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Why do we add sulphuric acid during the COD test?
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Kitani
Why do we add sulphuric acid during the COD test?
Two important factors are involved in this question. One is that concentrated sulphuric acid has a very high density compared to water. Density = 1.84 g/cc (water = 1.00 of course) and secondly the heat of hydration of conc. sulphuric acid is quite large.
So when you add acid to water it immediately begins sinking to the bottom of the vessel all the while becoming more diluted. A lot of heat is generated in the dilution reaction but it is spread out over the height of the water column and generally causes no trouble. (Just don’t start with hot water.) Preferably the water vessel should be relatively deep to allow distribution of the heat over a greater height.
In the reverse situation when water is added to conc. H2SO4, because of density differences, the water floats on the acid. Where the two are touching and mixing is poor, the temperature can get above boiling thus causing a steam explosion of boiling water and acid. Very dangerous.
The total heat generated is the same in either case but it is a question of distribution.
This exothermic reaction can also be quite serious with concentrated phosphoric acid and similar precautions need to be taken. Not true with concentrated HCl though, as even the most concentrated acid is only 38% HCl, so it is already diluted. Some temperature rise will occur but nothing serious. You cannot obtain 100% HCl as a liquid at room temperature and pressure.
Two important factors are involved in this question. One is that concentrated sulphuric acid has a very high density compared to water. Density = 1.84 g/cc (water = 1.00 of course) and secondly the heat of hydration of conc. sulphuric acid is quite large.
So when you add acid to water it immediately begins sinking to the bottom of the vessel all the while becoming more diluted. A lot of heat is generated in the dilution reaction but it is spread out over the height of the water column and generally causes no trouble. (Just don’t start with hot water.) Preferably the water vessel should be relatively deep to allow distribution of the heat over a greater height.
In the reverse situation when water is added to conc. H2SO4, because of density differences, the water floats on the acid. Where the two are touching and mixing is poor, the temperature can get above boiling thus causing a steam explosion of boiling water and acid. Very dangerous.
The total heat generated is the same in either case but it is a question of distribution.
This exothermic reaction can also be quite serious with concentrated phosphoric acid and similar precautions need to be taken. Not true with concentrated HCl though, as even the most concentrated acid is only 38% HCl, so it is already diluted. Some temperature rise will occur but nothing serious. You cannot obtain 100% HCl as a liquid at room temperature and pressure.
Most Chemical Oxygen Demand uses potassium dichromate for the oxidation of carbon compounds containing samples. These carbon compounds are oxidized to carbon dioxide by potassium dichromate in a strongly acidic environment, catalyzed by mercury and/or silver salts. Sulfuric acid is the acidifying agent.
Most Chemical Oxygen Demand uses potassium dichromate for the oxidation of carbon compounds containing samples. These carbon compounds are oxidized to carbon dioxide by potassium dichromate in a strongly acidic environment, catalyzed by mercury and/or silver salts. Sulfuric acid is the acidifying agent.
Two important factors are involved in this question. One is that concentrated sulphuric acid has a very high density compared to water. Density = 1.84 g/cc (water = 1.00 of course) and secondly the heat of hydration of conc. sulphuric acid is quite large.
So when you add acid to water it immediately begins sinking to the bottom of the vessel all the while becoming more diluted. A lot of heat is generated in the dilution reaction but it is spread out over the height of the water column and generally causes no trouble. (Just don’t start with hot water.) Preferably the water vessel should be relatively deep to allow distribution of the heat over a greater height.
In the reverse situation when water is added to conc. H2SO4, because of density differences, the water floats on the acid. Where the two are touching and mixing is poor, the temperature can get above boiling thus causing a steam explosion of boiling water and acid. Very dangerous.
The total heat generated is the same in either case but it is a question of distribution.
This exothermic reaction can also be quite serious with concentrated phosphoric acid and similar precautions need to be taken. Not true with concentrated HCl though, as even the most concentrated acid is only 38% HCl, so it is already diluted. Some temperature rise will occur but nothing serious. You cannot obtain 100% HCl as a liquid at room temperature and pressure.
Two important factors are involved in this question. One is that concentrated sulphuric acid has a very high density compared to water. Density = 1.84 g/cc (water = 1.00 of course) and secondly the heat of hydration of conc. sulphuric acid is quite large.
So when you add acid to water it immediately begins sinking to the bottom of the vessel all the while becoming more diluted. A lot of heat is generated in the dilution reaction but it is spread out over the height of the water column and generally causes no trouble. (Just don’t start with hot water.) Preferably the water vessel should be relatively deep to allow distribution of the heat over a greater height.
In the reverse situation when water is added to conc. H2SO4, because of density differences, the water floats on the acid. Where the two are touching and mixing is poor, the temperature can get above boiling thus causing a steam explosion of boiling water and acid. Very dangerous.
The total heat generated is the same in either case but it is a question of distribution.
This exothermic reaction can also be quite serious with concentrated phosphoric acid and similar precautions need to be taken. Not true with concentrated HCl though, as even the most concentrated acid is only 38% HCl, so it is already diluted. Some temperature rise will occur but nothing serious. You cannot obtain 100% HCl as a liquid at room temperature and pressure.
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Most Chemical Oxygen Demand uses potassium dichromate for the oxidation of carbon compounds containing samples. These carbon compounds are oxidized to carbon dioxide by potassium dichromate in a strongly acidic environment, catalyzed by mercury and/or silver salts. Sulfuric acid is the acidifying agent.
Most Chemical Oxygen Demand uses potassium dichromate for the oxidation of carbon compounds containing samples. These carbon compounds are oxidized to carbon dioxide by potassium dichromate in a strongly acidic environment, catalyzed by mercury and/or silver salts. Sulfuric acid is the acidifying agent.
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