Phosphate

Phosphates are in the narrow sense, the salts and esters of orthophosphoric acid ( H3PO4). In a broader sense, the condensates (polymers) of orthophosphoric acid and its esters, phosphates are called. The esters are described in phosphoric acid esters. Phosphorus is present in all of these compounds in the oxidation state (V). Oxygen compounds of phosphorus with other oxidation states are listed under phosphorus.

• 3.2 General

• 5.1 fertilizer
• 5.2 detergent additive
• 5.3 food additive
• 5.4 Other Uses

Variants

Primary, secondary and tertiary phosphates

The salts of the tribasic ortho- phosphoric acid ( H3PO4) can be divided into primary, secondary and tertiary phosphates. For monovalent cations M ' are the sum formulas according M'H2PO4, M'2H1PO4 and M'3PO4. Due to the partial neutralization of the phosphoric acid is obtained hydrogen or dihydrogen. These can react with both acids and bases with. Because of this property, many buffer solutions containing hydrogen phosphates.

Condensates

Phosphoric acid can be used in a condensation reaction ( dehydration ) form pyrophosphoric acid ( H4P2O7 ). Similarly, there are corresponding salts, the diphosphates ( pyrophosphates ) M'4P2O7. On continued reaction also poly-or cyclo - phosphates form. cyclo - phosphates are often called meta. Polyphosphates and metaphosphates are therefore the polymers of the salts of the phosphoric acid.

Triphosphate ( Na5P3O10 ) and meta-phosphates were used for water softening in detergents. As a food additive, for example, Triphosphate and diphosphate application.

Extraction

Phosphates of minerals such as apatite, Ca5 [( PO4 ) 3 ( OH, F, Cl) ] produced. The main deposits are located in northern Africa (Morocco, Togo, Western Sahara ), Jordan, United States (Florida ), Russia ( Kola Peninsula ), South Africa and China. In the past, the phosphate deposits found with the highest concentration ( Nauruit, which originated from guano ) on the Pacific island of Nauru. The original reserves are exhausted in 2003. 2004 new deposits were developed on Nauru. Saudi Arabia intends (2005) to promote the exploitation of its phosphate deposit in the coming years.

Resources of phosphates is limited, and most of them are contaminated with cadmium and / or radioactive heavy metals. Some phosphate deposits were previously as a source of uranium. It is estimated that the usable for fertilizer production phosphate deposits will be exhausted sooner than the world's oil reserves. The cadmium content of phosphate deposits is very different. Many developed countries have already introduced a limit for cadmium in fertilizers. So is the world's only known a deposit, which is below the EU limit ( Kola ). In developing countries, however, fertilization is carried out with cheaper cadmium- contaminated phosphate fertilizers.

The formerly practiced in developed countries use of basic slag ( a by-product of smelting iron ore ) is excluded due to the high chromium exposure for health reasons. A further possibility is to use the existing in sludge precipitated or biologically enriched phosphates or recover. In Germany and other countries of contaminated sewage sludge is burned in principle, since it also contains many heavy metals and endocrine disruptors in addition to the interesting phosphates. The phosphate extraction can thus be carried from the combustion residues.

Properties

In aqueous solution, phosphate anions exist in four forms. Under strongly basic conditions, the phosphate anion is mainly present as ( PO43 - ), while under weakly basic conditions dominates the hydrogen phosphate anion ( HPO42 ). Under mildly acidic conditions is mainly present the dihydrogen phosphate anion ( H2PO4 - ). In strongly acidic aqueous solution of phosphoric acid ( H3PO4) is the main form.

There are therefore three pH-dependent equilibrium reactions:

Under strongly alkaline conditions, such as at pH = 13 is PO43 - and HPO42 - in essence. If the solution is neutral (pH = 7.0 ) are H2PO4 ( 62%) and HPO42 ( 38%). At pH = 7.4, the ratio of the two components rotate at about 39% H2PO4 and 61% HPO42 -. Under strongly acidic conditions ( pH = 1) is dominant compared to H3PO4 H2PO4 -. HPO42 and PO43 - are virtually absent.

General

With the exception of the alkali and ammonium compounds are poorly soluble in water, most phosphates.

Phosphates can form compounds with heavy metals. This property makes the use of phosphates problematic because the phosphates can mobilize the heavy metals from the sludge.

For the most part contain deposits of phosphate compounds and heavy metals, such as Cadmium and uranium.

Importance for food

In the human diet phosphate plays an essential role in energy metabolism and bone structure. It combines with calcium to solid calcium apatite. The phosphate levels is important in connection with the calcium metabolism (see hyperphosphatemia ). The importance of phosphate for the occurrence of hyperactivity in children is considered to be refuted.

Use

Fertilizer

The main level of phosphate is used as fertilizers (see superphosphate, double superphosphate ). The suitability of phosphates for the fertilization was discovered by chance: fell in iron and steel production after the Thomas process as a by- product of the phosphate-rich basic slag, which turned out to be an excellent fertilizer.

Erosion of agricultural land phosphates get bound to clay minerals in rivers and lakes, and from there into the seas. In freshwater and marine ecosystems, they contribute significantly to eutrophication. Phosphates include a trigger of " blue-green algae blooms " ( cyanobacteria ) in the Baltic Sea.

Detergent additive

To soften water triphosphate can be used. On the use of phosphates in detergents is now omitted in parts of Europe, as they have finally led to eutrophication of bodies of water and tipping over. Zeolite A is used for this purpose as a replacement. In automatic dishwasher detergents, however Tripolyphosphate still be used as a water softener.

Food additive

Sodium phosphate ( E 339 ), potassium phosphate ( E 340 ), calcium phosphate ( E 341 ) and the salts of ortho- phosphoric acid diphosphate (E 450), triphosphate (E 451) and polyphosphate ( E 452 ) are approved as food additives and are used as preservatives, acidifiers, used as an acidity regulator and emulsifier. Phosphate is for non-alcoholic flavored drinks; uses (cola drinks in these as phosphoric acid ( E 338 ) ), sterilized and ultrahocherhitze milk, concentrated milk, milk and skimmed milk powder and as a processing aid (prevents the clumping together of pourable food ). Phosphates play a very important role in food production (especially in the meat industry ) and are components of the melting salt for processed cheese.

Other uses

Feed, corrosion inhibitors (see phosphating ); Flame retardants; Buffer substance for the neutral pH range (see above).

32P is a radioactive isotope of phosphorus is versatile used in the form of dihydrogen phosphate (or sodium phosphate) in research and in the nuclear medicine therapy especially in polycythemia vera ( radiophosphorus therapy).

Proof

Detection reactions of phosphates are described in phosphorus.