How phosphoric acid is prepared in laboratory and its properties

How phosphoric acid is prepared in laboratory can be seen as a way to get rid of this weak acid. The process begins with heating phosphoric acid, which causes it to break down into phosphorus and water vapor. These two gases are then reacted with calcium hydroxide and sodium carbonate respectively, in a series of addition-subtraction reactions that yield the triethyl phosphate product we all know. This in turn is decomposed by hydrolysis into chlorophosphonous acid and phosphoric anhydride, which is then used for further preparation of various chemicals like ethyldichlorophosphate.

Steps on how phosphoric acid is prepared in laboratory

1. Begin with an excess of phosphoric acid, concentrated to 85% H₃PO₄ by weight.

This excess is split between two reaction vessels. One half is heated to 60℃, while the other half is heated to ambient room temperature. It is important to note that heating phosphoric acid at this temperature will not cause the reaction to proceed any faster. The reaction is driven by a temperature differential between the two halves.

At 60℃, phosphoric acid decomposes into water and phosphorus:

H₃PO₄(l) → 3H₂O(g) + P(s)

At room temperature, the same reaction occurs but quickly comes to an equilibrium where each molecule of H₃PO₄ has only a small chance of being converted into H₂O and P before it finds another H₃PO₄ molecule to react with. The equilibrium constant for the reaction at these two temperatures is 7.92 x 10M at 60℃ and 2.83 x 10M at room temperature. That means that the reaction is much slower at 60C than it is at room temperature.

2. The water vapor produced by the first reaction is condensed and collected in the cold trap of a condenser cooled with liquid nitrogen. The Phosphorus is collected in a separate vessel filled with calcium hydroxide, a base commonly used to scrub sulfur compounds from smokestacks, and a third vessel filled with sodium carbonate, also known as soda ash or washing soda. This process works because byproducts of acid decomposition are usually weak bases, which react with bases to form salts.

3. After some time, the reaction vessel containing the water vapor is opened while the other two vessels are still cooled. The water evaporates and leaves behind phosphorous acid, a weak acid that dissolves in water:

H₃PO₄ → H₃PO₄(aq) + H+(aq)

The phosphorous acid dissolves in the water and reacts with the calcium hydroxide:

H₃PO₄(aq) + Ca(OH)₂(aq) → CaHPO₄(aq) + H₂O(l) + OH−(aq) [neutralization reaction]

The ammonium salt of phosphoric acid is precipitated and collected in a beaker filled with soda ash solution. This is a byproduct of the neutralization reaction and is considered dead weight.

After some time, after acid and base have been added to the three vessels, there is still a large excess of P in the waste phosphorous acid. The reaction vessel containing this excess P is removed from the apparatus and refilled with NaOH solution. After all three vessels have been refilled, only phosphorous nitrate remains inside of each one. This shows how phosphoric acid is prepared in laboratory.

Properties of phosphoric acid

1. Phosphoric acid is a weak base.

This is what keeps it from dissolving in water. The strong acid H₃PO₄ is also a base, but that is what will cause it to react with the calcium hydroxide and sodium carbonate to produce phosphorous acid.

2. Phosphoric acid can be made by adding ammonium chloride solution like this:

H₃PO₄(l) + 2NH₄Cl(aq) → H₃PO₄(aq) + H₂O(l) + Cl- (aq) [neutralization reaction]

The hydrochloric acid formed in this process can be used to scrub sulfur compounds off smokestacks, or used directly to make potassium sulfate.

3. Phosphoric acid is an important precursor to other chemicals.

It is used as a reactant in the synthesis of ethyldichlorophosphate, which can be used to make dyes and other organic compounds. Phosphoric acid is also used to make phosphorodithioic acid, which is the starting material for the production of dithianes and other compounds.

4. Phosphoric acid is used in the smell of food....

One well known use of phosphoric acid is as a flavor enhancer in foods. Certain flavoring compounds called organic acids are converted into phosphoric acids when the ammonium chloride solution for the neutralization reaction is added.

In conclusion, how phosphoric acid is prepared in laboratory, a weak acid is used in the decomposition of another weak acid. The decomposition process ends in the formation of an ammonium salt, which is collected in the waste phosphorous acid. Uses of these two byproducts are still being explored, but it will be important that they soon be developed into more useful products.