Indicators, pH Curves, Buffers

These flashcards cover key concepts related to acid-base titrations, indicators, and buffers, useful for exam preparation.

What are indicators in acid-base titrations?

Indicators are either weak acids or weak bases that change color at a particular pH, helping to signal the end of a titration.

What does the end-point of a titration correspond to?

The end-point corresponds to the volume of titrant added that causes a drastic change in the color of the indicator, indicating that the reaction is complete.

What is the equivalence point in titration?

The equivalence point corresponds to the addition of stoichiometric quantities of titrant, where the amounts of acid and base are in exact proportions.

What is the pH range of methyl orange as an indicator?

The pH range of methyl orange is approximately 3.1 to 4.4, making it suitable for detecting lower pH values in titrations.

In a strong acid-strong base titration, what is produced at the equivalence point?

The products are NaCl (aq) and water, resulting from the neutralization reaction.

At the equivalence point of a strong acid (HCl) and strong base (NaOH) titration, what is the pH?

At the equivalence point, pH = 7, indicating a neutral solution.

What is the characteristic feature of the titration curve for a strong acid-strong base titration?

A large rapid change in pH indicates the equivalence point, often visualized as a steep incline on the graph.

What happens to the pH as a weak acid is titrated with a strong base?

The curve begins at an acidic pH and shows a buffering region before the equivalence point, where pH changes more gradually.

What defines the buffering region in a weak acid-strong base titration?

The buffering region occurs at pH = pKa ± 1, where the weak acid and its conjugate base are present in significant amounts.

At the half-equivalence point of a weak acid-strong base titration, what should be true about the concentrations?

At this point, [Weak Acid] = [Conjugate Base], giving the solution its maximum buffering capacity.

What does phenolphthalein indicate in acid-base titrations?

Phenolphthalein is suitable for detecting neutralization at pH > 7, changing from colorless in acid to pink in alkaline conditions.

Phenolphthalein range

8.2-10

Phenolphthalein colour in acid vs alkali

Colourless in acid

Pink in alkali

Colour of methyl orange

Red in acid

Yellow in alkali

Screened methyl orange colour

Violet in acid

Green in alkali

Ph of screened methyl orange

3.1-4.4

Pk of each indicator

Methyl orange and screened methyl orange- 3.7

Phenolphthalein-9.3

What is the result of adding hydroxide ions to an acidic buffer solution?

The hydroxide ions react with the weak acid in the buffer, shifting the equilibrium left to maintain the pH.

How does an acidic buffer resist changes in pH when an acid is added?

The equilibrium shifts to the left to remove extra H⁺ ions, thus stabilizing pH.

What is the general form of a buffering action represented mathematically?

HA ⇋ H⁺ + A⁻ where HA is the weak acid and A⁻ is its conjugate base.

What is the function of the bicarbonate buffer system in the blood?

It maintains pH by balancing carbonic acid and bicarbonate ions, essential for physiological homeostasis.

How is the pH of a buffer affected if water is added?

The pH remains almost constant unless great amounts of water are added, demonstrating the buffer's capacity.

What is the Henderson-Hasselbalch equation used for?

To calculate the pH of a buffer solution based on the concentrations of the acid and its conjugate base, effectively guiding buffer preparation.

Assumptions of Henderson hasselbatch equation

Acid HA remains undissociated

All of the A comes from the salt

What does a pH of greater than 7 indicate in the context of buffers?

A pH greater than 7 indicates the presence of a solution that has undergone hydrolysis of the conjugate base.

Why is the sodium ethanoate and ethanoic acid combination used in buffers?

They form an effective acidic buffer that resists changes in pH, ideal for various biochemical applications.

What drives the equilibrium in a basic buffer when small amounts of acid are added?

The equilibrium shifts to the right to produce more salt and water, counteracting the effect of added H⁺ ions.

In a buffer system, what happens when a base is added?

The added hydroxide ions are neutralized by the weak acid, maintaining pH stability.

What pH value is typically ideal for biological buffers?

Biological buffers should ideally have a pK between 6.0 and 8.0 to function effectively in biological systems.

In amino acids, how do the side chain groups contribute to buffering?

They can either donate or accept hydrogen ions depending on the pH, providing versatility for buffering actions.

What is the purpose of the phosphate buffer system in biological fluids?

To stabilize internal pH, especially in renal and intracellular environments, crucial for physiological functions.

How does the blood buffer system respond to excess acidity?

The equilibrium shifts to absorb excess H⁺ ions, mitigating pH changes and protecting against acidosis.

What are the characteristics of Good's zwitterionic buffers?

They meet specific criteria such as being non-toxic, biocompatible, and effective within the physiological pH range.

What happens to pH after adding small amounts of a strong acid to a buffer?

The pH changes minimally as the buffer reacts to neutralize the added acid, demonstrating its capacity.

How do buffers work in controlling pH in biological systems?

They mitigate the impact of added acids or bases by shifting equilibrium to resist significant pH changes.

Criteria for buffers in blood

• Their pK, should reside between 6.0 to 8.0.

• They should exhibit high water solubility and minimal solubility in organic solvents.

• They should not permeate cell membranes.

• They should not exhibit any toxicity towards cells.

• The salt effect should be minimum, however, salts can be added as required.

• Ionic composition of the medium and temperature should have minimal effect of buffering capacity.

• Buffers should be stable and resistant to enzymatic degradation.

• Buffer should not absorb either in the visible or in the UV region.

Buffer systems in humans

Hydrogen carbonate

Phosphate

Amino acid

Acid base pairing for the biological systems

Carbonic acid- bicarbonate ion

Role of kidneys and lungs

Aid in the carbonate buffer system to lungs release co2 and kidneys to excrete excess h+ ions

Explain each system

See notes

What is the pKa value for acetic acid?

The pKa value for acetic acid is approximately 4.76, which is important for buffering calculations.

In the classic buffer calculation, what two components are essential to find pH?

The concentrations of weak acid and its conjugate base provide the necessary data for pH determination.

How does chemical equilibrium factor into buffering action?

Equilibrium positions adjust dynamically to maintain consistent pH despite external perturbations.

What is a unique property of protein buffers in physiological systems?

They can respond rapidly to pH changes due to the presence of charged amino acid side chains, enhancing metabolic stability.

What effect does temperature have on buffer systems?

Temperature can affect dissociation constants (Ka or Kb), altering buffer capacity and effectiveness.

In preparation for a buffer solution, what steps are followed?

Calculate moles needed, accurately weigh out components, mix thoroughly, and then dilute to the desired volume.

What is the main function of buffers in blood?

To stabilize pH during metabolic activities and respiratory gas exchange, maintaining homeostasis.

What does it mean when a buffer displays a high buffer capacity?

It indicates a greater concentration of buffering agents, thus resisting pH changes effectively even with significant fluctuations.

What role do the kidneys play in blood pH regulation?

They help control bicarbonate and carbonic acid levels to stabilize blood pH, ensuring proper metabolic function.

Why is understanding buffering important in biochemical applications?

Understanding buffering is crucial for maintaining physiological function and homeostasis, impacting health and biological research.

Define buffer

Solutions that can resist changes in pH when small quantities of acid or base are added.

What are the two types of buffers?

Acidic buffer

Basic buffer

For an acid conjugate base pair pkw= ?

kw=kax kb

Therefore larger ka and smaller kb, stronger acid

pkw= pka + pkb

Acidic buffer

Soln formed from a weak acid and salt of strong base

Explain the buffer action when dilute acid is added to an acidic buffer

When small amounts of acid are added, extra H+ ions are added

The position of the equilibrium moves to the left to resist the changes, keeping the pH almost constant

Explain the buffer action when a base is added to an acidic buffer

When small amount of base is added, extra OH- ions are added

The hydroxides react with the acid due to large reserve of acid and makes more salt and less acid

The position in the equilibrium moves to the right, to replace the H+ which reacted with OH- ions, keeping the pH constant

Basic buffer

Solution formed from weak base and its salt of a strong acid

Most common basic buffers

Ammonia in solution with ammonium salt

Additions to basic buffers

When small amounts of acid are added, the added hydrogen ions, H+, are removed as they react with the NH4OH (due to large excess of this) to form salt (NH4+) and H20.

The position of equilibrium moves from left to right to replace the hydroxide

When small amounts of an alkali are added, the hydroxide ions,

The position of equilibrium moves to the left hand side to nullify this

pH of acid/ base

pH= pKa + log salt/acid

pOH= pKb + log salt/ base