METRIC," in Watts's Dictionary of Ghemistry, vol. 1. p. 259, where the method is fully explained, and as an example, the preparation of a standard solution of sulphuric acid containing t grammes of hydrated sulphuric acid in 1 liter is given. It is obviously essential that the greatest care must be taken both with respect to the graduation of the measuring instruments and the strength and purity of the standard solutions, which must he protected from evaporation and other hurtful influences by. being kept in bottles of 1 or 2 liters' capacity, provided with well-ground stoppers.
Volumetric methods are usually classified as follows. according to.the principles on which they are based—(1) Analysis by saturation, when the quantity of a base or an acid Is measured by the quantity of acid or base which is required for exact saturation—a point to be determined by test-papers, tincture of litmus, etc. (2) Analysis by oxidation and reduction, when the quantity of the substance to be determined is found by the quan tity of chlorine, bromine, iodine, or oxygen to which it is equivalent (regarded as oxi dant), or by the quantity of chlorine, bromine, iodine, or oxygen which it requires to pass from a lower to a higher stage of oxidation. The chief oxidizing agents are per manganate of potash and bichromate of potash; while the reducing agents chiefly used are protoxide of iron and hyposulphite of soda. (3) Analysis by precipitation, when the determination of a substance is effected by precipitating it in Some insoluble and defi nite combination. Our limited space does not admit of our giving an example of more than one of these forms of analysis, and from its historic interest we shall select the last, In its application to the determination of silver. We shall borrow Mr. Sutton's account of this process. " Suppose," he observes, "that it is desirable to know the quantity of pure silver contained in a shilling. The coin is first dissolved in nitric acid, by which means a bluish solution containing silver, copper, and probably other metals, is obtained. It is a known fact that chlorine combines with silver in the presence of other metals to form chloride of silver,which is insoluble in nitric acid. The proportions in which the com
bination takes place are 35.5 of chlorine to every 108 of silver; consequently, if a standard solution of pure chloride of sodium is prepared by dissolving 58.5 grains of the salt—i.e., 1 eq. sodium ( 23) plus 1 eq. chlorine.( = 35.5) or 1 cq. chloride of sodium—in so much distilled water as will exactly make up 1000 grains by measure, every single grain of this solution will combine with 0.0108 of a grain of pure silver to form chloride of silver, which precipitates to the bottom of the vessel in which the mixture is made. In the process of adding the salt. solution to the silver, drop by drop, a point is at last reached when the precipitate ceases to form. Here the process must stop. On looking care fully at the graduated vessel from which the standard solution has been used, the oper ator sees at once the number of grains that have been necessary to produce the complete decomposition. For example, suppose the quantity used was 520 grains; all that is necessary to be done is to multiply 0.0108 grains by 520, which shows the amount of pure silver present to be 56.16 grains." By volumetric as compared with ordinary anal ysis, a large amount of time, labor, and therefore of expense, is saved; at the loss, how ever, often of due accuracy, unless the greatest care be taken that the standard solution's are of due strength, and the instruments accurately graduated. An analysis can thus be completed in a quarter of an hour that would formerly have occupied a day or more. Independently of its application to pure chemistry, it facilitates to a great extent the chemical analysis of urine (on which subject see the English translation of Neubauer and Vogel On the Thine, published by the New Syd. Soc.), of waters (on which see Parkes On Hygiene), of manures, soils, etc.; and its processes have been freely intro duced in the British Pharmacopoeia. The standard book on this subject is that of Mohr, a German chemist; the English reader may consult the text-books of Scott of Dublin, and Sutton of Norwich, and various memoirs in the Chemical News.