be rolled out of the tube into the vessel containing the main bulk of the solution. 3. By cooling the solution in a freezing mixture (ice and salt, ice and calcium chloride, or solid carbon dioxide and ether). It must be borne in mind that the rate of crystal formation is inversely proportional to the temperature ; cooling to very low temperatures may render the mass very viscous and thus considerably hinder crystallisation. In the latter case, the mixture should be allowed to warm slowly so that it may be given the opportunity to form crystals if it passes through an optimum temperature region for crystal formation. Once minute crystals have been formed, it is very probable that their size will be increased by keeping the mixture at a somewhat higher temperature. 4. By adding a few lumps of solid carbon dioxide ; this produces a number of cold spots here and there, and assists the formation of crystals. 5. If all the above methods fail, the solution should be left in an ice chest (or a refrigerator) for a prolonged period. The exercise of considerable patience is sometimes necessary so as to give the solution every opportunity to crystallise. The product of a chemical reaction, isolated by solvent extraction and subsequent removal of the solvent, which should normally be crystalline, is sometimes an oil, due to the presence of impurities. It is usually advisable to attempt to induce the oil to crystallise before purifying it by recrystallisation. Methods / and 2 (previous paragraph) may be applied; method 2 cannot always be used because of the difficulty of securing the necessary seed crystals, but should these be available, successful results will usually be obtained. Another procedure is to add a small quantity of an organic solvent in which the compound is sparingly soluble or insoluble, and then to rub with a stirring rod or grind in a mortar until crystals appear ; it may be necessary to continue the rubbing for an hour before signs of solidification are apparent. Another useful expedient is to leave the oil in a vacuum desiccator over silica gel or some other drying agent. If all the above methods fail to induce crystallisation, direct recrystallisation may be attempted: the solution should be boiled with decolourising carbon as this may remove some of the impurities responsible for the difficulty of crystal formation. Occasionally, conversion into a simple crystalline derivative is applicable ; subsequent regeneration of the original compound will usually yield a pure, crystalline solid. |
5楼2007-08-26 13:52:00
2楼2007-08-20 21:49:08
3楼2007-08-21 18:52:52
liuyuhong_0418
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4楼2007-08-25 12:03:49
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