_Standard Solutions._--If 40 _grm._ of pure caustic soda (NaHO) be dissolved in water, and a little tincture of litmus added, it will be coloured a bright blue. If hydrochloric acid be now added, drop by drop, the litmus will at last become purple, and a single drop more would turn it a bright red. At this point the liquid is neither acid nor alkaline, and if it be evaporated to dryness, nothing will be left but 58·5 _grm._ of common salt (NaCl), while 18 _grm._ of water will be formed and have escaped. We have therefore used exactly 36·5 _grm._ of pure HCl, and if we dissolve 40 _grm._ of caustic soda in 1 _litre_ of water, and 36·5 _grm._ of pure HCl in another, equal parts of these liquids will always exactly neutralise each other, forming nothing but common salt and water. It will be obvious that if we have a soda solution of the strength named, we can find the amount of hydrochloric acid in any solution of unknown strength, by seeing how much of it is required to neutralise, say, 10 _c.c._ (= 0·4 _grm._ soda) of the known solution. Instead of 40 _grm._ of caustic soda, we may take 56 _grm._ of potash to the _litre_, and it will exactly neutralise an equal volume of the hydrochloric solution containing 36·5 _grm._ If, again, we make a solution containing 49 _grm._ of pure sulphuric acid (SO_{4}H_{2}) per _litre_, it will neutralise an exactly equal volume of either the soda or the potash solution, thus being precisely equivalent to the HCl solution. Such solutions are called normal, and any normal acid solution will neutralise an equal volume of any normal alkali, and _vice versâ_. For many purposes normal solutions are too strong, and solutions containing 1/10 of the quantities required for normal solution are preferable; such solutions are called decinormal. All solutions containing known quantities of chemicals, and intended for use in volumetric analysis, are called Standard solutions.