MCQs on Buffers, pH Determination & Indicators

(GPAT / NIPER / AIIMS / ESIC / RRB / HSSC / DSSSB / Pharmacist Recruitment Pattern)

1. A buffer solution resists pH change mainly due to:

A. Increase in ionization of water
B. Neutralizes added acid or base
C. Prevention of hydrolysis
D. Maintenance of constant osmotic pressure

Answer: B

A buffer solution resists changes in pH because it contains a weak acid and its salt (or a weak base and its salt), which can neutralize small amounts of added acid or base.

• Added acid (H⁺) is neutralized by the basic component of the buffer.

• The added base (OH⁻) is neutralized by the acidic component of the buffer.

This maintains the pH nearly constant.

2. Which of the following is an acidic buffer?

A. NH₄OH + NH₄Cl
B. CH₃COOH + CH₃COONa
C. NaOH + NaCl
D. HCl + NaCl

Answer: B

Explanation:

An acidic buffer is made by mixing:

• a weak acid and

• It's salt with a strong base.

Here:

• CH₃COOH (acetic acid) is a weak acid.

• CH₃COONa (sodium acetate) is the salt of acetic acid with a strong base (NaOH).

Therefore, this mixture acts as an acidic buffer and maintains a pH below 7.

Why the other options are incorrect:

A. NH₄OH + NH₄Cl

• NH₄OH (ammonium hydroxide) is a weak base.

• NH₄Cl (ammonium chloride) is the salt of a strong acid.

This combination forms a basic (alkaline) buffer, not an acidic buffer.

3. The pH of a buffer containing equal concentrations of weak acid and its salt is equal to:

A. 7
B. pKa
C. pKb
D. 14

Answer: B

Explanation:

According to the pH = pKa + pKa+log⁡([Salt] / [Acid]),

When the concentrations of the weak acid and its salt are equal:

[Salt] = [Acid]

Therefore,

log⁡([Salt] / [Acid]) = log⁡(1) = 0

So the equation becomes:

pH=pKa

Hence, the pH of the buffer equals the pKa of the weak acid.

Why the other options are incorrect:

A. 7: A pH of 7 indicates a neutral solution, but buffer solutions can be acidic or basic depending on the weak acid/base used. Therefore, the pH is not always 7.

C. pKb: pKb is related to weak bases and basic buffers, not acidic buffers prepared from a weak acid and its salt.

D. 14: A pH of 14 indicates a highly basic solution, not a buffer containing a weak acid and its salt.

4. The Henderson–Hasselbalch equation for an acidic buffer is

pH = pKa + log [Salt] / [Acid]

A. Applicable to strong acids
B. Applicable at neutral pH
C. Incorrect because the log term is reversed
D. Correct

Answer: D

The correct answer is D. Correct

Explanation:

The Henderson–Hasselbalch equation for an acidic buffer is:

pH = pKa + log[Salt] / [Acid]

Where:

• [Salt] = concentration of the salt of the weak acid

• [Acid] = concentration of the weak acid

This equation is correctly written and is widely used to calculate the pH of acidic buffer solutions.

Why the other options are incorrect:

A. Applicable to strong acids: The Henderson–Hasselbalch equation is used for weak acids and their salts, not strong acids. Strong acids ionize completely and do not form effective buffer systems.

B. Applicable at neutral pH: The equation is not limited to neutral pH. It can be applied to acidic or basic buffer systems, depending on the weak acid/base involved.

C. Incorrect because the log term is reversed: The equation is correctly written.

If reversed, it would give an incorrect pH value.

5. Which buffer is commonly present in blood plasma?

A. Acetate buffer
B. Borate buffer
C. Phosphate buffer
D. Carbonate–bicarbonate buffer

Answer: D

Explanation:

The carbonate–bicarbonate buffer system is the major buffer present in blood plasma. It helps maintain the normal blood pH around 7.4 by regulating the hydrogen ion concentration.

The buffer system consists of:

• Carbonic acid (H₂CO₃) — weak acid

• Bicarbonate ion (HCO₃⁻) — conjugate base

It works according to the equilibrium:

H₂CO₃ ⇌ H⁺ + HCO₃⁻

This system is highly effective because the lungs regulate CO₂ and the kidneys regulate bicarbonate concentration.

6. Buffer capacity is maximum when:

A. pH = 7
B. Buffer is concentrated
C. Buffer is diluted
D. [Salt] = [Acid]

Answer: B

The correct answer is D. [Salt] = [Acid]

Explanation:

Buffer capacity is the ability of a buffer to resist changes in pH when small amounts of acid or base are added.

A buffer shows maximum capacity when the concentrations of the weak acid and its salt are equal:

[Salt] = [Acid]

At this condition:

pH = pK_a

The buffer can neutralize added acids and bases equally effectively, giving maximum buffering action.

Why the other options are incorrect:

A. pH = 7: Maximum buffer capacity does not depend on the pH being 7. Different buffers work best at different pH values depending on their pKa.

B. Buffer is concentrated: A concentrated buffer generally has greater total buffering ability than a dilute one, but the maximum buffer capacity condition specifically occurs when acid and salt concentrations are equal.

C. Buffer is diluted: Dilution decreases the number of buffer components present, reducing buffer capacity. Therefore, dilute buffers are less effective.

7. Which factor increases buffer capacity?

A. Diluted buffer
B. Decreased salt concentration,
C. Increase in temperature only
D. Increased total buffer concentration

Answer: D

Explanation:

Buffer capacity depends on the total concentration of the buffer components (weak acid/base and its salt). A higher concentration means more acid or base can be neutralized without a significant change in pH.

Therefore, increasing the total buffer concentration increases the buffer capacity.

Why the other options are incorrect:

A. Diluted buffer: Dilution lowers the concentration of buffer components, reducing the ability of the buffer to resist pH changes.

B. Decreased salt concentration: Reducing salt concentration disturbs the buffer ratio and decreases the buffering action.

C. Increase in temperature only: Temperature may affect the ionization of buffer components slightly, but simply increasing temperature does not necessarily increase buffer capacity.

8. The useful pH range of a buffer generally lies within:

A. pKa ± 7
B. pKa ± 5
C. pKa ± 1
D. pH 2–12

Answer: C

Explanation:

A buffer works most effectively when the pH is close to the pKa of the weak acid.

According to the Henderson–Hasselbalch equation:

pH = pKa + log[Salt] / [Acid]

A buffer is considered effective when the ratio of salt to acid lies between 10:1 and 1:10. This corresponds approximately to:

pH = pKa ± 1

Within this range, the buffer can effectively resist changes in pH.

Why the other options are incorrect:

A. pKa ± 7: This range is far too wide. No buffer can effectively maintain pH over such a broad interval.

B. pKa ± 5: This is also too broad for effective buffering action.

D. pH 2–12: No single buffer system can effectively operate over this entire pH range. Different buffers are required for different pH ranges.

9. In pharmaceutical dosage forms, buffers are used:

A. To improve appearance
B. To maintain solubility and stability
C. To maintain sterilization
D. To increase viscosity

Answer: B

Explanation:

Buffers are widely used in pharmaceutical dosage forms to maintain a desired pH. Proper pH helps:

• maintain the solubility of drugs,

• improve chemical stability,

• reduce degradation, and

• enhance therapeutic effectiveness.

Many drugs are stable only within a specific pH range, so buffers are important in formulations such as injections, syrups, eye drops, and tablets.

Why the other options are incorrect:

A. To improve appearance: Buffers are not mainly added to improve the appearance of pharmaceutical products.

C. To maintain sterilization: Sterilization is achieved by methods such as heat, filtration, or chemicals—not by buffers.

D. To increase viscosity: Viscosity is increased by thickening or suspending agents, not by buffer systems.

10. Which method is used to determine pH using hydrogen ion-selective electrodes?

A. Electrometry
B. Gravimetry
C. Colorimetry
D. Spectrophotometry

Answer: C

Explanation:

pH determination using hydrogen ion-selective electrodes is based on measuring electrical potential differences. This method is called electrometry.

A pH meter works using:

• a glass electrode sensitive to hydrogen ions (H⁺), and

• a reference electrode.

The potential developed is related to hydrogen ion concentration and, therefore to pH.

Why the other options are incorrect:

B. Gravimetry: Gravimetry involves measurement based on the mass or weight of substances, not electrical potential.

C. Colorimetry: Colorimetry determines concentration by measuring color intensity and is not based on hydrogen ion-selective electrodes.

D. Spectrophotometry: Spectrophotometry measures absorption of light by a substance at specific wavelengths, not electrode potential.

11. The glass electrode used in pH measurement by electrometry is sensitive to:

A. Hydroxyl ion
B. Chloride ion
C. Potassium ions
D. Hydrogen ions

Answer: C

Explanation:

The glass electrode used in pH measurement is specifically sensitive to the concentration of hydrogen ions (H⁺) in solution.

The electrode develops an electrical potential that depends on hydrogen ion activity, and this potential is used by the pH meter to determine the pH value.

Since pH is related to hydrogen ion concentration:

pH = -log [H⁺]

The glass electrode functions as a hydrogen ion-selective electrode.

Why the other options are incorrect:

A. Hydroxyl ion: Glass electrodes are not directly sensitive to hydroxyl ions (OH⁻).

B. Chloride ion: Chloride ions are measured using different ion-selective electrodes, not the standard glass pH electrode.

C. Potassium ions: Potassium-selective electrodes exist separately, but the glass electrode for pH measurement is not designed for potassium ions.

12. Which of these is NOT a suitable method to determine pH?

A. Polarimetric method
B. Colorimetric method
C. Electrometric method
D. Potentiometric method

Answer: A

The correct answer is A. Polarimetric method

Explanation:

The polarimetric method is not commonly used for determining pH. Polarimetry measures the rotation of plane-polarized light by optically active substances, such as sugars and certain organic compounds.

It does not measure hydrogen ion concentration and therefore is not suitable for pH determination.

Why are the other options correct methods for pH determination?

B. Colorimetric method: This method uses acid–base indicators that change color at different pH values. The observed color helps estimate the pH of the solution.

C. Electrometric method: Electrometric methods use electrodes (such as glass electrodes) to measure the electrical potential related to the hydrogen ion concentration.

D. Potentiometric method: Potentiometric determination of pH is based on measuring voltage differences between electrodes and is the principle used in pH meters. It is a type of electrometric method.

13. Which one is used in a colorimetric method for pH determination?

A. Conductivity meters
B. Electrodes only
C. pH meter
D. Indicators

Answer: D

The correct answer is D. Indicators

Explanation:

In the colorimetric method of pH determination, indicators are used. These are substances that change color depending on the pH of the solution.

Examples include:

• Phenolphthalein

• Methyl orange

• Litmus

The observed color is compared with a standard color chart to estimate the pH.

Why the other options are incorrect:

A. Conductivity meters: Conductivity meters measure the electrical conductivity of a solution, not its pH, by color change.

B. Electrodes only: Electrodes are used in electrometric or potentiometric methods, not in colorimetric methods.

C. pH meter: A pH meter is used in electrometric/potentiometric methods, where electrodes measure electrical potential differences.

14. Which one of the indicators is suitable for pH ranges around 7?

A. Phenolphthalein
B. Methyl orange
C. Thymol blue
D. Bromothymol blue

Answer: D

Explanation:

Bromothymol blue is suitable for pH values around neutral pH (approximately 6.0–7.6). Therefore, it is commonly used for solutions near pH 7.

Color change of bromothymol blue:

• Yellow in acidic medium

• Green near neutral pH

• Blue in alkaline medium

Why the other options are incorrect:

A. Phenolphthalein: Phenolphthalein changes color in the alkaline range (about pH 8.2–10), so it is not suitable around pH 7.

B. Methyl orange: Methyl orange works in the acidic range (about pH 3.1–4.4), far below neutral pH.

C. Thymol blue: Thymol blue has transition ranges mainly in strongly acidic and alkaline regions, making it less suitable specifically around pH 7.

15. Methyl orange changes color in the pH range:

A. 1.2–2.9
B. 3.1–4.4
C. 6.1–7.7
D. 8.1–12

Answer: B

Explanation:

Methyl orange is an acid–base indicator that changes color in the acidic pH range.

Its transition range is pH: 3.1 to 4.4

Color change:

• Red in acidic solution

• Orange in the transition range

• Yellow in alkaline solution

Therefore, methyl orange is mainly used in acid-base titrations involving strong acids.

Why the other options are incorrect:

A. 1.2–2.9: This range is too acidic and does not correspond to methyl orange.

C. 6.1–7.7: This range is closer to indicators like bromothymol blue.

D. 8.1–12: This is an alkaline range, more suitable for indicators such as phenolphthalein.

16. Phenolphthalein is colorless in which medium?

A. Alkaline
B. Acidic
C. Only in Neutral
D. Strongly alkaline

Answer: B

Explanation:

Phenolphthalein is an acid-base indicator that remains

• Colorless in an acidic medium

• Pink in alkaline medium

Its color change occurs approximately in the pH range of pH 8.2 to 10

Below this range (acidic solution), phenolphthalein remains colorless.

Why the other options are incorrect:

A. Alkaline: In an alkaline medium, phenolphthalein turns pink, not colorless.

C. Only in Neutral: Phenolphthalein is colorless not only at neutral pH but also throughout acidic conditions as well.

D. Strongly alkaline: In strongly alkaline solutions, phenolphthalein generally shows a deep pink color.

17. Which statement about buffer capacity is TRUE?

A. It does not depend upon concentration
B. It is highest at a low pH
C. Concentrated buffers resist pH changes better
D. Strong acid–strong base mixtures form the best buffers

Answer: C

Explanation:

Buffer capacity is the ability of a buffer solution to resist changes in pH when small amounts of acid or base are added.

A concentrated buffer contains a larger amount of buffering components (weak acid/base and its salt), so it can neutralize more added acid or base. Therefore, concentrated buffers have greater buffer capacity.

Why the other options are incorrect:

A. It does not depend upon concentration: This is incorrect because buffer capacity strongly depends on the concentration of buffer components.

B. It is highest at a low pH: Buffer capacity is not necessarily highest at a low pH. It is maximum when [Salt] = [Acid]

or when pH = pKa

D. Strong acid–strong base mixtures form the best buffers: Strong acids and strong bases do not form buffer systems because they ionize completely and cannot effectively resist pH changes.

18. Which pharmaceutical dosage form commonly uses phosphate buffer?

A. Aerosols
B. Ophthalmic dosage form
C. Ointments
D. Injectables

Answer: B

Explanation:

Phosphate buffers are commonly used in ophthalmic preparations (eye drops) because they help maintain a pH close to that of tears, improving:

• patient comfort,

• drug stability, and

• compatibility with eye tissues.

Phosphate buffer systems are effective and physiologically acceptable for ophthalmic use.

Why the other options are incorrect:

A. Aerosols: Buffers are generally not the primary components used in aerosol formulations.

C. Ointments: Most ointments are semisolid and non-aqueous, so buffer systems are less commonly required.

D. Injectables: Some injectables may contain buffers, but phosphate buffer is especially well known for use in ophthalmic formulations.

19. A buffer prepared from NH₄OH and NH₄Cl is:

A. A Universal buffer
B. A Neutral buffer
C. A Basic buffer
D. An Acidic buffer

Answer: C

Explanation:

A buffer made from:

• NH₄OH (ammonium hydroxide) — a weak base, and

• NH₄Cl (ammonium chloride)—its salt with a strong acid,

forms a basic (alkaline) buffer.

This buffer resists changes in pH in the alkaline range because:

• NH₄OH neutralizes added acids,

• NH₄⁺ ions from NH₄Cl help neutralize added bases.

Why the other options are incorrect:

A. A universal buffer: A universal buffer contains several buffering agents and works over a wide pH range. NH₄OH + NH₄Cl is only a basic buffer system.

B. A neutral buffer: A neutral buffer maintains a pH of around 7. The NH₄OH/NH₄Cl system generally has a pH above 7.

D. An Acidic buffer: Acidic buffers are made from a weak acid and its salt, such as acetic acid and sodium acetate. NH₄OH is a weak base, so this is not an acidic buffer.

20. The primary standard buffer used for a pH meter calibration is

A. Sodium chloride
B. Calcium carbonate
C. Sodium hydroxide
D. Potassium hydrogen phthalate

Answer: D

Explanation:

Potassium hydrogen phthalate (KHP) is commonly used as a primary standard buffer for calibrating pH meters because:

• it is highly pure,

• stable,

• non-hygroscopic, and

• produces a solution with a well-defined pH.

It is especially used for calibration in the acidic pH range.

Why the other options are incorrect:

A. Sodium chloride: Sodium chloride is a neutral salt and is not used as a standard buffer for pH calibration.

B. Calcium carbonate: Calcium carbonate is poorly soluble in water and unsuitable for accurate pH meter calibration.

C. Sodium hydroxide: Sodium hydroxide is a strong base and absorbs carbon dioxide from the air, making it unstable as a primary standard buffer.

21. Which ONE may affect pH measurement while using a glass electrode?

A. Ionic strength
B. Electrode condition
C. Temperature
D. All of the above

Answer: D

Explanation:

Several factors can affect pH measurement when using a glass electrode:

A. Ionic strength: Changes in ionic strength can influence the activity of hydrogen ions and affect electrode response.

B. Electrode condition: A dirty, aged, or damaged glass electrode may give inaccurate or unstable pH readings.

C. Temperature: Temperature affects both

• hydrogen ion activity, and

• electrode potential.

Therefore, pH measurements are temperature-dependent.

22. The pH of pure water at 25°C is pH = -log[H⁺]

A. 1
B. 5
C. 7
D. 12

Answer: C

Explanation:

Pure water undergoes slight ionization:

H₂O ⇌ H⁺ + OH⁻

At 25°C, the concentration of hydrogen ions in pure water is:

[H⁺]=1×10⁻⁷M

Using the pH formula:

pH = −log[H⁺]

Therefore:

pH = −log(10⁻⁷) = 7

Hence, pure water at 25°C is neutral with a pH of 7.

Why the other options are incorrect:

A. 1: A pH of 1 indicates a strongly acidic solution.

B. 5: A pH of 5 is acidic, not neutral.

D. 12: A pH of 12 indicates a strongly alkaline solution.

23. Which indicator is used for strong acid–strong base titration?

A. Phenolphthalein
B. Methyl orange
C. Both A and B
D. Congo red

Answer: C

Explanation:

In a strong acid–strong base titration, the pH changes very sharply near the equivalence point. Because of this wide and steep pH change, both:

• Phenolphthalein, and

• Methyl orange

can be used successfully as indicators.

• Phenolphthalein changes color in the alkaline range (pH 8.2–10).

• Methyl orange changes color in the acidic range (pH 3.1–4.4).

The sharp pH jump in strong acid–strong base titrations covers both transition ranges.

Why the other options are incorrect:

A. Phenolphthalein: Phenolphthalein alone can be used, but it is not the only suitable indicator.

B. Methyl orange: Methyl orange alone can also be used, but again, it is not the only suitable indicator.

D. Congo red:

Congo red is not commonly used for routine strong acid–strong base titrations because its transition range is less suitable.

24. In buffer preparation, the ratio of salt to acid mainly controls:

A. Density
B. Stability
C. Surface tension
D. pH

Answer: D

Explanation:

In a buffer solution, the ratio of salt to acid determines the pH according to the Henderson–Hasselbalch equation:

pH = pKa + log ([Salt] / [Acid]​)

• Increasing the salt concentration increases the pH.

• Increasing the acid concentration decreases the pH.

Thus, the salt-to-acid ratio mainly controls the pH of the buffer.

Why the other options are incorrect:

A. Density: Density depends mainly on the total concentration and nature of solutes, not specifically on the salt-to-acid ratio.

B. Stability: Buffers may improve formulation stability, but the ratio primarily affects pH rather than stability directly.

C. Surface tension: Surface tension is influenced mainly by surfactants and intermolecular forces, not by the buffer ratio.

25. Select the biological fluid that has a regulated pH around 7.35–7.45.

A. Urine
B. Saliva
C. Gastric juice
D. Blood

Answer: D. Blood

Explanation:

Human blood maintains a tightly regulated pH range of pH 7.35 to 7.45.

This slightly alkaline pH is essential for:

• normal enzyme activity,

• oxygen transport,

• metabolic processes, and

• proper functioning of body cells.

The carbonate–bicarbonate buffer system plays a major role in maintaining this pH.

Why the other options are incorrect:

A. Urine: Urine pH varies widely (about 4.5–8) depending on diet, metabolism, and health conditions.

B. Saliva: Saliva usually has a pH around 6.2–7.6 and is not as strictly regulated as blood.

C. Gastric juice: Gastric juice is highly acidic, with a pH around 1–3 due to hydrochloric acid.

26. The alkaline error in a glass electrode occurs:

A. At a neutral pH
B. At a high pH
C. At a very low pH
D. At a physiological pH

Answer: B At a high pH

Explanation:

The alkaline error (also called sodium error) in a glass electrode occurs at high pH values, usually above pH 9 or 10.

At very high pH:

• The concentration of hydrogen ions (H⁺) becomes very low.

• and the glass electrode may respond to other ions such as sodium ions (Na⁺).

This causes inaccurate pH readings.

Why the other options are incorrect:

A. At a neutral pH: Glass electrodes function accurately near neutral pH and do not show alkaline error there.

C. At a very low pH: At a very low pH, a different problem called "acid error" may occur, not "alkaline error."

D. At a physiological pH: Physiological pH (around 7.4) is within the normal, accurate operating range of a glass electrode.

27. Buffer action occurs due to:

A. Weak acid Complete ionization
B. Common ion effect
C. Hydrolysis
D. Hydroxilation

Answer: B Common ion effect

Explanation:

Buffer action mainly occurs due to the common ion effect.

A buffer contains:

• a weak acid and its salt, or

• a weak base and its salt.

Example of an acidic buffer:

CH₃COOH + CH₃COONa

Here, both substances provide a common ion:

CH₃COOH ⇌ H⁺ + CH₃COO⁻

The acetate ion from sodium acetate suppresses the ionization of acetic acid due to the common ion effect, helping the solution resist changes in pH.

Why the other options are incorrect:

A. Weak acid complete ionization: Weak acids do not ionize completely. In fact, partial ionization is essential for buffer action.

C. Hydrolysis: Hydrolysis may occur in some salt solutions, but it is not the main reason for buffer action.

D. Hydroxilation: “Hydroxilation” is not the principle responsible for buffer action in buffer solutions.

28. Which one is a limitation of the colorimetric determination of pH?

A. Expensive instruments
B. High accuracy
C. Subjective color comparison
D. Misture sensitivity

Answer: C Subjective color comparison

Explanation:

In the colorimetric method of pH determination, the pH is estimated by comparing the color of an indicator solution with a standard color scale.

This method has a limitation because

• Color perception varies from person to person,

• Slight color differences may be difficult to judge accurately.

Therefore, the result may be subjective and less accurate than electrometric methods.

Why the other options are incorrect:

A. Expensive instruments: Colorimetric methods are generally simple and inexpensive, often requiring only indicators and color charts.

B. High accuracy: High accuracy is actually an advantage of electrometric methods, not a limitation of colorimetry.

D. Misture sensitivity: This is not the standard or primary limitation commonly associated with colorimetric pH determination.

29. Which one is INCORRECT about pharmaceutical buffers?

A. They help to maintain drug stability
B. They reduce tissue irritation
C. They increase drug absorption
D. They help to maintain formulation pH

Answer: C. They increase drug absorption

Explanation:

Pharmaceutical buffers are mainly used to:

• maintain the desired pH,

• improve drug stability,

• reduce tissue irritation, and

• maintain solubility of drugs.

They are not primarily added to increase drug absorption.

Why are the other options correct?

A. They help to maintain drug stability: Correct. Many drugs are stable only within a specific pH range, so buffers help prevent degradation.

B. They reduce tissue irritation: Correct. Maintaining physiological pH in formulations such as eye drops and injections helps minimize irritation.

D. They help to maintain formulation pH: Correct. This is the main function of pharmaceutical buffers.

30. The Van Slyke equation is related to:

A. Surface tension
B. Solubility
C. Osmotic pressure
D. Buffer capacity

Answer: D. Buffer capacity

Explanation:

The Van Slyke equation is used to express and calculate the buffer capacity of a buffer solution.

Buffer capacity refers to the ability of a buffer to resist changes in pH when small amounts of acid or base are added.

The equation relates buffer capacity to:

• The concentration of buffer components, and

• The change in pH produced by the addition of an acid or base.

Why the other options are incorrect:

A. Surface tension: Surface tension is related to intermolecular forces at the liquid surface and is not described by the Van Slyke equation.

B. Solubility: Solubility describes how much solute dissolves in a solvent and is unrelated to the Van Slyke equation.

C. Osmotic pressure: Osmotic pressure is associated with solutions separated by semipermeable membranes and is not the focus of the Van Slyke equation.

31. A buffer solution shows the greatest resistance to pH change when:

A. Salt-to-acid ratio 1:1
B. Salt-to-acid ratio 1:10
C. Acid-to-salt ratio 10:1
D. Buffer is highly diluted

Answer: A. Salt-to-acid ratio 1:1

Explanation:

A buffer shows maximum resistance to pH change (maximum buffer capacity) when the concentrations of the weak acid and its salt are equal.

[Salt] = [Acid]

At this condition:

pH = pK_a

The buffer can neutralize added acids and bases most effectively.

Why the other options are incorrect:

B. Salt-to-acid ratio 1:10: This ratio makes the solution more acidic and reduces the overall buffering efficiency.

C. Acid-to-salt ratio 10:1. This also shifts the buffer away from the optimum buffering condition.

D. Buffer is highly diluted: Dilution decreases the concentration of buffering components and lowers buffer capacity.

32. Small amounts of HCl are added to an acetate buffer. The added H⁺ ions are consumed:

A. By Alkaline ions
B. By Acetic acid
C. By Acetate ions
D. By Water molecules

Answer: C

The correct answer is C. By Acetate ions

Explanation:

An acetate buffer contains:

• Acetic acid (CH₃COOH) — weak acid

• Acetate ions (CH₃COO⁻) from sodium acetate

When small amounts of HCl are added, the added hydrogen ions (H⁺) are neutralized by acetate ions:

CH₃COO⁻ + H⁺ → CH₃COOH

Thus, the pH changes only slightly because the buffer converts the added strong acid into weak acetic acid.

Why the other options are incorrect:

A. By Alkaline ions: The buffering action specifically involves acetate ions, not general “alkaline ions.”

B. By acetic acid: Acetic acid donates H⁺ ions; it does not consume added H⁺ ions.

D. By Water molecules: Water alone cannot effectively neutralize added H⁺ ions in a buffer system.

33. Which one buffer system maintains intracellular fluid pH?

A. Borate buffer
B. Bicarbonate buffer
C. Citrate buffer
D. Phosphate buffer

Answer: D. Phosphate buffer

Explanation:

The phosphate buffer system is an important buffer in intracellular fluid (ICF). It helps maintain the pH inside cells.

The system consists mainly of:

• Dihydrogen phosphate ion (H₂PO₄⁻) — weak acid

• Hydrogen phosphate ion (HPO₄²⁻) — conjugate base

Buffer equilibrium:

H₂PO₄⁻ ⇌ H⁺ + HPO₄²⁻

This buffer is effective in the physiological pH range and plays a major role inside cells and in renal tubular fluid.

Why the other options are incorrect:

A. Borate buffer: Borate buffer is mainly used in pharmaceutical and analytical applications, not as a major physiological intracellular buffer.

B. Bicarbonate buffer: The bicarbonate buffer system is the major buffer of blood plasma and extracellular fluid, not intracellular fluid.

C. Citrate buffer: Citrate buffers are commonly used in pharmaceutical formulations and laboratory preparations, not as the main intracellular buffer system.

34. The pH meter works on the principle of measuring:

A. Refractive index
B. Potential difference
C. Conductance
D. Turbidity

Answer: B Potential difference

Explanation:

A pH meter works by measuring the potential difference (voltage) developed between

• A glass electrode (sensitive to hydrogen ions), and

• A reference electrode.

This potential difference depends on the hydrogen ion concentration of the solution and is converted into pH.

The relationship is based on electrochemical principles.

Why the other options are incorrect:

A. Refractive index: Refractive index is used in optical measurements, not pH determination.

C. Conductance: Conductance measures the ability of a solution to conduct electricity, which is different from pH measurement.

D. Turbidity: Turbidity measures cloudiness or suspended particles in a liquid, not hydrogen ion concentration.

35. An ideal pharmaceutical buffer should:

A. Be highly reactive with drug molecules to enhance absorption

B. Alter the therapeutic activity of the drug for faster onset

C. Have a strong, unpleasant taste to discourage overdosing
D. Maintain desired pH with minimal tissue irritation

Answer: D Maintain desired pH with minimal tissue irritation

Explanation:

An ideal pharmaceutical buffer should:

• maintain the required pH of the formulation,

• improve drug stability and solubility,

• and minimize irritation to body tissues.

This is especially important in formulations such as:

• ophthalmic solutions,

• injections,

• oral liquids, and

• nasal preparations.

Maintaining an appropriate pH helps ensure patient comfort and product effectiveness.

Why the other options are incorrect:

A. Be highly reactive with drug molecules to enhance absorption: Buffers should generally be chemically compatible and non-reactive with drug molecules to avoid degradation or instability.

B. Alter the therapeutic activity of the drug for faster onset: Buffers are not intended to alter the therapeutic action of drugs. Their main role is pH control and stability maintenance.

C. Have a strong, unpleasant taste to discourage overdosing: An ideal pharmaceutical buffer should be palatable and acceptable to patients, especially in oral preparations.

An ideal pharmaceutical buffer should—

A. Maintain a constant pH during storage and administration

B. Be non-toxic and physiologically compatible

C. Have sufficient buffer capacity within the desired pH range

D. Be stable, easy to prepare, and sterilizable

36. Which indicator would you prefer for titration of weak acid against strong base?

A. Phenolphthalein
B. Methyl orange
C. Bromophenol blue
D. Congo red

Answer: A. Phenolphthalein

Explanation:

In the titration of a weak acid with a strong base, the equivalence point lies in the alkaline range (pH greater than 7).

Phenolphthalein is the preferred indicator because its color change occurs in the alkaline pH range:

pH: 8.2 to 10

Color change:

• Colorless in acidic medium

• Pink in alkaline medium

This makes it suitable for detecting the endpoint accurately.

Why the other options are incorrect:

B. Methyl orange: Methyl orange changes color in the acidic range (pH 3.1–4.4), which is not suitable for weak acid–strong base titrations.

C. Bromophenol blue: Its transition range is not ideal for the alkaline equivalence point of this titration.

D. Congo red: Congo red is not commonly used for such titrations because its transition range is less suitable and less sharp.

37. Which one best explains buffer failure?

A. Buffer becomes neutral automatically.

B. Indicators stop functioning.

C. Water ionization stops

D. Excess addition of acid/base exceeds buffer capacity

Answer: D. Excess addition of acid/base exceeds buffer capacity

Explanation:

A buffer can resist pH changes only up to a certain limit known as its buffer capacity.

When too much acid or base is added:

• the buffering components become exhausted,

• and the buffer can no longer neutralize the added H⁺ or OH⁻ ions effectively.

As a result, the pH changes rapidly. This is called buffer failure.

Why the other options are incorrect:

A. Buffer becomes neutral automatically

Buffers do not automatically become neutral. Their pH depends on the buffer components and their concentrations.

B. Indicators stop functioning: Indicators are unrelated to buffer failure. They only help detect pH changes.

C. Water ionization stops: Ionization of water never stops under normal conditions and is not the reason for buffer failure.

38. The pKa of acetic acid is 4.76. The pH of the acetate buffer having equal amount of acid and salt concentrations will be

A. 2.76
B. 4.76
C. 5.76
D. 6.76

Answer: B. 4.76

Explanation:

For an acidic buffer, the Henderson–Hasselbalch equation is:

pH = pKa + log ([Salt] / [Acid])

When the concentrations of acid and salt are equal:

[Salt] = [Acid]

Therefore:

log([Salt] / [Acid]) = log(1) = 0

So:

pH = pKa = 4.76

Hence, the pH of the acetate buffer is 4.76.

Why the other options are incorrect:

A. 2.76: This value is lower than the pKa and would occur only if acid concentration were much greater than salt concentration.

C. 5.76: This would occur if the salt concentration were ten times greater than the acid concentration.

D. 6.76: This value is too high and does not apply when acid and salt concentrations are equal.

39. Which parameter plays the most important role while selecting a buffer for injections?

A. Taste
B. Stability
C. Physiological compatibility
D. Odor

Answer C. Physiological compatibility

Explanation:

While selecting a buffer for injections, the most important consideration is physiological compatibility because injections are administered directly into the body tissues or bloodstream.

An ideal buffer for injections should:

• maintain a pH close to physiological pH,

• minimize tissue irritation and pain,

• be non-toxic, and

• be compatible with body fluids.

This ensures patient safety and comfort.

Why the other options are incorrect:

A. Taste: Taste is not important for injections because they are not administered orally.

B. Stability: Drug stability is important, but physiological compatibility is the primary concern for injectable preparations.

D. Odor: Odor has little significance in injectable dosage forms.

40. The relation between pH and hydrogen ion concentration is:

pH = -log[H⁺]

A. Linear relation
B. Directly proportional
C. Exponential
D. Inversely logarithmic

Answer: D. Inversely logarithmic

Explanation:

The relationship between pH and hydrogen ion concentration is given by: pH = −log[H⁺]

This means:

• as hydrogen ion concentration ([H⁺]) increases, pH decreases.

• and the relationship follows a logarithmic scale, not a linear one.

Thus, pH is inversely related to hydrogen ion concentration on a logarithmic basis.

For example:

• A tenfold increase in [H⁺] lowers the pH by 1 unit.

Why the other options are incorrect:

A. Linear relation: The relationship is logarithmic, not linear.

B. Directly proportional: If pH were directly proportional to ([H^+]), both would increase together, which is incorrect.

C. Exponential: The equation expresses a logarithmic relationship, not an exponential one.

Dr. Alok Singh