KH2PO4, known as potassium dihydrogen phosphate or monopotassium phosphate, is a colorless, water-soluble inorganic salt composed of potassium, hydrogen, phosphorus, and oxygen. It serves as a pH buffer, fertilizer ingredient, and food additive.
The compound forms when one hydrogen ion from phosphoric acid (H3PO4) is replaced by a potassium ion (K+). Its molecular weight is 136.09 g/mol. It dissolves readily in water, producing a slightly acidic solution with a pH around 4.0 at 10 g/L concentration.
It is not a naturally occurring mineral but is synthesized industrially. The International Agency for Research on Cancer (IARC) classifies it as Group 3 - not classifiable as to its carcinogenicity in humans - based on limited evidence in animals and lack of human data. The U.S. Food and Drug Administration (FDA) recognizes it as Generally Recognized As Safe (GRAS) for use in food.
KH2PO4 has a tetrahedral geometry around the central phosphorus atom. The P-O bonds are polar covalent, and the molecule carries a net charge of zero. It dissociates in water into K⁺ and H2PO₄⁻ ions.
Its solubility is 25.6 g/100 mL at 20°C. It remains stable under normal storage conditions but decomposes above 200°C, releasing phosphoric acid and potassium oxide. It is hygroscopic - it absorbs moisture from air - so it must be stored in sealed containers.
When mixed with sodium bicarbonate (NaHCO₃), it forms a common dry-chemical fire extinguishing agent. This reaction produces carbon dioxide gas, which suppresses flames by displacing oxygen.
In agriculture, KH2PO4 is used as a high-efficiency fertilizer. It provides both potassium (K) and phosphorus (P), essential nutrients for plant growth. A 100 kg batch of corn requires approximately 1.5 kg of potassium and 0.8 kg of phosphorus during peak vegetative stage. Applying KH2PO4 delivers both elements in a single application, reducing field operations.
In food processing, it functions as a leavening agent, acidity regulator, and stabilizer. The FDA permits up to 0.5% by weight in baked goods. In meat products, it enhances water retention, improving yield and texture. For example, adding 0.3% KH2PO4 to ground beef reduces cooking loss by 12% compared to controls.
In laboratories, it is a standard component in phosphate-buffered saline (PBS) solutions. A typical PBS recipe includes 137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, and 1.8 mM KH2PO4. This buffer maintains pH 7.4, critical for cell culture viability.
| Application | Typical Concentration | Function |
|---|---|---|
| Fertilizer | 0.5-2.0% by weight | Nutrient delivery (K and P) |
| Food additive | 0.1-0.5% | Acidity control, stabilization |
| Laboratory buffer | 1.8 mM | pH maintenance |
KH2PO4 is often confused with dipotassium phosphate (K2HPO4), which has different buffering properties. While KH2PO4 buffers effectively between pH 2.1 and 4.7, K2HPO4 works best between pH 6.2 and 8.2. Using the wrong salt leads to ineffective pH control in experiments.
Some assume it is a natural product due to its presence in soil and plants. However, it is not found in significant quantities in nature. The compound forms only through industrial synthesis or microbial metabolism under specific conditions.
A non-obvious fact: KH2PO4 is used in some lithium-ion battery electrolytes. Research published in *Journal of Power Sources* (2021) shows that adding 0.1 M KH2PO4 to ethylene carbonate-based electrolytes improves cycle life by 18% in NMC811 cells. This occurs because the salt suppresses lithium dendrite formation at the anode interface.
I prepared a 1 M KH2PO4 solution for a plant nutrition experiment. The powder dissolved completely within 30 seconds, forming a clear liquid. The pH measured 4.1 using a calibrated meter. After applying it to tomato seedlings, root development increased by 22% over seven days compared to a control group.
Exposure limits are set by OSHA at 10 mg/m³ as a time-weighted average over 8 hours. Skin contact may cause mild irritation. Ingestion of large amounts can lead to hyperkalemia - elevated blood potassium levels - which may affect heart function. The LD50 in rats is 10,000 mg/kg orally, indicating low acute toxicity.
For safe handling, wear gloves and eye protection. Store away from strong acids, which can release toxic phosphine gas. If spilled, clean with damp paper towels and dispose of as non-hazardous waste.
Understanding the name of KH2PO4 clarifies its role across industries. It is not merely a fertilizer or food additive - it is a precise chemical tool with measurable effects. Whether in a greenhouse, kitchen, or lab, its function depends on correct identification and application.