Calcium Phosphate

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Calcium Phosphate

OVERVIEW

The three forms of calcium phosphate (KAL-see-um FOSS-fate) all occur as tasteless, odorless, colorless to white crystalline or powdery solids.

Dibasic calcium phosphate, CaHPO₄, is also called calcium monohydrogen phosphate, dicalcium orthophosphate, or secondary calcium phosphate. It is usually found in the form of hydrate, such as CaHPO₄·2H₂O. It does not melt, instead decomposing when heated to 109°C (228°F).

Monobasic calcium phosphate, Ca(H₂PO₄)₂, is also known as calcium hypophosphite, calcium biphosphate, acid calcium phosphate, monocalcium orthophosphate, and primary calcium phosphate. It usually exists in the form of the hydrate Ca(H₂PO₄)₂·H₂O. It decomposes when heated to 200°C (400°F)

KEY FACTS

OTHER NAMES:

see Overview

FORMULA:

see Overview

ELEMENTS:

calcium, hydrogen, phosphorus, oxygen

COMPOUND TYPE:

Inorganic salts

STATE:

Solid

MOLECULAR WEIGHT:

136.06 to 310.20 g/mol

MELTING POINT:

see Overview; tribasic form: 1670°C (3040°F)

BOILING POINT:

Not applicable

SOLUBILITY:

See Overview

Tribasic calcium phosphate, Ca₃(PO₄)₂, may be called calcium orthophosphate, tricalcium orthophosphate, tertiary calcium orthophosphate, or tricalcium phosphate. Unlike the other two forms, the tribasic form contains no hydrogen atoms.

The dibasic and tribasic forms of calcium phosphate are insoluble in water and alcohol, but soluble in most acids. Monobasic calcium phosphate is soluble in water and acids, and insoluble in alcohol. All three compounds are non-flammable.

HOW IT IS MADE

Tribasic calcium phosphate can be obtained directly from rock and minerals such as apatite, a complex and impure form of calcium phosphate, or phosphorite, a mineral that contains calcium phosphate mixed with other compounds. The compound can also be produced synthetically by reacting phosphoric acid (H₃PO₄) with solid calcium hydroxide (Ca(OH)₂), or in the reaction between calcium chloride (CaCl₂) and sodium triphosphate (Na₅P₃O₁₀). Dibasic calcium phosphate can also be produced in the former of these synthetic reactions by using an aqueous solution of calcium hydroxide rather than the solid compound. Monobasic calcium phosphate is produced synthetically by adding excess phosphoric acid to a solution of either dibasic or tribasic calcium phosphate and allowing the solution to evaporate.

COMMON USES AND POTENTIAL HAZARDS

Each form of calcium phosphate has its own uses. The dibasic form is used as a nutrient and mineral supplement in animal foods and in certain processed foods, especially cereals. In addition to its nutritional value, dibasic calcium phosphate acts as a dough conditioner, stabilizer, and thickener in foods. The compound is also used in dental products to provide replacement for hydroxyapatite lost to decay or other factors.

Interesting Facts

The forms of calcium phosphate discussed here are only three of many ways in which calcium, phosphorus, and oxygen combine to form calcium phosphate-type compounds. Another such compound is known as hydroxyapatite (Ca₅(PO₄)₃(OH)), which makes up about 70 percent of an adult tooth.
Calcium phosphate is one of the two most common compounds found in kidney stones. The second of these compounds is calcium oxalate (CaC₂O₄).

The most common application of monocalcium phosphate is in fertilizers, where its function is to provide the phosphorus that plants need to remain healthy and grow properly. It is also used as a food additive in baking powders and wheat flours where it serves to retain the proper acidity of the food product to which it is added. It is also used, like the dibasic form, as a nutritional supplement in processed foods.

Tricalcium phosphate has the largest range of uses of the three forms of the compound, the most important of which are in the manufacture of fertilizers, the production of ceramic materials, and the preparation of other compounds of phosphorus. The compound is also used:

As a clarifying agent in the purification of sugar solutions;
As a mordant in the dyeing of cloth;
In the preparation of dental products;
As a stabilizer for plastics;
As a food additive to prevent a powdery product from caking (becoming compacted);
As a nutritional supplement; and
As a means of removing radioactive strontium (stron-tium-90) from milk that has been contaminated by the element.

Words to Know

AQUEOUS: Consisting of some material dissolved in water.
MORDANT: A substance used in dyeing and printing that reacts chemically with both a dye and the material being dyed to help hold the dye permanently to the material.
SYNTHESIS: a chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.
WATER OF HYDRATION: Water that has combined with a compound by some physical means.

While none of the forms of calcium phosphate is flammable, they can all irritate the skin, eyes, and respiratory system. Tricalcium phosphate is the most hazardous of the family of compounds. Direct contact with the eyes may cause severe symptoms, including irritation and burning, resulting in damage to the cornea. The compound is also poisonous if taken internally. It can cause stomach pain, vomiting, and low blood pressure. These problems occur when one is exposed to relatively large amounts of the compound. In the quantities present in foods and other products with which consumers come into contact, the compound poses little risk to users.