Grow Monoammonium Phosphate Crystals (MAP) That Resemble Emeralds


Monoammonium Phosphate (MAP)
Monoammonium phosphate (MAP) or ammonium dihydrogen phosphate (ADP) yields crystals that resemble emeralds or quartz.

Crystal growing kits that you buy usually contain monoammonium phosphate because it’s safe, reliable, and quickly forms crystals. It’s easy to grow monoammonium phosphate crystals without a kit. The chemical forms clear needles or spikes that resemble quartz crystals. Adding a bit of food coloring and controlling growing conditions yields rectangular green crystals that resemble emeralds.

Monoammonium Phosphate Crystal Materials

All you need is the chemical, hot water, a spoon, and a container:

  • Monoammonium phosphate [(NH4)(H2PO4)]
  • Hot water
  • Spoon
  • Glass or bowl

You can use very hot tap water, but freshly boiled water dissolves more of the salt, resulting in a saturated solution and quick crystal growth. The pure chemical is safe enough for food (but don’t eat it), so you can use kitchen utensils. Of course, wash them after the project before using them with food.

Where to Find Monoammonium Phosphate

Monoammonium phosphate (MAP) has multiple names. It’s known equally well as ammonium dihydrogen phosphate (ADP). Other names include ammonium phosphate monobasic and monoammonium orthophosphate. The chemical finds use in fertilizer, dry fire extinguishers, food additives, and for growing piezoelectric crystals for electronics and birefringent crystals for optics. Theoretically, you might find it as a garden fertilizer, but if it isn’t sold under its chemical name, you’ll have to read product labels.

The best source of monoammonium phosphate is online, from a store like Amazon, because they sell the pure chemical. Be sure to search under both “monoammonium phosphate” and “ammonium dihydrogen phosphate.” The product should be a white or colorless crystalline powder. It’s around $10 for a pound, which is more than enough to grow a lot of crystals.

Grow Monoammonium Phosphate Crystals

  1. Stir 6 tablespoons of monoammonium phosphate into 1/2 cup very hot water (roughly 80-85 grams in 100 ml water).
  2. Add food coloring. Use green or a mixture of green and blue for “emeralds”. Natural ammonium phosphate crystals are clear and resemble quartz.
  3. Stir the liquid to dissolve all of the salt and then set the container in a place where it won’t be disturbed.
  4. You’ll see crystals within a day. You’ll either get a bed of thin crystal needles or a few large single crystals. The form of the crystals depends on the rate of cooling. Quickly cooling the liquid (as in a refrigerator or on a cold window sill) yields the needles. Slow cooling promotes large crystal growth. One way to slow the cooling rate is to place your container of crystal-growing liquid inside a large pan of hot water. The pan of water insulates the smaller container so it reaches room temperature more slowly.
  5. Either keep the crystals in the liquid or scrape them out of the container to dry and keep.

Growing a Big Crystal

You may get a perfect, large emerald shape immediately. If you don’t, select one (or more) of the best crystals from the first batch. It doesn’t need to be large, but does need to have the shape you desire.

  1. Scrape your selection out of the container and set it on a paper towel. It will be a seed crystal for further growth.
  2. Meanwhile, make a fresh batch of crystal-growing liquid.
  3. Let it cool and then place the seed crystal in the container. The reason you let the liquid cool is so it won’t dissolve your seed! If you add more than one seed, make sure they aren’t touching each other or the side of the container. Let this crystal grow until you are satisfied with its size.
  4. Take a spoon or butter knife and scrape the monoammonium phosphate “emerald” out of the container. Set it on a paper towel to try. The resulting crystal is stable and can be displayed. Handling the crystal is safe, but moisture from your hands could cause dye onto your fingers. You can coat a dry crystal with clear nail polish to seal it.
DAP Crystal
Monoammonium phosphate (dihydrogen ammonium phosphate) naturally forms clear crystals. (photo: Damien Miller)

Tips for Success

  • If you have trouble dissolving the powder in the water, it probably isn’t hot enough. There are two solutions. One is to heat the liquid in the microwave or on the stove so the chemical dissolves. The other option is to pour the liquid through a coffee filter or paper towel. Discard the solid and use the liquid to grow the crystals.
  • A single chemical often produces shapes of crystals. Monoammonium phosphate grows tetragonal crystals, but their forms include needles, spikes, rectangular prisms, pyramids, and other configurations. To some extent, random chance affects what you’ll get. But, you can repeat the project with different rates of cooling to see different crystal forms.

Safety Information

  • Monoammonium phosphate has low toxicity. It’s used as a food additive in baked goods as a dough conditioner and leavening agent.
  • It’s safe to pour the crystal-growing solution down the drain.
  • The powdered chemical may cause skin irritation. If it gets in the air, eye and lung irritation may occur. So, it’s best to either wear gloves or else rinse your hands immediately after contact with the powdered chemical or crystal solution.

References

  • Lide, David R. (1998). Handbook of Chemistry and Physics (87 ed.). Boca Raton, Florida: CRC Press. ISBN 0-8493-0594-2.
  • Nutrien (2015). (MAP) Monoammonium Phosphate Safety Data Sheet.
  • Van Wazer, John R. (1958). Phosphorus And Its Compounds – Volume I: Chemistry. New York: Interscience Publishers, Inc.

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