CTC activity and iodine number are not two ways of saying the same thing. CTC activity is a gravimetric mass-gain percentage measured on a vapour stream under ASTM D3467; iodine number is milligrams of iodine adsorbed per gram from aqueous solution at a 0.02 N residual filtrate concentration, under ASTM D4607. They probe different pore populations, sit under different ASTM subcommittees, and there is no published conversion between them. Our own published data shows it: across SURSORB PAC-900, PAC-1050 and PAC-1250 — the same coconut-shell pellet at three activation levels — minimum CTC activity rises from 50 % to 75 %, a 50 % relative increase, while minimum iodine number rises from 950 to 1250 mg/g, a 32 % increase. Over the same three grades apparent density falls from 480 ± 30 g/L to 380 ± 30 g/L.
The short version
- CTC activity (ASTM D3467-04(2020)) is the ratio, in percent, of the mass of CCl4 adsorbed at saturation to the mass of the sample. Commercial carbons span roughly 20 %–90 %.
- Iodine number (ASTM D4607-14(2021)) is milligrams of iodine adsorbed per gram from aqueous solution at a 0.02 N residual. ASTM calls it a relative indicator of porosity, not a capacity.
- No reliable CTC-to-iodine conversion exists. The only published conversions run CTC to butane activity, and the two of them — 2.57 and 2.55 — disagree.
- A datasheet CTC number may never have been measured with CCl4. EUROCARB has stated publicly that it moved its clearance test to butane under the Montreal Protocol and reports CTC activity derived by correlation.
- Across SURSORB PAC-900/1050/1250, CTC rises 50 %, iodine rises 32 %, and apparent density falls from 480 to 380 g/L. The numbers move together, not proportionally.
What does CTC activity actually measure?
Carbon tetrachloride activity, measured under ASTM D3467, is how much CCl₄ vapour a carbon holds at saturation expressed as a percentage of the carbon’s own mass. ASTM D3467 defines carbon tetrachloride activity as “the ratio (in percent) of the weight of CCl4 adsorbed” to the weight of the sample when saturated under the conditions the method lays down. The measurement is gravimetric: weigh the carbon, saturate it in a CCl4-laden air stream, run to constant weight, weigh again. The number is the mass gain, and it correlates with the pore volume of the sample. ASTM’s stated significance is narrow — D3467 determines how far the activation process has been carried. It is a quality-control index for the producer.
The current designation is ASTM D3467-04(2020), four pages, Book of Standards Volume 15.01, under Subcommittee D28.04. The endpoint is constant weight, not a fixed clock. That matters, because some datasheets — including two of ours — print CTC with a “(60 min)” qualifier, which is a timed variant rather than the run-to-saturation endpoint of D3467. If a sheet qualifies CTC with a duration, ask which test was run.
ASTM attaches an explicit limitation to the result: the activity “does not necessarily provide an absolute or relative measure of the effectiveness” of that carbon on other adsorbates or at other operating conditions. That sentence is the whole argument of this article, written by the committee that owns the method.
What does iodine number actually measure?
Iodine number, measured under ASTM D4607, is the milligrams of iodine a carbon adsorbs per gram from aqueous solution, standardised to a residual iodine concentration of 0.02 N in the filtrate. The carbon is treated with dilute hydrochloric acid, boiled, cooled, contacted with iodine solution, and the residual iodine back-titrated with sodium thiosulfate against a starch indicator. That 0.02 N normalisation point is what makes results from different laboratories comparable at all.
ASTM D4607 describes what it delivers as “the relative activation level of unused or reactivated carbons” — a relative indicator of porosity. ASTM is explicit that the relationship between iodine number and surface area varies with raw material, processing route and pore distribution, so iodine approximates surface area for some carbons, not as a rule. The AWWA B604 foreword goes the other way, calling iodine number “indicative of the total surface area of a carbon,” which is the water industry’s working convention rather than a physical identity.
The current designation is ASTM D4607-14(2021), five pages, Volume 15.01 — but under Subcommittee D28.02, not D28.04. Iodine number is governed by a different group of people from the one that governs CTC, ball-pan hardness, apparent density, moisture, ash and butane. The two numbers were never designed as a matched pair. And D4607 carries the same species of disclaimer as D3467: iodine number “does not necessarily indicate the carbon’s adsorption capacity for other species.”
Which pore population does each one probe?
Iodine is adsorbed predominantly in micropores; carbon tetrachloride reaches into mesopores and large micropores. That is the physical reason the two numbers do not track each other. Published work on activated carbon characterisation states that the iodine molecule is “mainly adsorbed in micropores” while carbon tetrachloride is adsorbed in the larger pores. The IUPAC 2015 technical report fixes the vocabulary: micropores are below 2 nm internal width, mesopores 2–50 nm, macropores above 50 nm. Anyone quoting a tighter numeric window for “the pores iodine sees” is extrapolating; we will not print one, because none is verifiable from a primary source.
Iodine is adsorbed mainly in micropores; carbon tetrachloride reaches mesopores and large micropores. Two probes, two pore populations, two different numbers.
Why gas-phase buyers ask for CTC and water buyers ask for iodine
Because each index was built in the phase its buyers work in. CTC activity is measured from a vapour stream and reflects the pore range vapour-phase organic loading uses, so solvent recovery, vapour control and gas-phase odour work grew up specifying it. Iodine number is measured from aqueous solution and reflects micropore development, and the water supply chain standardised on it: ANSI/AWWA B604-18, the US standard for granular activated carbon in water treatment, lists iodine number among its physical parameters at clause 4.2.9. There is no deeper theory than convention plus phase-appropriateness. A carbon can be developed hard in the micropore range and read high on iodine while offering only moderate CTC, or be opened out and read the other way. Both are legitimate carbons; neither number tells you which your process wants.
Is there a conversion between CTC activity and iodine number?
No. There is no published, reliable conversion between carbon tetrachloride activity and iodine number, and we will not print one. The reasons are on the record from four independent directions:
| Source | What it says |
|---|---|
| ASTM D3467 | CTC activity does not necessarily measure effectiveness on other adsorbates or at other conditions |
| ASTM D4607 | Iodine number does not necessarily indicate capacity for other species |
| Water Conditioning & Purification (Nowicki et al., 2008) | These popular tests “do not differentiate the total pore volume” |
| EUROCARB | “the correlation between Butane testing and CTC testing is not linear” |
The 2008 WC&P piece puts it bluntly: buying on an iodine or butane number alone, across different vendors, is described there as often not good judgement.
The butane factors disagree — and that is the proof
The only conversions anyone has published in this family run CTC against butane activity, and there are two of them:
- CTC activity number = 2.57 × butane activity — GTZ Proklima, Selection and safe use of alternatives to CTC, 1st edition, July 2009.
- CTC activity = 2.55 × butane activity — Scholz, Technologies 2023, 11(6), 153.
Two published sources, one relationship, two different constants. A physical conversion — grams to kilograms, g/cm³ to kg/m³ — does not have competing values. A fitted correlation does. The 2.55/2.57 discrepancy is the cleanest available evidence that these are regression outputs on some population of carbons, not conversions. ASTM D5742 itself is described as including experimental correlation data relating butane and carbon tetrachloride activity values — correlation being the operative word.
One caution: ASTM D5228 butane working capacity and ASTM D5742 butane activity are different measurements. D5742 is a mass-gain ratio in the same form as CTC. D5228 is volumetric — butane adsorbed at saturation minus butane retained after a specified purge. The 2.55 and 2.57 factors relate to D5742 activity. Applying them to a working-capacity number is an error.
A CTC number that was never measured as CTC
This is the part that should change how you read a datasheet. Carbon tetrachloride is restricted under the Montreal Protocol, and ASTM positions D5742 butane activity as “an environmentally preferable alternative to the carbon tetrachloride activity test.” Suppliers have acted on that. EUROCARB states publicly that it has replaced D3467 with D5742 as its quality-control clearance test and that it will “continue to report CTC activity on all relevant documentation” — values obtained by correlation from the butane result. That is an honest, on-the-record disclosure, and we cite it approvingly rather than critically. But follow the logic: a CTC figure on a datasheet may be a butane measurement passed through a correlation factor whose two published versions do not agree.
So the useful question to a supplier is not “what is your CTC?” It is: which test did you run, to which designation and revision, and is the CTC figure on this sheet measured or derived?
What does 60 CTC mean?
A CTC activity of 60 means the carbon adsorbs 60 % of its own mass in carbon tetrachloride at saturation — 60 g of CCl4 on 100 g of carbon — under ASTM D3467 conditions. SURSORB PAC-1050 is specified at CCl4 adsorption capacity minimum 60 %; SURSORB CAG at CTC adsorption minimum 50 %.
Note the word minimum. Almost every CTC line on almost every datasheet in this industry is a floor, not a result. A grade specified at minimum 60 % might clear at 62 or at 71, and comparing two suppliers’ minima compares two commercial commitments, not two carbons. The measured value for your lot belongs on a certificate of analysis — a different document with different obligations, covered in what a certificate of analysis should contain.
What one carbon family shows
SURSORB PAC is the only Suracsh family publishing CTC and iodine number for the same carbons — a clean natural experiment: same precursor, same pellet form, three activation levels.
| Parameter | PAC-900 | PAC-1050 | PAC-1250 |
|---|---|---|---|
| CCl4 adsorption capacity, % min | 50 | 60 | 75 |
| Iodine number, mg/g min | 950 | 1050 | 1250 |
| Apparent density, g/L | 480 ± 30 | 450 ± 30 | 380 ± 30 |
| Ball pan hardness, min | 98 | 98 | 96 |
| Crushing strength, kg min | 4 | 4 | 3.5 |
One carbon family, three activation levels. CTC activity rises by half; iodine number rises by less than a third; apparent density falls by a fifth. The three numbers move together but not proportionally.
Read the arithmetic. From PAC-900 to PAC-1250, CTC goes 50 → 75: 25 percentage points, but a 50 % relative rise. Iodine goes 950 → 1250, a 32 % rise. If the two indices were interchangeable proxies for “how good is this carbon,” those percentages would match. They do not — CTC moves more than half again as fast as iodine across the same three carbons. Further activation burns out more skeleton and widens pores; micropore-filled iodine capacity improves with diminishing returns while the mesopore and large-micropore volume CCl4 fills keeps opening up.
The density is the tell
Apparent density falls from 480 ± 30 g/L on PAC-900 to 380 ± 30 g/L on PAC-1250 — about a fifth, in the opposite direction to both capacity indices. That is not a defect; it is the mechanism. More pore volume per gram means less mass in a litre of packed bed. Ball pan hardness follows the same logic, easing from minimum 98 to minimum 96, crushing strength from minimum 4 kg to minimum 3.5 kg.
The commercial consequence gets missed constantly: you buy carbon by mass and you fill vessels by volume. A 20 % drop in apparent density means roughly 20 % fewer kilograms in the same vessel, so a higher-CTC carbon may deliver less total capacity per vessel than its index suggests. ASTM D2854 says as much in its own significance statement — packed density is essential when designing vessels and when procuring material. If you are sizing a bed rather than comparing grades, capacity per unit volume governs; our guide to reading a datasheet works through the unit traps.
All three numbers are indices, not properties
Including surface area. The IUPAC technical report on physisorption — Thommes and co-authors, Pure and Applied Chemistry 2015 — is unusually direct about what BET area means for a microporous solid, which is what activated carbon is. A Type I isotherm is the microporous case, and for it the report states that the BET area “represents an apparent surface area,” which “may be regarded as a useful adsorbent ‘fingerprint’.” It warns that extreme caution is needed in the presence of micropores. ISO 9277:2022, the BET standard itself, agrees by construction: BET theory is described there as applicable to Type II and Type IV isotherms, with microporous Type I solids handled as a special case in an annex.
So the honest summary of the three numbers on the front of every carbon datasheet is this:
| Index | Method | What it is | What it is not |
|---|---|---|---|
| CTC activity, % | ASTM D3467 | Mass gain at CCl4 saturation; tracks pore volume | A capacity for your adsorbate |
| Iodine number, mg/g | ASTM D4607 | Iodine adsorbed at 0.02 N residual; relative activation level | A surface area, or a capacity |
| BET area, m²/g | ISO 9277 | An apparent surface area; a fingerprint | A real accessible surface for a microporous carbon |
Three fingerprints. Useful for telling one lot from another, for confirming that activation ran to completion, and for holding a supplier to a floor. Not sufficient for predicting service life on your gas stream. For that you need the contaminant, the concentration, the temperature, the humidity, the empty-bed contact time, and a capacity test at conditions that resemble yours.
The full specification for every grade named here is published in HTML at suracsh.us, and every datasheet downloads without a form or an NDA.
Sources
- ASTM D3467 — Carbon Tetrachloride Activity of Activated Carbon
- ASTM D3467-04 definition, ANSI Webstore
- ASTM D4607 — Iodine Number of Activated Carbon
- ASTM D5742 — Butane Activity of Activated Carbon
- ASTM D5228 — Butane Working Capacity of Activated Carbon
- ASTM D2854 — Apparent Density of Activated Carbon
- Thommes et al., Physisorption of gases (IUPAC Technical Report), Pure Appl. Chem. 2015
- ISO 9277:2022 — Specific surface area of solids by gas adsorption, BET method
- Iodine and carbon tetrachloride adsorption in micropores and mesopores
- ANSI/AWWA B604 — Granular Activated Carbon, preview
- EUROCARB — Replacement of the CTC test for carbon activity
- GTZ Proklima — Selection and safe use of alternatives to CTC, July 2009
- Scholz, Technologies 2023, 11(6), 153
- Nowicki et al., Standard Activated Carbon Test Methods, Water Conditioning & Purification, 2008
- Carbon tetrachloride activity range for commercial carbons
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Questions
What does 60 CTC mean on an activated carbon datasheet?
It means the carbon gained 60 % of its own mass in carbon tetrachloride when saturated under ASTM D3467. A 100 g sample adsorbs 60 g of CCl4. Most datasheets print it as a minimum, so 60 is a floor, not a measured result for your lot.
Can you convert CTC activity to iodine number?
No. There is no reliable published conversion between carbon tetrachloride activity and iodine number, and neither ASTM method claims transferability to other adsorbates. The two published CTC-to-butane factors, 2.55 and 2.57, disagree with each other, which shows those are fitted approximations rather than conversions.
Why does gas-phase carbon quote CTC and water carbon quote iodine number?
CTC activity is measured from a vapour stream and is adsorbed in mesopores and large micropores, which matches solvent-recovery and vapour duty. Iodine number is measured from aqueous solution and reflects micropore development, which is what AWWA B604 and water-treatment purchasing have standardised on.
Is CTC activity still tested with carbon tetrachloride?
Not always. CCl4 is restricted under the Montreal Protocol, and ASTM D5742 butane activity is positioned as an environmentally preferable alternative. At least one supplier, EUROCARB, has publicly stated that it clears product on butane and reports CTC activity derived by correlation.