Classification and basistiy rule
Phosphorus acid molecules represent a tetrahedron. The key rule determining their properties states that only hydrogen atoms within hydroxyl groups (–OH) can dissociate and be replaced by a metal. Hydrogen bound directly to a phosphorus atom (P–H bond) does not participate in salt formation.
Based on this rule, three main acids are distinguished:
- Hypophosphorous acid ($H_3PO_2$). Contains one –OH group and two P–H bonds. It is monobasic. Forms salts known as hypophosphites.
- Phosphorous acid ($H_3PO_3$). Contains two –OH groups and one P–H bond. It is dibasic. Forms salts known as phosphites.
- Orthophosphoric (phosphoric) acid ($H_3PO_4$). All three hydrogen atoms are part of –OH groups. This is a tribasic acid that forms phosphates.
Dissociation of orthophosphoric acid
Phosphoric acid dissociates stepwise in aqueous solutions. In the first step, it behaves as a medium-strength acid: at a concentration of 0.1 mol/L, the degree of dissociation reaches about 20%. This stage yields a hydrogen cation and a dihydrogen phosphate ion ($H_2PO_4^-$).
In the second step, where the second proton is cleaved, it becomes a weak acid. The third dissociation step is extremely weak—in strength at this stage, the compound barely surpasses ordinary water.
Solubility of phosphates
The solubility of phosphoric acid salts in water follows a clear pattern. Neutral salts (trisubstituted phosphates) are practically insoluble in water. The only exceptions are salts of singly charged ions, such as alkali metals. Acidic salts (mono- and disubstituted), however, dissolve significantly better.
This difference is related to crystal lattice structure: upon transition from $PO_4^{3-}$ to $H_2PO_4^-$, the anion charge decreases, making ionic bonds less strong and facilitating dissolution.
| Salt | Formula | Salt type | Solubility at 20 °C (mg/L) |
|---|---|---|---|
| Calcium phosphate | $Ca_3(PO_4)_2$ | Insoluble | 0,2 |
| Calcium hydrogen phosphate | $CaHPO_4$ | Slightly soluble | 70 |
| Calcium dihydrogen phosphate | $Ca(H_2PO_4)_2$ | Readily soluble | 15 000 |
Chemical properties: hydrolysis and heating
Soluble neutral phosphates undergo severe hydrolysis. An interesting example is trisubstituted ammonium phosphate—it simply does not exist in aqueous solution. The ammonium ion ($NH_4^+$) possesses stronger acidic properties than the hydrogen phosphate ion. Consequently, ammonium donates its proton to the phosphate, resulting in the release of ammonia ($NH_3$).
Upon heating solid salts, their stability depends on the type. Neutral s-block metal salts tolerate heating well. Acidic salts, however, are thermally unstable and undergo dehydration (condensation) reactions:
- Hydrogen phosphates lose water and convert into diphosphates (pyrophosphates).
- Dihydrogen phosphates undergo stepwise changes: first converting into acidic diphosphates, and upon further heating, forming polymeric metaphosphates.
Production and biological role
In industry, phosphoric acid is produced by double decomposition (exchange) reactions. Natural minerals (apatite or phosphorite containing $Ca_3(PO_4)_2$) are treated with sulfuric acid. Depending on the exact reactant ratio, pure orthophosphoric acid or valuable acidic salts—fertilizers (precipitate or superphosphate)—can be obtained.
From a biological perspective, for normal biochemical reactions, living organisms require exclusively "inorganic" phosphorus, where a phosphorus-oxygen bond is present. Organophosphorus compounds (with a P–C bond) are mostly extremely toxic and are used in agriculture as pesticides and insecticides.
Briefly: answers to topical questions
What anions are formed at each of the three dissociation steps of orthophosphoric acid?
During the three dissociation steps of orthophosphoric acid, dihydrogen phosphate, hydrogen phosphate, and phosphate ions are sequentially formed.
- Dihydrogen phosphate ion ($\text{H}_2\text{PO}_4^-$) is formed at the first step of proton abstraction from a neutral molecule.
- Hydrogen phosphate ion ($\text{HPO}_4^{2-}$) is formed at the second step.
- Phosphate ion ($\text{PO}_4^{3-}$) is formed at the third step.
The main contribution to the concentration of hydrogen ions is made only by the first step, because at each subsequent stage the degree of dissociation drops sharply due to electrostatic attraction of the abstracted proton to the negatively charged anion.
What role do hydrogen phosphates and dihydrogen phosphates play in maintaining the acid-base balance of blood?
Hydrogen phosphates and dihydrogen phosphates form the phosphate buffer system, which maintains constant blood pH (7.40 ± 0.05) and the optimal ratio of bicarbonate buffer components.
- Hydrogen phosphate ion ($HPO_4^{2-}$)—an alkaline component that neutralizes excess protons during medium acidification.
- Dihydrogen phosphate ion ($H_2PO_4^-$)—an acidic component that neutralizes excess bases.
In blood, these components are present in a 1:4 ratio. When hydrogen ion concentration increases, they are bound by hydrogen phosphate to form dihydrogen phosphate, the excess of which is then excreted by the kidneys. This system also plays a significant role in the intracellular regulation of acid-base balance.
Which enzymes carry out the transfer of phosphate groups to organic substrates in the cell?
The transfer of phosphate groups to organic substrates is carried out by kinases (phosphotransferases), which belong to the transferase class.
- Protein kinases (e.g., protein kinase A, MEK kinase, MAPK kinase) phosphorylate proteins at amino acid residues (threonine, tyrosine).
- Nucleoside monophosphate kinases and nucleoside diphosphate kinases participate in nucleotide synthesis, converting monophosphates into diphosphates and triphosphates.
- Pyridoxal kinase transfers a phosphate group to pyridoxal.
Kinases catalyze phosphate group transfer reactions from one compound to another, most commonly using an ATP molecule as a donor.
In which diseases do pathological deposits of insoluble calcium phosphates occur in body tissues?
Pathological deposition of calcium salts occurs in dystrophic calcification of tissues and in certain kidney diseases.
- Tuberculosis—calcification develops in areas of caseous necrosis.
- Atherosclerosis—dystrophic calcification develops in atherosclerotic plaques.
- Malignant tumors (e.g., sarcomas)—calcification may be a sign of secondary changes.
- Nephrocalcinosis—calcification of the renal parenchyma; medullary nephrocalcinosis is associated with hyperparathyroidism, medullary sponge kidney, renal tubular acidosis, hypervitaminosis D, and primary hyperoxaluria, while cortical nephrocalcinosis is associated with acute cortical necrosis, chronic glomerulonephritis, and primary hyperoxaluria.
- Nephrolithiasis—formation of calculi in the renal collecting system.
Dystrophic calcification also develops in zones of infarctions, old thrombi, fiber-altered heart valves, and parasite capsules.
To quickly determine the basicity of a phosphorus acid from its formula, subtract one from the number of oxygen atoms. $H_3PO_2$ (2 oxygens − 1) = monobasic. $H_3PO_3$ (3 − 1) = dibasic. $H_3PO_4$ (4 − 1) = tribasic.
Class and exam questions
Why is phosphorous acid ($H_3PO_3$) dibasic if it contains three hydrogen atoms?
In the phosphorous acid molecule, only two hydrogen atoms are bonded to oxygen (forming hydroxyl groups). The third hydrogen is bonded directly to phosphorus, so it cannot dissociate in solution or be replaced by a metal.
Does normal ammonium phosphate exist in water?
No, it decomposes in aqueous solution. The ammonium cation acts as a stronger acid than the hydrogen phosphate anion, thus donating a proton to it to form ammonia.
Why do acidic calcium salts dissolve better than normal salts?
The lower the charge of the anion (dihydrogen phosphate has a charge of -1, while tertiary phosphate has -3), the weaker the crystal lattice of the salt. A less durable structure is much more easily destroyed by water molecules.
What else to study on this topic
- Polymerization processes: formation of diphosphoric, triphosphoric, and metaphosphoric acids.
- Writing neutralization reaction equations for various phosphorus acids with an excess of potassium hydroxide.
- Features of stoichiometry calculations in the industrial synthesis of superphosphate and precipitate.
- Toxicological mechanisms of poisoning by organophosphorus compounds.
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