Aqua Destillata (U. S. P.)—Distilled Water.
FORMULA: H2O. MOLECULAR WEIGHT: 17.96.
Preparation.—"Water, one thousand volumes (1000 vol.), to make eight hundred volumes (800 vol.). Distill the water from a suitable apparatus provided with a block-tin or glass condenser. Collect the first one hundred volumes (100 vol.), and throw this portion away. Then collect eight hundred volumes (800 vol.) and keep the distilled water in glass-stoppered bottles, rinsed with hot distilled water immediately before being filled"—(U. S. P.).
Description and Tests.—"A colorless, limpid liquid, without odor or taste, and perfectly neutral to litmus paper. The transparency of distilled water should not be affected, nor should any color be imparted to it, by test solutions of hydrogen sulphide or ammonium sulphide (absence of metallic impurities), or by those of barium chloride (sulphates), silver nitrate (chlorides), ammonium oxalate (calcium), or mercuric chloride (ammonia); nor should its transparency be affected when mixed with twice its volume of calcium hydrate T.S. (absence of carbonic acid). It should give no reaction for nitrates or nitrites when tested as described under Water (see Aqua). When 1000 Cc. of distilled water are evaporated on a water-bath to dryness, no residue should remain. On heating 100 Cc. of distilled water acidulated with 10 Cc. of diluted sulphuric acid, to boiling, and subsequently adding 1 Cc. of centi-normal potassium permanganate V.S., the color of the liquid should not be completely destroyed by boiling for 10 minutes, nor by afterwards setting the vessel aside, well covered, for 10 hours (absence of organic or other oxidizable matters)"—(U. S. P.).
The demand of the U. S. P. that 1000 Cc. of distilled water, evaporated on a water-bath to dryness, should leave no residue, is probably impossible to strictly comply with. The amount of residue which good distilled water may leave upon evaporation, should not exceed 1 part in 100,000 (Am. Jour. Pharm., 1896, p. 187). Distilled water is apt to become contaminated with fresh water algae (confervae).
In many pharmaceutical and chemical processes, distilled water is very essential, while in others, pure spring or river, or rain water, will be sufficient. The reason for throwing away the first 100 parts which are distilled over, is that any volatile principles which may be present, as ammonia, carbonic acid gas, etc., and which may pass over with the first portions, may be removed. The last 100 parts, left in the still, contain the residual impurities, and if allowed to pass over might give to the product an empyreumatic odor and taste. Carbon dioxide will be absorbed by distilled water if the latter be exposed to the atmosphere for any considerable time. In this case lime-water will occasion turbidity in the distilled water. Distilled water is nearly always employed in making the medicated waters, in diluting acids, and in making solutions of various salts, as the alkaloidal salts, silver nitrate, permanganate of potassium, etc. For other uses, see Aqua.