CH 223 Chemical of the Week
Phosphoric Acid
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PHOSPHORIC ACID, H3PO4
About 10 million tons of phosphoric acid, H3PO4, are produced in the United States each year. Most of the acid (about 80%) is used in the production of agricultural fertilizers, with the remainder being used for detergent additives (about 10%), cleaners, insecticide production, and cattle feed additives. The commercial method of preparation is the addition of sulfuric acid to phosphate rock.
3 H2SO4(l) + Ca3(PO4)2(s) + 6 H2O(l) 
2 H3PO4(s) + 3 CaSO4.2 H2O(s)
Pure anhydrous phosphoric acid is a white solid that melts at 42.35°C to form a viscous liquid. In solution, phosphoric acid behaves as a triprotic acid, having three ionizable hydrogen atoms. The hydrogen ions are lost sequentially.
| H3PO4(aq) | |
H+(aq) + H2PO4-1(aq) | Ka1 = 7.5 × 10-3 |
| H2PO4-1(aq) | |
H+(aq) + HPO4-2(aq) | Ka2 = 6.2 × 10- 8 |
| HPO4-2(aq) | |
H+(aq) + PO4-3(aq) | Ka3 = 1.7 × 10-12 |
Phosphoric acid is not a particularly strong acid as indicated by its first dissociation constant. It is a stronger acid than acetic acid but weaker than sulfuric acid and hydrochloric acid. Each successive dissociation step occurs with decreasing ease. Thus, the ion H2PO4-1 is a very weak acid and HPO42 -1 is an extremely weak acid.
Salts of phosphoric acid can be formed by replacing one, two, or three of the hydrogen ions. For example, NaH2PO4, sodium dihydrogen phosphate, can be formed by reacting one mole of phosphoric acid with one mole of sodium hydroxide.
| H3PO4(aq) + NaOH(aq) | |
NaH2PO4(aq) + H2O(l) | |
| net ionic form: | H3PO4(aq) + OH-1(aq) | |
H2PO4-1(aq) + H2O(l) |
Similarly, Na2HPO4, disodium hydrogen phosphate, and Na3PO4, trisodium phosphate, could be formed by the reaction of one mole of H3PO4 with two and three moles of NaOH, respectively. (Be sure you are able to write net ionic equations for these processes.)
Salts containing the anion H2PO4-1 are weakly acidic. The tendency of this ion to dissociate is greater than its tendency to hydrolyze, that is, its Ka2 is larger than its Kb.
| H2PO4(aq) | |
H+(aq) + HPO41-(aq) | Ka2 = 6.2 × 10-8 |
| H2PO4-1(aq) + H2O(l) | |
H3PO4(aq) + OH-1(aq) | Kb = Kw/Ka1 = 1.3 × 10-12 |
Because H2PO4-1 is weakly acidic and of low toxicity, it is used as the acid in some baking powders. These baking powders contain NaH2PO4 and NaHCO3 (sodium bicarbonate). The leavening action of baking powders results from the production of carbon dioxide gas by an acid-base reaction between these two ingredients.
H2PO4-1(aq) + HCO3-1(aq) HPO4-2(aq) + H2O(l) + CO2(g)
In the reaction between them, H2PO4-1 acts as the Brønsted acid, and HCO3-1 acts as the base. A comparison of the ionization constants for these two ions reveals that H2PO4-1 is a stronger acid than HCO3-1.
| H2PO4-1(aq) | |
H+(aq) + HPO4-2(aq) | Ka2 = 6.2 × 10-8 |
| HCO3-1(aq) | |
H+(aq) + CO3-2(aq) | Ka2 = 4.8 × 10-11 |
Salts containing the anion HPO42 -1 are weakly basic. The tendency of this ion to hydrolyze is greater than its tendency to dissociate.
| HPO4-2(aq) | |
H+(aq) + PO4-3(aq) | Ka3 = 1.7 × 10-12 |
| HPO4-2(aq) + H2O(l) | |
H2PO4-1(aq) + OH-1(aq) | Kb = Kw/Ka2 = 1.6 × 10-17 |
Solutions containing the phosphate ion, PO4-3, are quite basic. This ion has no acidic hydrogen, and its base ionization constant (hydrolysis constant) is relatively large.
| PO4-3(aq) + H2O(aq) | |
HPO4-2(aq) + OH-1(aq) | Kb = 5.9 × 10-13 |
As a result, solutions of soluble phosphates tend to have the same slippery, "soapy" feel as solutions of strong bases, such as NaOH or KOH.
Phosphoric acid is used primarily in the manufacture of fertilizers, detergents, and pharmaceuticals. In the steel industry, it is used to clean and rust-proof the product. It is also used as a flavoring agent in carbonated beverages (read the ingredients list on a can of Coca-Cola), beer, jams and jellies, and cheeses. In foods, phosphoric acid provides a tart, acidic flavor.
In the manufacture of detergents, phosphoric acid is used to produce water softeners. Water softeners remove Ca2+ and Mg2+ ions from "hard" water. If not removed, these hard-water ions react with soap and form insoluble deposits that cling to laundry and the washing machine. Phosphates produced from phosphoric acid are used extensively as water softeners (builders) in detergents. The most widely used phosphorus compound in solid detergent mixtures is sodium tripolyphosphate, Na5P3O10. As a water softener, sodium tripolyphosphate binds to Ca2+ and Mg2+, forming soluble chemical species, called complexes or chelates. These complexes prevent the Ca2+ and Mg2+ from reacting with soap and forming deposits.
Most phosphoric acid is used in the production of fertilizers. Phosphorus is one of the elements essential for plant growth. Organic phosphates are the compounds which provide the energy for most of the chemical reactions that occur in living cells. Therefore, enriching soils with phosphate fertilizers enhances plant growth.
Increasing the phosphate concentration in surface waters also enhances the growth of aquatic plant life. Run-off from fertilized farm lands can stimulate plant growth in lakes and streams. Waste water that contains phosphates from detergents can have the same effect. Lakes that are rich in plant nutrients suffer from accelerated eutrophication. When the lush aquatic plant growth in a nutrient-rich lake dies, the decomposition of the dead plant material consumes dissolved oxygen. This consumption reduces the level of dissolved oxygen to a point where it is insufficient to support animal life. To reduce the threat of lake eutrophication, many localities have banned the use of phosphates in detergents. In some cases, the phosphates have been replaced by carbonates. In others, new detergents have been developed that do not react with the Ca2+ and Mg2+ ions of hard water.
EXERCISE: Give an example of another triprotic acid and report its three Ka values. Give the source of your information.
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