O,O-Dimethyl phosphorodithioate is a small molecule with two distinct careers. In one it is a reactive intermediate: a sulphur nucleophile that carries a phosphorus centre, used to build organophosphate actives. In the other it is the acid half of a dithiophosphate collector, the flotation reagent class known generically as an aerofloat. Understanding which career a buyer wants explains almost every difference in specification and handling. This guide covers the chemistry, both application routes and the handling that the sulphide content demands. The specification is on the dimethyl phosphorodithioate product page.
Identity and properties
| Property | Value | Practical significance |
|---|---|---|
| Chemical name | O,O-Dimethyl phosphorodithioate; dimethyl dithiophosphoric acid | Two methoxy groups, one P=S and one P-SH on the same phosphorus |
| Structural formula | (CH3O)2P(=S)SH | The acid hydrogen is what makes it both an acid and a ligand precursor |
| CAS | 756-80-9 | – |
| Formula / molecular weight | C2H7O2PS2 / about 158 | Small and highly functional – acid, sulphur donor and ligand former in one molecule |
| Appearance | Dark brown to pale purple liquid | The colour arises from the P=S chromophore and trace polysulphides; it is not an impurity indicator in itself |
| Assay | 92% min | The balance is related sulphur species and hydrolysis products, which is normal for this chemistry |
| Acidity | Strong (the P-SH proton is readily lost) | It is handled and shipped as the acid; salts are made as needed rather than stored |
| Water behaviour | Hydrolyses on contact with water | Releases methanol and sulphide species, including hydrogen sulphide |
| Odour | Pungent, sulphidic | An odour warning, but not a reliable guide to concentration – use gas detection |
The dialkyl dithiophosphate family
The dialkyl dithiophosphoric acids share a common structure and differ only in the alkyl group. That single variable governs both the physical properties and the industrial behaviour, and it is why the dimethyl member is chosen for some duties and explicitly not for others.
| Member | Alkyl group | Character | Typical use |
|---|---|---|---|
| Dimethyl | CH3 | Shortest chain, most polar, least hydrophobic when converted to a collector | Reactive intermediate; selective flotation where a weak hydrophobiser is wanted |
| Diethyl | C2H5 | Common commercial collector grade | General sulphide flotation |
| Di-isopropyl / di-isobutyl | C3 / C4 branched | Stronger collecting power, good selectivity | Sulphide flotation, particularly where froth control matters |
| Di-aryl (cresyl, etc.) | Aromatic | Very different solubility and selectivity | Specific mineral separations |
| Long-chain (C6-C8) | – | The basis of lubricant antiwear additives as metal salts | Lubricant and grease additive chemistry |
The practical rule: shorter chains give greater polarity and lower hydrophobicity. In flotation that means less collecting power but more selectivity; in synthesis it means a more reactive, more polar intermediate. The dimethyl grade is therefore chosen when you want reactivity or selectivity, not when you want brute collecting force.
Route one: organophosphate intermediate
| Step | What happens | Why the specification matters |
|---|---|---|
| The acid as a nucleophile | The sulphur attacks an electrophilic centre on the partner molecule, forming a P-S-C bond | The free acid content and the amount of oxidised sulphur species change the selectivity |
| Salt formation (optional) | The ammonium or alkali salt can be formed first, giving a cleaner reaction in aqueous or two-phase systems | Residual water and free acid affect the salt ratio |
| Oxidation (optional) | Controlled oxidation converts P=S to P=O in some routes | Pre-existing oxidised impurities carry through |
| Work-up | By-product sulphide and unreacted acid are removed | Assay consistency determines the charge calculation |
Route two: dithiophosphate collectors
| Point | Detail |
|---|---|
| Conversion | The acid is neutralised with sodium or potassium hydroxide to give the dithiophosphate salt – the commercial collector |
| Mechanism | The dithiophosphate anion chemisorbs on sulphide mineral surfaces through the sulphur atoms, leaving the alkyl groups facing outward to create a hydrophobic surface |
| Selectivity | Dithiophosphates are generally more selective than xanthates – they float chalcopyrite, galena and sphalerite in preference to pyrite, which is why they are used where pyrite rejection matters |
| Where the dimethyl grade fits | Short chain means weaker hydrophobicity – appropriate for cleaner, more selective separations and often blended with a stronger collector |
| Dosage | Typically higher than a xanthate for the same recovery, because the molecule is smaller and less surface active |
| pH sensitivity | Dithiophosphates work over a useful pH range but decompose in strongly acidic or strongly oxidising pulps |
Synthesis and its impurity signature
| Route | Chemistry | Impurities it leaves |
|---|---|---|
| P2S5 + methanol | Phosphorus pentasulphide reacted with methanol | Free sulphur, hydrogen sulphide, partially reacted species – the classical reason for a 92% assay rather than higher |
| PCl3 + methanol, then sulphur or H2S | Trichloride route with a subsequent sulphurisation step | Residual chloride, which matters if the product is going into a catalyst-sensitive system |
Both routes generate hydrogen sulphide and require scrubbing. The colour of the finished acid – dark brown to pale purple – is characteristic and comes from the polysulphide and P=S chromophores; a significant shift in colour usually indicates either a different route or degradation in storage.
Handling: the hydrogen sulphide problem
| Hazard | What causes it | Control |
|---|---|---|
| Hydrogen sulphide release | Hydrolysis on contact with water, and thermal decomposition. H2S is lethal at concentrations well below its odour perception limit after olfactory fatigue | Ventilation, fixed gas detection, no confined-space entry without testing, and never assume the nose is a detector |
| Corrosivity | Strongly acidic plus sulphidic | Polyethylene, GRP or 316L stainless; never carbon steel |
| Hydrolysis | Water contact | Sealed containers, dry handling, short storage times |
| Exothermic salt formation | Neutralisation with alkali | Controlled addition and cooling when making the collector salt |
| Environmental | Toxic to aquatic life; sulphur load in effluent | Bunded storage, spill containment, effluent treatment |
Buying notes
| Item | What to fix in writing |
|---|---|
| Assay and basis | 92% minimum, with the analytical method |
| Colour range | Accepted appearance, since the colour is both characteristic and a degradation indicator |
| Chloride | Relevant only if the route used is the trichloride one and chlorides matter downstream |
| Free sulphur | Often not specified, but it affects flotation performance and filterability |
| Packaging | PE drums or IBC compatible with an acidic sulphidic liquid |
| Remaining shelf life | Agreed on delivery – the material degrades in storage |
| Documentation | Batch COA, SDS, and the transport classification for the destination |
Frequently asked questions
Is this the same as a commercial dithiophosphate collector?
Not directly – the collector is the sodium or potassium salt of this acid, and commercial collectors are usually the diethyl, di-isopropyl or di-isobutyl homologues. The dimethyl acid is the intermediate from which a dimethyl collector would be made, or a reagent in its own right.
Why is the assay only 92% and not higher?
Because the product is a reactive, hydrolysable acid made by a route that leaves related sulphur species and hydrolysis products. A 92% specification is normal commercial practice for this material, and the figure has to be used as-is when calculating charges.
Can it be stored long term?
Not advisable. It hydrolyses with water contact and degrades with heat, and the degradation generates hydrogen sulphide. Order to consumption rate.
What is the main safety issue?
Hydrogen sulphide. The material is an acid and a sulphide donor at the same time, and H2S is odourless at concentrations that are already dangerous after a short exposure. Gas detection, not the nose, is the control.