Found this interesting info on our favourite acids:
HYPOPHOSPHOROUS ACID
Boil water with phosphuret of barium. Part of the water is decomposed; a portion of the phosphorus combines with the hydrogen of the water, forming phosphureted hydrogen which escapes, and other portions produce hypophosphorous and phosphoric acids, by combining with its oxygen. These acids unite with baryta, the phosphate of baryta, which is insoluble, being left while the hypophosphite of baryta may be removed in solution by water.
To the solution of the hypophosphite gradually add diluted sulphuric acid, till no further precipitate appears; filter, that the sulphate of baryta formed in this manner may be separated; the solution contains the hypophosphorous acid.
The solution of the acid may be concentrated till it becomes viscid and crystalline. When subjected to an increased heat, the acid and water react upon each other; part of the acid acquiring oxygen from the water and also from another portion of acid which is decomposed, and producing phosphoric acid; while the phosphorus of the decomposed acid, along with the hydrogen of the water, form phosphureted hydrogen. All its salts are soluble; and it is a powerful deoxidating agent.
PHOSPHOROUS ACID
Phosphorous Acid is formed when phosphorus is inflamed in a smaller quantity of air than is necessary for its complete combustion, and even when phosphorus is exposed to the air at natural temperatures. It is mixed, however, with phosphoric or metaphosphoric acid. The process best adapted for preparing it in a pure state, was pointed out by Sir H. Davy. A piece of dry phosphorus is put into a tube retort, and bichloride of mercury in powder placed over it. On exposing the retort to heat, the phosphorus, as it rises in vapor through the bichloride, takes one proportion of chlorine from it, and a limpid fluid condenses in the receiver, a compound of chlorine and phosphorus. On mixing it with water. they mutually decompose each other, the chlorine combining with the hydrogen of a portion of the water, and forming hydrochloric acid, while the phosphorus takes the oxygen, and is converted into phosphorous acid; by heating the liquid till it becomes of a thick consistence, all the hydrochloric acid and most of the water are driven oft and the phosphorous acid, still combined with a portion of water, becomes a solid crystalline mass on cooling.
It has a sour taste and a disagreeable fetid smell. It reddens the vegetable blues, and when exposed to heat it is decomposed, a portion of phosphureted hydrogen being disengaged, and phosphoric acid remaining. It is a powerful deoxidating agent, and precipitates gold, silver, mercury, arid platinum in the metallic form.
HYDRIODIC ACID
Hydriodic acid gas is formed by the action of one part of phosphorus with ten of iodine and four of water. The iodine and water are put first into a very small glass-retort or flask, the phosphorus added, and a gentle heat applied with a spirit lamp. In a very short time, a brisk reaction commences, a slight explosion generally taking place within the retort from the heat produced inflaming a portion of phosphorus, and also from the disengagement of a little phosphureted hydrogen. Dense vapors are at the same time disengaged, and the hydriodic acid gas may be collected by displacement after these have been expelled. Water absorbs it as rapidly as hydrochloric acid gas, end it cannot be kept long over mercury, as this metal begins to act upon it whenever they come into contact, the mercury combining with the iodine, and leaving hydrogen gas. Phosphureted hydrogen is disengaged in considerable quantity towards the end of the operation; when it begins to come, it is recognized by the acid gas with which it is mixed producing with the air a whiter colored vapor than previously; the process should then be stopped to prevent it from accumulating. Fifty or a hundred grains of iodine, with the proper quantity of phosphorus and water, will be found quite sufficient, using a retort capable of containing about five or six ounces of water. Constant attention must be paid to this operation while it is going on.
A number of complicated changes take place during the preparation of this gas, from the reaction of the different substances mixed together and part of the newly formed products. Small cubical crystals may frequently be seen in the neck of the flask or retort employed; they consist of hydriodic acid and Phosphureted hydrogen, and are rapidly decomposed by water with effervescence, this fluid combining with the hydriodic acid. The hydriodic acid gas is produced by the iodine combining with the hydrogen of a portion of water which is decomposed, the oxygen uniting with the phosphorus.
Other processes for the preparation of hydriodic acid gas have been recommended, but there is none by which the beginner can prepare a small quantity more easily than that which has been described. There are several circumstances with respect to the Phosphureted hydrogen, and the other products that are formed at different periods of the process, according to the temperature applied, and the proportion of water employed, which require further investigation.
M. F. D’Arcet has proposed to prepare hydriodic acid gas in a very pure form by heating a solution of hypophosphorous acid, till it begins to give off Phosphureted hydrogen by decomposing water, after which it is to be cooled, and mixed with an equal weight of iodine. On subjecting the mixture to a gentle heat, the water affords hydrogen to the iodine, producing hydriodic acid gas, while its oxygen is transferred to the hypophosphorous acid.
