c0deblue
May 8th, 2004, 04:11 PM
A very early patent may provide clues to a possible method of obtaining high strength HNO<SUB>3</SUB> without distillation, heat, vacuum, or lab equipment of any kind apart from a suitable glass container. The use of phosphoric acid as a dehydrating agent for HNO<SUB>3</SUB> has been touched on in a few past posts, but the possibilities don't seem to have been really explored.
Background:
Patent # 1,008,690, awarded to Eberhard Brauer November 14, 1911, discloses methods for concentrating HNO<SUB>3</SUB> using phosphoric acid or arsenic acid as substitutes for H<SUB>2</SUB>SO<SUB>4</SUB> in a simple (non-reflux) distillation process. But it is what the patent states about the properties of phosphoric acid - not the described method itself - that suggests avenues for further investigation.
Quoting from the patent:
"Sulphuric acid as a dehydrating agent in concentrating dilute nitric acid by distillation possesses this disadvantage, that in addition to the water with which it chemically combines to a considerable extent it also unites with the nitric acid itself, with the result that the boiling point of the mixture is remarkably increased whereby a considerable decomposition of the nitric acid takes place in addition to an increased expenditure of heat, as compared with the simple distillation. ...
The use of acids of the phosphoric acid group, especially phosphoric and arsenic acid, as compared with the above, possesses the following advantages: They do not enter into any chemical combination with the nitric acid, but merely combine with the water. The production of a nitric acid that contains but little water, from a mixture of diluted nitric acid and phosphoric or arsenic acid, therefore does not require any kind of decomposition of any chemical compound, as is the case when employing sulphuric acid as a concentrating agent. ... It is, therefore, neither necessary to employ more heat than required for mere distillation, which means a savings in fuel, nor is there any fear of a decomposition of the nitric acid taking place. Hence it is possible to obtain as a product a nitric acid of a high percentage and free from nitric oxid."
The patent goes on to describe the progression of a simple distillation employing 1.7 S.G. (about 85%) phosphoric acid in a 2:1 ratio with 65% nitric acid. HNO<SUB>3</SUB> yield and concentrations are given as follows:
"On heating, nitric acid of nearly 100% strength is distilled off, the concentration decreasing as the distillation is continued. On stopping the distillation when 80% of the absolute nitric acid in the diluted nitric acid used have passed over, a nitric acid of 92% strength is obtained."
The patent also gives HNO<SUB>3</SUB> yields and concentrations for distillation using arsenic acid (of undisclosed concentration). These data are shown here for reference, but arsenic acid should probably be avoided due to the difficulties of obtaining it and dangers associated with its use:
"... During the distillation, the first fraction which passes at 97 percent strength is 30% of the pure nitric acid contained in the original diluted nitric acid. The first 86 percent of the quantity of pure nitric acid in the original dilute nitric acid used, when collected together, will result in an acid of 94 percent strength. The next 14 percent of the quantity will still contain 47 percent. Ultimately, pure water will pass. When so conducting the process that the distillation is stopped only when nothing but pure water passes, the entire quantity of nitric acid employed will be recovered in the form of nitric acid of 86.9 percent strength."
The idea:
The patent states that phosphoric acid "chemically combines" with water but not with nitric acid. I take this to mean that the phosphoric acid forms a true solution with water, but only a mechanical mixture with HNO<SUB>3</SUB>. Even though the method disclosed in the patent exploits this characteristic in connection with a distillation process, it seems logical that there should be an equal if not greater advantage if gravity separation rather than distillation is used as the concentrating method.
In a batch distillation process the pot solution strength continually declines as the product distills over, resulting in an ever-diminishing distillate concentration. If the reasoning above is valid, this should not be true of the proposed gravity separation - after thorough mixing, the phosphoric acid (at a suitable ratio) should simply take up all the water in the dilute HNO<SUB>3</SUB> and then slowly separate into a distinct layer, leaving high-concentration HNO<SUB>3</SUB> to be drawn off.
A rough approximation:
Initial estimates show that at the 2:1 ratio called for in the patent, the density of the resulting phosphoric acid solution (after taking up the water contained in the initial charge of dilute HNO<SUB>3</SUB>) is too close to the 1.51 SG density of pure HNO<SUB>3</SUB> to facilitate much separation. However, the differential increases to more workable levels as the ratio of phosphoric acid to nitric acid is increased.
At a 3:1 H<SUB>3</SUB>PO<SUB>4</SUB> to HNO<SUB>3</SUB> ratio, for example, the density of the phosphoric acid after taking up all the water is (very roughly) 1.6 SG, corresponding to a solution strength of about 76%. This 1.6/1.51 differential ought to be sufficient to effect distinct separation if allowed to stand (protected from light) for a few days. Naturally, waiting time could be shortened considerably if a centrifuge or acid-proof mechanical separator were available, but the object here is to come up with a method that doesn't require fancy equipment.
Note: The above approximation is based on the use of the commonly available ortho-phosphoric acid H<SUB>3</SUB>PO<SUB>4</SUB>, 85% strength. Results would differ for other acid forms and concentrations.
Obtaining the product:
If initial concentrations are known, it is a simple matter to work out the end product volumes and mark the separation container to facilitate drawing off. If concentrations are not known exactly, the demarcation line should be somewhat visible at the right viewing angle due to a slight difference in refractive index of the respective layers. The upper layer will be the concentrated HNO<SUB>3</SUB> end product, which can be carefully siphoned off. Alternatively, the bottom component could be drained off if the vessel is fitted with a bottom outlet, although in this case some adulteration of the HNO<SUB>3</SUB> will occur due to its contact with the phosphoric-acid-wetted-vessel-walls. Obviously, using a tall narrow vessel will make the process easier. The recovered phosphoric acid (which is valuable at about twice the cost of sulphuric acid) is reconcentrated to its original 1.7 S.G. by heating in the usual way (a hydrometer is helpful).
While admittedly not a very efficient or speedy process, a method such as this would have the advantage not only of being a completely non-decomposing process (provided initial mixing is done with care to minimize dehydration heating), but of opening new avenues of experimentation for those with limited equipment budgets.
Comments please, but be gentle if I'm completely off base! :)
Background:
Patent # 1,008,690, awarded to Eberhard Brauer November 14, 1911, discloses methods for concentrating HNO<SUB>3</SUB> using phosphoric acid or arsenic acid as substitutes for H<SUB>2</SUB>SO<SUB>4</SUB> in a simple (non-reflux) distillation process. But it is what the patent states about the properties of phosphoric acid - not the described method itself - that suggests avenues for further investigation.
Quoting from the patent:
"Sulphuric acid as a dehydrating agent in concentrating dilute nitric acid by distillation possesses this disadvantage, that in addition to the water with which it chemically combines to a considerable extent it also unites with the nitric acid itself, with the result that the boiling point of the mixture is remarkably increased whereby a considerable decomposition of the nitric acid takes place in addition to an increased expenditure of heat, as compared with the simple distillation. ...
The use of acids of the phosphoric acid group, especially phosphoric and arsenic acid, as compared with the above, possesses the following advantages: They do not enter into any chemical combination with the nitric acid, but merely combine with the water. The production of a nitric acid that contains but little water, from a mixture of diluted nitric acid and phosphoric or arsenic acid, therefore does not require any kind of decomposition of any chemical compound, as is the case when employing sulphuric acid as a concentrating agent. ... It is, therefore, neither necessary to employ more heat than required for mere distillation, which means a savings in fuel, nor is there any fear of a decomposition of the nitric acid taking place. Hence it is possible to obtain as a product a nitric acid of a high percentage and free from nitric oxid."
The patent goes on to describe the progression of a simple distillation employing 1.7 S.G. (about 85%) phosphoric acid in a 2:1 ratio with 65% nitric acid. HNO<SUB>3</SUB> yield and concentrations are given as follows:
"On heating, nitric acid of nearly 100% strength is distilled off, the concentration decreasing as the distillation is continued. On stopping the distillation when 80% of the absolute nitric acid in the diluted nitric acid used have passed over, a nitric acid of 92% strength is obtained."
The patent also gives HNO<SUB>3</SUB> yields and concentrations for distillation using arsenic acid (of undisclosed concentration). These data are shown here for reference, but arsenic acid should probably be avoided due to the difficulties of obtaining it and dangers associated with its use:
"... During the distillation, the first fraction which passes at 97 percent strength is 30% of the pure nitric acid contained in the original diluted nitric acid. The first 86 percent of the quantity of pure nitric acid in the original dilute nitric acid used, when collected together, will result in an acid of 94 percent strength. The next 14 percent of the quantity will still contain 47 percent. Ultimately, pure water will pass. When so conducting the process that the distillation is stopped only when nothing but pure water passes, the entire quantity of nitric acid employed will be recovered in the form of nitric acid of 86.9 percent strength."
The idea:
The patent states that phosphoric acid "chemically combines" with water but not with nitric acid. I take this to mean that the phosphoric acid forms a true solution with water, but only a mechanical mixture with HNO<SUB>3</SUB>. Even though the method disclosed in the patent exploits this characteristic in connection with a distillation process, it seems logical that there should be an equal if not greater advantage if gravity separation rather than distillation is used as the concentrating method.
In a batch distillation process the pot solution strength continually declines as the product distills over, resulting in an ever-diminishing distillate concentration. If the reasoning above is valid, this should not be true of the proposed gravity separation - after thorough mixing, the phosphoric acid (at a suitable ratio) should simply take up all the water in the dilute HNO<SUB>3</SUB> and then slowly separate into a distinct layer, leaving high-concentration HNO<SUB>3</SUB> to be drawn off.
A rough approximation:
Initial estimates show that at the 2:1 ratio called for in the patent, the density of the resulting phosphoric acid solution (after taking up the water contained in the initial charge of dilute HNO<SUB>3</SUB>) is too close to the 1.51 SG density of pure HNO<SUB>3</SUB> to facilitate much separation. However, the differential increases to more workable levels as the ratio of phosphoric acid to nitric acid is increased.
At a 3:1 H<SUB>3</SUB>PO<SUB>4</SUB> to HNO<SUB>3</SUB> ratio, for example, the density of the phosphoric acid after taking up all the water is (very roughly) 1.6 SG, corresponding to a solution strength of about 76%. This 1.6/1.51 differential ought to be sufficient to effect distinct separation if allowed to stand (protected from light) for a few days. Naturally, waiting time could be shortened considerably if a centrifuge or acid-proof mechanical separator were available, but the object here is to come up with a method that doesn't require fancy equipment.
Note: The above approximation is based on the use of the commonly available ortho-phosphoric acid H<SUB>3</SUB>PO<SUB>4</SUB>, 85% strength. Results would differ for other acid forms and concentrations.
Obtaining the product:
If initial concentrations are known, it is a simple matter to work out the end product volumes and mark the separation container to facilitate drawing off. If concentrations are not known exactly, the demarcation line should be somewhat visible at the right viewing angle due to a slight difference in refractive index of the respective layers. The upper layer will be the concentrated HNO<SUB>3</SUB> end product, which can be carefully siphoned off. Alternatively, the bottom component could be drained off if the vessel is fitted with a bottom outlet, although in this case some adulteration of the HNO<SUB>3</SUB> will occur due to its contact with the phosphoric-acid-wetted-vessel-walls. Obviously, using a tall narrow vessel will make the process easier. The recovered phosphoric acid (which is valuable at about twice the cost of sulphuric acid) is reconcentrated to its original 1.7 S.G. by heating in the usual way (a hydrometer is helpful).
While admittedly not a very efficient or speedy process, a method such as this would have the advantage not only of being a completely non-decomposing process (provided initial mixing is done with care to minimize dehydration heating), but of opening new avenues of experimentation for those with limited equipment budgets.
Comments please, but be gentle if I'm completely off base! :)