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 The Weston cell ran every national standards lab — NIST, NPL, PTB — until the Josephson voltage standard finally superseded it in 1990.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). OSHA PEL 5 µg/m³ as Cd; ACGIH TLV 10 µg/m³ TWA. 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.