This protocol outlines the gas-saturation method used to produce a pure 3,000 ppm Chlorine Dioxide Solution (CDS) for water purification. By using gas diffusion into distilled water rather than direct mixing, this method yields a clean solution free from unreacted raw precursors and chemical byproducts.

 

Equipment

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• 10 ml measuring cylinder/syringe (for accurate liquid measurements)

• 500 ml borosilicate glass wide-mouth reaction bottle with a gas-tight GL45 lab cap (HDPE or PTFE liner)

• Borosilicate glass test tube (sized to sit upright inside the 500 ml bottle without tipping)

• 100 ml amber glass storage bottles with HDPE/PET chemical-resistant screw caps

• Optional: Chlorine dioxide test strips (e.g., LaMotte) or high-precision photometric equipment (e.g., MARA Quantics) for concentration testing

 

Note: Standard home jars, rubber seals, or metal caps must be avoided. Chlorine dioxide gas reacts with rubber and corrodes metals, which degrades the solution and introduces contaminants.

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Ingredients & Reactants

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• 500 ml distilled water 

• 10 ml Citric Acid solution (50%)

• 10 ml Sodium Chlorite solution (25% or 28%)

 

Preparation & Saturation Protocol

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1. Prepare the Water Base: Measure 500 ml of distilled water into the 500 ml borosilicate glass reaction bottle.

2. Prepare the Test Tube (Outside the Bottle):

   • Stand the clean, dry borosilicate glass test tube upright in a test tube rack or small beaker on your workbench.

   • Add 10 ml of 50% Citric Acid solution into the test tube first.

   • Add 10 ml of Sodium Chlorite solution (25% or 28%) directly into the same test tube.

3. Observe Activation & Insert Test Tube:

   • Observe the liquid inside the test tube rapidly turning a deep amber color, signaling active chlorine dioxide gas generation.

   • Using clean hands or forceps, carefully lower the filled test tube into the 500 ml reaction bottle so it stands upright within the distilled water base. (Ensure no water enters the test tube, and no reactant spills into the surrounding water).

4. Seal Immediately:

   • Immediately close and tightly seal the outer 500 ml bottle with its gas-tight lab cap to prevent the chlorine dioxide gas from escaping.

5. Saturation Phase: Place the sealed bottle in a dark location at ambient temperature (approx. 20°C / 68°F), completely shielded from direct sunlight and UV exposure. Light causes the chlorine dioxide molecule to degrade.

 

Evaluating Reaction Completeness

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The gas will gradually diffuse out of the test tube and saturate into the surrounding 500 ml of distilled water. At 20°C, full saturation typically requires 12 to 24 hours.

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Visual indicators help determine when saturation is complete:

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• Incomplete Saturation: The liquid inside the test tube remains darker yellow/amber than the surrounding water in the bottle. Gas generation and absorption are still occurring.

• Equal Saturation: Both the test tube liquid and the surrounding water share an identical yellow hue.

• Full Saturation: The surrounding water in the bottle exhibits a deeper yellow color than the depleted fluid inside the test tube. The 500 ml water volume has successfully absorbed the active gas up to the target concentration (~3,000 ppm).

 

Handling the Residual Reactant Fluid

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The fluid remaining inside the test tube must never be consumed or added to the saturated water. It contains unreacted precursors and reaction byproducts.

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• Air Disinfection: The test tube can be removed and placed in a 10 m² room, allowing remaining gas to off-gas safely for room air purification.

• Surface Disinfection: The fluid can be diluted and used for non-critical surface sanitisation.

 

Storage & Preservation

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Once saturation is complete:

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1. Carefully open the main bottle, remove the test tube, and discard/repurpose the residual reactant fluid.

2. Transfer the saturated CDS into 100 ml amber glass storage bottles equipped with HDPE or PET caps, filling them close to the top to minimise head space.

3. Store all bottles in a dark, cool environment (refrigeration is recommended to prolong gas retention).

 

Why 100 ml Bottles? Using smaller storage bottles minimises the internal air space ("headspace"). Each time a large bottle is opened, dissolved chlorine dioxide gas escapes into the empty space (similar to carbonated beverages losing fizz), causing the concentration to drop over time.

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Dosing & Application Principles

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• Standard Water Purification: Add 1 ml of 3,000 ppm CDS per 1 Litre of clear water.

• Turbid / Cloudy Water: Increase dosing incrementally depending on water clarity and biological load. Allow a minimum contact time of 30 minutes before consumption.

 

Technical & Safety Notes

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• Material Compatibility: Use only HDPE, PET, or PTFE caps and seals. Avoid all metals and soft rubbers.

• Verifying Concentration: While visual color provides a practical field guide, precise verification can be confirmed using LaMotte chlorine dioxide test strips or MARA equipment.

• Labelling: Clearly label all storage bottles with "Chlorine Dioxide Solution (CDS) 3,000 ppm" along with the preparation date. Keep strictly out of reach of children and pets.