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

Les tampons sont des solutions qui visent à maintenir un pH constant dans un système, indépendamment de l’ajout d’acides ou de bases forts. En l’absence de tampons, l’ajout d’un acide fort ou d’une base à une solution modifie considérablement le pH. Les tampons sont cruciaux dans la vie quotidienne. Par exemple, le sang agit comme un tampon et est capable de maintenir un pH entre 7,35 et 7,45, car un pH supérieur à 7,8 ou inférieur à 6,8 peut entraîner la mort.
Alors, comment fonctionne un tampon ? Un tampon est essentiellement un acide ou une base faible et sa base conjuguée ou un acide en équilibre les uns avec les autres. Prenons, par exemple, un tampon à base d’acide acétique et d’acétate de sodium. La solution tampon contient un acide faible et son sel conjugué, qui se dissocie pour former l’acétate de base conjuguée. La solution est acide car le Ka de l’acide acétique est supérieur au Kb de la base conjuguée.
N’oubliez pas que la constante de dissociation, Ka ou Kb, définit la force de l’acide ou de la base, respectivement, car il s’agit de la constante d’équilibre pour la dissociation du composé dans l’eau. La constante de dissociation peut également être représentée par pKa, ou le log négatif de Ka, où plus le pKa est petit, plus l’acide est fort. Il en va de même pour pKb, où plus le pKb est petit, plus la base est forte.
Pour en revenir au tampon d’acétate, si nous ajoutons une base forte, comme l’hydroxyde de sodium, les ions hydroxyde réagissent avec les ions hydronium présents dans la solution. Cela fait réagir plus d’acide acétique avec l’eau afin de revenir à l’équilibre, formant ainsi plus d’ions hydronium. Si nous ajoutons un acide fort, comme l’acide chlorhydrique, davantage d’ions hydronium se forment en solution. Ces ions réagissent avec les ions acétate pour former plus d’acide acétique. Dans les deux cas, il y a très peu de changement dans le pH de la solution.
Gardez à l’esprit que chaque tampon a une plage de pH spécifique où il peut tamponner une solution. Nous montrons ici une courbe de titrage pour notre tampon d’acide acétique. La plage de mise en mémoire tampon est mise en surbrillance. Cette région est déterminée par l’équation d’Henderson-Hasselbalch, qui nous dit que le pH du tampon est déterminé par le Ka de l’acide faible et le rapport de la base conjuguée à l’acide faible.
Loin de ce pH, l’acide faible ou la base du tampon est épuisé et il ne peut pas tamponner le pH. Ainsi, lorsque vous choisissez un tampon pour votre application, choisissez-en un dont le pKa est proche du pH souhaité.
Enfin, tous les tampons ont une limite à leur capacité tampon, c’est-à-dire la quantité d’acide ou d’une base qui peut être ajoutée à la solution tampon avant que le pH ne change de manière significative. Par exemple, si nous ajoutons trop d’acide à notre solution tampon d’acide acétique et d’acétate de sodium, nous protonifierons tous les ions acétate et accumulerons beaucoup d’ions hydronium, abaissant ainsi le pH.
De même, si nous ajoutons trop de base, nous déprotonerons tout l’acide acétique et accumulerons beaucoup d’ions hydroxyde, augmentant ainsi le pH. Ainsi, la capacité tampon est influencée par la concentration de l’acide faible et du sel conjugué. Ainsi, un tampon en acétate de 1 molaire a une capacité tampon plus élevée qu’un tampon en acétate de 0,1 molaire.
Dans cet atelier, vous préparerez et examinerez des tampons avec une grande variété de plages de pH. Vous utiliserez ensuite vos tampons pour déterminer le pKa d'un indicateur de pH, appelé rouge neutre, et explorerez le changement du pKa du rouge neutre lorsque la protéine de liaison à la riboflavine est introduite.

tampons

<h4>Tampons</h4>
Lorsqu’un acide d’Arrhenius (HA) est ajouté à l’eau, il se dissocie en sa base conjuguée (A-) et en un cation hydrogène (H+).
HA + H2 O →H+(aq) + A-(aq)
La quantité d’ions hydrogène présents dans la solution détermine l’acidité de la solution, où plus d’ions hydrogène indiquent un pH plus faible ou plus acide. De même, lorsqu’une base Arrhenius forte (BOH) est ajoutée à l’eau, elle se dissocie en son acide conjugué (B+) et en un ion hydroxyde (OH-).
BOH + H2 O →B+(aq) + OH-(aq)
En général, l’ajout d’acides ou de bases forts à une solution modifie considérablement le pH car l’acide ou la base réagit avec les molécules d’eau en solution, augmentant la concentration d’ions hydrogène ou d’ions hydroxyde. Cependant, ce changement de pH peut être atténué à l’aide d’un tampon. Les tampons sont des solutions qui aident à maintenir un pH constant dans un système, indépendamment de l’ajout d’acides ou de bases forts.
Le plus souvent, les composants d’un tampon sont la paire conjuguée acide-base d’un acide faible ou d’une base faible. Pour cette raison, les acides ou les bases forts qui se dissocient complètement dans l’eau font de très mauvais tampons et les acides faibles ou les bases qui se dissocient partiellement font de meilleurs tampons. Lorsque le tampon est présent, l’acide ou la base forte ne réagit pas avec les molécules d’eau présentes en solution et réagit plutôt avec la base faible. Cela se traduit par peu ou pas de changement dans le pH de la solution.
<h4>L’effet ionique commun</h4>
Un tampon fonctionne par le biais d’un phénomène appelé effet d’ion commun. L’effet ionique commun se produit lorsqu’un ion donné est ajouté à un mélange à l’équilibre qui contient déjà l’ion donné. Lorsque cela se produit, l’équilibre s’éloigne de la formation d’une plus grande partie de cet ion.
Par exemple, l’acide acétique (CH3COOH) se dissocie légèrement dans l’eau, formant l’ion acétate (CH3COO-) et l’ion hydrogène.
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
Si une plus grande quantité d’ion acétate est ajoutée à partir d’acétate de sodium soluble, la position d’équilibre se déplace vers la gauche pour former plus d’acide acétique non dissocié, et la concentration d’ions hydrogène diminue. Ici, l’ion commun – l’acétate – supprime la dissociation de l’acide acétique.
Un tampon doit contenir des concentrations élevées de composants acides (HA) et basiques (A-) pour tamponner une solution. Si la quantité d’ions hydrogène ou hydroxyde ajoutée au tampon est faible, ils provoquent la conversion d’une petite quantité d’un composant du tampon en l’autre. Tant que la concentration d’ions ajoutés est beaucoup plus faible que les concentrations de HA et A- déjà présentes dans le tampon, les ions ajoutés auront peu d’effet sur le pH puisqu’ils sont consommés par l’un des composants du tampon. Lorsque la concentration d’hydrogène ou d’hydroxyde dépasse les concentrations de l’acide et de sa base conjuguée, l’effet tampon est perdu et le pH change.
<h4>Équation d’Henderson-Hasselbalch</h4>
La constante de dissociation, Ka, d’un acide faible est calculée à l’aide des concentrations de l’acide non dissocié Ha, et des concentrations des ions hydrogène et de la base conjuguée, A-.
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" align-texte centre><img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" align-texte centre><img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" font-size>Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chimie et réactivité chimique. Belmont, Californie : Brooks/Cole, Cengage Learning.
	<li style="font-size :17px">Silberberg, M.S. (2009). Chimie : la nature moléculaire de la matière et du changement. Boston, MA : McGraw Hill.</li>

Tampons

Tampons
Lorsqu’un acide d’Arrhenius (HA) est ajouté à l’eau, il se dissocie en sa base conjuguée (A-) et en un cation hydrogène (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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Buffer pH and the Common Ion Effect

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