Understanding the structure and uses of citric acid is essential for B. Pharm students preparing for exams and practical formulation work. This concise, SEO-friendly overview covers citric acid structure, molecular formula (C6H8O7), stereochemistry, pKa values, and common pharmaceutical roles such as buffering agent, chelator, pH adjuster, preservative, and effervescent excipient. You’ll also encounter its analytical tests, USP monograph, stability concerns, and interactions with metal ions and active pharmaceutical ingredients. These focused insights and keywords like citric acid structure, uses of citric acid, citric acid MCQs, and pharmaceutical excipient help reinforce learning and exam readiness. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which of the following is the correct molecular formula of citric acid?
- C6H8O7
- C3H4O3
- C7H6O6
- C4H6O5
Correct Answer: C6H8O7
Q2. Citric acid is formally named as which IUPAC name?
- 2-Hydroxypropane-1,2,3-tricarboxylic acid
- 1,2,3-Trihydroxypropane-1,2,3-tricarboxylic acid
- Propane-1,2-dicarboxylic acid
- 2,3-Dihydroxybutanedioic acid
Correct Answer: 2-Hydroxypropane-1,2,3-tricarboxylic acid
Q3. How many dissociable acidic protons does citric acid have?
- One
- Two
- Three
- Four
Correct Answer: Three
Q4. Which set best represents the approximate pKa values of citric acid?
- 3.1, 4.8, 6.4
- 2.0, 4.0, 6.0
- 1.2, 2.8, 4.5
- 5.0, 6.5, 8.0
Correct Answer: 3.1, 4.8, 6.4
Q5. Citric acid is most commonly produced industrially by which method?
- Distillation of citrus juice
- Chemical synthesis from acetic acid
- Fermentation using Aspergillus niger
- Electrochemical oxidation of propylene
Correct Answer: Fermentation using Aspergillus niger
Q6. Which of the following describes the stereochemistry of citric acid?
- Optically active with one chiral center
- Optically active with two chiral centers
- Optically inactive (achiral) molecule
- Racemic mixture of enantiomers
Correct Answer: Optically inactive (achiral) molecule
Q7. Citric acid acts as an anticoagulant in blood collection primarily because it:
- Oxidizes hemoglobin
- Chelates calcium ions
- Denatures clotting proteins
- Inhibits platelet aggregation directly
Correct Answer: Chelates calcium ions
Q8. Which citrate salt is commonly used as an alkalinizing agent and systemic buffer?
- Monosodium citrate
- Disodium citrate
- Trisodium citrate
- Calcium citrate
Correct Answer: Trisodium citrate
Q9. In effervescent tablet formulations, citric acid is combined with what to generate carbon dioxide?
- Sodium chloride
- Sodium bicarbonate
- Calcium carbonate
- Magnesium stearate
Correct Answer: Sodium bicarbonate
Q10. Which property explains why citric acid is widely used as a buffering agent in pharmaceutical formulations?
- It is a strong monoprotic acid
- It has multiple pKa values allowing buffer systems across several pH ranges
- It is completely non-ionizable
- It acts as an oxidizing agent
Correct Answer: It has multiple pKa values allowing buffer systems across several pH ranges
Q11. Which of the following describes a key role of citric acid in topical formulations?
- Acts as a primary preservative by disrupting membranes
- Functions as a chelating agent to stabilize formulations by binding metal ions
- Serves as a major emulsifier
- Acts as a strong oxidant to sterilize the preparation
Correct Answer: Functions as a chelating agent to stabilize formulations by binding metal ions
Q12. Which analytical method is commonly used to quantify citric acid content in a pharmaceutical sample?
- Acid–base titration with standardized NaOH
- Gravimetric precipitation with silver nitrate
- Flame photometry
- Polarimetry
Correct Answer: Acid–base titration with standardized NaOH
Q13. The citrate ion has what overall charge at physiological pH (~7.4)?
- Neutral (0)
- −1
- −2
- −3
Correct Answer: −3
Q14. Which statement is true about the solubility of citric acid?
- It is insoluble in water and highly soluble in organic solvents
- It is highly soluble in water and sparingly soluble in ethanol
- It is soluble only in nonpolar solvents
- It sublimes on heating instead of dissolving
Correct Answer: It is highly soluble in water and sparingly soluble in ethanol
Q15. Which pharmaceutical application uses citric acid as an effervescent acidifier and taste enhancer?
- Sustained-release matrix tablets
- Effervescent vitamin tablets
- Topical ointments
- Injectable suspensions
Correct Answer: Effervescent vitamin tablets
Q16. Which USP-related statement about citric acid is correct?
- There is no USP monograph for citric acid
- USP recognizes both anhydrous and monohydrate forms of citric acid
- USP only accepts citric acid produced synthetically
- USP requires citric acid to be used only as an active pharmaceutical ingredient
Correct Answer: USP recognizes both anhydrous and monohydrate forms of citric acid
Q17. Citric acid can stabilize vitamin C in formulations primarily by:
- Acting as a strong oxidant
- Chelating trace metals that catalyze oxidation
- Raising pH to alkaline levels
- Forming covalent bonds with ascorbic acid
Correct Answer: Chelating trace metals that catalyze oxidation
Q18. The citrate anion is a key intermediate in which metabolic pathway?
- Glycolysis
- Pentose phosphate pathway
- TCA (Krebs) cycle
- Fatty acid beta-oxidation
Correct Answer: TCA (Krebs) cycle
Q19. Which functional groups are present in citric acid?
- Three carboxyl groups and one hydroxyl group
- Two carboxyl groups and two amine groups
- One carboxyl group and three hydroxyl groups
- Three hydroxyl groups only
Correct Answer: Three carboxyl groups and one hydroxyl group
Q20. Which of the following is a common pharmaceutical citrate used as an anticoagulant in blood storage?
- Potassium citrate
- Calcium citrate
- Sodium citrate
- Magnesium citrate
Correct Answer: Sodium citrate
Q21. Which reaction describes formation of citric acid esters used in specialty applications?
- Hydrogenation of citric acid
- Esterification with alcohols under acid catalysis
- Nitration of aromatic citrates
- Oxidative cleavage of citric acid
Correct Answer: Esterification with alcohols under acid catalysis
Q22. Citric acid’s buffering capacity is most effective near which pH region?
- pH 1–2
- pH 3–7
- pH 8–10
- pH 11–13
Correct Answer: pH 3–7
Q23. Which of the following makes citric acid useful in solubilizing calcium salts?
- Its ability to form insoluble complexes with calcium
- Its chelating action forming soluble calcium-citrate complexes
- Its strong basicity
- Its oxidative power
Correct Answer: Its chelating action forming soluble calcium-citrate complexes
Q24. Regarding stability, citric acid-containing formulations are most at risk from:
- Hydrolysis in strongly acidic media only
- Microbial growth because citric acid is nutritive
- Interactions with alkaline excipients and moisture uptake
- Photodegradation under visible light only
Correct Answer: Interactions with alkaline excipients and moisture uptake
Q25. In IR spectroscopy of citric acid, which major absorption is expected for carboxylic acid groups?
- Strong C=O stretch near 1700 cm−1
- Sharp C–H stretch at 3000 cm−1 only
- NO2 stretch at 1500 cm−1
- Strong C–Cl stretch at 800 cm−1
Correct Answer: Strong C=O stretch near 1700 cm−1
Q26. Which statement about citric acid toxicity and regulatory status is correct?
- Citric acid is classified as highly toxic and banned in foods
- Citric acid is generally recognized as safe (GRAS) for food and pharmaceutical use
- Citric acid can only be used topically due to toxicity
- Citric acid is a controlled substance
Correct Answer: Citric acid is generally recognized as safe (GRAS) for food and pharmaceutical use
Q27. Which citric acid derivative is commonly used as an alkalinizing agent in oral solutions?
- Monopotassium citrate
- Trisodium citrate
- Calcium citrate
- Citric acid anhydride
Correct Answer: Trisodium citrate
Q28. In tablet formulation, citric acid may improve disintegration by:
- Acting as a direct compression lubricant
- Reacting with carbonate to produce effervescence and promote breakup
- Increasing tablet hardness by forming covalent bonds
- Reducing tablet porosity permanently
Correct Answer: Reacting with carbonate to produce effervescence and promote breakup
Q29. Which of the following is a correct description of citric acid monohydrate vs anhydrous?
- Monohydrate contains one molecule of water per citric acid molecule; anhydrous contains none
- Anhydrous contains one water molecule; monohydrate contains none
- They are identical in water content
- Monohydrate is a gas, anhydrous is a solid
Correct Answer: Monohydrate contains one molecule of water per citric acid molecule; anhydrous contains none
Q30. Which metal ion interaction is most relevant to citric acid’s pharmaceutical function?
- Formation of insoluble lead-citrate precipitates only
- Chelation of divalent metal ions such as Ca2+ and Mg2+
- Ionic bonding exclusively with alkali metals to form covalent salts
- No significant interaction with metal ions
Correct Answer: Chelation of divalent metal ions such as Ca2+ and Mg2+
Q31. Which process best describes a common analytical test to distinguish citric acid monohydrate from anhydrous form?
- Determination of refractive index
- Thermogravimetric analysis to detect bound water
- Colorimetry for hydroxyl content
- Measurement of magnetic susceptibility
Correct Answer: Thermogravimetric analysis to detect bound water
Q32. Citric acid can accelerate the hydrolysis of which class of drugs in solution?
- Stable quaternary ammonium salts
- pH-sensitive ester prodrugs and ester-containing APIs
- Metal nanoparticle suspensions
- Non-ionizable hydrophobic compounds only
Correct Answer: pH-sensitive ester prodrugs and ester-containing APIs
Q33. Which of the following is a common pharmaceutical use of calcium citrate?
- As a primary antibiotic
- As a calcium supplement and antacid component
- As an opioid antagonist
- As a vaccine adjuvant exclusively
Correct Answer: As a calcium supplement and antacid component
Q34. In formulating parenteral solutions, which citric acid-related precaution is important?
- Avoid chelating ions that are required for physiological function in blood products
- Use citric acid to sterilize solutions by itself
- Replace buffers with strong mineral acids when possible
- Citric acid is inert and needs no evaluation
Correct Answer: Avoid chelating ions that are required for physiological function in blood products
Q35. Which chromatographic technique is commonly used to separate and quantify citric acid in complex mixtures?
- Size-exclusion chromatography with UV detection at 254 nm
- Ion-exclusion or ion chromatography with conductivity or UV detection
- Normal-phase silica chromatography with nonpolar eluents
- Gas chromatography without derivatization
Correct Answer: Ion-exclusion or ion chromatography with conductivity or UV detection
Q36. The role of citric acid in preventing metal-catalyzed oxidation in formulations is primarily because it:
- Reduces oxygen concentration physically
- Forms stable complexes with transition metals, reducing catalytic activity
- Acts as an oxidizing agent to consume radicals
- Polymerizes to form a protective film
Correct Answer: Forms stable complexes with transition metals, reducing catalytic activity
Q37. In stability studies, which factor most commonly affects citric acid-containing products?
- Degradation by UV light only
- Moisture uptake leading to hydrolysis or effervescence reactions
- Spontaneous polymerization at room temperature
- Gas evolution under anaerobic conditions only
Correct Answer: Moisture uptake leading to hydrolysis or effervescence reactions
Q38. Which statement about citric acid’s role in controlled drug release is accurate?
- Citric acid is used as a matrix former for sustained-release by itself
- Acidic micro-environments generated by citric acid can influence polymer erosion and drug release rates
- Citric acid universally retards drug release regardless of polymer type
- Citric acid cannot be used in combination with hydrophilic polymers
Correct Answer: Acidic micro-environments generated by citric acid can influence polymer erosion and drug release rates
Q39. Which of the following is a true statement about citric acid and taste-masking?
- Citric acid only makes formulations more bitter
- Citric acid can enhance sourness but also mask unpleasant tastes when used appropriately
- Citric acid is tasteless and does not affect flavor
- Citric acid neutralizes sweeteners completely
Correct Answer: Citric acid can enhance sourness but also mask unpleasant tastes when used appropriately
Q40. Which monitoring parameter is most important when using citric acid as a preservative adjunct?
- Ambient air pressure
- Formulation pH and water activity
- Molecular weight of citric acid
- Magnetic susceptibility of the container
Correct Answer: Formulation pH and water activity
Q41. Which structural fragment correctly represents citric acid backbone?
- A three-carbon chain with three carboxyl groups and one hydroxyl on the central carbon
- A six-carbon aromatic ring with two carboxyl groups
- A linear four-carbon dicarboxylic acid with two hydroxyls
- A branched five-carbon ketone
Correct Answer: A three-carbon chain with three carboxyl groups and one hydroxyl on the central carbon
Q42. During titration of citric acid, why are multiple buffering regions observed?
- Because citric acid is a polyprotic acid with multiple dissociation steps
- Because citric acid reacts with water to form gas
- Because citric acid is insoluble in titration media
- Because citric acid is non-ionizable
Correct Answer: Because citric acid is a polyprotic acid with multiple dissociation steps
Q43. Which excipient compatibility concern is often evaluated with citric acid?
- Potential to form insoluble salts with basic APIs
- Formation of covalent bonds with polymers at room temperature
- Explosive decomposition when mixed with sugars
- None—citric acid is universally compatible
Correct Answer: Potential to form insoluble salts with basic APIs
Q44. Which of the following is NOT a common pharmaceutical use of citric acid?
- pH adjustment and buffering
- Effervescence in tablets and powders
- Primary active antiviral agent in systemic infections
- Chelating agent to stabilize formulations
Correct Answer: Primary active antiviral agent in systemic infections
Q45. Which of these formulations often contain citric acid to enhance drug solubility or stability?
- Effervescent granules
- Oral syrups and solutions
- Topical creams where metal ions need chelation
- All of the above
Correct Answer: All of the above
Q46. For quality control, which impurity is commonly tested in citric acid raw material?
- Residual solvents only
- Reducing sugars and fermentation by-products
- Heavy hydrocarbons
- Nitrogen-containing impurities above 50%
Correct Answer: Reducing sugars and fermentation by-products
Q47. How does citric acid improve absorption of certain minerals in supplements?
- By converting minerals into insoluble salts
- By chelating minerals into soluble citrate complexes that are better absorbed
- By oxidizing minerals to a higher oxidation state
- By acting as a surfactant to emulsify minerals
Correct Answer: By chelating minerals into soluble citrate complexes that are better absorbed
Q48. Which of the following best describes the environmental origin of industrial citric acid?
- Extracted directly from citrus fruit juice for the entire global supply
- Mostly produced by microbial fermentation of carbohydrate substrates
- Synthesized via petroleum-derived benzene ring transformation
- Harvested from marine algae cultures exclusively
Correct Answer: Mostly produced by microbial fermentation of carbohydrate substrates
Q49. In a formulation containing citric acid and a basic excipient, what is a likely immediate chemical consequence?
- Complete inertness with no reaction
- Neutralization reaction producing a citrate salt and potential CO2 if carbonate is present
- Polymerization of citric acid to a high-molecular-weight resin
- Spontaneous combustion
Correct Answer: Neutralization reaction producing a citrate salt and potential CO2 if carbonate is present
Q50. Which practice is most appropriate when storing citric acid raw material in a pharmaceutical warehouse?
- Store in a hot, humid environment to maintain monohydrate form
- Store in a cool, dry place in tightly closed containers to prevent moisture uptake
- Store under strong alkaline conditions
- Expose to open air to avoid pressure buildup
Correct Answer: Store in a cool, dry place in tightly closed containers to prevent moisture uptake
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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