Ceric Ammonium Sulfate Preparation and Standardization

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🔬 Preparation and Standardization of 0.1 M Ceric Ammonium Sulfate (Ce(SO4​)2​⋅2(NH4​)2​SO4​⋅2H2​O) Solution

Challenging the Premise: Molarity Notation

The notes specify “0⋅1 M” Ceric Ammonium Sulfate Solusation (Solution). In chemical convention, the notation is properly 0.1 M (zero point one molar). I will use the correct notation.

I. Materials Required (Clarified and Challenged)

The notes list the following reagents (from image 2):

Note’s Entry	Clarified Chemical Name and Grade	Chemical Formula	Challenge/Rationale
Cerric Ammonium Sulphate AR grade	Ceric Ammonium Sulfate (Analytical Reagent Grade)	Ce(SO4​)2​⋅2(NH4​)2​SO4​⋅2H2​O	The term is “Ceric,” not “Cerric.” The dihydrate salt is commonly used.
Sulphuric Acid AR grade	Concentrated Sulfuric Acid (Analytical Reagent Grade)	H2​SO4​	Essential for stabilizing the Ce4+ ion.
Sodium Hydroxide Flake AR Grade	Sodium Hydroxide (Pellets/Flakes, AR Grade)	NaOH	Used for As2​O3​ dissolution, which is incorrect in the note (it mentions NH4​OH).
Arsenious Trioxide AR Grade	Arsenic Trioxide (Primary Standard Grade)	As2​O3​	Used as the primary standard for standardization.
Ferroin Sulphate Solusation	Ferroin Sulfate Solution	Fe(phen)3​SO4​	An oxidation-reduction indicator for the titration.
Osmic Acid	Osmium Tetroxide Solution	OsO4​	Used as a catalyst for the slow Ce4+/As(III) reaction.

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II. Preparation of 0.1 M Ceric Ammonium Sulfate Solution

(Reference: Image 2, points 3, 4, 5, 6)

1. Calculate the Required Mass: The MW (Molecular Weight) of Ce(SO4​)2​⋅2(NH4​)2​SO4​⋅2H2​O is ≈632.55 g/mol. To prepare 1000 mL of a 0.1 M solution, the required mass (m) is:m=Molarity×MW×Volume (L)m=0.1 mol/L×632.55 g/mol×1.0 L≈63.26 gramsThe note mentions “about 65 gms,” which is an acceptable approximation.
2. Dissolution:
   • Weigh accurately 63.26 grams of Ceric Ammonium Sulfate.
   • Dissolve the salt in a mixture of 30 mL of concentrated Sulfuric Acid (H2​SO4​) and approximately 500 mL of distilled water.
   • The note indicates to heat the mixture gently to aid dissolution. Ce(IV) salts often hydrolyze in water, so H2​SO4​ is crucial to maintain acidity and stability.
3. Dilution and Filtration:
   • Cool the solution to room temperature.
   • Filter the solution, if necessary (if it is turbid or milky), to remove any insoluble material.
   • Dilute the clear solution with distilled water in a volumetric flask to exactly 1000 mL (1 Liter).
4. Maturation:
   • Allow the prepared solution to stand for one day before standardization. This ensures full dissolution and chemical equilibrium.

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III. Standardization of Ceric Ammonium Sulfate Solution

The Ce(IV) solution is standardized against a primary standard, Arsenic Trioxide (As2​O3​), which is reduced to As(V) by the Ce(IV) ions.

A. Preparation of the Primary Standard (Arsenic Trioxide)

(Reference: Image 2, points 1 and 2)

1. Weighing:
   • Weigh accurately about 0.2 grams of primary standard grade Arsenic Trioxide (As2​O3​), which has been previously dried at 105∘C for 1 hour.
2. Dissolution and Neutralization (Correction):
   • Transfer the As2​O3​ to a 500 mL conical flask.
   • The notes state to dissolve in 25 mL of 0.8 N NH4​OH solution. As2​O3​ is acidic and reacts slowly with Sodium Hydroxide (NaOH) to form soluble sodium arsenite (NaAsO2​). The note’s use of NH4​OH (Ammonium Hydroxide) is chemically incorrect for dissolution and should be challenged.
   • Correction: Dissolve the As2​O3​ in 25 mL of a 0.8 N Sodium Hydroxide (NaOH) solution. Swirl gently until dissolved.
   • Dilute with 100 mL of water.
   • Neutralize the solution with Diluted Sulfuric Acid (H2​SO4​) to a pH of 7−8 (using litmus paper or pH meter).

B. Titration Procedure

(Reference: Image 1, points 1-5)

1. Acidification:
   • Add 20 mL of Diluted Sulfuric Acid (H2​SO4​) to the neutralized As2​O3​ solution. This ensures a highly acidic environment required for the ceric oxidation.
2. Catalyst and Indicator Addition:
   • Add 0.1 mL of Osmic Acid (OsO4​) solution (the catalyst).
   • Add 0.1 mL of Ferroin Sulfate indicator solution. The solution should turn pink (from the indicator).
3. Titration:
   • Titrate the solution with the prepared Ceric Ammonium Sulfate solution ($\text{Ce}(\text{IV)$ from a burette.
4. End Point:
   • The End Point is reached when the pink color of the solution changes abruptly to a very pale blue or colorless.

C. Calculation of Molarity

The reaction is:

As2​O3​+4H2​O→2H3​AsO4​+4H++4e−

Ce4++e−→Ce3+

Overall:

As2​O3​+4Ce4++2H2​O→2H3​AsO4​+4Ce3++4H+

(Reference: Image 1, points 6-8)

1. Equivalency Factor: The note states: “Each of 0.004946 gm of Arsenic Trioxide is equivalent to 1 mL of 0.1 M Cerric Ammonium Sulphate.” This factor is derived from MW(As2​O3​)/4e−=197.84 g/mol/4=49.46 g/mol. A 0.1 M solution corresponds to 0.1 millimoles per mL. 0.1 Molarity Factor=(49.46 g/mol/1000)×0.1 equiv./L=0.004946 g/mL. (This confirms the note’s factor is correct.)
2. Molarity Formula: The final formula in the note is a bit unclear, but the correct calculation for the exact molarity (M) is based on the equivalency:M=Volume of Ce(IV) used (mL)×0.004946×10Weight of As2​O3​ (g)​Where:
   • W: Weight of As2​O3​ taken (in grams).
   • V: Volume of Ce(IV) solution used in the titration (in mL).
   • 0.004946: The mass (in grams) of As2​O3​ equivalent to 1 mL of a 0.1 M Ce(IV) solution.
   • 10: A factor to convert the 0.1 M equivalency to a 1.0 M base for simple ratio calculation.
   Exact Molarity(M)=Volume of Ce(IV) used (V)×0.04946Weight of As2​O3​​