Common Osmium Salts and Compounds in Water
Osmium can occur in different chemical forms in water, and the identity of a compound matters when interpreting measurements or assessing treatment. The compounds below are representative specialist reagents, not a list of forms known to be prevalent in natural or industrial waters. For broader context, see osmium.
For the broader water assessment, see osmium in water.
Representative compounds and their distinctions
| Compound | Formula | Relevance to water assessment |
|---|---|---|
| Potassium hexachloroosmate(IV) | K2[OsCl6] | An ionic complex salt containing a chlorinated osmium complex. Its behavior should not be inferred from that of an osmium oxide. |
| Potassium osmate(VI) dihydrate | K2OsO4·2H2O | A different oxo salt supplied as a hydrate. The hydration state affects formula mass and comparisons reported by mass. |
| Osmium tetroxide | OsO4 | A molecular oxide, not a salt. It is water-soluble, volatile and hazardous; its properties must not be assigned to all osmium compounds. |
Manufacturer listings identify potassium hexachloroosmate(IV) and potassium osmate(VI) dihydrate. The NIOSH Pocket Guide entry for osmium tetroxide documents its hazards and physical properties. A measured solubility value should be tied to the exact compound, including its hydrate where applicable, and to the stated test conditions. The identities in this table alone do not establish numerical solubilities or justify soluble/insoluble classifications for the salts.
Speciation and reporting concentrations
A result reported as total osmium does not identify which compound or oxidation state is present. Comparing total and filtered-sample concentrations can help distinguish operationally defined particle-associated and filter-passing fractions, but filtration alone does not establish molecular speciation. Where compound identity affects a treatment decision, the analytical approach should address the forms of interest and the sample matrix. Sampling and interpretation should also consider whether an osmium form might be present outside the liquid phase.
Check the reporting basis when comparing data. A concentration in mg/L as Os is the mass of osmium per litre; it is not the same as mg/L of an osmium compound. For an identified compound, the osmium mass fraction is nOs × MOs / Mcompound, where nOs is the number of osmium atoms per formula unit and the compound molar mass includes any water of hydration. The compound concentration multiplied by this fraction gives the equivalent concentration as Os.
Treatment assessment and phase accounting
Treatment options depend on the species and water matrix. Separation of particle-associated material, and assessment of adsorption, ion exchange or membrane processes for dissolved forms, may be relevant, but their performance cannot be assumed for an unspecified osmium compound. Bench-scale or otherwise representative, species-specific evaluation is needed before drawing removal conclusions; there is no universal filter or resin performance figure for osmium.
A lower measured liquid concentration demonstrates a change in that sampled phase, not necessarily recovery or elimination of osmium. Interpretation should account for relevant liquid, solid and air phases, especially where a volatile compound such as OsO4 is identified. The central distinction is between measured concentration change, the chemical form present and the ultimate destination of the osmium.
Interpreting water-solubility data
Thermo Scientific lists potassium osmate(VI) dihydrate as soluble in water. This qualitative description does not specify a saturation concentration or test temperature. For potassium hexachloroosmate(IV), the identity specification cited above is not a quantitative solubility measurement; obtain data for the exact reagent and solution conditions before using it in a design calculation.
For comparison, NIOSH reports osmium tetroxide solubility as 6% at 77 °F (25 °C). This is an oxide value, not a value for an osmium salt. Retain the source's reporting basis rather than converting an unspecified percentage directly to mg/L. Water solubility does not remove the need to assess volatility and exposure pathways.
Worked concentration conversion
Using the CIAAW standard atomic weight of osmium, 190.23, and the manufacturer's molar mass of 368.45 g/mol for potassium osmate(VI) dihydrate:
mg/L as Os = mg/L of pure dihydrate × 190.23 ÷ 368.45.
The calculated osmium mass fraction is approximately 51.63%. Thus, 100 mg/L of pure dihydrate corresponds to 51.63 mg/L as Os if all its osmium is present in the measured fraction. One mmol/L of elemental osmium corresponds to 190.23 mg/L as Os. Correct for reagent purity and use the exact hydrate formula. These are composition calculations, not solubility limits or preparation instructions.
Concentration and recovery mass balance
Where a validated separation transfers osmium into a smaller collected volume, calculate concentration from the mass actually recovered:
Recovered concentration = feed concentration × feed volume × recovered mass fraction ÷ recovered volume.
For example, 1,000 L at 1 mg/L as Os contains 1 g of osmium. If 90% is recovered in 10 L, the resulting concentration is 90 mg/L as Os, a concentration factor of 90. This is an illustrative mass balance, not a recommended osmium process or guaranteed recovery.
Removal from treated water, recovery into a collected stream and product purity are separate results. Include residues, wash liquids and any relevant air losses in the assessment. Where osmium tetroxide may be present, open evaporation or aeration must not be assumed to provide safe concentration or removal: competent process-safety assessment and suitable containment are essential.