Name: separating mixtures with solvent extraction
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 2 min 55 s
Channel: JoVE

萃取是一种从混合物中分离成分的技术。您已经熟悉提取。每次泡茶时，您都会从茶叶中提取水溶性化合物，如咖啡因和香料，进入热水中。这是固液萃取的一个例子。
液-液萃取是另一种类型的萃取，其中混合物溶解在两个不混溶的液相中。溶剂必须是不混溶的，这意味着它们不会混合并且是单独的相。液-液萃取在分液漏斗中进行，因此密度较大的溶剂将沉降到底部，而密度较低的溶剂将位于顶部。然后，每种溶质化合物将转移到它最易溶解的阶段。因此，选择具有不同极性的溶剂很重要。
一般来说，非极性溶质会分配到有机相中，而极性溶质会分配到水相中。分配系数 K 是有机相中溶质浓度与水相中溶质浓度的比值。溶质分离后，收集两个不同的相。请记住，在两个阶段中，每个组分都可能有残留。
酸碱萃取是一种特殊类型的液-液萃取，它根据溶解度差异分离酸性化合物和碱性化合物。回想一下，酸是溶解在水中时提供质子的化合物，而碱是接受质子的化合物。
为了分离酸性化合物，在混合物中加入碱。碱会接受来自酸性化合物的质子，使其成为离子的。相反，添加酸会将质子转移到碱性化合物中。在这两种情况下，离子化合物将转移到水相中，而中性化合物将分配到有机相中。离子化合物被收集，然后去质子化或再质子化以将其转化回其原始化合物。
在本实验中，您将分离纤维素、苯甲酸和咖啡因的混合物。首先，您将使用萃取和过滤来分离纤维素。然后，您将进行酸碱萃取以分离苯甲酸和咖啡因。

萃取

萃取
萃取是有机化学中用于分离目标化合物的常用技术。在萃取过程中，溶质从一个相转移到另一个相，以将其与未反应的起始材料或杂质分离。萃取还用于促进从难以蒸发去除的反应溶剂（例如高沸点溶剂）中分离溶质。<span style="color: rgb（0， 0， 0）;字体系列:Roboto、RobotoDraft、Helvetica、Arial、sans-serif;字体大小:13px;字体样式:normal;font-variant-ligatures: 正常;font-variant-caps:正常;字体粗细:400;letter-spacing: 正常;孤儿: 2;文本对齐: start;文本缩进:0px;文本转换:无;white-space:预包装;寡妇:2;字间距:0px;-webkit-text-stroke-width: 0px;背景色:rgb（255， 255， 255）;text-decoration-style: 初始;text-decoration-color:初始;显示: inline ！important;float: 无;> 一般来说，有三种类型的提取。首先，在固液萃取中，溶质从固相转移到液相。在液-液萃取中，溶质从一种液体转移到另一种液体。在酸碱萃取中，溶质转化为离子化合物并从有机相转移到水相。萃取的一个常见例子是冲泡咖啡或茶。含有咖啡因、植物风味和气味的固相被热水提取到液相中。
液-液萃取
液-液萃取将溶解在水相中的有机化合物转移到有机溶剂中，或者用于将未反应的反应物、盐和其他水溶性杂质转移到水相中，同时将感兴趣的有机化合物留在有机相中。不混溶液体是指即使充分混合，也永远不会形成均匀溶液的液体。相反，不混溶的液体会分离成不同的相，如油和水。
液-液萃取将溶解在水相中的有机化合物转移到有机溶剂中。为了进行液-液萃取，首先将含有溶质的水溶液加入分液漏斗中。然后，将非水溶性有机溶剂添加到分液漏斗中。当漏斗中的内容物充分混合时，有机化合物基于其在有机相中的溶解度高于水相，因此分配到有机相中。
由于这两种溶剂是不混溶的，因此两种液体形成离散层，密度较大的液体在底部，密度较低的液体在顶部。一旦这两个相重新沉淀成两层，通过打开分离漏斗底部的旋塞并让一层流出来将它们分开。去除溶质的液体称为抽余液，而获得溶质的液体称为提取物。
有机化合物根据其在每个相中的溶解度在两层之间分配。当溶质的化学势在两相中相同时，达到平衡。溶质的分配系数 K 是有机层中样品的浓度除以水相中的浓度之比。分配系数是一个常数，取决于溶质和萃取中使用的溶剂对。分配系数是溶质对两种溶剂中每一种的偏好的表达式。分配系数大的溶质更容易被提取到有机溶剂层中。分配系数小的溶质喜欢过渡到水相。
确定用于液-液萃取的溶剂对是至关重要的一步。选择要使用的溶剂时，请考虑以下几点:首先，溶质在溶剂中的溶解度必须高于在水中的溶解度。因此，了解潜在溶剂对中溶质的分配系数是必要的。其次，溶剂对必须与水不混溶，并且在混合时不会形成均匀的溶液。第三，溶剂必须是惰性的，不会与溶质发生反应。溶剂还应具有挥发性，以便可以很容易地从溶质中去除。不混溶于水的有机溶剂通常具有非极性或低极性。
了解溶剂的密度以确定顶层和底层的身份也很重要。除氯化有机溶剂外，大多数有机液体的密度低于水，并且会沉降到分液漏斗的底部。
酸碱萃取
酸碱萃取是一种根据有机化合物的酸碱特性分离有机化合物的液-液萃取。如果溶质是酸或碱，则其电荷会随着 pH 值的变化而变化。一般来说，大多数有机化合物是中性的，因此在有机溶剂中的溶解度比在水中更强。但是，如果有机化合物变成离子，那么它就会变得更易溶于水。这在将有机酸或碱化合物从有机相提取到水相中很有用。
酸碱萃取通过将溶质转化为水溶性盐形式来利用这一特性，从而改变其溶解度。有机化合物及其盐的溶解度必须有很大的不同，才能使该技术有效。
例如，考虑含有有机羧酸、胺和中性化合物的混合物。由六个或更多碳原子组成的羧酸不溶于水，完全溶于有机溶剂。然而，它们的共轭碱（一种离子化合物）是水溶性的，不溶于有机溶剂。由至少七个碳原子组成的胺不溶于水，但溶于有机溶剂。该胺（一种离子化合物）的共轭酸是水溶性的，不溶于有机溶剂。
当与碱反应时，羧酸被中和成盐形式。混合物中的其他化合物保持中性。一旦羧酸转化为盐，它就会分配到水相中，而中性化合物则保留在有机相中。
酸碱萃取还用于分离两种弱酸或两种 pKa 差异显著的弱碱。在酸的情况下，具有较小 pKa 值的相对较强的酸使用弱碱中和为盐。弱碱不能有效地与较弱的酸反应，只有较强的酸才会转化为盐。然后，在萃取过程中将盐萃取到水相中。弱碱的过程类似，其中相对较强的碱被使用弱酸中和为盐。
引用
<ol>
	<li style="font-size:17px">哈里斯，哥伦比亚特区（2015 年）。定量化学分析。纽约州纽约市:W.H. Freeman and Company。</li>
</ol>

萃取
萃取是有机化学中用于分离目标化合物的常用技术。在提取过程中，溶质被转移......

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JoVE Lab Manual

Chemistry

化学

Extraction

separating mixtures with solvent extraction

Solvent extraction is a common way to separate and isolate a target compound in organic chemistry. In this process, a solute moves from one phase to another so it can be separated from unreacted starting materials or impurities. Extraction is also useful when a product is dissolved in a reaction solvent that is hard to remove by evaporation, such as a solvent with a high boiling point.

There are three main types of extraction. In solid-liquid extraction, the solute moves from a solid phase into a liquid phase. In liquid-liquid extraction, the solute moves from one liquid into another. In acid-base extraction, the solute is changed into an ionic compound and moved from an organic phase into an aqueous phase. A familiar example is brewing coffee or tea, where hot water extracts caffeine, plant flavors, and odors from the solid material.

Liquid-liquid extraction is used when an organic compound is dissolved in water or when water-soluble impurities need to be removed from an organic product. It works with immiscible liquids, which are liquids that do not form one uniform solution, even after mixing. Oil and water are a common example. To carry out this extraction, the aqueous solution is placed in a separatory funnel, and a non-water-soluble organic solvent is added. After mixing, the compound partitions into the phase where it is more soluble.

Because the two liquids are immiscible, they separate into layers after settling. The denser liquid forms the bottom layer, and the less dense liquid stays on top. The layers are then separated by opening the stopcock at the bottom of the separatory funnel and letting one layer drain out. The layer that loses the solute is called the raffinate, and the layer that gains the solute is called the extract. At equilibrium, the solute has the same chemical potential in both phases.

The partition coefficient, K, describes how a solute divides between the two layers. It is the concentration in the organic layer divided by the concentration in the aqueous layer. This value depends on the solute and the solvent pair used. A large partition coefficient means the solute prefers the organic layer. A small partition coefficient means the solute prefers the aqueous layer. Choosing the right solvent pair is important, so the solute should be more soluble in that solvent than in water, the two solvents should be immiscible, and the solvents should be inert and volatile. The water-immiscible organic solvent is usually non-polar or low in polarity.

Knowing density is also important because it tells which layer is on top and which is on the bottom. Most organic liquids are less dense than water and rise to the top, although chlorinated organic solvents are an exception and can settle to the bottom. Acid-base extraction is a special type of liquid-liquid extraction that separates organic compounds based on acid-base behavior. When pH changes, the charge on an acid or base can change too. Neutral organic compounds are usually more soluble in organic solvents, but ionic forms are more soluble in water.

This property lets chemists turn an organic acid or base into a water-soluble salt. The salt and the neutral compound must have very different solubilities for the method to work well. For example, a mixture may contain an organic carboxylic acid, an amine, and a neutral compound. Carboxylic acids with six carbons or more are insoluble in water and soluble in organic solvents, but their conjugate bases are water-soluble and insoluble in organic solvents. Amines with at least seven carbons are insoluble in water and soluble in organic solvents, while their conjugate acids are water-soluble and insoluble in organic solvents.

When a base is added, the carboxylic acid is neutralized to its salt form, while the other compounds remain neutral. The salt then moves into the aqueous phase, and the neutral compounds stay in the organic phase. Acid-base extraction can also separate two weak acids or two weak bases when their pKa values are far enough apart. For weak acids, the stronger acid with the smaller pKa is converted to a salt using a weak base, while the weaker acid is not reacted much. The same idea applies to weak bases, but a weak acid is used to neutralize the stronger base.

Separating Mixtures with Solvent Extraction

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Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how solvent extraction separates mixtures in organic chemistry, including liquid-liquid extraction, separatory funnels, partition coefficients, and acid-base extraction.

Separating Mixtures with Solvent Extraction | JoVE K12

通过这段 3 分钟的教师指南，准备教授 萃取。涵盖 JoVE Lab Manual 上 Chemistry 实验的关键准备步骤。

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在这段 19 分钟的学生实验流程视频中，逐步学习 萃取 的操作步骤。属于 JoVE Lab Manual 上的 Chemistry 系列。

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Separating Mixtures with Solvent Extraction

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