Sodium Phosphate

OVERVIEW

The three forms of sodium phosphate are formed when one or more of the three hydrogen atoms in phosphoric acid (H3PO4) are replaced by sodium atoms. When one hydrogen is replaced, the monobasic form is produced; replacing two hydrogen atoms results in the formation of the dibasic form; and replacing all three hydrogens results in the formation of tribasic sodium phosphate. All three forms of sodium phosphate are colorless to white crystalline solids or white powders. All may occur as hydrates, such as monobasic sodium phosphate monohydrate and dihydrate (NaH₂PO₄·H₂O and NaH₂PO₄·₂H₂O); dibasic sodium phosphate dihydrate, heptahydrate, and dodecahydrate (Na₂HPO₄·₂H₂O, Na₂HPO₄·₇H₂O, and Na₂HPO₄·₁₂H₂O); and tribasic sodium phosphate dodecahydrate (Na₃PO₄·₁₂H₂O).

KEY FACTS

OTHER NAMES:

See Overview

FORMULA:

Monobasic: NaH₂PO₄; Dibasic: Na₂HPO₄; Tribasic: Na₃PO₄

ELEMENTS:

Sodium, phosphorus, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

119.98 to 163.94 g/mol

MELTING POINT:

Not applicable

BOILING POINT:

Not applicable

SOLUBILITY:

Soluble in water; insoluble in ethyl alcohol

The three salts are also known by a number of other names, such as: NaH₂PO₄: primary sodium phosphate, primary sodium orthophosphate, sodium biphosphate, MSP; Na₂HPO₄: secondary sodium phosphate, secondary sodium orthophosphate, disodium phosphate, disodium hydrogen phosphate, DSP; Na₃PO₄: tertiary sodium phosphate, tertiary sodium orthophosphate, trisodium phosphate, TSP.

HOW IT IS MADE

All forms of sodium phosphate are made by treating phosphoric acid with a sodium compound to replace one or more of the hydrogen atoms in the acid. For example, reacting phosphoric acid with sodium hydroxide will make monobasic sodium phosphate:

H₃PO₄ + NaOH → NaH₂PO₄ + H₂O

Reacting phosphoric acid with sodium carbonate will produce the dibasic form of the compound:

H₃PO₄ + Na₂CO₃ → Na₂HPO₄ + H₂O + CO₂

And treating phosphoric acid with an excess of sodium hydroxide will result in the formation of the tribasic form of the compound:

H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O

Other methods of preparation are available for all three forms of sodium phosphate.

COMMON USES AND POTENTIAL HAZARDS

The three forms of sodium phosphate have somewhat different uses. Monobasic sodium phosphate is used as a food additive to maintain proper acidity and in baking powders, as a food supplement to provide the phosphorus needed in a person's daily diet, in the treatment of boiler water to reduce the formation of scale on the inner surface of the boiler, and as a feed supplement for cattle and other farm animals.

Dibasic sodium phosphate is used as a food additive to maintain emulsions and proper acidity of food products, in the manufacture of fertilizers, as a food supplement for humans and farm animals, in the treatment of silk, for fireproofing of wood and paper products, to treat boiler water, in the production of detergents, as a raw material in the manufacture of ceramics, as a mordant for dyeing, and as a cathartic and laxative.

Tribasic sodium phosphate is used in a variety of cleaning agents, such as detergents, industrial cleaning products, and metal cleaners; to treat boiler water; for the tanning of leather; in the manufacture of textiles and paper products; in the purification of sugar; as a dietary supplement for humans and farm animals; in paint removers; and for various photographic purposes.

Interesting Facts

• Chicken processors often dip whole chickens in a solution of TSP before treating and packaging them. The compound kills bacteria and reduces the risk of food poisoning.

All three forms of sodium phosphate are mild skin, eye, and respiratory system irritants. Tribasic sodium phosphate has somewhat more serious health consequences than the monobasic or dibasic forms. Exposure to the compounds may produce redness, itching, burns, pain, and blisters on the skin; redness, pain, and burning of the eyes; and a burning sensation, coughing, shortness of breath, and a sore throat if ingested. Taken in larger amounts, the tribasic form may cause vomiting, nausea, and diarrhea and may induce shock and collapse. In such cases, immediate medical assistance is required.

Words to Know

CATHARTIC

A substance that promotes bowel movements.

EMULSION

A temporary mixture of two liquids that normally do not dissolve in each other.

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.

FOR FURTHER INFORMATION

"Sodium Phosphate Dibasic Heptahydrate." Ted Pella, Inc. http://www.tedpella.com/msds_html/19545msd.htm (accessed on November 10, 2005).

"Sodium Phosphate, Monobasic." Cornell University. http://msds.ehs.cornell.edu/msds/msdsdod/a9/m4481.htm#Section3 (accessed on November 10, 2005).

"Sodium Phosphates." Agricultural Marketing Service, U.S. Department of Agriculture. http://www.ams.usda.gov/nop/NationalList/TAPReviews/sodiumphosphates.pdf (accessed on November 10, 2005).

"Trisodium Phosphate." Household Products Database, National Institutes of Health. http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=brands&id=3007033 (accessed on November 10, 2005).

"Trisodium Phosphate (Anhydrous)." International Chemical Safety Cards. http://www.inchem.org/documents/icsc/icsc/eics1178.htm (accessed on November 10, 2005).

"USDA Approves Phosphate to Reduce Salmonella in Chicken." Environmental Nutrition (February 1993): 3.