Ferrous Ammonium Sulfate Solution, 0.4 N, 1 Gallon

• What it is. This ferrous ammonium sulfate solution is a 0.4 N reductometric titrant, supplied in a 1-gallon size. It is prepared by dissolving ferrous ammonium sulfate hexahydrate (Mohr's salt) in water acidified with sulfuric acid, providing iron(II) for redox titrations.
• Chemical identity. Ferrous ammonium sulfate hexahydrate, (NH4)2Fe(SO4)2·6H2O (CAS 7783-85-9, 392.14 g/mol), is a double salt of iron(II) sulfate and ammonium sulfate. It crystallizes readily and resists air oxidation better than plain ferrous sulfate, so it is the preferred source of iron(II) for standard solutions.
• Molar equals normal. Each iron(II) ion gives up one electron when oxidized to iron(III), so for ferrous ammonium sulfate the equivalent weight equals the formula weight and normality equals molarity. A 0.4 N solution is therefore 0.4 M, about 156.9 g of the hexahydrate per liter (calculated).
• How it works as a titrant. Iron(II) is oxidized by dichromate, permanganate, cerium(IV), chlorine, and other oxidizers. These reactions let FAS determine oxidizing agents directly or back-titrate the excess oxidant left after a sample reaction, a common approach for organic matter and oxidizable species.
• Why sulfuric acid is included. Acid suppresses iron(II) hydrolysis and slows its oxidation by air. Even so, ferrous solutions slowly lose strength as iron(II) converts to iron(III), so standardize against potassium dichromate or another primary oxidant before critical work and periodically thereafter.
• Back-titration in oxidation methods. In many methods, a sample reacts with a measured excess of potassium dichromate in acid, then the unreacted dichromate is titrated with FAS. The amount of dichromate consumed by the sample is calculated from the difference between the blank and sample titrations.
• Why a 0.4 N strength. A relatively strong FAS titrant keeps volumes practical when samples or oxidant excesses are large, and it serves as a convenient stock for preparing weaker working solutions by accurate dilution with deionized water.
• Endpoint detection. FAS titrations use a redox indicator such as ferroin or diphenylamine sulfonate, or a potentiometric endpoint with a platinum electrode. Choose the indicator your method specifies for the oxidant being titrated.
• Handling at a glance. The solution is acidic and can irritate skin and eyes. Wear safety glasses and nitrile gloves, keep it away from oxidizers, and collect iron- and acid-containing waste for proper disposal.

Technical Specifications

Applications / Common Uses

• Dichromate back-titration: Titrates excess dichromate after sample oxidation.
• Oxidant assays: Determines permanganate, cerium(IV), and other oxidizers.
• Chlorine determination: Serves as the reductant in titrimetric chlorine methods.
• Working-solution stock: Dilutes to weaker FAS titrants.
• Teaching labs: Demonstrates redox titration with a stable iron(II) source.

Storage, Handling & Shipping

Store the solution tightly closed at room temperature, away from oxidizers and direct light, and keep the headspace small to limit air contact. Re-standardize before critical use, since iron(II) slowly oxidizes to iron(III) over time.

• Incompatible materials: Strong oxidizers, strong bases, and reactive metals.
• Personal protection: Safety glasses and nitrile gloves.
• Spill response: Absorb with inert material, neutralize residue with sodium bicarbonate, and rinse with water.
• First aid: Eyes or skin: rinse with water for several minutes. Ingestion: rinse mouth and seek medical advice.

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