Selected Iridium Salts in Water: Speciation and Treatment
Iridium may enter industrial water systems through specialist materials, process chemicals or residues. The compounds listed below are selected iridium reagents, not a list of species commonly found in natural waters. For an overview of the element, see iridium. When assessing an iridium-bearing stream, engineers should distinguish the reagent added from the chemical forms present after it contacts water.
For the broader chemistry and treatment context, see iridium in water.
Selected chloride-containing compounds
| Compound | Formula | Classification and water-related note |
|---|---|---|
| Iridium(III) chloride hydrate | IrCl3·xH2O | Hydrated iridium(III) chloride; the value of x can vary. |
| Potassium hexachloroiridate(III) hydrate | K3[IrCl6]·xH2O | Potassium salt containing a chloride-coordinated iridium(III) complex. The referenced hydrated product is described as water-soluble. |
| Hydrogen hexachloroiridate(IV) hydrate | H2[IrCl6]·xH2O | Related acid containing a chloride-coordinated iridium(IV) complex; it is not a neutral salt. |
These formulas identify supplied compounds, not their measured concentrations or definitive forms in a water sample. In particular, a solubility statement for a hydrated product should not be applied to an anhydrous form, and no single numerical solubility should be assumed for all iridium chlorides or chloroiridates.
Solubility in water
The Alfa Aesar precious-metal compounds catalogue describes its iridium(III) chloride hydrate as soluble in water, but its anhydrous iridium(III) chloride as insoluble in water. These qualitative product descriptions are not numerical saturation limits. Confirm the exact product, hydration state, temperature and solution composition before preparing a solution or assessing a waste stream. A water-soluble starting reagent does not guarantee that all iridium stays dissolved after dilution or treatment.
Speciation after dilution
Chloride ligands and the oxidation state of iridium affect the species that may be present. Dilution and the receiving water's composition can change the balance between dissolved complexes and other forms. Consequently, the nominal formula of an added salt or acid cannot, by itself, establish which iridium species remain in solution. Identifying individual species requires an analytical approach suited to that question; a total-iridium result alone cannot provide the answer.
Measurement and treatment assessment
Specify whether a reported concentration is expressed as mg/L as Ir or as mg/L of compound. These are different mass bases, and a variable hydrate content further complicates conversion from a weighed reagent to compound concentration. Total and filtered measurements can help distinguish operationally defined fractions, but neither measurement proves the identity of a dissolved complex or a retained particle.
Treatment assessment should address the measured form and the actual water matrix. Particle-removal steps may be relevant where iridium is associated with removable solids; dissolved complexes require a separate assessment of suitable removal or separation options. Membrane and ion-exchange resin performance cannot be assumed to be universal across chloride concentrations, oxidation states and operating conditions. Include captured solids and any concentrate streams in the iridium mass balance when comparing treatment options.
Concentration: mg/L as Ir versus mg/L of reagent
Using the CIAAW standard atomic weight of 192.217 for iridium, 1 mmol/L of iridium corresponds to 192.217 mg/L as Ir. For a compound of known composition, multiply its concentration in mg/L by the iridium mass fraction: the number of Ir atoms per formula unit multiplied by 192.217, divided by the compound's molar mass in g/mol. Include water of crystallization when calculating the molar mass.
For variable hydrates such as IrCl3·xH2O, use the batch's certified iridium mass fraction when available. As an illustrative calculation only, a reagent certified to contain 40.0% Ir would contribute 40.0 mg/L as Ir when 100 mg of reagent is added per litre of final mixture. This is a mass contribution, not proof of complete dissolution. The assumed 40.0% is not a specification for any product in the table.
Removal and concentration for recovery
Removal from treated water transfers iridium to another stream unless it is recovered as a separate material. Measure iridium in the feed, treated water, solids, spent media and concentrates to assess the mass balance. For a liquid recovery stream, its iridium concentration equals the recovered iridium mass divided by that stream's volume.
For example, 1,000 L of feed at 1 mg/L as Ir contains 1,000 mg of iridium. If 90% of that mass is transferred into 10 L of concentrate, the concentrate contains 900 mg, or 90 mg/L as Ir. The concentration factor is 90. This is an illustrative mass balance, not a predicted treatment performance; actual separation, losses and achievable concentrate volume require testing with the specific water.
Compound references
The formula identities for iridium(III) chloride hydrate and hydrogen hexachloroiridate(IV) hydrate are listed in Material Matters. The potassium hexachloroiridate(III) hydrate product listing identifies the hydrated complex and describes it as soluble in water.