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Sulfuric acid and phosphoric acid are both very important chemicals and are connected in many ways. The wet process production of phosphoric acid requires sulfuric acid and is the greatest connection. Diluted sulfuric acid and phosphoric acid can both be used as catalysts for the same chemical reactions. They have such similar properties that the United Nations and North American identification number for warnings and hazards are the same. Having similar properties leads to other major connections as well.
The majority of the sulfuric acid consumed is used in the fertilization industry. This is not because sulfuric acid is good for plants but because it is used to produce phosphoric acid. In the wet process, phosphorus-rich rocks are mined and mixed with sulfuric acid. This releases gas and phosphoric acid in lower concentrations. The phosphoric acid is eventually added to soil to make fertilizer that is rich in phosphorus.
Common identification techniques, like color or odor, are useless for separating sulfuric acid and phosphoric acid. The properties are so similar, they are often classified in the same category of chemicals. Both are colorless liquids that are explosive when added improperly to water. They are also corrosive and are strong oxidizing agents when exposed to materials.
Acids act as catalysts, meaning they help a reaction go faster by drawing certain groups of atoms together. Sulfuric acid and phosphoric acid have a similar size, shape, and electric charge. They act nearly identically as catalysts — in fact, many chemistry classes will mimic industrial processes that normally call for sulfuric acid by mixing phosphoric acid instead, which is safer. Organisms often use phosphoric acid, making it much safer than sulfuric acid when digested.
A property of acids, called disassociation, determines the number of times an acid breaks ups. Sulfuric acid is diprotonic, meaning it separates two times, and phosphoric acid is triprotonic, meaning it separates three times. This allows both to be used as buffers, meaning they can stabilize the acid properties of a solution. Sulfuric acid and phosphoric acid are used because they disassociate multiple times.
In biology, sulfuric acid can often perform the same tasks as phosphoric acid. This connection comes from ancient organisms that used sulfuric acid; modern organisms use phosphoric acid because it is less corrosive. Experiments are done to test this connection and understand why sulfuric acid is better at certain tasks, like breaking down food. In some instances, sulfuric acid has become toxic to organisms and phosphoric acid works much better. In other experiments, the results are surprising because sulfuric acid does not cause harm and it is does the task quicker than phosphoric acid.