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

L'estrazione è una tecnica per separare i componenti di una miscela. Hai già familiarità con l'estrazione. Ogni volta che metti in infusione il tè, estrai composti idrosolubili, come la caffeina e gli aromi, dalle foglie di tè nell'acqua calda. Questo è un esempio di estrazione solido-liquido.
L'estrazione liquido-liquido è un altro tipo di estrazione in cui la miscela viene disciolta in due fasi liquide immiscibili. I solventi devono essere immiscibili, il che significa che non si mescolano e sono fasi separate. Le estrazioni liquido-liquido vengono eseguite in un imbuto separatore in modo che il solvente più denso si depositi sul fondo e il solvente meno denso si depositi sopra. Quindi, ogni composto soluto si trasferirà alla fase in cui è più solubile. Pertanto, è importante scegliere solventi con polarità diverse.
In generale, i soluti non polari si dividono nella fase organica e i soluti polari si dividono nella fase acquosa. Il coefficiente di ripartizione, K, è il rapporto tra la concentrazione di soluto nella fase organica e quella nella fase acquosa. Una volta che i soluti si sono separati, le due diverse fasi vengono raccolte. Tieni presente che è probabile che ci siano residui di ogni componente in entrambe le fasi.
L'estrazione acido-base è un tipo speciale di estrazione liquido-liquido che separa i composti acidi da quelli basici in base alle differenze di solubilità. Ricordiamo che un acido è un composto che dona un protone quando disciolto in acqua, e una base è un composto che accetta un protone.
Per separare i composti acidi, viene aggiunta una base alla miscela. La base accetterà un protone dal composto acido, rendendolo ionico. Al contrario, l'aggiunta di un acido trasferirebbe un protone a un composto basico. In entrambi i casi, il composto ionico si trasferirà alla fase acquosa e i composti neutri si divideranno nella fase organica. Il composto ionico viene raccolto e quindi deprotonato o riprotonato per riconvertirlo nel suo composto originale.
In questo laboratorio, separerai una miscela di cellulosa, acido benzoico e caffeina. Innanzitutto, utilizzerai l'estrazione e la filtrazione per isolare la cellulosa. Quindi, eseguirai un'estrazione acido-base per separare l'acido benzoico e la caffeina.

estrazione

estrazione
L'estrazione è una tecnica comune utilizzata in chimica organica per isolare un composto bersaglio. Nel processo di estrazione, un soluto viene trasferito da una fase all'altra per separarlo dai materiali di partenza non reagiti o dalle impurità. L'estrazione viene utilizzata anche per facilitare l'isolamento di un soluto da un solvente di reazione difficile da rimuovere per evaporazione, come un solvente con un alto punto di ebollizione.<span style="color: rgb(0, 0, 0); famiglia di caratteri: Roboto, RobotoDraft, Helvetica, Arial, sans-serif; dimensione del carattere: 13px; stile del carattere: normale; legature-varianti-carattere: normale; caratteri-varianti-maiuscole: normale; peso del carattere: 400; spaziatura tra lettere: normale; orfani: 2; allineamento del testo: inizio; rientro del testo: 0px; trasformazione del testo: nessuna; spazio bianco: pre-avvolgimento; vedove: 2; spaziatura tra le parole: 0px; -webkit-testo-larghezza-tratto: 0px; colore di sfondo: RGB (255, 255, 255); stile-decorazione-testo: iniziale; testo-decorazione-colore: iniziale; Display: in linea !important; galleggiante: nessuno;" > In generale, ci sono tre tipi di estrazioni. In primo luogo, nell'estrazione solido-liquido, il soluto viene trasferito da una fase solida a una fase liquida. Nell'estrazione liquido-liquido, un soluto viene trasferito da un liquido all'altro. Nell'estrazione acido-base, un soluto viene trasformato in un composto ionico e trasferito da una fase organica a una acquosa. Un esempio comune di estrazione è la preparazione del caffè o del tè. La fase solida, che contiene la caffeina, gli aromi e gli odori delle piante, viene estratta dall'acqua calda nella fase liquida.
Estrazione Liquido-Liquido
Un'estrazione liquido-liquido trasferisce un composto organico disciolto in una fase acquosa a un solvente organico, oppure viene utilizzata per trasferire reagenti non reagiti, sali e altre impurità idrosolubili alla fase acquosa, lasciando il composto organico di interesse nella fase organica. I liquidi immiscibili sono liquidi che non formano mai una soluzione omogenea, anche se accuratamente miscelati. Invece, i liquidi immiscibili si separano in diverse fasi, come l'olio e l'acqua.
Un'estrazione liquido-liquido trasferisce un composto organico disciolto in una fase acquosa in un solvente organico. Per eseguire un'estrazione liquido-liquido, in primo luogo, la soluzione acquosa contenente il soluto viene aggiunta a un imbuto separatore. Quindi, un solvente organico non solubile in acqua viene aggiunto all'imbuto separatore. Quando il contenuto dell'imbuto è ben miscelato, il composto organico si divide nella fase organica in base alla sua maggiore solubilità nella fase organica rispetto alla fase acquosa.
Poiché i due solventi sono immiscibili, i due liquidi formano strati discreti, con il liquido denso sul fondo e il liquido meno denso sulla parte superiore. Una volta che le due fasi si sono stabilizzate in due strati, vengono separate aprendo il rubinetto sul fondo dell'imbuto separatore e lasciando defluire uno strato. Il liquido a cui è stato rimosso il soluto è chiamato raffinato, mentre il liquido che ha guadagnato il soluto è chiamato estratto.
Il composto organico viene suddiviso tra i due strati in base alla sua solubilità in ciascuna fase. L'equilibrio è raggiunto quando il potenziale chimico del soluto è lo stesso nelle due fasi. Il coefficiente di ripartizione per un soluto, K, è il rapporto tra la concentrazione del campione nello strato organico divisa per la concentrazione nella fase acquosa. Il coefficiente di ripartizione è una costante che dipende sia dal soluto che dalla coppia di solventi utilizzati nell'estrazione. Il coefficiente di ripartizione è un'espressione della preferenza del soluto per ciascuno dei due solventi. I soluti con un coefficiente di ripartizione elevato hanno una maggiore tendenza ad essere estratti nello strato di solvente organico. I soluti con piccoli coefficienti di partizione preferiscono passare alla fase acquosa.
Determinare quale coppia di solventi utilizzare per un'estrazione liquido-liquido è un passaggio fondamentale. Quando si sceglie quali solventi utilizzare, considerare quanto segue: Innanzitutto, il soluto deve essere più solubile nel solvente che in acqua. Pertanto, è necessario conoscere il coefficiente di partizione del soluto in una potenziale coppia di solventi. In secondo luogo, la coppia solvente deve essere immiscibile in acqua e non formare una soluzione omogenea quando viene miscelata. In terzo luogo, i solventi devono essere inerti e non reagire con il soluto. Il solvente deve anche essere volatile in modo da poter essere rimosso facilmente dal soluto. Il solvente organico immiscibile in acqua possiede generalmente una polarità non polare o bassa.
È anche fondamentale conoscere le densità dei solventi per determinare l'identità degli strati superiore e inferiore. La maggior parte dei liquidi organici ha una densità inferiore a quella dell'acqua, ad eccezione dei solventi organici clorurati, e si deposita sul fondo dell'imbuto separatore.
Estrazione acido-base
L'estrazione acido-base è un tipo di estrazione liquido-liquido che separa i composti organici in base alle loro proprietà acido-base. Se un soluto è un acido o una base, la sua carica cambia al variare del pH. Generalmente, la maggior parte dei composti organici sono neutri e quindi più solubili in solventi organici che in acqua. Tuttavia, se il composto organico diventa ionico, diventa più solubile in acqua. Questo è utile per estrarre un acido organico o un composto basico da una fase organica a una fase acquosa.
L'estrazione acido-base sfrutta questa proprietà trasformando il soluto nella sua forma salina solubile in acqua, modificandone così la solubilità. La solubilità del composto organico e del suo sale deve essere notevolmente diversa affinché la tecnica sia efficace.
Ad esempio, considera una miscela contenente un acido carbossilico organico, un'ammina e un composto neutro. Gli acidi carbossilici costituiti da sei o più atomi di carbonio sono insolubili in acqua e completamente solubili in solventi organici. Tuttavia, le loro basi coniugate (un composto ionico) sono solubili in acqua e insolubili in solventi organici. Un'ammina composta da almeno sette atomi di carbonio è insolubile in acqua ma solubile in solventi organici. L'acido coniugato di quell'ammina (un composto ionico) è solubile in acqua e insolubile in solventi organici.
Quando reagisce con una base, l'acido carbossilico viene neutralizzato nella sua forma salina. Gli altri composti della miscela rimangono neutri. Una volta che l'acido carbossilico si è trasformato in sale, si dividerà nella fase acquosa, mentre i composti neutri rimangono nella fase organica.
L'estrazione acido-base viene utilizzata anche per separare due acidi deboli o due basi deboli con una differenza significativa nel loro pKa. Nel caso degli acidi, l'acido relativamente più forte, avente un piccolo valore di pKa, viene neutralizzato in un sale utilizzando una base debole. La base debole non reagisce in modo efficiente con l'acido più debole e solo l'acido più forte viene trasformato in un sale. Quindi, il sale viene estratto nella fase acquosa durante l'estrazione. Il processo segue in modo simile per le basi deboli, dove la base relativamente più forte viene neutralizzata in un sale utilizzando un acido debole.
referenze
<ol>
	<li style="dimensione del carattere:17px">Harris, D.C. (2015). Analisi chimica quantitativa. New York, NY: W.H. Freeman e compagnia.</li>
</ol>

Estrazione

Estrazione
L'estrazione è una tecnica comune utilizzata in chimica organica per isolare un composto bersaglio. Nel processo di estrazione, un soluto viene trasferito ...

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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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Video completi e incentrati sul curriculum sia per gli istruttori che per gli studenti nei corsi di laboratorio di Chimica introduttiva.

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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

Preparati a insegnare Estrazione con questa guida per istruttori di 3 minuti. Copre i passaggi chiave di preparazione per il laboratorio Chemistry su JoVE Lab Manual.

Preparazione dell'istruttore per Estrazione in Chemistry - Oltre 8.0K visualizzazioni | JoVE Lab Manual

Segui le procedure passo dopo passo di Estrazione in questo video di protocollo studente di 19 minuti. Parte della collezione Chemistry su JoVE Lab Manual.

Protocollo studente per Estrazione in Chemistry - Oltre 67K visualizzazioni | JoVE Lab Manual

Separating Mixtures with Solvent Extraction

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