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What Is the Connection between Sulfuric Acid and Phosphoric Acid?

By Richard Nelson
Updated May 21, 2024
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The most important connection between sulfuric acid and phosphoric acid is usually that one is required to make the other: phosphoric acid is created when sulfuric acid interacts with tricalcium phosphate, a chemical compound widely present in the soil. In many of the most crucial respects, the acids are very similar, though the sulfuric version tends to be much more strongly acidic and is often more corrosive and potentially hazardous as a consequence. Researchers and students often use the weaker phosphoric version in simulated labs and tests because it behaves almost identically in most circumstances without being as potent. Particularly when diluted, sulfuric acid and phosphoric acid can both be used as catalysts for the same chemical reactions. They have such parallel properties that their United Nations and North American identification numbers for warnings and hazards are the same.

Chemical and Physical Similarities

The two acids have very similar chemical structures when viewed on the microscopic level. The primary difference between them is a hydrogen atom as well as the core acidic element. The chemical formula for sulfuric acid, for instance, is H2SO4; it is made entirely of hydrogen, sulfur, and oxygen. Phosphoric acid, by contrast, carries the formula H3PO4. It has an additional hydrogen and contains phosphorous instead of sulfur. Phosphorous and sulfur are really closely related, though. When sulfur interacts with the right components and under the right circumstances, it can actually turn into phosphorous. This connection impacts almost every arrangement of the element.

Scientists have a couple of different ways of immediately differentiating acids, some of which are simpler than others. Color, consistency, and odor are usually some of the first hints, but in all of these respects, sulfuric and phosphoric acids are identical. In fact, the properties are so similar that these two 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 most materials, particularly metals.

Use in Titration

Acids act as catalysts, which means that 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.

Use in Manufacturing and Industry

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.

Corrosiveness Considerations

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.

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Discussion Comments
By anon1007448 — On Jul 12, 2022

The article above states, "When sulfur interacts with the right components and under the right circumstances, it can actually turn into phosphorous." This statement is completely false. Unless a nuclear reaction is occurring, the sulfur cannot become phosphorous. A chemical reaction may occur in which the sulfur is replaced by phosphorous.

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