Cadmium Sulfate
Properties
| State | Solid (white crystalline powder; hygroscopic) |
| Color | White |
| Solubility | Very soluble in water (755 g/L at 20°C); slightly soluble in ethanol |
| Melting Point | 1000°C (anhydrous) |
| Boiling Point | Decomposes above 1000°C |
About Cadmium Sulfate
Cadmium sulfate's claim to fame is metrology: it was the electrolyte in the Weston cell, the international voltage standard from 1911 to 1990. Edward Weston designed the cell in 1893 around three components — a cadmium-mercury amalgam negative electrode (about 12.5 wt% Cd in Hg), a saturated solution of CdSO4 with crystals of CdSO4·8/3H2O present to keep it saturated against temperature drift, and a paste of Hg2SO4 over a mercury pool as the positive electrode — and the resulting EMF of 1.01864 V at 20 °C was reproducible cell-to-cell to better than a few microvolts. That extraordinary stability came from a quirk of cadmium-sulfate chemistry: the saturated-cell version uses the unusual 8/3-hydrate (CdSO4·8/3H2O, sometimes written 3CdSO4·8H2O), which has a relatively flat solubility curve near room temperature and so buffers the cell against thermal drift. The Weston cell ran every national standards lab — NIST, NPL, PTB — until the Josephson voltage standard finally superseded it in 1990. Outside metrology, CdSO4 shows up as the cadmium-source salt in cadmium electroplating baths for aerospace fasteners (where Cd's combination of low-friction, galvanically-compatible-with-aluminum corrosion protection is still irreplaceable in some specs) and as a precursor for cadmium-yellow pigments via metathesis with sodium sulfide. It's a colorless, hygroscopic, very water-soluble solid with a molar mass of 208.474 g/mol anhydrous, and it's an IARC Group 1 carcinogen with the same renal and skeletal toxicity as every other soluble cadmium compound.
Where you'll encounter it
If you've ever calibrated an old analog voltmeter or worked through a teaching electrochemistry course on EMF reference standards, the saturated Weston cell — wet, glass-bodied, painstakingly thermostatted — was the gold standard before solid-state references took over. In an aerospace plating shop, CdSO4 is one of the standard make-up salts for the sulfate-bath cadmium-plating process used on landing-gear bolts and high-strength steel fasteners, where MIL-STD-870 and AMS-QQ-P-416 still call out cadmium plating despite OEMs working hard to replace it with zinc-nickel. In a pigment-restoration lab, CdSO4 is also the oxidative degradation product that conservators look for on the surface of old cadmium-yellow paintings, the white bloom that marks where CdS has weathered.
Common Uses
- Electrolyte in the Weston voltage-reference cell (1.01864 V at 20 °C)
- Make-up salt for sulfate-bath cadmium electroplating of aerospace fasteners
- Precursor for cadmium-sulfide pigments via metathesis with Na2S
- Cadmium-source reagent in fluorescent-screen and CdS-photocell production
- Trace-metal calibration standard in atomic-absorption spectroscopy
Safety Information
Acutely toxic and a confirmed human carcinogen. GHS H300+H330 (fatal if swallowed or inhaled), H340 (mutagenic), H350 (IARC Group 1 carcinogen), H360 (reproductive toxicity), H410 (very toxic to aquatic life). Occupational exposure limits for cadmium are among the lowest set for any metal. CdSO4 is fully water-soluble (755 g/L), which makes it more bioavailable than CdS or CdTe and a worse contamination risk in a spill. Renal cortex is the principal target tissue; biological half-life 10–30 years. Use a fume hood, double nitrile gloves, splash goggles; dispose as RCRA D006 hazardous waste.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.