Formula: NH4H2PO4 This compound, also known by its abbreviated name ADP, together with its chemical sibling, potassium dihydrogen phosphate(KH2PO4, abbreviated names is KDP) are one of the standard compounds in crystal growing kits. It is safe and produces large elongated crystals easily.
- Crystal system: tetragonal
- Crystal shape: obeliscs (tetragonal columns with pyramidal tips)
- Color: colorless
- Stability on air: stable
A common fertilizer, providing both nitrogen and phosphorus to plants. Fertilizer-grade compound has low purity, containing mix of NH4H2PO4 and (NH4)2HPO4 as well as miscellaneous insoluble fillers, so purification by filtering and recrystallization is required.
Solubility at room temperature is 40.4 g/100 mL, it grows significantly with temperature.
I used the regular slow evaporation method. This compound is a good nutrient for bacteria, so infestation of the growing vessel was a constant problem. Boiling and filtering solution helps for a while; adding few drops of iodine tincture also seem to reduce infestation, but not forever.
Apparently, shape of crystals depends on pH of the solution. At low pH (acidic) long needles with small tip and slightly curved faces are growing. Increasing pH by seem to produce wider crystals with flatter sides. To reduce pH, I added several ml of 10% NH3 solution to 400ml of slightly supersaturated solution of compound.
The compound is safe and occasionally used in food industry.