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

to roztwory, które działają w celu utrzymania stałego pH w systemie niezależnie od dodania mocnych kwasów lub zasad. W przypadku braku dodanie mocnego kwasu lub zasady do roztworu znacznie zmienia pH. są kluczowe w życiu codziennym. Na przykład krew działa jak bufor i jest w stanie utrzymać pH między 7,35 a 7,45, ponieważ pH powyżej 7,8 lub poniżej 6,8 może spowodować śmierć.
Jak więc działa bufor? Bufor jest zasadniczo słabym kwasem lub zasadą i jego sprzężoną zasadą lub kwasem w równowadze ze sobą. Weźmy na przykład bufor wykonany z kwasu octowego i octanu sodu. Roztwór buforowy zawiera słaby kwas i jego sprzężoną sól, która dysocjuje, tworząc sprzężony octan zasadowy. Roztwór jest kwaśny, ponieważ Ka kwasu octowego jest wyższy niż Kb sprzężonej zasady.
Pamiętaj, że stała dysocjacji, Ka lub Kb, określa odpowiednio moc kwasu lub zasady, ponieważ jest to stała równowagi dla dysocjacji związku w wodzie. Stała dysocjacji może być również reprezentowana jako pKa lub ujemny logarytm Ka, gdzie im mniejsze pKa, tym silniejszy kwas. To samo dotyczy pKb, gdzie im mniejsze pKb, tym silniejsza podstawa.
Wracając do buforu octanowego, jeśli dodamy mocną zasadę, taką jak wodorotlenek sodu, jony wodorotlenkowe reagują z jonami hydroniowymi obecnymi w roztworze. Powoduje to, że więcej kwasu octowego reaguje z wodą, aby powrócić do równowagi, tworząc w ten sposób więcej jonów hydroniowych. Jeśli dodamy mocny kwas, taki jak kwas solny, w roztworze powstaje więcej jonów hydroniowych. Jony te reagują z jonami octanowymi, tworząc więcej kwasu octowego. W obu przypadkach zmiana pH roztworu jest bardzo niewielka.
Należy pamiętać, że każdy bufor ma określony zakres pH, w którym może buforować roztwór. Tutaj pokazujemy krzywą miareczkowania dla naszego buforu kwasu octowego. Zakres buforowania jest podświetlony. Obszar ten jest określony równaniem Hendersona-Hasselbalcha, które mówi nam, że pH buforu jest określone przez Ka słabego kwasu i stosunek sprzężonej zasady do słabego kwasu.
Daleko od tego pH słaby kwas lub zasada buforu jest wyczerpana i nie może buforować pH. Tak więc, wybierając bufor do swojego zastosowania, wybierz taki, którego pKa jest bliskie pożądanemu pH.
Wreszcie, wszystkie mają ograniczenie swojej zdolności buforowej, co oznacza ilość kwasu lub zasady, którą można dodać do roztworu buforowego, zanim pH ulegnie znaczącej zmianie. Na przykład, jeśli dodamy zbyt dużo kwasu do naszego roztworu buforowego kwasu octowego i octanu sodu, sprotonujemy wszystkie jony octanowe i zgromadzimy dużo jonów hydroniowych, obniżając w ten sposób pH.
Podobnie, jeśli dodamy zbyt dużo zasady, zdeprotonujemy cały kwas octowy i zgromadzimy dużo jonów wodorotlenkowych, zwiększając w ten sposób pH. Tak więc na zdolność buforową ma wpływ stężenie słabego kwasu i soli sprzężonej. Tak więc 1-molowy bufor octanowy ma większą pojemność buforową niż 0,1-molowy bufor octanu.
W tym laboratorium przygotujesz i zbadasz o szerokim zakresie pH. Następnie użyjesz swoich, aby określićpK a wskaźnika pH, zwanego neutralną czerwienią, i zbadać zmianę pKa neutralnej czerwieni po wprowadzeniu białka wiążącego ryboflawinę.

bufory

<h4></h4>
Po dodaniu kwasu Arrheniusa (HA) do wody dysocjuje on na sprzężoną zasadę (A-) i kation wodoru (H+).
HA + H2O → H+(aq) + A-(aq)
Ilość jonów wodorowych obecnych w roztworze określa kwasowość roztworu, gdzie więcej jonów wodorowych wskazuje na niższe lub bardziej kwaśne pH. Podobnie, gdy do wody dodaje się silną zasadę Arrheniusa (BOH), dysocjuje ona na sprzężony kwas (B+) i jon wodorotlenkowy (OH-).
BOH + H2O → B+(aq) + OH-(aq)
Ogólnie rzecz biorąc, dodanie mocnych kwasów lub zasad do roztworu dramatycznie zmienia pH, ponieważ kwas lub zasada reaguje z cząsteczkami wody w roztworze, zwiększając stężenie jonów wodorowych lub jonów wodorotlenkowych. Jednak tę zmianę pH można złagodzić za pomocą buforu. to roztwory, które działają w celu utrzymania stałego pH w układzie, niezależnie od dodania mocnych kwasów lub zasad.
Najczęściej składnikami buforu są sprzężona para kwasowo-zasadowa słabego kwasu lub słabej zasady. Z tego powodu mocne kwasy lub zasady, które całkowicie dysocjują w wodzie, tworzą bardzo słabe, a słabe kwasy lub zasady, które częściowo dysocjują, tworzą lepsze. Gdy bufor jest obecny, mocny kwas lub zasada nie reaguje z cząsteczkami wody obecnymi w roztworze, a zamiast tego reaguje ze słabym kwasem / sprzężoną zasadą. Powoduje to niewielką lub żadną zmianę pH roztworu.
<h4>Wspólny efekt jonowy</h4>
Bufor działa poprzez zjawisko zwane wspólnym efektem jonowym. Wspólny efekt jonowy występuje, gdy dany jon jest dodawany do mieszaniny w stanie równowagi, która już zawiera dany jon. Kiedy tak się dzieje, równowaga przesuwa się od tworzenia większej ilości tego jonu.
Na przykład kwas octowy (CH3COOH) nieznacznie dysocjuje w wodzie, tworząc jon octanowy (CH3COO-) i jon wodorowy.
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
Jeśli z rozpuszczalnego octanu sodu doda się więcej jonu octanu, pozycja równowagi przesuwa się w lewo, tworząc więcej niezdysocjowanego kwasu octowego, a stężenie jonów wodorowych maleje. W tym przypadku powszechny jon - octan - hamuje dysocjację kwasu octowego.
Bufor musi zawierać wysokie stężenia zarówno kwaśnych (HA), jak i zasadowych (A-) składników buforujących roztwór. Jeśli ilość jonów wodoru lub wodorotlenku dodanych do buforu jest mała, powodują one przekształcenie niewielkiej ilości jednego składnika buforowego w drugi. Dopóki stężenie dodanych jonów jest znacznie mniejsze niż stężenia HA i A- już obecnych w buforze, dodane jony będą miały niewielki wpływ na pH, ponieważ są zużywane przez jeden ze składników buforu. Gdy stężenie wodoru lub wodorotlenku przekracza stężenie kwasu i jego sprzężonej zasady, efekt buforowania zostaje utracony, a pH ulegnie zmianie.
<h4>Równanie Hendersona-Hasselbalcha</h4>
Stałą dysocjacji,Ka, słabego kwasu oblicza się na podstawie stężeń niezdysocjowanego kwasu HA oraz stężeń jonów wodorowych i sprzężonej zasady A-.
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="Równanie stałej dysocjacji kwasu Ka=[H+][A-]/[HA], wzór chemiczny." data-altupdatedat="1754907201000">
Wyższe wartości Ka reprezentują mocniejsze kwasy, podczas gdy mniejsze wartości Ka reprezentują słabsze kwasy. Aby określić stężenie jonów wodorowych, równanie jest przestawiane. W tej formie jasne jest, że stosunek gatunków kwasowych do zasadowych jest ważny przy określaniu stężenia jonów wodorowych, a co za tym idzie, pH.
<img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" alt="Równanie dysocjacji kwasów, [H+] = Ka × [HA]/[A-], pojęcie równowagi chemicznej." data-altupdatedat="1754907201000">
Przyjęcie ujemnego logarytmu wspólnego obu stron daje równanie Hendersona-Hasselbalcha.
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="równanie Hendersona-Hasselbalcha; pH, pKa, roztwór buforowy, równowaga kwasowo-zasadowa, wzór chemiczny." data-altupdatedat="1754907201000">
Równanie Hendersona-Hasselbalcha umożliwia bezpośrednie obliczenie pH buforu, bez konieczności uprzedniego obliczania stężenia jonów wodorowych.
Na przykład można go użyć do określenia pH buforu o pojemności 1 litra po dodaniu 0,02 mola mocnej zasady. Mocna zasada dysocjuje całkowicie, więc stężenie dodanych jonów hydroksylowych wynosi 0,02 M. Spowoduje to zmniejszenie stężenia kwasu o 0,02. Zakładając, że pierwotne stężenie składników kwasowych (HA) i zasadowych (A-) wynosi po 0,5 M, nowe stężenie zasady wzrasta o 0,02 M do 0,52 M, podczas gdy stężenie kwasu spada o 0,02 M do 0,48 M. ZnającpK a składnika kwasowego buforu, możemy podstawić te nowe stężenia składników do równania Hendersona-Hasselbalcha w celu określenia pH.
Jest to przydatne do określania pojemności buforowej lub ilości mocnego kwasu lub mocnej zasady, którą można dodać do buforu bez znaczącego wpływu na pH.
<h4>Pojemność buforowa</h4>
Pojemność buforowa jest miarą zdolności buforu do opierania się zmianom pH. Zdolność ta zależy od stężenia składników buforowych, czyli kwasu i jego sprzężonej zasady. Wyższe stężenie buforu ma większą pojemność buforową. Oznacza to, że musiałaby zostać dodana większa ilość jonów wodorowych lub mocniejszy kwas, aby zakłócić równowagę i zmienić pH buforu.
Na pojemność bufora mają również wpływ względne stężenia składników bufora. Bufor jest bardziej skuteczny, gdy stężenia składników są podobne. Jeśli stosunek składników buforowych jest podobny, to stosunek stężenia składników nie zmienia się znacząco po dodaniu kwasu lub zasady; Należy dodać duże ilości kwasu lub zasady, aby zrównoważyć proporcje i zmienić pH.
pH buforu różni się od jego pojemności buforowej. Zakres pH to zakres, w którym bufor jest skuteczny. Zazwyczaj mają użyteczny zakres w granicach 1 jednostki pHpK a kwaśnego składnika buforu.
<h4>Odwołania</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemia i reaktywność chemiczna. Belmont, Kalifornia: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Chemia: molekularna natura materii i zmiana. Boston, Massachusetts: McGraw Hill.</li>
</ol>

Bufory

Po dodaniu kwasu Arrheniusa (HA) do wody dysocjuje on na sprzężoną zasadę (A-) i kation wodoru (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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synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

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

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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

Przygotuj się do nauczania Bufory dzięki temu 8-minutowemu przewodnikowi dla instruktorów. Obejmuje kluczowe kroki przygotowawcze dla laboratorium Chemistry na JoVE Lab Manual.

Przygotowanie instruktora do Bufory w Chemistry - Ponad 8.0K wyświetleń | JoVE Lab Manual

Postępuj zgodnie z procedurami Bufory krok po kroku w tym 15-minutowym filmie z protokołem studenckim. Część kolekcji Chemistry na JoVE Lab Manual.

Protokół studencki dla Bufory w Chemistry - Ponad 25K wyświetleń | JoVE Lab Manual

Buffer pH and the Common Ion Effect

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