Isothiazolinone preservatives are the default answer to a narrow but commercially important problem: how to keep a water-based product free of bacteria, yeast and mould for a year without heat, without formaldehyde, and at a cost that does not show up in the formulation budget. The CMIT/MIT mixture – 5-chloro-2-methyl-4-isothiazolin-3-one (CAS 26172-55-4) blended with 2-methyl-4-isothiazolin-3-one (CAS 2682-20-4), mixture CAS 55965-84-9 – is the most widely used member of that family. This article covers the chemistry, the manufacturing route, why the two components are blended at all, how the product degrades and how to recognise degradation from a certificate of analysis, how it compares with the alternatives, and what a buyer should verify.
1. What the Product Is
CMIT/MIT-14 is an aqueous solution containing 14.0 – 14.5% total active isothiazolinone, of which roughly three quarters is the chlorinated component CMIT and one quarter the unchlorinated MIT, stabilised with magnesium salts at pH 2.0 – 4.0. It is a technical-grade biocide for industrial preservation, not a cosmetic or pharmaceutical ingredient. The concentrated solution is yellow to yellow-green, has a density of 1.27 – 1.33 g/mL at 20 °C, and is corrosive and a skin sensitiser as supplied.
| Identity item | CMIT | MIT |
|---|---|---|
| IUPAC name | 5-chloro-2-methyl-4-isothiazolin-3-one | 2-methyl-4-isothiazolin-3-one |
| CAS number | 26172-55-4 | 2682-20-4 |
| Molecular formula | C4H4ClNOS | C4H5NOS |
| Molecular weight | 149.60 | 115.15 |
| InChIKey | DHNRXBZYEKSXIM-UHFFFAOYSA-N | BEGLCMHJXHIJLR-UHFFFAOYSA-N |
| Canonical SMILES | CN1C(=O)C=C(S1)Cl | CN1C(=O)C=CS1 |
| Role in the blend | higher intrinsic activity, faster kill, more reactive and less stable | lower intrinsic activity, more hydrolytically stable, carries the long-term protection |
The mixture itself has the registration CAS 55965-84-9. There is no single molecular formula for the blend – the formula often printed in catalogue databases, C8H9ClN2O2S2 with a “molecular weight” of about 264.8, is simply the sum of the two component formulas and their molar masses (149.60 + 115.15 = 264.75). It is a useful arithmetic artefact, not a molecule. Treating it as a molecular weight leads to errors whenever a molar concentration or a molar ratio is calculated, which is one reason the CMIT:MIT ratio on specification sheets is quoted by mass.
2. Mode of Action
Isothiazolinones act by a two-step mechanism at the cell. The ring first reacts with nucleophilic thiol groups, notably the cysteine residues of enzymes and of coenzyme A, forming a mixed disulfide and opening the ring. The second step involves further reaction with cellular nucleophiles, including iron-sulfur centres and amine groups, which disrupts metabolic and energy-generating pathways. The result is rapid inhibition of growth rather than a slow static effect, which is why these actives are described as having a fast kill rate at very low concentrations.
Three practical consequences follow from this mechanism:
- They are consumed by the substrate, not just by microbes. Any nucleophile in the formulation – thiols, sulfites, thiosulphates, some amine-neutralised resins, some reducing agents – will react with the active and reduce the effective dose. This is the single most common cause of “the preservative failed” claims.
- The two components are not equivalent in reactivity. CMIT carries a chlorine at the 5-position and is far more reactive toward nucleophiles than MIT. It therefore delivers the fast kill and takes the damage; MIT survives longer and provides the residual protection.
- Blending them is the invention. CMIT alone is too reactive and degrades too quickly to hold a product for a year; MIT alone is too slow and not potent enough against some bacteria. The mixture gives an initial rapid reduction in viable count and then a lower but persistent concentration of the more stable component.
3. Manufacturing Route
Industrial production starts from a dithiodipropionate ester or the corresponding amide and closes the ring with a chlorinating agent, which simultaneously supplies the chlorine for the 5-position of CMIT.
| Stage | Chemistry | What has to be controlled |
|---|---|---|
| 1. Ester/amide build-up | methyl acrylate or a related propionate is converted to N-methyl-3,3′-dithiodipropionamide by methylamine ammonolysis of the corresponding ester | residual ester and monoamide; these carry through to the final product as non-biocidal organic load |
| 2. Cyclisation / chlorination | the diamide is cyclised with a chlorinating agent such as chlorine or thionyl chloride, forming the isothiazolinone ring and chlorinating the 5-position | chlorine stoichiometry and temperature: these define the CMIT:MIT ratio and the DCMIT level |
| 3. Quench and phase separation | reaction mass is quenched into water and the product separated from salt and acid residues | pH, salt load, residual acidity |
| 4. Stabilisation | magnesium chloride / magnesium nitrate stabiliser added, pH adjusted to 2.0 – 4.0 | the stabiliser level and pH together determine drum stability |
| 5. Standardisation and filtration | adjusted to total active 14.0 – 14.5% and CMIT:MIT ratio, filtered | assay, ratio, colour, absence of particulate matter |
The over-chlorination product is DCMIT, the 4,5-dichloro analogue of MIT. It contributes little to preservation, has poorer solubility and a less favourable toxicological profile, so it is controlled at ≤0.1% in a well-run process. A rising DCMIT value is the clearest single indicator that the chlorination step has drifted.
4. Why the Ratio Is the Key Specification
The specification of a 14% grade typically reads: CMIT 10.0 – 12.0%, MIT 3.0 – 5.0%, total active 14.0 – 14.5%, CMIT:MIT ratio 2.5 – 3.5 by mass. Read the three lines together and the real delivery window is much narrower than the individual limits suggest.
| Scenario | Ratio (mass) | Total active | CMIT | MIT |
|---|---|---|---|---|
| Lower corner | 2.5 | 14.0% | 10.00% | 4.00% |
| Nominal middle | 3.0 | 14.25% | 10.69% | 3.56% |
| Upper corner | 3.5 | 14.5% | 11.28% | 3.22% |
| Not deliverable | any | – | 12.0% | 3.0% |
The last row is the trap: 12.0% CMIT with 3.0% MIT is 15.0% total, which violates the 14.0 – 14.5% range. Individual limits on a specification sheet are not independent, and a certificate that reports values near the individual limits without checking the total is either mislabelled or measured inconsistently.
4.1 Mass ratio versus molar ratio – a 30% error waiting to happen
Because the two components have different molecular weights, mass ratio and molar ratio diverge substantially. A nominal “3:1” mixture understood as a molar ratio corresponds to a mass ratio of 3 x 149.60 / 115.15 = 3.90, which lies outside the 2.5 – 3.5 window printed on the sheet. Understood as a mass ratio of 3.0, the same product is a molar ratio of about 2.31.
| Basis | Value for a “3:1” product | CMIT mass fraction | MIT mass fraction |
|---|---|---|---|
| Mass ratio 3.0 | 3.00 mass/mass | 75.0% of active | 25.0% of active |
| Molar ratio 3.0 | 3.90 mass/mass | 79.6% of active | 20.4% of active |
Two suppliers quoting “3:1” against different bases are describing products that differ by roughly 6% in CMIT content and by 20 – 30% in the ratio figure itself. Always ask which basis the ratio is on, and reconcile against the individual component assays rather than the ratio line alone.
4.2 Concentration conversion for dosing
Dosing is expressed on active ingredient, but the product is sold by weight and measured by volume. Working from a density of 1.30 g/mL and a total active of 14.25%: one litre contains 1,300 g of product and therefore 185 g of active, of which about 143 g is CMIT (0.96 mol/L) and 43 g is MIT (0.37 mol/L). A 200 kg drum contains 28.5 kg of active. Errors of 10% are common when the biocide is measured in a jug instead of being weighed, and a 10% dose error is larger than the entire difference between two competing grades.
5. Stability and Degradation
The molecule is stable in acid and unstable in alkali. That single sentence explains the pH specification, the storage recommendation and most formulation failures.
| Factor | Effect on the active | Practical control |
|---|---|---|
| pH above about 8 | ring opening and hydrolysis; loss accelerates steeply with pH and temperature | keep the concentrate at pH 2.0 – 4.0; check the pH of the finished product before choosing this family |
| Temperature above 40 °C | faster hydrolysis and colour formation | store below 35 °C; add the biocide late in the process, below 40 °C |
| Reducing agents (sulfites, bisulfites, thiosulphates, ascorbate) | dechlorination of CMIT to MIT; CMIT falls, MIT rises, total falls more slowly | never add reductant and biocide to the same stage; if a reductant is present, expect a ratio shift on the COA of the finished product |
| Amines and amine-neutralised resins | nucleophilic attack, consumption of active | dose after neutralisation and test efficacy, not just concentration |
| Iron and other transition metals | catalytic degradation and colour darkening | stainless steel or plastic contact; avoid carbon steel drums and iron pipework |
| Freezing | phase separation and local concentration effects | do not store below 5 °C; if frozen, warm and homogenise before use and re-assay |
| Light | photodegradation and colour drift | opaque or original containers, warehouse storage |
5.1 Reading degradation from a certificate
Three patterns are diagnostic in a finished-product or aged-sample analysis. A fall in CMIT with a smaller fall in MIT and a ratio shifted toward the lower limit points to reduction or nucleophilic consumption – the batch met a reductant or an amine. A proportional fall in both components with an unchanged ratio points to simple hydrolysis, which means pH or temperature is out of range. A rise in DCMIT in incoming material points upstream, to the chlorination step at the biocide plant, and should be rejected at goods-in rather than discovered in a failed batch.
6. Grades and Product Forms
| Form | Total active | Typical use | Handling note |
|---|---|---|---|
| Concentrate (14%) | 14.0 – 14.5% | bulk users, in-plant dilution, emulsion producers | corrosive; weigh, do not measure by volume |
| Dilute (2.5 – 3%) | 2.5 – 3.0% | smaller formulators, direct addition without dilution step | still a sensitiser; same PPE |
| Low-activity (1.5%) | 1.5% | legacy formulations, cosmetics-adjacent rinse-off systems | registration status differs by market; verify before use |
| MIT-only (CMIT-free) | varies, typically 10 – 50% or as a solution | where the chlorinated component is restricted or a slower, more stable profile is wanted | different hazard and regulatory profile |
The trade names most buyers recognise – Kathon, Acticide, Mergal, Preventol, Biochek and similar – refer to these same active blends sold under different registrations and stabiliser systems. Comparing them on the certificate, not on the trade name, is the only reliable approach.
7. Applications and How the Dose Is Set
Required use levels are determined by a preservative efficacy test on the real formulation, not by a catalogue figure, because the dose depends on pH, on the availability of nucleophiles in the system, on the contamination load during manufacture, and on the required preservation period. What can be stated generally is where the family is used and what the constraints are.
| Application | Why CMIT/MIT fits | Main constraint |
|---|---|---|
| Water-based decorative and industrial paints | broad-spectrum, fast, cost-effective at low dose, compatible with most binder systems | amine-neutralised binders consume active; test at the real pH |
| Polymer emulsions and latex | protects the emulsion in the drum and through the paint’s wet state | redox initiator residues and residual monomer can react |
| Adhesives, sealants, construction chemicals | works in systems that cannot be heated | some thickener systems bind the active |
| Metalworking fluids | fast knockdown of bacteria and the odour they produce | high pH of many coolants shortens active life; often blended with other actives |
| Paper and board, coating colours | slime control in wet-end and process water | high organic load and high pH demand a higher dose |
| Oilfield water systems | control of sulphate-reducing bacteria | requires a programme, not a single product, and field-specific approval |
| Detergents and cleaners | preserves aqueous concentrates during storage | surfactant micelles and high pH reduce availability |
8. Comparison with Alternative Preservatives
| Active | CAS | Best property | Main weakness | Typical position |
|---|---|---|---|---|
| CMIT/MIT | 55965-84-9 (mixture) | fast, broad-spectrum, very low cost per treated tonne | sensitising, unstable above pH 8, consumed by nucleophiles | default in-can preservative for acidic to neutral systems |
| BIT | 2634-33-5 | stable over a wide pH range, low sensitisation relative to CMIT/MIT | slower, weaker against some bacteria, needs partners for full spectrum | preferred where alkalinity or a milder hazard profile is required |
| OIT | 26530-20-1 | strong against fungi and yeasts | poor water solubility, strong sensitiser, mainly used in dry-film protection | film preservative rather than in-can alone |
| Bronopol | 52-51-7 | very effective against bacteria, good in slightly alkaline systems | formaldehyde-releasing chemistry, nitrosamine concerns with amines, instability at alkaline pH | replacement candidate where CMIT/MIT is restricted |
| DBNPA | 10222-01-2 | extremely fast knockdown, low persistence | hydrolyses quickly; not a long-term preservative | process sanitisation, not storage protection |
| Glutaraldehyde | 111-30-8 | powerful, cheap, wide industrial use | strong sensitiser, vapour irritant, restricted in some markets | water treatment and metalworking, not consumer products |
| Zinc pyrithione | 13463-41-7 | excellent against fungi and algae | insoluble, restricted in some regions, zinc discharge concerns | dry-film and marine, in combination |
| IPBC | 55406-53-6 | outstanding fungicide for coatings | narrow bacterial spectrum, cost | paired with a bactericide |
The decision logic usually runs in this order. If the formulation is acidic to neutral and the hazard profile is acceptable, CMIT/MIT at low dose is the cheapest effective answer. If the formulation is alkaline, or if the destination market restricts isothiazolinones in the target use category, move to BIT or a BIT/CMIT blend. If the problem is fungi rather than bacteria, add a film preservative such as OIT or IPBC instead of increasing the in-can dose. If the problem is a fouled process line rather than a stored product, use a fast knockdown agent such as DBNPA and accept that it will not persist.
9. Safety, Regulation and Transport
Both components are potent skin sensitisers, and sensitisation is permanent. The concentrate is corrosive because of its low pH, and it is toxic to aquatic organisms. Engineering controls – closed transfer, local exhaust ventilation, splash guards – come first, followed by chemical-resistant gloves and eye protection. Never decant into beverage bottles or unlabelled containers; ingestion incidents involving decanted biocides are a recurring pattern in poison centre reports.
Regulatory status is use-category-specific rather than product-specific, and this is where buyers most often go wrong:
- Cosmetics: the European cosmetics regulation permits CMIT/MIT only in rinse-off products at very low levels and has prohibited leave-on use since 2017; several other markets have followed with their own restrictions, and MIT separately has been restricted further. A 14% industrial concentrate is not a cosmetic ingredient.
- Biocidal products: in the European Union the mixture is regulated under the Biocidal Products Regulation and is authorised through product family approvals at member-state level. Industrial use without a valid authorisation for the specific product type is not permitted.
- United States: FIFRA registration applies to the formulated product and its labelling; a technical concentrate supplied for reformulation does not transfer approval to the customer’s product.
- Other markets: most require notification or registration of the formulated product, and several restrict isothiazolinones in consumer-adjacent categories more tightly than in industrial ones.
Transport classification follows the concentrate as supplied: a corrosive liquid requiring the corresponding packaging group and labelling on the SDS. Do not assume the classification of a diluted grade applies to the 14% concentrate; always use the SDS supplied with the actual batch.
10. Purchasing Checklist
| Check | Acceptance criterion | Why |
|---|---|---|
| Total active | 14.0 – 14.5%, method stated | defines value per kilogram |
| CMIT and MIT individually | consistent with the total and the ratio | catches mislabelled or blended-down material |
| Ratio basis | mass or molar, stated explicitly | a 30% apparent difference between suppliers is often only a basis difference |
| DCMIT | ≤0.1% | over-chlorination indicator |
| pH | 2.0 – 4.0 | stability of the concentrate in the drum |
| Density | 1.27 – 1.33 g/mL at 20 °C | volume-to-mass conversion for dosing |
| Gardner colour | ≤5 | oxidation or metal contamination during storage |
| Appearance | clear, no sediment, no turbidity | phase separation or salt precipitation |
| Documentation | COA per batch, SDS in destination language, registration status statement | customs and product-registration requirements |
Three questions worth asking every supplier: which method was used for CMIT and MIT (HPLC with UV detection at the isothiazolinone chromophore is standard, but sample preparation matters), what is the maximum storage time in the customer’s warehouse before re-assay, and which stabiliser system is used. The last one matters because magnesium chloride and magnesium nitrate systems behave differently at low temperature and in hard water.
11. Packaging and Storage
Standard packaging is 25 kg and 220 kg HDPE drums and 1,000 kg IBCs; 5 kg and 25 kg jerricans are available for trials. Storage should be in the closed original container, out of sunlight, between 5 °C and 35 °C, away from alkalis, amines, reducing agents, food, feed and oxidisers. Do not store on carbon steel or in iron-containing vessels. Under these conditions a shelf life of 12 months from the date of manufacture is typical; opened containers should be resealed immediately and used within the campaign. Because both components degrade faster at higher temperature, a warehouse in a hot climate should be treated as a shorter storage life, not as an equivalent one.
12. FAQ
What is the difference between CMIT/MIT-14 and a 1.5% cosmetic grade?
Only the dilution, the stabiliser level and the regulatory framing. The actives are the same. A 14% concentrate is a technical product for industrial formulation, not a cosmetic ingredient, and the restriction profile for cosmetics applies to what ends up in the finished consumer product.
Why does my finished product show a lower CMIT:MIT ratio than the incoming biocide?
Because CMIT reacted first. Reduction, nucleophilic attack by amines or thiols, and alkaline hydrolysis all remove CMIT faster than MIT. A ratio that has drifted downward is evidence of consumption, and it usually means the effective dose is lower than intended.
Can I use this in a product at pH 9?
Not as a sole preservative with confidence. Hydrolysis above about pH 8 is fast enough that protection will not survive the intended shelf life. BIT or a BIT blend is the usual answer in alkaline systems.
Is there a formaldehyde-releasing concern?
No. Isothiazolinones do not release formaldehyde; that concern belongs to a different family (Bronopol, DMDM hydantoin, quaternium-15, and similar). The relevant hazard here is skin sensitisation and aquatic toxicity.
How do I convert a target ppm on active to grams of product?
Divide by the active fraction. At 14% active, 10 ppm in a 1,000 kg batch needs 10 g of active, which is 10 / 0.14 = 71 g of product. Weigh it; do not measure by volume.
Can the product freeze in winter shipping?
It should not be allowed to freeze. If it does, warm it to room temperature, homogenise and re-assay before use, because phase separation can leave the top and bottom of the drum at different concentrations.
References and Further Reading
- Identity data (formulas, molecular weights, InChIKeys, canonical SMILES) verified against PubChem records: CMIT CID 33344, MIT CID 39800.
- All ratio and concentration arithmetic on this page is derived from the component molar masses: molar ratio 3:1 corresponds to a mass ratio of 3 x 149.60 / 115.15 = 3.90; a mass ratio of 3.0 corresponds to a molar ratio of 2.31.
- Density conversion: 1.30 g/mL x 14.25% = 185 g active per litre, split as about 143 g/L CMIT (0.96 mol/L) and 43 g/L MIT (0.37 mol/L).
- Regulatory statements are summarised for orientation only. Registration status, permitted product types and labelling must be confirmed for the destination market and the specific use category before commercial use.
This article is technical orientation for industrial buyers. It is not a formulation approval, a registration opinion, or a safety assessment for any specific product. Always work from the SDS of the actual batch and from the efficacy testing of the actual formulation.
For the specification and packaging of our 14% grade see the CMIT/MIT-14 product page. For family selection see industrial preservative selection, and for troubleshooting an existing formulation see stability and formulation compatibility.