Aqua

It is customary for distilled water alone to be used for 'aqua' by the best dispensing chemists, but in Great Britain Insurance Act dispensing has resulted in a rule being formulated to the effect that dispensers shall use tap-water for 'aqua.' Distilled water is distinguished from natural water by containing no solids, and it should be free from ammonia and other impurities. Supplies of distilled water are generally obtained from wholesale houses, but it should never be stocked without examination, in case it be (as has happened) the condensed vapour from steam-heating pipes or steam boilers, consequently liable to contain impurities, of which ammonia and nitrites are the most common, the latter giving very strange results in mixtures containing iodides. Moreover, such distilled water has a tendency to become viscous, owing to the formation of thread-like organisms. The best plan is to distil water as it is required, adding to every 5 gallons of water in the still 10 grains of potassium permanganate (or a drachm of potassium bichromate) and a drachm of sulphuric acid. This ensures the destruction of organic matter, the products of which remain in the still.

For uniformity and elegance in dispensing distilled water is necessary.

Aqua Fontana

The variation of calcareous matter in different natural waters is alone a sufficient reason for ex-eluding them from the dispensing-counter, because some mixtures compounded with such water differ in appearance according to the amount of calcareous matter contained in the water. The following are examples of what may be expected when distilled water and tap-water are used indifferently.

Tinct. lavand. co. gives a bright mixture with distilled, but a muddy one with tap, water. Tinct. cardamom, co. produces with distilled water a reddish-brown colour, but with tap-water a brilliant crimson, as if ammonia had been added.

Liq. arsenicalis gives a precipitate of calcium carbonate with tap-water, which will probably be regarded by any nervous patient who knows what is in the mixture as the arsenic imperfectly dissolved, if it has been previously obtained without such a deposit owing to the use of distilled water.

Liquor hydrargyri perchloridi gave a lot of trouble until the 1898 Pharmacopoeia removed the ammonium chloride from it. The older solution contained a double salt which reacted with earthy carbonates, giving a precipitate of mercuric oxide, so that when diluted with tap-water the old solution deposited. The late Mr. W. Martindale showed that this does not happen with solution of mercuric chloride alone, and his suggestion was adopted by the B.P. authorities.

Such prescriptions as the following are occasionally seen at the dispensing-counter:

It is sometimes advisable to boil distilled water, so as to get rid of carbonic acid, which it may contain, and which causes precipitates in Nos. I. and II. In dispensing No. III. the rectified spirit should be mixed with the water before the acetate of lead is dissolved in it. The spirit furthers the expulsion of the air and carbonic-acid gas, and so prevents precipitation of lead as carbonate.

If the infusion of gentian in No. IV. be made with tap-water, the result will be that the calcium bicarbonate present in tap-water, which is only deposited on prolonged boiling, reacts with the ammonium carbonate with precipitation of calcium carbonate, which makes the mixture slightly turbid.