Name: buffer ph and the common ion effect
Uploaded: 2020-03-26T07:22:25.000Z
Duration: 4 min 35 s
Channel: JoVE

I tamponi sono soluzioni che lavorano per mantenere un pH costante in un sistema indipendentemente dall'aggiunta di acidi o basi forti. In assenza di tamponi, l'aggiunta di un acido forte o di una base a una soluzione modifica significativamente il pH. I tamponi sono fondamentali nella vita di tutti i giorni. Ad esempio, il sangue funge da tampone ed è in grado di mantenere un pH compreso tra 7,35 e 7,45 poiché un pH superiore a 7,8 o inferiore a 6,8 può causare la morte.
Quindi, come funziona un buffer? Un tampone è essenzialmente un acido o una base debole e la sua base o acido coniugato in equilibrio tra loro. Prendiamo, ad esempio, un tampone fatto di acido acetico e acetato di sodio. La soluzione tampone contiene un acido debole e il suo sale coniugato, che si dissocia per formare l'acetato di base coniugato. La soluzione è acida perché il Ka dell'acido acetico è superiore al Kb della base coniugata.
Ricorda che la costante di dissociazione, Ka o Kb, definisce la forza dell'acido o della base, rispettivamente, in quanto è la costante di equilibrio per la dissociazione del composto in acqua. La costante di dissociazione può anche essere rappresentata come pKa, o il log negativo di Ka, dove più piccolo è il pKa, più forte è l'acido. Lo stesso vale per pKb, dove più piccolo è il pKb, più forte è la base.
Tornando al tampone acetato, se aggiungiamo una base forte, come l'idrossido di sodio, gli ioni idrossido reagiscono con gli ioni idronio presenti nella soluzione. Questo fa sì che più acido acetico reagisca con l'acqua per tornare all'equilibrio, formando così più ioni idronio. Se aggiungiamo un acido forte, come l'acido cloridrico, si formano più ioni idronio in soluzione. Questi ioni reagiscono con gli ioni acetato per formare più acido acetico. In entrambi i casi, il pH della soluzione è molto piccolo.
Tieni presente che ogni tampone ha un intervallo di pH specifico in cui può tamponare una soluzione. Qui mostriamo una curva di titolazione per il nostro tampone di acido acetico. L'intervallo di buffering è evidenziato. Questa regione è determinata dall'equazione di Henderson-Hasselbalch, che ci dice che il pH del tampone è determinato dal Ka dell'acido debole e dal rapporto tra la base coniugata e l'acido debole.
Lontano da questo pH, l'acido debole o la base del tampone è esaurito e non può tamponare il pH. Quindi, quando selezioni un tampone per la tua applicazione, scegline uno il cui pKa sia vicino al pH desiderato.
Infine, tutti i tamponi hanno un limite alla loro capacità tampone, ovvero la quantità di acido o base che può essere aggiunta alla soluzione tampone prima che il pH cambi in modo significativo. Ad esempio, se aggiungiamo troppo acido alla nostra soluzione tampone di acido acetico e acetato di sodio, protoneremo tutti gli ioni acetato e accumuleremo molti ioni idronio, abbassando così il pH.
Allo stesso modo, se aggiungiamo troppa base, deprotoneremo tutto l'acido acetico e accumuleremo molti ioni idrossido, aumentando così il pH. Pertanto, la capacità tampone è influenzata dalla concentrazione dell'acido debole e del sale coniugato. Quindi, un tampone acetato 1 molare ha una capacità tampone maggiore rispetto a un tampone acetato 0,1 molare.
In questo laboratorio, preparerai ed esaminerai tamponi con un'ampia varietà di intervalli di pH. Utilizzerai quindi i tuoi tamponi per determinare il pKa di un indicatore di pH, chiamato rosso neutro, ed esplorerai la variazione del pKa del rosso neutro quando viene introdotta la proteina legante la riboflavina.

buffer

<h4>Buffer</h4>
Quando un acido arrhenius (HA) viene aggiunto all'acqua, si dissocia nella sua base coniugata (A-) e in un catione idrogeno (H+).
HA + H2O → H+(aq) + A-(aq)
La quantità di ioni idrogeno presenti nella soluzione determina l'acidità della soluzione, dove più ioni idrogeno indicano un pH più basso o più acido. Allo stesso modo, quando una forte base di Arrhenius (BOH) viene aggiunta all'acqua, si dissocia nel suo acido coniugato (B+) e in uno ione idrossido (OH-).
BOH +H 2O → B+(aq) + OH-(aq)
Generalmente, l'aggiunta di acidi o basi forti a una soluzione cambia drasticamente il pH perché l'acido o la base reagisce con le molecole d'acqua in soluzione, aumentando la concentrazione di ioni idrogeno o ioni idrossido. Tuttavia, questa variazione di pH può essere mitigata utilizzando un tampone. I tamponi sono soluzioni che lavorano per mantenere un pH costante in un sistema, indipendentemente dall'aggiunta di acidi o basi forti.
Molto spesso, i componenti di un tampone sono la coppia acido-base coniugata di un acido debole o di una base debole. Per questo motivo, gli acidi o le basi forti che si dissociano completamente in acqua formano tamponi molto poveri e gli acidi deboli o le basi che si dissociano parzialmente sono tamponi migliori. Quando il tampone è presente, l'acido o la base forte non reagisce con le molecole d'acqua presenti in soluzione e reagisce invece con l'acido/base coniugato debole. Ciò si traduce in una variazione minima o nulla del pH della soluzione.
<h4>L'effetto ionico comune</h4>
Un tampone funziona attraverso un fenomeno chiamato effetto ionico comune. L'effetto dello ione comune si verifica quando un dato ione viene aggiunto a una miscela all'equilibrio che contiene già lo ione dato. Quando ciò accade, l'equilibrio si sposta dalla formazione di più di quello ione.
Ad esempio, l'acido acetico (CH3COOH) si dissocia leggermente in acqua, formando lo ione acetato (CH3COO-) e lo ione idrogeno.
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
Se viene aggiunta più ione acetato dall'acetato di sodio solubile, la posizione di equilibrio si sposta a sinistra per formare più acido acetico non dissociato e la concentrazione di ioni idrogeno diminuisce. Qui, lo ione comune – l'acetato – sopprime la dissociazione dell'acido acetico.
Un tampone deve contenere alte concentrazioni di entrambi i componenti acidi (HA) e basici (A-) per tamponare una soluzione. Se la quantità di ioni idrogeno o idrossido aggiunti al tampone è piccola, provocano la conversione di una piccola quantità di un componente tampone nell'altro. Finché la concentrazione di ioni aggiunti è molto più piccola delle concentrazioni di HA e A- già presenti nel tampone, gli ioni aggiunti avranno scarso effetto sul pH poiché vengono consumati da uno dei componenti del tampone. Quando la concentrazione di idrogeno o idrossido supera le concentrazioni dell'acido e della sua base coniugata, l'effetto tampone viene perso e il pH cambia.
<h4>Equazione di Henderson-Hasselbalch</h4>
La costante di dissociazione, Ka, di un acido debole è calcolata utilizzando le concentrazioni dell'acido non dissociato HA, e le concentrazioni degli ioni idrogeno e della base coniugata, A-.
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="Equazione della costante di dissociazione acida Ka=[H+][A-]/[HA], formula chimica." data-altupdatedat="1754907201000">
Valori Ka più alti rappresentano acidi più forti, mentre valori Ka più bassi rappresentano acidi più deboli. Per determinare la concentrazione di ioni idrogeno, l'equazione viene riorganizzata. In questa forma, è chiaro che il rapporto tra specie acide e specie basiche è importante nel determinare la concentrazione di ioni idrogeno e, per estensione, il pH.
<img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" alt="Equazione di dissociazione acida, [H+] = Ka × [HA]/[A-], concetto di equilibrio chimico." data-altupdatedat="1754907201000">
Prendendo il logaritmo comune negativo di entrambi i lati si ottiene l'equazione di Henderson-Hasselbalch.
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="equazione di Henderson-Hasselbalch; pH, pKa, soluzione tampone, equilibrio acido-base, formula chimica." data-altupdatedat="1754907201000">
L'equazione di Henderson-Hasselbalch consente di calcolare direttamente il pH del tampone, senza dover prima calcolare la concentrazione di ioni idrogeno.
Ad esempio, può essere utilizzato per determinare il pH di un tampone da 1 L dopo aver aggiunto 0,02 moli di una base forte. La base forte si dissocia completamente, quindi la concentrazione di ioni ossidrile aggiunti è di 0,02 M. Ciò ridurrà la concentrazione dell'acido di 0,02. Supponendo che la concentrazione originale dei componenti acido (HA) e base (A-) sia ciascuno di 0,5 M, la nuova concentrazione di base aumenta di 0,02 M a 0,52 M, mentre la concentrazione di acido diminuisce di 0,02 M a 0,48 M. Conoscendo il pKa della componente acida del tampone, possiamo sostituire queste nuove concentrazioni di componenti nell'equazione di Henderson-Hasselbalch per determinare il pH.
Questo è utile per determinare la capacità del tampone, o la quantità di acido forte o base forte che può essere aggiunta a un tampone senza influire in modo significativo sul pH.
<h4>Capacità tampone</h4>
La capacità del tampone è la misura della capacità di un tampone di resistere alle variazioni di pH. Questa capacità dipende dalla concentrazione dei componenti tampone, ovvero l'acido e la sua base coniugata. Una concentrazione di tampone più elevata ha una maggiore capacità tampone. Ciò significa che dovrebbe essere aggiunta una maggiore quantità di ioni idrogeno, o un acido più forte, per interrompere l'equilibrio e modificare il pH del tampone.
La capacità tampone è influenzata anche dalle concentrazioni relative dei componenti tampone. Il tampone è più efficace quando le concentrazioni dei componenti sono simili. Se il rapporto dei componenti tampone è simile, il rapporto di concentrazione dei componenti non cambia in modo significativo quando viene aggiunto acido o base; è necessario aggiungere grandi quantità di acido o base per compensare il rapporto e modificare il pH.
Il pH del tampone differisce dalla sua capacità tampone. L'intervallo di pH è l'intervallo in cui il tampone è efficace. Tipicamente, i tamponi hanno un intervallo utilizzabile entro 1 unità di pH del pKa della componente acida del tampone.
<h4>Referenze</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimica e reattività chimica. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Chimica: la natura molecolare della materia e il cambiamento. Boston, MA: McGraw Hill.</li>
</ol>

Buffer

Buffer
Quando un acido arrhenius (HA) viene aggiunto all'acqua, si dissocia nella sua base coniugata (A-) e in un catione idrogeno (H+).
HA + H2O → H+(aq) + ...

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Buffers

buffer ph and the common ion effect

Buffer solutions help keep pH steady when acids or bases are added. In water, an Arrhenius acid (HA) dissociates into hydrogen ions, H+, and a conjugate base, A-. More hydrogen ions make a solution more acidic and lower its pH. An Arrhenius base (BOH) dissociates into a conjugate acid, B+, and hydroxide ions, OH-. Strong acids and strong bases usually change pH a lot because they react completely in water and raise the amount of H+ or OH-.

Buffers reduce those pH changes. A buffer usually contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Weak acids and weak bases are better buffers because they dissociate only partially in water. When a buffer is present, added strong acid or base reacts with the buffer components instead of causing a large shift in pH.

The way buffers work is called the common ion effect. A common ion is an ion already present in an equilibrium mixture. When more of that ion is added, the equilibrium shifts away from making more of it. A good example is acetic acid, CH3COOH, which dissociates slightly in water to form H+ and acetate, CH3COO-. If sodium acetate adds more acetate ion, the equilibrium shifts left and less hydrogen ion remains in solution.

A buffer needs high amounts of both the acidic form, HA, and the basic form, A-. Small amounts of added H+ or OH- are used up by one buffer component and converted into the other. If the added acid or base is much smaller than the amounts already in the buffer, the pH changes very little. Once the concentration of added H+ or OH- becomes larger than the buffer components, the buffer effect is lost and the pH shifts.

The Henderson-Hasselbalch equation helps calculate buffer pH. It comes from the acid dissociation constant, Ka, which uses the concentrations of HA, H+, and A-. A larger Ka means a stronger acid, while a smaller Ka means a weaker acid. After rearranging and taking the negative log, the equation shows that the ratio of acid to base controls pH, so buffer pH can be found without first calculating hydrogen ion concentration.

For example, a 1 L buffer with 0.5 M HA and 0.5 M A- can change after 0.02 moles of strong base are added. The strong base dissociates completely, so 0.02 M OH- is present. That amount lowers the acid concentration to 0.48 M and raises the base concentration to 0.52 M. The new values can be placed into the Henderson-Hasselbalch equation with the acid pKa to find the pH. This also helps estimate buffer capacity, or how much strong acid or strong base a buffer can absorb without a large pH change.

Buffer capacity depends on concentration. A more concentrated buffer has a greater ability to resist pH change. It also depends on how similar the acid and base concentrations are. Buffers work best when those concentrations are close, because the ratio changes only a little when acid or base is added. Buffer pH is not the same as buffer capacity. The effective pH range is usually within 1 pH unit of the acid component's pKa.

Buffer pH and the Common Ion Effect

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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 buffers resist pH change through the common ion effect, the Henderson-Hasselbalch equation, and buffer capacity in high school chemistry.

Buffer pH and the Common Ion Effect | JoVE K12

Preparati a insegnare Buffer con questa guida per istruttori di 8 minuti. Copre i passaggi chiave di preparazione per il laboratorio Chemistry su JoVE Lab Manual.

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

Segui le procedure passo dopo passo di Buffer in questo video di protocollo studente di 15 minuti. Parte della collezione Chemistry su JoVE Lab Manual.

Protocollo studente per Buffer in Chemistry - Oltre 25K visualizzazioni | JoVE Lab Manual

Buffer pH and the Common Ion Effect

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