How to Measure the Iodine Number of Recycled Carbon Black?

The iodine adsorption number (IAN, commonly called iodine number) is a core quality parameter for recycled carbon black (rCB). It directly reflects the microporous surface area, adsorption capacity, and reinforcement performance of pyrolysis-derived carbon materials, and serves as the primary benchmark for grading rCB and comparing it with standard virgin carbon black grades such as N330, N550 and N660.

Measurement of rCB iodine number follows globally standardized titration protocols. ASTM D1510 and ISO 1304:2016 are the most widely accepted industry standards, both offering two operation modes: manual starch-indicator titration (Method A) and automated potentiometric titration (Method B) as the arbitration reference method.

1. Test Principle

The method is based on the selective adsorption of iodine molecules onto the microporous surfaces of carbon black particles. A calibrated iodine solution is mixed with a dried rCB sample to reach adsorption equilibrium. After separating the solid carbon material, the remaining free iodine in the clear supernatant is titrated with a standard sodium thiosulfate solution.

By calculating the difference between blank and sample titration volumes, the mass of iodine adsorbed per gram of rCB is derived, reported in units of mg iodine per gram of carbon black (mg/g, equivalent to g/kg).

Notably, iodine number mainly corresponds to micropore surface area. Results can be influenced by volatile content, solvent-extractable pyrolysis residues and sample aging — factors that are especially relevant for rCB produced from waste tire or plastic pyrolysis.

2. Required Reagents & Apparatus

Reagents

• Standard iodine solution (0.0473 N per ASTM D1510 / 0.02364 mol/L per ISO 1304), prepared with potassium iodide at a fixed mass ratio
• Standard sodium thiosulfate titrant (0.0394 N for ASTM D1510 procedure)
• 10 g/L starch indicator solution
• Deionized or distilled water
• Certified carbon black reference material for system calibration

Apparatus

• Forced-air drying oven with stable temperature control at 125 °C
• Desiccator with effective drying agent
• Analytical balance with readability of 0.1 mg or better
• Mechanical reciprocating shaker (240 strokes per minute)
• Centrifuge capable of ≥ 1000 r/min
• Class A glass burette or automatic potentiometric titrator with platinum electrode
• Sealed glass reaction vials, volumetric pipettes and Erlenmeyer flasks

3. Step-by-Step Test Procedure

Step 1: Sample Pre-treatment

1. Place a representative rCB specimen in the drying oven and dry at 125 °C for 1 hour, following the loss-on-heating specification of ISO 1126.
2. Transfer the dried sample immediately to a desiccator and cool to room temperature before weighing.

For rCB with high residual pyrolysis oil or volatile content, thorough drying is essential; incomplete drying will cause a measurable downward bias in iodine number results.

Step 2: Sample Weighing & Reagent Addition

1. Weigh an appropriate mass of dried rCB into a clean, dry sealed vial. Sample mass is selected according to expected iodine value, typically 0.25–0.5 g for commercial medium-grade rCB.
2. Pipette exactly 25 mL of standard iodine solution into the vial and seal the cap immediately to prevent iodine volatilization.

Step 3: Adsorption & Solid-Liquid Separation

1. Fix the vial on the mechanical shaker and agitate for exactly 1 minute at 240 strokes per minute to ensure full contact between rCB particles and iodine solution.
2. Transfer the vial to the centrifuge without delay. Centrifuge at ≥ 1000 r/min for 1–3 minutes until all carbon black solids settle completely and the supernatant becomes optically clear.

Step 4: Titration

1. Pipette exactly 20 mL of the clear supernatant into an Erlenmeyer flask, avoiding disturbance of the settled sediment.
2. Titrate the supernatant with standard sodium thiosulfate solution:
   • Manual titration (Method A): Add starch indicator when the solution turns pale yellow; stop titration at the point where the blue color completely disappears.
   • Potentiometric titration (Method B): Use an automatic titrator with a platinum indicator electrode to detect the equivalence point automatically. This method has higher precision and is the reference method for dispute resolution.
3. Perform a blank test following the identical procedure, using 25 mL of iodine solution with no rCB added.

4. Calculation Formula

The iodine adsorption number (IAN, mg/g) is calculated as follows:

IAN = (V_B − V_S) × (25 / 20) × c × 253.81 / m

Where:

• V_B = sodium thiosulfate volume consumed in blank titration, mL
• V_S = sodium thiosulfate volume consumed in sample titration, mL
• 25/20 = correction factor for the aliquot ratio of total iodine solution to tested supernatant
• c = exact normality of the sodium thiosulfate solution
• 253.81 = molar mass of iodine, g/mol
• m = mass of the dried rCB sample, g

For automated Method B with scaled-up sample and reagent volumes, the simplified calculation defined in the corresponding standard may be applied.

5. Special Considerations for Recycled Carbon Black

Unlike virgin carbon black produced under controlled furnace processes, pyrolysis-derived rCB has inherent material characteristics that require extra attention to ensure accurate and repeatable results:

1. Interference from volatiles and extractables — Residual pyrolysis oils and organic deposits on rCB surfaces can block micropores and lower measured iodine values. For true quality evaluation, testing should be performed on finished, purified rCB rather than raw pyrolysis char.
2. Inorganic impurity effects — Ash, residual steel fines and mineral fillers from feedstock tires do not adsorb iodine but can cause supernatant turbidity. rCB processed with magnetic separation and precision classification yields more reliable test data.
3. Sample homogeneity — rCB often contains hard agglomerates. Samples must be thoroughly homogenized before weighing to avoid deviation from uneven particle size distribution.
4. Aging effect — Surface oxidation during storage gradually reduces iodine number. Testing should be conducted on fresh samples, and results should be documented with test date and sample history.

6. Industrial Value for rCB Production

In the global rCB industry, iodine number is the primary grading index. Commercial rCB products typically cover a range of 30–80 mg/g, corresponding to virgin carbon black grades from N660 to N330. Proper ultra-fine grinding and air classification processing — such as the systems provided by JACAN Powder Equipment — preserves the native porous structure of pyrolysis char, enabling producers to achieve stable, grade-specific iodine values suitable for high-value rubber, tire and polymer filler applications.