Potassium Sulfate
Properties
| State | Solid (white crystalline powder) |
| Color | White |
| Solubility | Soluble in water (111 g/L at 20°C) |
| Melting Point | 1069°C |
| Boiling Point | 1689°C |
About Potassium Sulfate
Potassium sulfate, K2SO4 at 174.259 g/mol, is the chloride-free potash that sits at the premium end of the global fertilizer market. Where muriate of potash (KCl) is cheap and works for grain crops, sulfate of potash (SOP) is what tobacco growers, citrus growers, viticulturalists, and potato farmers buy because the chloride in KCl burns leaves and degrades flavor and storage quality. K2SO4 supplies both potassium (44.9% K2O equivalent) and sulfur (18% S) in one application, which matters more every year as atmospheric sulfur deposition has dropped following Clean Air Act controls. About 7 million tonnes are produced annually, primarily by the Mannheim process — KCl plus H2SO4 at 600°C in cast-iron muffle furnaces, with HCl gas captured as a coproduct that's resold for steel pickling. Some SOP comes from natural langbeinite (K2SO4·2MgSO4) at the Carlsbad New Mexico mines, and the Great Salt Lake operates as a solar evaporation source. In the lab, K2SO4 is the boiling-point elevator added to concentrated H2SO4 in Kjeldahl nitrogen digestions — it raises the digest temperature enough to break down tough proteins fully without losing N as NH3 to the atmosphere. Glassmakers use it as a fining agent to drive bubbles out of molten soda-lime glass during the refining stage. The compound forms no hydrates at ordinary conditions and is the rare anhydrous sulfate that won't pick up moisture.
Where you'll encounter it
If you've ever rolled a Cuban cigar wrapper, sliced into an Idaho russet potato, or sipped a Napa Valley Cabernet, the potassium that grew the plant likely came from K2SO4 rather than KCl. In a Kjeldahl protein lab, the standard digestion mixture is 10 g K2SO4 plus 0.4 g CuSO4 (the catalyst) plus 25 mL concentrated H2SO4 added to a 500 mg sample — the K2SO4 is what raises the boiling point of the H2SO4 from 338°C to closer to 380°C, which is the temperature window that breaks every peptide bond in 30-45 minutes of digestion. In a viticulture extension service, the typical K2SO4 application for established Cabernet vines is 200-400 lb/acre per year split between bud break and veraison, dialed in based on petiole tissue tests showing K below 1.0%.
Common Uses
- Premium SOP fertilizer for tobacco, citrus, grapes, potatoes, and chloride-sensitive crops
- Kjeldahl nitrogen digestion boiling-point elevator for protein assays
- Fining agent in soda-lime container glass and fiber glass melting tanks
- Sulfate source in flash powder for stage-flash pyrotechnic effects
- Salt bridge electrolyte in undergraduate galvanic cell demonstrations
- Gypsum and alum coproduct in fertilizer manufacturing
- Reference electrolyte in some specialty electrochemical reference electrodes
Safety Information
GHS: not classified as hazardous. Oral LD50 in rats over 6600 mg/kg — essentially as benign as table salt for acute exposure. The practical handling concerns are dust irritation in bulk fertilizer terminals (use a dust mask for sustained exposure to powdered SOP) and the standard hyperkalemia risk that comes with any bulk potassium salt in livestock feed at grossly excessive doses. K2SO4 is not hygroscopic, doesn't form hydrates at room conditions, and doesn't react with the metals or plastics commonly used in fertilizer handling equipment. Compatible with PVC, polyethylene, stainless steel, and most coated carbon steels. Spent solutions can be drain-disposed in jurisdictions that allow potassium and sulfate discharge, which is most agricultural ones.
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.