phosphoric acid

[lín suān]

Inorganic acid

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Phosphoric acid, also known as orthophosphoric acid, is a common Inorganic acid , chemical formula is H three PO four And the molecular weight is 97.995. Phosphoric acid is not easy to volatilize and decompose, almost none Oxidizability , has the common property of acid, is a ternary weak acid, and its acid ratio hydrochloric acid 、 sulphuric acid 、 nitric acid Weak, but more than acetic acid 、 boric acid Isointense. Phosphoric acid is easily exposed to air Deliquescence , heating will cause water loss inorganpyrophosphate And further water loss Metaphosphate 。 Phosphoric acid is mainly used in pharmaceutical, food, fertilizer and other industries, including antirust ， food additives , Dental and orthopedic surgery ，EDIC Corrosive , electrolyte, flux, dispersant, industrial Corrosive , fertilizer raw materials and components, household cleaning products, can also be used as chemical reagents.

Chinese name: phosphoric acid^[5]
Foreign name: Phosphoric acid^[5]
Alias: orthophosphoric acid
chemical formula: H three PO four^[5]
molecular weight: ninety-seven point nine nine five^[5]
CAS login number: 7664-38-2^[5]
EINECS login number: 231-633-2^[5]
Melting point: 42 ℃ (anhydrous)
Boiling point: 261 ℃ (anhydrous)

Water solubility: one hundred (infinitely miscible)
Density: 1.874g/mL (liquid)
Appearance: Transparent colorless liquid
Application: It is mainly used in pharmaceutical, food, fertilizer and other industries, and can also be used as chemical reagent
Security description: S1/2；S7；S16；S24/25；S26；S36/37/39；S45^[5]
Hazard symbol: C^[5]
Hazard description: R34^[5]
UN dangerous goods number: three thousand four hundred and fifty-three^[5]

catalog

six test method
seven Hazard category

A brief history of research

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Following German businessmen Portland After the discovery of phosphorus and the production of phosphorus by German chemist Konkel, British chemist Boyle He also independently produced phosphorus. He was also the first chemist to study the properties and compounds of phosphorus. In his paper "A New Experiment of Observed Cold Light" published in 1682, he wrote“ phosphorus White smoke is generated after combustion, and the solution generated after the interaction of white smoke and water is acidic. " The white smoke is Phosphoric anhydride (Phosphorus pentoxide), and the solution generated by the reaction with water is phosphoric acid. However, he did not carry out further research on phosphoric acid.

The first chemist who studied phosphoric acid was a French chemist Lavoisier 。 In 1772, he made an experiment by burning phosphorus in a mercury sealed bell jar. From the experimental results, it is concluded that a certain amount of phosphorus can be burned in a certain volume of air; When phosphorus burns, it generates white flakes of anhydrous phosphorus, like fine snow; After combustion, the air in the bottle remains about 80% of the original capacity; Phosphorus is about 2.5 times heavier after combustion than before combustion; The white powder tablet is dissolved in water to form phosphoric acid. Lavoisier also proved that phosphoric acid can be prepared by reacting concentrated nitric acid with phosphorus.

More than a hundred years later, German chemist Libich did many experiments in agricultural chemistry to reveal the value of phosphorus and phosphoric acid to plant life. The role of organic chemistry in agriculture and physiology, written by Li Bixi in 1840, scientifically demonstrated the problem of soil fertility and pointed out the role of phosphorus on plants. At the same time, he also further explored the application of phosphoric acid and phosphate as fertilizer, from which the production of phosphoric acid entered the era of large-scale.

Physical and chemical properties

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Phosphoric acid stereo structure formula

Orthophosphoric acid is a phosphoric acid composed of a single phosphorus oxygen tetrahedron. In phosphoric acid molecule, the P atom is sp three Hybrid, three hybrid orbitals form three oxygen atoms σ bond The other P-O bond is composed of σ Bond and two d-p bonds from oxygen to phosphorus. σ The coordination bond is a pair of phosphorus atoms Lone pair electron To the empty orbital of oxygen atom. D ← p The coordination bond is p of oxygen atom y 、p z Two pairs of lone pair electrons in orbit and d of phosphorus atom xz 、d yz The empty tracks are overlapped. Because the 3d energy level of phosphorus atom is higher than that of oxygen atom. 2p of energy level The energy is much higher, and the molecular orbital formed is not very effective, so the number of P-O bonds is triple, but from Bond energy and Key length It is between Single bond and double bond between. Pure H three PO four And its crystal hydrate hydrogen bond There is, which may be the reason why the concentrated phosphoric acid solution is viscous.

physical property

Specific conductance

Normal temperature (25 ℃), H three PO four When the concentration of solution is 45% - 47%, the specific conductance is the maximum.^[2]

Solubility

Phosphoric acid and water can be mixed in an infinite proportion, but at the same time, the equilibrium reaction of dehydration hydration occurs with water. When the water content is lower than 5%, the dehydration gradually starts to generate pyrophosphoric acid.^[6]

Crystallizing point

The crystallization point of phosphoric acid is related to the water content. The crystallization point of anhydrous phosphoric acid is 42 ℃, and the mixed system of 62.5% phosphoric acid/37.5% water reaches a low eutectic point, and the crystallization point is - 85 ℃.^[6]

Generally, the mass fraction of commercially available phosphoric acid is 85%, and its crystallization point (freezing point) is 21 ℃. When it is lower than this temperature, it will precipitate hemihydrate (ice) crystals. Of course, phosphoric acid usually does not form (ice) crystals at temperatures above 10 ℃ or even below. This is because phosphoric acid has the feature of supercooling, that is, when the actual phosphoric acid on the market is lower than 21 ℃, it will deviate from its (ice) crystallization point and will not form (ice) crystals immediately. However, as long as such a low temperature is maintained for a period of time, phosphoric acid is easy to form crystals (ice) in a static state.

Phosphoric acid crystal belongs to physical change and Non chemical change 。 Chemical properties can't It is changed by crystallization, that is, the property of phosphoric acid is can't If it is affected by crystallization, as long as the temperature is given to melt or heated water is diluted and dissolved, it can still be Normal use 。

Crystallization characteristics: high phosphoric acid concentration, high purity and high crystallinity. According to experience, when the temperature is about 4 ℃ and the concentration is more than 85%, its crystallinity will increase. If it is accidentally mixed with crystalline phosphoric acid, it will cause immediate infection of phosphoric acid that did not form crystalline (ice), and the phosphoric acid will form crystalline (ice), and the phosphoric acid will form crystalline (ice) abnormally fast, leading to the formation of crystalline (ice) in most of the phosphoric acid storage containers. After the phosphoric acid (ice) crystal is formed, the phosphoric acid in the upper part becomes dilute, and the needle shaped crystal (ice) pure phosphoric acid is deposited in the lower part. According to experience, 75% phosphoric acid is also difficult to form (ice) crystals at a lower temperature (around 4 ℃), so it is recommended to use 75% phosphoric acid at a lower temperature.

Phosphoric acid (ice) crystal is like water ice, which is its own physical property and inherent attribute, and cannot be changed. Only proper preservation and treatment can prevent (ice) crystal formation. Commercial phosphoric acid contains 85% H three PO four A thick, concentrated solution of. Crystallization from concentrated solution forms semi hydrate 2H three PO four ·H two O (melting point 302.3K).

boiling point

Commercial phosphoric acid is a mixture of water, and its boiling point is related to the water content. The lower the water content, the higher the boiling point. The boiling point of 85% phosphoric acid is 158 ℃. The boiling point of anhydrous phosphoric acid is 261 ℃, and dehydration reaction occurs at this time to generate pyrophosphoric acid with higher boiling point.^[6]

chemical property

Phosphoric acid is ternary Intermediate acid , in three steps ionization , not easy volatilization , difficult to decompose, certain Oxidizability 。 have Commonness of acid 。^[3]

p K a1 ：2.12

p K a2 ：7.20

p K a3 ：12.36

（1） Concentrated phosphoric acid can be co heated with sodium chloride to generate hydrogen chloride gas (and potassium iodide 、 Sodium bromide And other similar reactions), which belongs to high boiling point acid to low boiling point acid:

Principle: non-volatile acid Volatile acid

（2） Phosphate radical Ions have strong coordination ability It can form soluble complexes with many metal ions. As Fe 3+ And PO four 3- Colorless soluble complex [Fe (PO four ) two ] 3- And [Fe (HPO four ) two ] - , use this property to analyze PO commonly used in chemistry four 3- Masking Fe 3+ Ions and concentrated phosphoric acid can dissolve tungsten, zirconium, silicon, iron silicide, etc. and form with them Complex 。

（3） When phosphoric acid is subjected to strong heat dehydration , generated in turn Polyphosphoric acid ， inorganpyrophosphate 、 Triphosphate And polymetaphosphoric acid. Triphosphoric acid has a chain structure, and poly metaphosphoric acid has a ring structure.

(4) It should be noted that, Concentrated hot phosphoric acid can corrode silicon dioxide , generating Heteropolyacid 。 Concentrated hot phosphoric acid can also decompose most minerals, such as chromite, rutile, ilmenite, etc.

phosphate

phosphate There are three categories: Orthosalt (including

）, acid salt monohydrogen phosphate (containing

）And dihydrogen phosphate (including

）。

The relationship between the three types of salt is:

(1) Solubility

Normal salt and monohydrogen salt: except for a few salts such as potassium, sodium and ammonium, the rest are insoluble in water, but soluble in strong acids.

Dihydrosalts: all are soluble in water.

(2) Mutual transformation

Drip lye into phosphoric acid. With the increase of lye, dihydrogen phosphate, monohydrogen phosphate and phosphate are successively generated.

Drop strong acid into the phosphate solution. With the increase of acid, monohydrogen phosphate, dihydrogen phosphate and phosphoric acid are successively generated.

（3） Ion coexistence Questions for

①

、

And

Cannot coexist.

②

、

And

Cannot coexist.

③

And

Cannot coexist (combined generation

）。

④

And

Coexistence,

and

Coexistence.

test

The reaction of phosphate and excessive ammonium molybdate in concentrated nitric acid solution produces yellowish ammonium molybdate phosphate crystals, which is a characteristic reaction for identifying phosphate ions:

application area

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Agriculture: Phosphoric acid is important for production phosphate fertilizer （ calcium superphosphate 、 Potassium dihydrogen phosphate Raw materials for the production of feed nutrients（ Calcium dihydrogen phosphate ）Raw materials.

Industry: Phosphoric acid is an important chemical raw material. Its main functions are as follows:

1. Treat the metal surface and generate insoluble phosphate film on the metal surface to protect the metal from corrosion.

2. And nitric acid It is mixed as a chemical polishing agent to improve the smoothness of metal surface.

3. Raw materials for production of washing products and pesticides phosphate 。

4. Production contains phosphorus Flame retardant Raw materials.

Food: Phosphoric acid is one of the food additives Acidizing agent 、 Yeast nutrient Coke contains phosphoric acid. Phosphate is also an important food additive, which can be used as a nutrient enhancer.

Medical Science: Phosphoric acid can be used to produce phosphorus containing drugs, such as Sodium glycerophosphate Etc.

security information

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Safety terminology	S1/2：Keep locked up and out of reach of children. Keep it locked and away from children.
	S7：Keep container tightly closed. Store in a tightly closed container.
	S16：Keep away from sources of ignition - No smoking. Keep away from ignition sources and no smoking.
	S24/25：Avoid contact with skin and eyes. Avoid skin and eye contact.
	S26：In case of contact with eyes, rinse immediately with plenty of water and seek medical advice. After eye contact, immediately flush with plenty of water and seek medical advice.
	S36/37/39：Wear suitable protective clothing, gloves and eye/face protection. Wear appropriate protective clothing, gloves and eye/face protection.
	S45：In case of accident or if you feel unwell, seek medical advice immediately (show the lable where possible). In case of an accident or feeling unwell, seek medical advice immediately (show the label if possible).
Risk terminology	R34：Causes burns. Causes burns.

Toxicological data

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acute toxicity	LD fifty	1530mg/kg (rat by mouth)
acute toxicity	LD fifty	2740mg/kg (rabbit percutaneous)
thrill	Rabbit transdermal	595mg (24h), severe irritation.
thrill	Rabbit transocular	119mg, severe irritation.
Subacute and chronic toxicity	Long term inhalation of animals	10.6mg/m three The serum protein content increased and the liver glycogen decreased.
Ecotoxicity	TLm	138mg/L (96h) (Mosquito fish)^[4]

test method

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GB 3149-92 Method

Mixed indicator: 0.1% Thymol blue 3 parts of solution and 0.1% Phenolphthalein Mix 2 parts of solution evenly.

1. Gravimetric method

Also called arbitration method, take 5g of sample (accurate to 0.0002g) and put it into a 100mL beaker, add 10mL of hydrochloric acid, cover the watch glass, boil for 10min, cool it, transfer it into a 500mL volumetric flask, add 10mL of hydrochloric acid, dilute it with water, and shake it well. Take 50mL of the solution and put it into a 500mL volumetric flask, then dilute it with water, shake it up, and it is the sample solution. Take 20mL of sample solution and put it into a 400mL beaker, dilute it to 100mL with water, add 50mL of quinomycolone test solution (TS-202), cover a watch glass, heat it on a hot plate until the temperature in the cup reaches 75 ℃± 5 ℃, keep it for half a minute or keep it warm in a water bath until the solution is layered (it cannot be heated by open fire, even when adding reagents or heating, it cannot be stirred and mixed evenly to avoid the formation of lumps), Rotate 3~4 times during cooling. Filter with No. 4 glass crucible with constant weight at 175~185 ℃ (or 240~260 ℃) in advance. First filter the supernatant, wash the precipitation with decanting method for 3~4 times, about 20mL of water each time. Filter the washing solution through the crucible, then transfer the precipitation to the glass crucible, continue to wash with water for 5~6 times, place the crucible in 175~185 ℃: bake in an oven for 45min (or bake in an oven at 240~260 ℃ for 15min), The glass crucible is cooled in a dryer to room temperature and weighed. Carry out blank test as per the determination procedure.

The phosphoric acid content is calculated as follows:

H three PO four (%)=0.04428×( G two - G one )/ G × 50/500 × 20/500 × 100 formula G 1. Amount of sample taken, g

G two ——Precipitation amount obtained when determining the sample, g;

G one ——Sediment weight measured in blank, g;

0.04428 - each gram of quinoline phosphomolybdate is equivalent to the mass of phosphoric acid, g.^[1]

2. Volumetry

Carry out the above gravimetric determination procedure to "rotate 3~4 times during cooling", and the following operations shall be carried out according to the following determination procedure. Filter with a filter covered with filter paper, absorbent cotton or paper pulp. First filter the supernatant. Wash the sediment for 3 to 4 times with pouring method, using about 25-30mL of water each time. Then transfer the sediment to the filter and continue to wash with water until it is free of acid (take about 20mL of washing solution, add one drop of mixed indicator and one drop of 0 25mol/L sodium hydroxide Liquid, until the color is similar to that of the same body of water). Transfer the sediment into the original beaker, and add carbon dioxide 100mL of water, stir well and precipitate, add about 0.5ml/L sodium hydroxide 10mL of standard solution, fully stir until the precipitation is dissolved, add 5 drops of mixed indicator, titrate with 0.25mol/L hydrochloric acid standard solution, and the end point is when the solution turns from purple to yellowish.

Carry out blank test with the same determination procedure.

The phosphoric acid content is calculated as follows:

H three PO four (%)= c ( V one - V two )×0.003769/ G ×50/500×20/500×100

Where c ——Concentration of sodium hydroxide standard solution, mol/L;

V one ——Sample consumption sodium hydroxide Volume of standard solution, mL;

V two ——Sample consumption hydrochloric acid Volume of standard solution, mL;

V ——In the blank test, 1mL hydrochloric acid standard solution is equivalent to sodium hydroxide Volume of standard solution, mL;

0.003769 -- mass of millimole phosphoric acid, g;

G is the amount of sample taken, g.

The allowable error for parallel determination of this method is within 0~2%.

FAO/WHO titration method Take 1.00g of sample and put it into a glass stoppered flask, add about 100mL of water, and then add Thymolphthalein Test solution (TS-248) 0.5mL, 1mol/L sodium hydroxide Liquid titration, each mL is equivalent to H three PO four 0.049g。^[1]

Hazard category

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Skin corrosion/irritation, Category 1B;

Serious eye injury/eye irritation, category 1.^[7]

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