Precipitation titrations are a classic and essential analytical method for B.Pharm students to master. These titrations, which rely on the formation of an insoluble product, are fundamental for the quantitative analysis of various ions, particularly halides like chloride. Understanding the principles behind methods like those of Mohr, Volhard, and Fajans is crucial for quality control assays, such as the estimation of sodium chloride, ensuring the purity and correct composition of pharmaceutical products.
- Precipitation titrations are based on the formation of a(n):
- Soluble complex
- Insoluble precipitate
- Colored solution
- Gaseous product
- Which of the following is the most common titrant used in precipitation titrations?
- Sodium hydroxide
- Hydrochloric acid
- Silver nitrate
- EDTA
- In Mohr’s method, the indicator used is:
- Ferric alum
- Potassium chromate
- Fluorescein
- Eosin
- The Volhard method is an example of which type of titration?
- Direct titration
- Back titration
- Non-aqueous titration
- Acid-base titration
- Fajans method uses which type of indicator?
- A redox indicator
- An acid-base indicator
- A metal-ion indicator
- An adsorption indicator
- The end point in Mohr’s method is the formation of a:
- White precipitate
- Red-brown precipitate of silver chromate
- Red-colored soluble complex
- Color change on the precipitate surface
- The Volhard method is typically carried out in what type of medium?
- Neutral
- Strongly alkaline
- Acidic
- Buffered at pH 10
- The indicator in the Volhard method is:
- Potassium chromate
- Fluorescein
- Ferric alum (Fe³⁺)
- Starch
- In Fajans method, the color change occurs on the:
- Surface of the precipitate
- Bulk of the solution
- Indicator itself in solution
- Titrant
- Mohr’s method is suitable for the determination of:
- Iodide ions
- Thiocyanate ions
- Chloride and bromide ions
- Silver ions
- The titrant used in the Volhard method for the determination of silver is:
- Silver nitrate
- Potassium thiocyanate
- Sodium chloride
- EDTA
- Fluorescein is a suitable adsorption indicator for the titration of chloride ions with silver nitrate because it is a(n):
- Weaker acid than the analyte
- Anionic dye that adsorbs to the precipitate after the equivalence point
- Cationic dye
- Indicator that complexes with silver
- Why must the pH be neutral or slightly alkaline for Mohr’s method?
- In acidic solutions, chromate is converted to dichromate.
- In acidic solutions, silver chloride dissolves.
- In alkaline solutions, silver nitrate precipitates.
- The indicator only works at low pH.
- The end point in the Volhard method is the formation of a:
- Red precipitate
- White precipitate
- Red-colored soluble complex
- Blue-colored solution
- Which method is particularly useful for the analysis of halides in an acidic solution?
- Mohr’s method
- Fajans method
- Volhard’s method
- Acid-base titration
- The principle of Fajans method relies on the:
- Co-precipitation of an indicator
- Formation of a secondary precipitate
- Adsorption of the indicator onto the precipitate
- A change in the solution’s redox potential
- A titration curve for a precipitation titration typically plots pAg (negative log of Ag⁺ concentration) against the:
- Temperature
- pH
- Volume of titrant added
- Time
- In the estimation of sodium chloride, the analyte is the:
- Silver ion
- Nitrate ion
- Chloride ion
- Sodium ion
- What is the name of the red-colored complex formed at the end point of the Volhard titration?
- Ferric chloride
- Silver thiocyanate
- Ferric thiocyanate
- Ferrous chromate
- For Fajans method to work effectively, the precipitate formed should have a:
- Small, crystalline structure
- Large surface area (colloidal nature)
- Dark color
- High solubility
- In Mohr’s method, the indicator precipitate (silver chromate) is _______ soluble than the analyte precipitate (silver chloride).
- more
- less
- equally
- not
- When determining chloride ions by the Volhard method, what step is necessary to prevent the reaction of silver chloride with the thiocyanate titrant?
- Heating the solution
- Adding an indicator
- Filtering off the AgCl precipitate or adding nitrobenzene
- Making the solution alkaline
- Eosin is a suitable adsorption indicator for the titration of:
- Chloride ions
- Bromide and iodide ions
- Silver ions
- Fluoride ions
- Titrations involving silver nitrate are often called:
- Acidimetric titrations
- Alkalimetric titrations
- Argentometric titrations
- Complexometric titrations
- The primary layer of ions on the surface of a precipitate before the equivalence point in the titration of NaCl with AgNO₃ is composed of:
- Ag⁺ ions
- Cl⁻ ions
- NO₃⁻ ions
- Na⁺ ions
- Why is Mohr’s method not suitable for titrating iodides?
- Silver iodide is too soluble.
- Silver iodide strongly adsorbs the chromate indicator.
- Iodide ions react with the indicator.
- The reaction is too slow.
- The primary standard used to standardize silver nitrate solutions is:
- Silver metal
- Potassium hydrogen phthalate
- Sodium chloride
- Sodium carbonate
- After the equivalence point in Fajans method, the surface of the precipitate becomes:
- Positively charged, attracting the anionic indicator
- Negatively charged, repelling the anionic indicator
- Neutral
- Dissolved
- The addition of nitric acid in the Volhard method helps to:
- Prevent the precipitation of ferric hydroxide
- Increase the solubility of the silver halide
- Act as a catalyst
- Buffer the solution
- Which of the following methods relies on the formation of a colored secondary precipitate for end point detection?
- Fajans method
- Volhard method
- Mohr’s method
- Potentiometry
- The main source of error in Mohr’s method is the:
- Adsorption of the indicator
- Amount of titrant needed to produce enough indicator precipitate to be visible
- Co-precipitation of impurities
- Instability of the titrant
- The “Modified Volhard’s” method often involves:
- Using a different indicator
- Adding an immiscible organic liquid like nitrobenzene
- Performing the titration at a high temperature
- Using a different titrant
- For an adsorption indicator to work, the indicator ion must have the same charge as the:
- Titrant ion
- Analyte ion
- Precipitate
- Solvent
- Which of the following conditions is a limitation of the Mohr’s method?
- It must be performed in a narrow pH range.
- It can only be used for colored solutions.
- It is a back-titration method.
- It does not use an indicator.
- The formation of a precipitate is governed by the:
- Henderson-Hasselbalch equation
- Nernst equation
- Solubility product constant (Ksp)
- Beer-Lambert law
- In the Volhard method, an excess of ______ is added to the halide solution.
- Potassium thiocyanate
- Ferric alum
- Standard silver nitrate solution
- Nitric acid
- The sharpness of the end point in a precipitation titration is greatest when the precipitate is:
- Highly soluble
- Highly insoluble
- Colored
- Colloidal
- Which method is named after the Latin word for silver, ‘argentum’?
- Argentometric titration
- Acid-base titration
- Redox titration
- Gravimetric analysis
- The precipitate formed during the estimation of sodium chloride by an argentometric titration is:
- NaNO₃
- AgCl
- Ag₂CrO₄
- Fe(SCN)₃
- Before the equivalence point in Fajans method, the indicator anion is ______ from the negatively charged precipitate surface.
- adsorbed
- repelled
- precipitated
- complexed
- If the pH is too high (>10) in Mohr’s method, what unwanted precipitate will form?
- Silver chloride
- Silver chromate
- Silver hydroxide
- Sodium hydroxide
- The titration of thiocyanate (SCN⁻) ions can be performed using:
- Mohr’s method
- Fajans method
- Volhard’s method
- The direct addition of KSCN
- Adsorption indicators are typically:
- Strong inorganic acids
- Weak organic dyes
- Metal salts
- Nonpolar solvents
- Which statement best describes the equivalence point in a precipitation titration?
- The point where the indicator changes color.
- The point where the titrant and analyte are present in stoichiometrically equivalent amounts.
- The point where the solution becomes saturated.
- The point of maximum precipitation rate.
- Which of these methods does not use silver nitrate as the primary titrant?
- Mohr’s method
- Fajans method
- The back-titration step of the Volhard method
- The direct titration of silver ions in the Volhard method
- To avoid photodecomposition of the silver chloride precipitate, Fajans method should be performed:
- In bright sunlight
- In the absence of an indicator
- In diffuse light and with protection from direct sunlight
- At high temperatures
- The reaction between silver ions and thiocyanate ions produces:
- Ag₂S
- Ag(SCN)₂⁻
- AgSCN
- Ag₂SCN
- A key requirement for a successful precipitation titration is that the reaction must be:
- Slow and incomplete
- Rapid and quantitative
- Reversible
- Endothermic
- In the Volhard method, why is nitric acid used instead of other acids like HCl?
- Nitric acid is a better solvent.
- HCl would react with the silver nitrate titrant to form AgCl.
- Nitric acid is required to form the colored complex.
- Nitric acid is less corrosive.
- The main advantage of Fajans method is its:
- Suitability for acidic solutions
- High accuracy and sharp end point
- Use of a simple indicator
- Applicability to all types of ions
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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