Free base, sulfate or copper chelate: choosing an 8-hydroxyquinoline form by where it has to work
In short
- The three forms are the same chemistry with three different solubilities — none, slightly, and freely — and that is usually the whole selection.
- The free base is amphoteric. Its published pKa values, ≈5.0 and ≈9.9, mean pH decides whether it dissolves, so your formulation's pH can be a purchasing decision.
- The copper chelate's water solubility of none is the feature. It stays in the substrate it was applied to, which is what a mildew-proofing agent has to do.
- The sulfate is not a like-for-like substitution by mass. It carries two 8-HQ units per sulfuric acid — 388.39 g/mol against 145.16 — so about 75% of its weight is the active molecule.
- Two lines on the copper chelate's specification limit chloride and sulfate. Those are residues of the copper salt it was made from, so they describe a process route.
Most of what looks like a product range here is one molecule. 8-Hydroxyquinoline chelates metals and is active against bacteria and fungi, and the free base, the sulfate and the copper chelate all do that. What separates them is not what they do but where they will dissolve, and that turns out to decide almost every buying question that follows.
Same chemistry, three solubilities
| Free base | Sulfate | Copper chelate | |
|---|---|---|---|
| CAS | 148-24-3 | 134-31-6 | 10380-28-6 |
| Also called | 8-Quinolinol, Oxine | Oxyquinoline Sulfate (INCI), Quinosol | Oxine-copper |
| Molecular weight | 145.16 | 388.39 | — |
| Appearance | White to off-white crystalline powder | Pale yellow crystalline powder | Yellow-green crystalline powder |
| Water solubility | Slightly soluble | Freely soluble | None |
| Thermal | Melts 73–75 °C, boils 267 °C | — | Decomposes 270 °C |
Read the bold row first. Almost every application question resolves against it.
The free base's solubility is a pH question
Slightly soluble in water understates what the free base actually does, and the published pKa values explain why: ≈5.0 for the protonated ring nitrogen and ≈9.9 for the phenolic –OH.
Two ionisable groups on opposite ends of the pH scale make the molecule amphoteric. Below about pH 5 the nitrogen is protonated and the compound dissolves as a cation. Above about pH 10 the phenol is deprotonated and it dissolves as an anion. In between — which is where most formulations live — it is neutral, and neutral is where it is least soluble in water and most soluble in organic solvents: ethanol, acetone, chloroform, benzene.
So "slightly soluble" is a statement about a particular pH, not a fixed property, and it has a direct consequence. A free base that dissolved cleanly in your acid-side process can come out of solution when the pH is neutralised. If the product ends up near neutral pH and has to stay in solution, the free base is the wrong form and no amount of stirring changes that.
It also explains where the free base genuinely belongs: analytical chelation, synthesis where it is an intermediate rather than an ingredient, and non-aqueous systems.
The sulfate exists to put the same chemistry into water
The sulfate salt is freely soluble in water. That is the entire reason it exists, and it is worth being blunt about what it costs.
The formula is (C9H7NO)2·H2SO4 — two 8-hydroxyquinoline units per sulfuric acid, at 388.39 g/mol against the free base's 145.16. Two units of 145.16 in 388.39 is about 74.7% of the mass; the rest is sulfate.
That number matters because substituting one form for the other on a like-for-like mass basis silently changes the dose by a quarter. If a formulation is specified at a percentage of the free base and someone switches to the sulfate at the same percentage, the active content drops by roughly 25%. It is an easy conversion and an easy one to skip.
The sulfate's own applications follow from solubility: aqueous antiseptic and preservative systems, including cosmetic use, where its INCI name is Oxyquinoline Sulfate.
The copper chelate is insoluble on purpose
Solubility in water: none reads like a limitation on a specification sheet. For this product it is the specification.
Oxine-copper is used as a fungicide and mildew-proofing agent on textiles, rope, paint, paper and wood. Every one of those is an application where the active has to stay where it was applied and survive contact with water — rain, washing, humidity — without leaching out. A water-soluble biocide in an exterior coating is a biocide that leaves in the first storm.
Its thermal limit is worth noting alongside: it decomposes at 270 °C rather than melting. There is no molten processing window. Anything that requires the material to melt is asking it to break down instead.
Two impurity lines that describe a process route
The copper chelate's published specification carries two limits that look like housekeeping and are not:
| Parameter | Specification |
|---|---|
| Assay | 98.00% min. |
| Loss on drying | 0.50% max. |
| Hydrochloride | 0.01% max. |
| Sulfate (SO42−) | 0.10% max. |
The chelate is made by bringing 8-hydroxyquinoline together with a soluble copper salt. Which salt determines which anion is left behind to be washed out, and the two limits sit ten-fold apart — chloride an order of magnitude tighter than sulfate.
That asymmetry is informative in two directions. It suggests where the residues come from, and it says which one the specification treats as more serious. Chloride is the one to care about downstream: in a coating or on a metal-adjacent substrate, residual chloride is a corrosion risk that the copper content does nothing to offset. If your application is a paint or a treatment on or near metal, the chloride figure on the certificate — as a number, not as "conforms" — is worth having.
Choosing, in one pass
| If the material has to… | Form | Because |
|---|---|---|
| Dissolve in an aqueous formulation | Sulfate | Freely soluble; remember the 75% mass conversion |
| Stay in a coating, textile or timber and resist washing out | Copper chelate | Insoluble in water by design |
| React onward, or work in an organic solvent | Free base | Soluble in ethanol, acetone, chloroform; melts at 73–75 °C |
| Dissolve at strongly acid or strongly alkaline pH | Free base | Amphoteric — pKa ≈5.0 and ≈9.9 |
| Survive melt processing | None of the three | The chelate decomposes at 270 °C; the free base boils at 267 °C |
What to ask for on the certificate
The assay method, and whether the figure is normalised area percent or determined against a standard.
On the copper chelate, the chloride and sulfate figures as numbers. "Conforms" against 0.01% is compatible with 0.009% and with 0.0001%, and those are different materials for a coating.
On the sulfate, whether the assay is reported as the salt or as free-base equivalent. This is the same 75% question arriving on a certificate instead of in a formulation, and it is the single most common place for a dose to go wrong on this product family.
Loss on drying on all three, because all three are hygroscopic to some degree and a wet lot is a diluted lot.
Related
- 8-Hydroxyquinoline derivatives — the family overview
- 8-Hydroxyquinoline, CAS 148-24-3 — properties and supply
- 8-Hydroxyquinoline Sulfate, CAS 134-31-6 — Oxyquinoline Sulfate
- Copper 8-Hydroxyquinoline (Oxine-Copper), CAS 10380-28-6
Sources. Every value on this page is quoted from the four product pages linked above: CAS numbers, molecular weights, appearance, solubility, melting and boiling points, decomposition temperature, the pKa figures, the INCI name, and the copper chelate's full specification. The 75% figure is arithmetic on two published molecular weights. The reading of the chloride and sulfate limits as route residues is an inference from the specification, offered as a question to ask a producer rather than a statement about any particular plant.