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

Os tampões são soluções que trabalham para manter um pH constante em um sistema, independentemente da adição de ácidos ou bases fortes. Na ausência de tampões, a adição de um ácido forte ou de uma base a uma solução altera significativamente o pH. Os amortecedores são cruciais na vida cotidiana. Por exemplo, o sangue atua como um tampão e é capaz de manter um pH entre 7,35 e 7,45, pois um pH acima de 7,8 ou abaixo de 6,8 pode causar a morte.
Então, como funciona um buffer? Um tampão é essencialmente um ácido ou base fraca e sua base conjugada ou ácido em equilíbrio entre si. Tomemos, por exemplo, um tampão feito de ácido acético e acetato de sódio. A solução tampão contém um ácido fraco e seu sal conjugado, que se dissocia para formar o acetato de base conjugado. A solução é ácida porque o Ka do ácido acético é maior que o Kb da base conjugada.
Lembre-se de que a constante de dissociação, Ka ou Kb, define a força do ácido ou base, respectivamente, pois é a constante de equilíbrio para a dissociação do composto em água. A constante de dissociação também pode ser representada como pKa, ou o log negativo de Ka, onde quanto menor o pKa, mais forte é o ácido. O mesmo vale para pKb, onde quanto menor o pKb, mais forte é a base.
Voltando ao tampão acetato, se adicionarmos uma base forte, como hidróxido de sódio, os íons hidróxido reagem com os íons hidrônio presentes na solução. Isso faz com que mais ácido acético reaja com a água para retornar ao equilíbrio, formando assim mais íons hidrônio. Se adicionarmos um ácido forte, como o ácido clorídrico, mais íons hidrônio são formados em solução. Esses íons reagem com os íons acetato para formar mais ácido acético. Em ambos os casos, há muito pouca alteração no pH da solução.
Lembre-se de que cada tampão tem uma faixa de pH específica onde pode tamponar uma solução. Aqui mostramos uma curva de titulação para nosso tampão de ácido acético. O intervalo de buffer é realçado. Esta região é determinada pela equação de Henderson-Hasselbalch, que nos diz que o pH do tampão é determinado pelo Ka do ácido fraco e pela razão entre a base conjugada e o ácido fraco.
Longe desse pH, o ácido fraco ou a base do tampão se esgotam e não podem tamponar o pH. Portanto, ao selecionar um tampão para sua aplicação, escolha um cujo pKa esteja próximo do pH desejado.
Finalmente, todos os tampões têm um limite para sua capacidade tampão, ou seja, a quantidade de ácido ou uma base que pode ser adicionada à solução tampão antes que o pH mude significativamente. Por exemplo, se adicionarmos muito ácido à nossa solução tampão de ácido acético e acetato de sódio, protonaremos todos os íons acetato e acumularemos muitos íons hidrônio, diminuindo assim o pH.
Da mesma forma, se adicionarmos muita base, desprotonaremos todo o ácido acético e acumularemos muitos íons hidróxido, aumentando assim o pH. Assim, a capacidade tampão é influenciada pela concentração do ácido fraco e do sal conjugado. Portanto, um tampão de acetato de 1 molar tem uma capacidade de tamponamento maior do que um tampão de acetato de 0,1 molar.
Neste laboratório, você preparará e examinará tampões com uma ampla variedade de faixas de pH. Em seguida, você usará seus tampões para determinar o pKa de um indicador de pH, chamado vermelho neutro, e explorará a mudança no pKa do vermelho neutro quando a proteína de ligação à riboflavina é introduzida.

buffers

<h4>Buffers</h4>
Quando um ácido de Arrhenius (HA) é adicionado à água, ele se dissocia em sua base conjugada (A-) e um cátion hidrogênio (H+).
HA + H2O → H+(aq) + A-(aq)
A quantidade de íons de hidrogênio presentes na solução determina a acidez da solução, onde mais íons de hidrogênio indicam um pH mais baixo ou mais ácido. Da mesma forma, quando uma forte base de Arrhenius (BOH) é adicionada à água, ela se dissocia em seu ácido conjugado (B+) e um íon hidróxido (OH-).
BOH + H2O → B+(aq) + OH-(aq)
Geralmente, a adição de ácidos ou bases fortes a uma solução altera drasticamente o pH porque o ácido ou base reage com as moléculas de água em solução, aumentando a concentração de íons de hidrogênio ou íons de hidróxido. No entanto, essa mudança no pH pode ser mitigada usando um tampão. Os tampões são soluções que trabalham para manter um pH constante em um sistema, independentemente da adição de ácidos ou bases fortes.
Na maioria das vezes, os componentes de um tampão são o par ácido-base conjugado de um ácido fraco ou base fraca. Por esta razão, ácidos ou bases fortes que se dissociam completamente na água são tampões muito pobres e ácidos ou bases fracas que se dissociam parcialmente são melhores tampões. Quando o tampão está presente, o ácido ou base forte não reage com as moléculas de água presentes na solução e, em vez disso, reage com o ácido/base conjugado fraco. Isso resulta em pouca ou nenhuma alteração no pH da solução.
<h4>O efeito do íon comum</h4>
Um buffer funciona por meio de um fenômeno chamado efeito de íon comum. O efeito do íon comum ocorre quando um determinado íon é adicionado a uma mistura em equilíbrio que já contém o íon dado. Quando isso acontece, o equilíbrio se afasta da formação de mais desse íon.
Por exemplo, o ácido acético (CH3COOH) se dissocia ligeiramente na água, formando o íon acetato (CH3COO-) e o íon hidrogênio.
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
Se mais do íon acetato for adicionado a partir do acetato de sódio solúvel, a posição de equilíbrio muda para a esquerda para formar mais ácido acético não dissociado e a concentração de íons de hidrogênio diminui. Aqui, o íon comum - acetato - suprime a dissociação do ácido acético.
Um tampão deve conter altas concentrações dos componentes ácidos (HA) e básicos (A-) para tamponar uma solução. Se a quantidade de íons de hidrogênio ou hidróxido adicionada ao tampão for pequena, eles farão com que uma pequena quantidade de um componente do tampão se converta no outro. Desde que a concentração de íons adicionados seja muito menor do que as concentrações de HA e A- já presentes no tampão, os íons adicionados terão pouco efeito sobre o pH, uma vez que são consumidos por um dos componentes do tampão. Quando a concentração de hidrogênio ou hidróxido excede as concentrações do ácido e sua base conjugada, o efeito tampão é perdido e o pH muda.
<h4>Equação de Henderson-Hasselbalch</h4>
A constante de dissociação, Ka, de um ácido fraco é calculada usando as concentrações do ácido não dissociado HA e as concentrações dos íons de hidrogênio e da base conjugada, A-.
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="Equação da constante de dissociação ácida Ka=[H+][A-]/[HA], fórmula química." data-altupdatedat="1754907201000">
Valores mais altosde K a representam ácidos mais fortes, enquanto valores menoresde K a representam ácidos mais fracos. Para determinar a concentração de íons de hidrogênio, a equação é reorganizada. Nesta forma, fica claro que a proporção de espécies ácidas para espécies de base é importante para determinar a concentração de íons de hidrogênio e, por extensão, o pH.
<img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" alt="Equação de dissociação ácida, [H+] = Ka × [HA]/[A-], conceito de equilíbrio químico." data-altupdatedat="1754907201000">
Tomando o logaritmo comum negativo de ambos os lados resulta na equação de Henderson-Hasselbalch.
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="Equação de Henderson-Hasselbalch; pH, pKa, solução tampão, equilíbrio ácido-base, fórmula química." data-altupdatedat="1754907201000">
A equação de Henderson-Hasselbalch permite o cálculo do pH do tampão diretamente, sem ter que calcular a concentração de íons de hidrogênio primeiro.
Por exemplo, pode ser usado para determinar o pH de um tampão de 1 L após a adição de 0,02 moles de uma base forte. A base forte se dissocia completamente, então a concentração de íons hidroxila adicionados é de 0,02 M. Isso diminuirá a concentração do ácido em 0,02. Assumindo que a concentração original dos componentes ácido (HA) e base (A-) é cada 0,5 M, a nova concentração de base aumenta em 0,02 M para 0,52 M, enquanto a concentração de ácido diminui em 0,02 M para 0,48 M. Conhecendo o pKa do componente ácido do tampão, podemos substituir essas novas concentrações de componentes na equação de Henderson-Hasselbalch para determinar o pH.
Isso é útil para determinar a capacidade do tampão, ou quanto ácido forte ou base forte pode ser adicionado a um tampão sem afetar significativamente o pH.
<h4>Capacidade do buffer</h4>
A capacidade do tampão é a medida da capacidade de um tampão de resistir à mudança de pH. Essa capacidade depende da concentração dos componentes do tampão, ou seja, do ácido e de sua base conjugada. Uma concentração de tampão mais alta tem uma capacidade de tampão maior. Isso significa que uma quantidade maior de íons de hidrogênio, ou um ácido mais forte, teria que ser adicionada para interromper o equilíbrio e alterar o pH do tampão.
A capacidade do buffer também é afetada pelas concentrações relativas dos componentes do buffer. O tampão é mais eficaz quando as concentrações dos componentes são semelhantes. Se a proporção do componente do tampão for semelhante, a proporção de concentração do componente não muda significativamente quando ácido ou base é adicionado; grandes quantidades de ácido ou base devem ser adicionadas para compensar a proporção e alterar o pH.
O pH do tampão difere de sua capacidade tampão. A faixa de pH é a faixa na qual o tampão é eficaz. Normalmente, os tampões têm uma faixa utilizável dentro de 1 unidade de pH do pKa do componente ácido do tampão.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Química e Reatividade Química. Belmont, CA: Brooks / Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Química: A Natureza Molecular da Matéria e da Mudança. Boston, MA: Colina McGraw.</li>
</ol>

Buffers

Buffers
Quando um ácido de Arrhenius (HA) é adicionado à água, ele se dissocia em sua base conjugada (A-) e um cátion hidrogênio (H+).
HA + H2O → H+(aq) + ...

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

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Buffer pH and the Common Ion Effect

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