Volhard’s and modified Volhard’s methods MCQs With Answer

Introduction

Volhard’s and modified Volhard’s methods are classical analytical techniques in titrimetric analysis for determining halides (chloride, bromide, iodide) and sometimes silver. These methods use silver nitrate to precipitate halides and then quantify the unreacted silver by titration with thiocyanate (KSCN) in the presence of ferric ion indicator, producing a characteristic red ferric thiocyanate complex at the endpoint. Key terms for B. Pharm students include back titration, silver nitrate, potassium/ammonium thiocyanate, ferric alum indicator, precipitation titration, endpoint detection, interferences and sample preparation. Mastery of these methods aids in excipient and drug analysis, quality control and formulation work. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the basic principle of Volhard’s method?

• Direct titration of halide with silver nitrate using chromate indicator
• Back titration: precipitate halide with excess AgNO3, then titrate excess Ag+ with thiocyanate using ferric ion indicator
• Titration of halide with standard potassium dichromate solution
• Neutralization titration of halide with standard base

Correct Answer: Back titration: precipitate halide with excess AgNO3, then titrate excess Ag+ with thiocyanate using ferric ion indicator

Q2. Which indicator is used in Volhard’s titration to detect the endpoint?

• Potassium chromate
• Phenolphthalein
• Ferric ion (ferric alum) producing a red thiocyanate complex
• Methyl orange

Correct Answer: Ferric ion (ferric alum) producing a red thiocyanate complex

Q3. In Volhard’s method the characteristic red color at endpoint arises from which complex?

• Ag(SCN) precipitate
• Fe(SCN)2+ (ferric thiocyanate) complex
• AgClO4 complex
• Fe(CN)6 3- complex

Correct Answer: Fe(SCN)2+ (ferric thiocyanate) complex

Q4. Why is nitric acid often added during Volhard titrations?

• To oxidize thiocyanate to thiourea
• To keep the medium acidic and prevent precipitation of silver hydroxide or basic silver salts
• To react with halides to form halogen gas
• To complex ferric ion and suppress color

Correct Answer: To keep the medium acidic and prevent precipitation of silver hydroxide or basic silver salts

Q5. Which of the following halides gives the least soluble silver salt and precipitates most readily with AgNO3?

• Chloride (Cl-)
• Bromide (Br-)
• Iodide (I-)
• Fluoride (F-)

Correct Answer: Iodide (I-)

Q6. What is the role of potassium/ammonium thiocyanate in Volhard titration?

• It precipitates halides as thiocyanates
• It complexes with ferric ion to suppress color
• It titrates the excess silver ion by forming AgSCN until free SCN- forms red Fe(SCN)2+
• It reduces silver to metallic silver

Correct Answer: It titrates the excess silver ion by forming AgSCN until free SCN- forms red Fe(SCN)2+

Q7. Which type of titration is Volhard’s method classified as?

• Direct titration
• Back (indirect) titration
• Redox titration
• Complexometric titration

Correct Answer: Back (indirect) titration

Q8. Modified Volhard’s method differs from the classical Volhard mainly in which procedural step?

• Using chromate instead of ferric alum as indicator
• Filtering off the precipitated silver halide before titrating the filtrate to avoid turbidity
• Direct titration of halide with KSCN
• Replacing AgNO3 with AgCl standard solution

Correct Answer: Filtering off the precipitated silver halide before titrating the filtrate to avoid turbidity

Q9. Which interference complicates Volhard titration by reacting with silver and consuming Ag+?

• Sulfate ion (SO4 2-)
• Ammonia (NH3) forming [Ag(NH3)2]+ complex
• Potassium ion (K+)
• Nitrate ion (NO3-)

Correct Answer: Ammonia (NH3) forming [Ag(NH3)2]+ complex

Q10. For accurate Volhard titration of chloride in a drug sample, what precaution is important?

• Ensure sample contains cyanide to sharpen endpoint
• Remove interfering organic matter and ensure acidic medium to avoid AgOH formation
• Evaporate all water to dryness before titration
• Add excess silver carbonate to buffer the solution

Correct Answer: Remove interfering organic matter and ensure acidic medium to avoid AgOH formation

Q11. Which halide cannot be accurately determined by Volhard’s method without special care due to its solubility or complex behavior?

• Chloride
• Bromide
• Iodide (due to some solubility and oxidation tendencies)
• None; all halides behave identically

Correct Answer: Iodide (due to some solubility and oxidation tendencies)

Q12. Why is ferric alum used in small amount only as indicator in Volhard titration?

• Because ferric alum precipitates with halides
• Because excess ferric ion would complex with thiocyanate and distort titration
• Because ferric alum acts as primary titrant
• Because ferric alum neutralizes nitric acid

Correct Answer: Because excess ferric ion would complex with thiocyanate and distort titration

Q13. During the thiocyanate titration of excess Ag+, AgSCN formation is important. Which statement is correct?

• AgSCN is highly soluble and does not interfere
• AgSCN precipitates and removes Ag+ until SCN- is in slight excess, then Fe(SCN)2+ forms giving color
• AgSCN reacts with ferric ion to form colorless complex
• AgSCN decomposes to Ag and SCN2-

Correct Answer: AgSCN precipitates and removes Ag+ until SCN- is in slight excess, then Fe(SCN)2+ forms giving color

Q14. Which sample type is Volhard’s method particularly useful for in pharmaceutical analysis?

• Highly colored or turbid solutions without filtration
• Solid drug substances or tablets where halide content is to be determined after dissolution and filtration
• Determination of metal complexes only
• Analysis of strong oxidizers like permanganate

Correct Answer: Solid drug substances or tablets where halide content is to be determined after dissolution and filtration

Q15. Compared to Mohr’s method, Volhard’s method is preferred when:

• Sample is alkaline and chromate indicator works well
• Sample is acidic, turbid or contains interfering colored substances and back titration avoids endpoint masking
• Direct titration is desired with silver nitrate
• Chloride concentration is extremely high beyond titrant range

Correct Answer: Sample is acidic, turbid or contains interfering colored substances and back titration avoids endpoint masking

Q16. What is the stoichiometric reaction between Ag+ and SCN- during titration?

• Ag+ + SCN- → AgSCN (s)
• Ag+ + 2 SCN- → [Ag(SCN)2]-
• Ag+ + SCN- → Ag + SCN radical
• Ag+ + SCN- → AgCN + S

Correct Answer: Ag+ + SCN- → AgSCN (s)

Q17. Which of the following is a limitation of Volhard’s method in pharmaceutical analysis?

• It cannot determine bromide or iodide
• Interference from substances that complex silver or react with thiocyanate can cause errors
• It requires no reagents besides water
• It always gives instantaneous and unambiguous endpoints in colored solutions

Correct Answer: Interference from substances that complex silver or react with thiocyanate can cause errors

Q18. In a Volhard titration, if the endpoint red color appears gradually and fades, what is the likely cause?

• Indicator is too concentrated
• Thiocyanate solution is contaminated or iron indicator is insufficient; slow complex formation or competing reactions
• Sample contains no halide so endpoint is immediate and permanent
• Silver nitrate used was too fresh

Correct Answer: Thiocyanate solution is contaminated or iron indicator is insufficient; slow complex formation or competing reactions

Q19. Why should glassware be rinsed with distilled water and acidified solution in Volhard titration?

• To introduce chloride contamination
• To remove traces of halides and prevent adsorption of silver on glass surfaces and minimize contamination
• To coat the glass with silver
• To neutralize nitric acid

Correct Answer: To remove traces of halides and prevent adsorption of silver on glass surfaces and minimize contamination

Q20. Which precaution reduces loss of thiocyanate indicator sensitivity on storage?

• Store KSCN in open sunlight
• Protect thiocyanate solutions from light and oxidizing agents, and prepare fresh standardized titrant
• Add strong oxidizer to stabilize
• Store in alkaline solution to prevent decomposition

Correct Answer: Protect thiocyanate solutions from light and oxidizing agents, and prepare fresh standardized titrant

Q21. What is the effect of organic matter in a sample on Volhard titration?

• No effect; organics are inert in this titration
• They may adsorb silver halide or react with silver/thiocyanate, causing turbidity or endpoint errors
• They precipitate ferric ion and sharpen the endpoint
• They convert thiocyanate to thiourea giving a sharper color

Correct Answer: They may adsorb silver halide or react with silver/thiocyanate, causing turbidity or endpoint errors

Q22. Which gas evolution must be avoided during Volhard titration by controlling acidity?

• Chlorine gas from chloride oxidation
• Carbon dioxide from carbonate decomposition that could form silver carbonate
• Nitrogen gas from nitrate decomposition
• Oxygen evolution due to overoxidation of thiocyanate

Correct Answer: Carbon dioxide from carbonate decomposition that could form silver carbonate

Q23. If 1.00 mL of 0.100 M AgNO3 is added to a chloride-containing sample and after precipitation 0.025 mL of 0.100 M KSCN is required to titrate the excess Ag+, what does this indicate qualitatively?

• All Ag+ remained in solution and no chloride was present
• Majority of Ag+ was consumed by chloride precipitation; small excess Ag+ titrated by KSCN indicates significant chloride present
• Thiocyanate reacted with chloride directly
• Ferric ion oxidized silver to silver oxide

Correct Answer: Majority of Ag+ was consumed by chloride precipitation; small excess Ag+ titrated by KSCN indicates significant chloride present

Q24. How does modified Volhard improve accuracy for colored or turbid pharmaceutical samples?

• It changes the indicator to phenolphthalein
• It separates the precipitate by filtration or centrifugation and titrates the clear filtrate to avoid masking
• It doubles the amount of silver nitrate added
• It dilutes the sample with organic solvent

Correct Answer: It separates the precipitate by filtration or centrifugation and titrates the clear filtrate to avoid masking

Q25. Which reagent must be avoided in samples prior to Volhard titration because it forms complexes with Fe3+ and interferes with indicator?

• EDTA or other strong chelating agents
• Sodium chloride
• Potassium nitrate
• Sodium sulfate

Correct Answer: EDTA or other strong chelating agents

Q26. In pharmaceutical QC, why might Volhard’s method be chosen over an instrumental halide assay?

• It requires no chemicals and is fully automated
• It is simple, cost-effective, and suitable for routine halide determination where instrumentation is unavailable
• It gives structural information about organic impurities
• It measures volatile impurities more accurately

Correct Answer: It is simple, cost-effective, and suitable for routine halide determination where instrumentation is unavailable

Q27. What is a common standardization practice for thiocyanate titrant used in Volhard titration?

• Standardize against a primary standard silver solution or by back titration using known Ag+ amount
• Assume concentration from bottle label without standardization
• Standardize against phenolphthalein
• Standardize by measuring pH

Correct Answer: Standardize against a primary standard silver solution or by back titration using known Ag+ amount

Q28. Which precaution improves precision when performing many Volhard titrations in a QC lab?

• Use different titrant strengths for each run
• Maintain constant acid concentration, temperature, and freshly standardized thiocyanate titrant
• Vary ferric indicator volume randomly
• Store all reagents in direct sunlight to accelerate reactions

Correct Answer: Maintain constant acid concentration, temperature, and freshly standardized thiocyanate titrant

Q29. During Volhard titration, a persistent brown/black coloration appears. What is the likely cause?

• Formation of metallic silver or silver oxide from contamination or over-reduction
• Proper formation of ferric thiocyanate endpoint
• Excess nitrate reacting with thiocyanate to produce red color
• Complete absence of halide in sample

Correct Answer: Formation of metallic silver or silver oxide from contamination or over-reduction

Q30. What is the main analytical advantage of performing a back titration (as in Volhard) rather than direct titration for certain samples?

• Back titration always uses less reagent
• Back titration allows separation of a precipitate first and measurement of residual reagent in clear solution, avoiding endpoint masking from turbidity or color
• Back titration eliminates need for indicators
• Back titration produces no waste

Correct Answer: Back titration allows separation of a precipitate first and measurement of residual reagent in clear solution, avoiding endpoint masking from turbidity or color

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