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

Tamponlar, güçlü asitlerin veya bazların eklenmesinden bağımsız olarak bir sistemde sabit bir pH'ı korumak için çalışan çözeltilerdir. Tamponların yokluğunda, bir çözeltiye güçlü bir asit veya bir bazın eklenmesi pH'ı önemli ölçüde değiştirir. Tamponlar günlük yaşamda çok önemlidir. Örneğin, kan bir tampon görevi görür ve 7.8'in üzerinde veya 6.8'in altında bir pH ölüme neden olabileceğinden, pH'ı 7.35 ile 7.45 arasında tutabilir.
Peki, bir tampon nasıl çalışır? Bir tampon, esasen zayıf bir asit veya baz ve onun eşlenik bazı veya birbiriyle dengede olan asittir. Örneğin, asetik asit ve sodyum asetattan yapılmış bir tampon alın. Tampon çözeltisi, zayıf bir asit ve eşlenik baz asetatı oluşturmak üzere ayrışan eşlenik tuzunu içerir. Çözelti asidiktir çünkü asetik asidin KA'sı, eşlenik bazın KB'sinden daha yüksektir.
Ayrışma sabitinin, Ka veya Kb'nin, bileşiğin sudaki ayrışması için denge sabiti olduğu için sırasıyla asit veya bazın gücünü tanımladığını unutmayın. Ayrışma sabiti aynı zamanda pKa veya Ka'nın negatif günlüğü olarak da temsil edilebilir, burada pKa ne kadar küçükse, asit o kadar güçlü olur. Aynısı pKb için de geçerlidir, burada pKb ne kadar küçükse, baz o kadar güçlü olur.
Asetat tamponuna dönersek, sodyum hidroksit gibi güçlü bir baz eklersek, hidroksit iyonları çözeltide bulunan hidronyum iyonları ile reaksiyona girer. Bu, dengeye dönmek için daha fazla asetik asidin su ile reaksiyona girmesine ve böylece daha fazla hidronyum iyonu oluşturmasına neden olur. Hidroklorik asit gibi güçlü bir asit eklersek, çözeltide daha fazla hidronyum iyonu oluşur. Bu iyonlar, daha fazla asetik asit oluşturmak için asetat iyonları ile reaksiyona girer. Her iki durumda da çözelti pH'ında çok az değişiklik olur.
Her tamponun, bir çözeltiyi tamponlayabileceği belirli bir pH aralığına sahip olduğunu unutmayın. Burada asetik asit tamponumuz için bir titrasyon eğrisi gösteriyoruz. Arabelleğe alma aralığı vurgulanır. Bu bölge, tamponun pH'ının zayıf asidin KA'sı ve eşlenik bazın zayıf aside oranı ile belirlendiğini söyleyen Henderson-Hasselbalch denklemi ile belirlenir.
Bu pH'tan çok uzakta, tamponun zayıf asidi veya bazı tükenir ve pH'ı tamponlayamaz. Bu nedenle, uygulamanız için bir tampon seçerken, pKa'sı istenen pH'a yakın olan bir tampon seçin.
Son olarak, tüm tamponların tamponlama kapasitelerinin bir sınırı vardır, bu da pH önemli ölçüde değişmeden önce tampon çözeltisine eklenebilecek asit veya baz miktarı anlamına gelir. Örneğin, asetik asit ve sodyum asetat tampon çözeltimize çok fazla asit eklersek, tüm asetat iyonlarını protonlaştıracağız ve çok sayıda hidronyum iyonu biriktireceğiz, böylece pH'ı düşüreceğiz.
Benzer şekilde, çok fazla baz eklersek, tüm asetik asidi protondan arındıracağız ve çok sayıda hidroksit iyonu biriktireceğiz, böylece pH'ı artıracağız. Bu nedenle, tamponlama kapasitesi, zayıf asit ve eşlenik tuzun konsantrasyonundan etkilenir. Bu nedenle, 1 molar asetat tamponu, 0.1 molar asetat tamponundan daha yüksek bir tamponlama kapasitesine sahiptir.
Bu laboratuvarda, çok çeşitli pH aralıklarına sahip tamponları hazırlayacak ve inceleyeceksiniz. Daha sonra, nötr kırmızı olarak adlandırılan bir pH göstergesinin pKa'sını belirlemek için tamponlarınızı kullanacak ve riboflavin bağlayıcı protein eklendiğinde nötr kırmızının pKa'sındaki değişikliği keşfedeceksiniz.

arabellek

<h4>Arabellek</h4>
Suya bir Arrhenius asidi (HA) eklendiğinde, eşlenik bazına (A-) ve bir hidrojen katyonuna (H+) ayrışır.
HA + H2O → H+(sulu) + A-(sulu)
Çözeltide bulunan hidrojen iyonlarının miktarı, çözeltinin asitliğini belirler, burada daha fazla hidrojen iyonu daha düşük veya daha asidik bir pH'ı gösterir. Benzer şekilde, suya güçlü bir Arrhenius bazı (BOH) eklendiğinde, eşlenik asidine (B+) ve bir hidroksit iyonuna (OH-) ayrışır.
BOH + H2O → B+(sulu) + OH-(sulu)
Genel olarak, bir çözeltiye güçlü asitlerin veya bazların eklenmesi, pH'ı önemli ölçüde değiştirir, çünkü asit veya baz, çözeltideki su molekülleri ile reaksiyona girerek hidrojen iyonlarının veya hidroksit iyonlarının konsantrasyonunu arttırır. Bununla birlikte, pH'daki bu değişiklik bir tampon kullanılarak hafifletilebilir. Tamponlar, güçlü asitlerin veya bazların eklenmesinden bağımsız olarak bir sistemde sabit bir pH'ı korumak için çalışan çözeltilerdir.
Çoğu zaman, bir tamponun bileşenleri, zayıf bir asit veya zayıf bazın eşlenik asit-baz çiftidir. Bu nedenle, suda tamamen ayrışan güçlü asitler veya bazlar çok zayıf tamponlar oluşturur ve kısmen ayrışan zayıf asitler veya bazlar daha iyi tamponlar yapar. Tampon mevcut olduğunda, güçlü asit veya baz, çözeltide bulunan su molekülleri ile reaksiyona girmez ve bunun yerine zayıf asit/eşlenik baz ile reaksiyona girer. Bu, çözeltinin pH'ında çok az veya hiç değişiklik olmamasına neden olur.
<h4>Ortak İyon Etkisi</h4>
Bir tampon, ortak iyon etkisi adı verilen bir fenomen aracılığıyla çalışır. Ortak iyon etkisi, belirli bir iyon, verilen iyonu zaten içeren dengedeki bir karışıma eklendiğinde ortaya çıkar. Bu olduğunda, denge o iyondan daha fazlasını oluşturmaktan uzaklaşır.
Örneğin, asetik asit (CH3COOH) suda hafifçe ayrışarak asetat iyonu (CH3COO-) ve hidrojen iyonu oluşturur.
CH3COOH(aq) H2O ⇔ H+(aq) + CH3COO-(aq)
Çözünür sodyum asetattan daha fazla asetat iyonu eklenirse, denge konumu daha fazla ayrışmamış asetik asit oluşturmak için sola kayar ve hidrojen iyonlarının konsantrasyonu azalır. Burada, ortak iyon - asetat - asetik asidin ayrışmasını baskılar.
Bir tampon, bir çözeltiyi tamponlamak için hem asidik (HA) hem de bazik (A-) bileşenlerin yüksek konsantrasyonlarını içermelidir. Tampona eklenen hidrojen veya hidroksit iyonlarının miktarı küçükse, bir tampon bileşeninin az miktarda diğerine dönüşmesine neden olurlar. Eklenen iyonların konsantrasyonu, tamponda zaten mevcut olan HA ve A- konsantrasyonlarından çok daha küçük olduğu sürece, eklenen iyonlar, tampon bileşenlerinden biri tarafından tüketildikleri için pH üzerinde çok az etkiye sahip olacaktır. Hidrojen veya hidroksit konsantrasyonu, asit ve eşlenik bazının konsantrasyonlarını aştığında, tamponlama etkisi kaybolur ve pH değişir.
<h4>Henderson-Hasselbalch Denklemi</h4>
Zayıf bir asidin ayrışma sabiti, Ka, ayrışmamış asit HA'nın konsantrasyonları ve hidrojen iyonlarının ve eşlenik bazın A- konsantrasyonları kullanılarak hesaplanır.
<img src="/CDNSource/lm/labs/45/45_Concepts_4.jpg" alt="Asit ayrışma sabiti denklemi Ka=[H+][A-]/[HA], kimya formülü." data-altupdatedat="1754907201000">
Daha yüksek Ka değerleri daha güçlü asitleri temsil ederken, daha küçük Ka değerleri daha zayıf asitleri temsil eder. Hidrojen iyonlarının konsantrasyonunu belirlemek için denklem yeniden düzenlenir. Bu formda, asit türlerinin baz türlerine oranının, hidrojen iyonlarının konsantrasyonunun ve buna bağlı olarak pH'ın belirlenmesinde önemli olduğu açıktır.
<img src="/CDNSource/lm/labs/45/45_Concepts_5.jpg" alt="Asit ayrışma denklemi, [H+] = Ka × [HA]/[A-], kimyasal denge kavramı." data-altupdatedat="1754907201000">
Her iki tarafın negatif ortak logaritmasını almak, Henderson-Hasselbalch denklemi ile sonuçlanır.
<img src="/CDNSource/lm/labs/45/45_Concepts_6.jpg" alt="Henderson-Hasselbalch denklemi; pH, pKa, tampon çözelti, asit-baz dengesi, kimyasal formül." data-altupdatedat="1754907201000">
Henderson-Hasselbalch denklemi, önce hidrojen iyonlarının konsantrasyonunu hesaplamak zorunda kalmadan doğrudan tampon pH'ının hesaplanmasını sağlar.
Örneğin, 0.02 mol güçlü bir baz eklendikten sonra 1 L'lik bir tamponun pH'ını belirlemek için kullanılabilir. Güçlü baz tamamen ayrışır, bu nedenle eklenen hidroksil iyonlarının konsantrasyonu 0.02 M'dir. Bu, asit konsantrasyonunu 0.02 oranında azaltacaktır. Asit (HA) ve baz (A-) bileşenlerinin orijinal konsantrasyonunun her birinin 0,5 M olduğunu varsayarsak, yeni baz konsantrasyonu 0,02 M ila 0,52 M artarken, asit konsantrasyonu 0,02 M ila 0,48 M azalır. Tamponun asit bileşeninin pKa'sını bilerek, pH'ı belirlemek için bu yeni bileşen konsantrasyonlarını Henderson-Hasselbalch denklemine koyabiliriz.
Bu, tampon kapasitesinin belirlenmesinde veya pH'ı önemli ölçüde etkilemeden bir tampona ne kadar güçlü asit veya güçlü baz eklenebileceğinin belirlenmesinde yararlıdır.
<h4>Tampon Kapasitesi</h4>
Tampon kapasitesi, bir tamponun pH değişimine direnme yeteneğinin ölçüsüdür. Bu yetenek, tampon bileşenlerinin konsantrasyonuna, yani asit ve eşlenik bazına bağlıdır. Daha yüksek bir tampon konsantrasyonu, daha büyük bir tampon kapasitesine sahiptir. Bu, dengeyi bozmak ve tamponun pH'ını değiştirmek için daha fazla miktarda hidrojen iyonu veya daha güçlü bir asit eklenmesi gerektiği anlamına gelir.
Tampon kapasitesi, tampon bileşenlerinin nispi konsantrasyonlarından da etkilenir. Tampon, bileşenlerin konsantrasyonları benzer olduğunda daha etkilidir. Tampon bileşen oranı benzerse, asit veya baz eklendiğinde bileşen konsantrasyon oranı önemli ölçüde değişmez; oranı dengelemek ve pH'ı değiştirmek için büyük miktarlarda asit veya baz eklenmelidir.
Tamponun pH'ı, tampon kapasitesinden farklıdır. pH aralığı, tamponun etkili olduğu aralıktır. Tipik olarak, tamponlar, tamponun asit bileşeninin pKa'sının 1 pH birimi içinde kullanılabilir bir aralığa sahiptir.
<h4>Başvuru</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Kimya ve Kimyasal Reaktivite. Belmont, CA: Brooks/Cole, Cengage Öğrenme.</li>
	<li style="font-size:17px">Silberberg, M.S. (2009). Kimya: Maddenin ve Değişimin Moleküler Doğası. Boston, MA: McGraw Tepesi.</li>
</ol>

Arabellek

Arabellek
Suya bir Arrhenius asidi (HA) eklendiğinde, eşlenik bazına (A-) ve bir hidrojen katyonuna (H+) ayrışır.
HA + H2O → H+(sulu) + ...

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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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Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

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

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Buffer pH and the Common Ion Effect

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