Aluminum Sulfate Octadecahydrate: Lab Handling & Usage Guide

Aluminum sulfate octadecahydrate is a widely used inorganic reagent with the formula Al₂(SO₄)₃·18H₂O, valued in research and industry for its coagulating, mordanting, and pH-buffering properties.

Chemical Formula

Al₂(SO₄)₃·18H₂O

Molecular Weight

666.43 g/mol

Appearance

White crystalline solid

Solubility in Water

Freely soluble; aqueous solutions are acidic (pH ~3–4 at 1% w/v)

Storage Condition

Store in a cool, dry, well-ventilated area in a tightly closed container

CAS Number

7784-31-8

Aladdin SKU

A100190

Physical and Chemical Properties

Aluminum sulfate octadecahydrate (Al₂(SO₄)₃·18H₂O, CAS 7784-31-8, MW 666.43 g/mol) is a white, odorless crystalline solid that is freely soluble in water. Upon dissolution, it hydrolyzes to produce aluminum hydroxide colloids and sulfuric acid, rendering aqueous solutions distinctly acidic (pH approximately 3–4 at 1% w/v). The compound is hygroscopic and may lose water of crystallization upon prolonged exposure to warm, dry air. It is incompatible with strong bases, which precipitate aluminum hydroxide, and with strong oxidizers. Thermal decomposition above approximately 770 °C yields aluminum oxide, sulfur dioxide, and sulfur trioxide.

Because of its hydrated nature, care should be taken when weighing: use freshly opened reagent or verify water content if high accuracy is required, as partial efflorescence can alter the effective molecular weight of the sample.

Safety and Handling

Personal Protective Equipment (PPE): When handling aluminum sulfate octadecahydrate, wear appropriate PPE including chemical-splash safety goggles, nitrile gloves, and a laboratory coat. For operations that may generate dust (e.g., weighing large quantities), use a dust mask rated at least N95 or work within a fume hood.

Skin and Eye Contact: The compound is a mild irritant. In case of skin contact, wash thoroughly with soap and water for at least 15 minutes. In case of eye contact, irrigate immediately with copious amounts of water for at least 15 minutes and seek medical attention.

Inhalation: Avoid inhaling dust. If inhalation occurs, move to fresh air immediately. Seek medical advice if symptoms persist.

Storage: Store in a tightly closed container in a cool (15–25 °C), dry, well-ventilated area, away from incompatible materials (strong bases, strong oxidizers). Keep away from moisture to prevent caking.

Disposal: Dispose of in accordance with local, regional, and national regulations. Dilute aqueous solutions may be neutralized before drain disposal where permitted.

Solution Preparation Protocol

Materials required: Aluminum sulfate octadecahydrate (Aladdin A100190), analytical balance, volumetric glassware, Type I or Type II deionized water, magnetic stirrer and stir bar, pH meter (optional).

General procedure for a 1% w/v stock solution (example: 100 mL):

1. Weigh 1.00 g of aluminum sulfate octadecahydrate on a tared weighing boat using an analytical balance.
2. Transfer the solid to a 100 mL beaker containing approximately 70 mL of deionized water at room temperature.
3. Stir magnetically until the solid is completely dissolved (the compound dissolves readily with mild agitation).
4. Transfer the solution quantitatively to a 100 mL volumetric flask and make up to the mark with deionized water. Mix by inversion.
5. Check the pH if required (expected pH 3–4). Adjust with dilute H₂SO₄ or dilute NaOH only if the experimental protocol specifies a target pH, noting that adjustment will alter the aluminum speciation.
6. Label the flask with compound name, concentration, date of preparation, and preparer's initials. Store at room temperature and use within 30 days.

Note: For applications requiring accurate aluminum concentration (e.g., ICP calibration standards), verify purity and water content from the Certificate of Analysis (CoA) and correct the weighed mass accordingly.

Key Laboratory Applications

Aluminum sulfate octadecahydrate is a versatile reagent employed across multiple research and applied science disciplines:

• Water and wastewater treatment research: Acts as a primary coagulant, destabilizing colloidal suspensions through charge neutralization and sweep flocculation. Used to study turbidity removal, phosphorus removal, and sludge characteristics in bench-scale jar tests.
• Histology and staining: A key component of alum hematoxylin formulations (e.g., Harris hematoxylin, Mayer hematoxylin). Functions as a mordant, forming a coordination complex (lake) between hematoxylin dye and tissue proteins, enabling nuclear staining in H&E protocols.
• Soil chemistry and agriculture research: Used to acidify soil or growth media and to study aluminum toxicity mechanisms in plants. Applied to raise soil exchangeable aluminum levels in ecotoxicology studies.
• Adjuvant research (immunology): Alum-based adjuvants (aluminum salts) are extensively studied to enhance immune responses in vaccine formulations. Aluminum sulfate is used to precipitate protein antigens as alum-adsorbed preparations in research settings.
• Paper and textile research: Used as a sizing agent in paper manufacturing studies and as a mordant in textile dyeing research to fix dyes to natural fibers.
• pH and buffer studies: Its controlled hydrolysis behavior makes it useful as a model compound for studying aluminum speciation and polymerization in aqueous systems.

Quality Verification and Troubleshooting

Verifying reagent quality: Upon receipt, inspect the reagent for appearance (white crystalline solid, free-flowing). Significant yellowing, caking, or off-odor may indicate degradation or contamination. Always consult the Certificate of Analysis (CoA) supplied with Aladdin product A100190 for lot-specific purity, water content, and heavy metal limits before use in critical applications.

Common issues and remedies:

• Incomplete dissolution: Ensure water temperature is at least 15 °C and stir adequately. Do not use cold water (<10 °C), as solubility decreases significantly.
• Turbid solutions: Turbidity indicates partial hydrolysis to Al(OH)₃. This may occur if solution pH is raised above ~5.5, or if the solution is aged. Prepare fresh and maintain acidic pH.
• Unexpected pH values: Confirm water quality (use freshly prepared deionized water with conductivity <1 µS/cm). Contamination from carbonate in reagent-grade water can buffer and raise pH unexpectedly.
• Weighing inconsistency: Efflorescence (loss of water of crystallization) can reduce the actual hydrate content. If high-precision aluminum concentration is critical, use ICP-OES or gravimetric analysis to verify solution concentration after preparation.

Shelf life: When stored properly in a sealed container away from moisture and heat, aluminum sulfate octadecahydrate is stable for at least 2 years. Verify the expiry date on the Aladdin product label before use.

Frequently asked questions

What is the difference between aluminum sulfate octadecahydrate and anhydrous aluminum sulfate?

Aluminum sulfate octadecahydrate (Al₂(SO₄)₃·18H₂O, MW 666.43 g/mol) contains 18 molecules of water of crystallization, making it the stable crystalline hydrate at ambient conditions. Anhydrous aluminum sulfate (MW 342.15 g/mol) contains no water. When preparing solutions, the correct molecular weight must be used; substituting one form for the other without adjusting mass will result in significant concentration errors.

Can aluminum sulfate octadecahydrate solutions be autoclaved for sterilization?

Autoclaving aluminum sulfate solutions is generally not recommended. The elevated temperature and pressure may accelerate hydrolysis of Al³⁺ ions, increasing turbidity and shifting pH unpredictably. For sterile applications, filter sterilization through a 0.22 µm membrane is the preferred method.

What pH range is optimal for aluminum sulfate to function as a coagulant?

Aluminum sulfate is most effective as a coagulant in the pH range of approximately 6.5 to 7.5, where aluminum hydroxide precipitates as a gelatinous floc that efficiently enmeshes suspended particles. Outside this range, particularly below pH 5 or above pH 8, aluminum remains in soluble ionic or aluminate forms with reduced coagulation efficiency.

How should I dispose of aluminum sulfate solutions after laboratory use?

Dispose of aluminum sulfate solutions according to your institution's chemical waste management policy and applicable local regulations. Dilute solutions may be neutralized to pH 6–9 and disposed of via the sanitary sewer where local regulations permit. Concentrated solutions or those containing other hazardous materials must be collected as chemical waste for licensed disposal.

Is aluminum sulfate octadecahydrate suitable for biological media preparation?

Aluminum sulfate octadecahydrate can be used in specific biological media formulations where aluminum is a required micronutrient or where acidification is needed, such as in certain plant growth studies or microbiology experiments. However, it is not a standard component of common bacterial or mammalian cell culture media. Always verify purity grade (check the CoA for A100190) and ensure the concentration used is within the non-toxic range for the organism being studied.

References

1. Coagulation and Flocculation in Water and Wastewater Treatment, IWA Publishing. General reference for aluminum sulfate coagulation chemistry.
2. Bancroft JD, Gamble M (Eds.). Theory and Practice of Histological Techniques, 6th ed. Churchill Livingstone, 2008. Reference for alum hematoxylin staining protocols.
3. World Health Organization. Aluminium in Drinking-water: Background document for development of WHO Guidelines for Drinking-water Quality. WHO/SDE/WSH/03.04/53. Geneva, 2003.
4. Exley C. The coordination chemistry of aluminium in neurodegenerative disease. Coordination Chemistry Reviews, 2012. General reference for aluminum speciation in aqueous systems.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.