VOLUMETRIC ANALYSIS? in chemis try, analysis consisting in submitung the sub stance to be estimated to certain characteristic reactions—the chemist employing for such re actions liquids of lcnown strength, and, from the quantity of liquid employed to induce the reaction, determining the weight of the sub stance to be estimated by means of the laws of equivalence. The idea of this method sug gested itself first to Gay-Lussac in considering how most readily to determine the amount of silver in an alloy of silver and copper; but the method itself did not come into general use till a considerable time afterward. The liquid reagents of known strength are called standard solutions; and the amount employed may be es timated either by weight or by volume, but the latter being the easiest of application, is uni versally employed—hence the name Volu metric Analysis. In order that a reaction may be applicable in Volumetric Analysis, it must satisfy these two conditions: (1) It must not occupy much time; (2) the termination of the reaction must be easily recognized and unmis takable to the eye. These conditions litnit the number of volumetric processes. In addition to the ordinary chemical apparatus, Volumetric Analysis requires graduated glass vessels of different kinds for measurement of the stand ard solutions. Of these the most essential are: (1) Pipettes, glass vessels of the form of Figs. 1 and 2, intended for the delivery of the stand ard solution: Fig. 1 shows a vessel provided with a single mark upon the neck, while Fig. 2 shows one divided and graduated through its whole length into cubic centitnetres (c.c.), ac cording to French scale; (2) flasks graduated for the contents in various sizes from one tenth of a litre to five litres, and used for prep aration of standard solutions; (3) burettes, or graduated tubes for measuring liquids used in an analysis. The best burette for general pur poses is known as Mohr's burette (see Fig. 3) ; and its lower part is attached to an India-rub ber tube and spring-clamp or clip (Quetsch Hahn). Its principal advantages over other forms are, that its constant upright position en ables the operator at once to read off the num ber of degrees of standard (or test) solution used for any analysis, while the quantity of fluid to be delivered can be most accurately regulated by the pressure of the thumb and finger on the clamp; moreover, as it is not held in the hand, no error is likely to arise in the measurement from the heat of the operator's hand. Its greatest drawback is that it cannot be used for those test-solutions which decom pose India-rubber.
The standard solutions, lcnown also as test or titrated solutions (from French titre, which signifies the standard of a coin), may be divided into: (1) Such as are immediately prepared by weighihg a substance of known composition, dissolving it, and diluting it to the required vol tune ; (2) such as are prepared by approximate mixture and subsequent exact analysis. The greatest care is indispensable regarding both the graduation of the measuring instruments and the strength and purity of the standard solutions, which must be protected from evap oration and other hurtful influences by being kept in bottles of one or two litres capacity, provided with well-ground stoppers.
Voltunetric 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 meas ured by the quantity of acid or base 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 quantity of chlorine, bromine, io dine or oxygen to which it is equivalent (re garded as oxidant) ; or by the quantity of chlor ine, bromine, iodine or oxygen which it requires to pass from a lower to a higher stage of oxida tion: the chief oxidizing agents are permangan ate 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 definite combination. We have space for an example of only one of these three forms; and, selecting the last for its historic in terest in its application to the determination of silver, we give Sutton's account of the process: "Suppose that it is desirable to know the quantity of pure silver contained in a coin. 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 lcnown fact that chlorine combines with silver in the presence of other metals to form chlor ide of silver, which is insoluble in nitric acid. The proportions in which the combination 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 eq. chloride of soditun — in so much distilled water as will exactly malce up 1,000 grains by meas ure, 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 so lution 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 operator sees at once the number of grains that have been necessary to produce the complete decom position. For instance, suppose the quantity used was 520 grains; all that is necessary to be done is to multiply 0.0108 grain by 520, which shows the amount of pure silver present to be 56.16 grains. By volumetric analysis as com pared with ordinary analysis, much time, labor and expense are saved; at the loss, however, often of due accuracy, unless the greatest care be talcen that the standard solutions are of proper strength, and the instruments accurately (graduated. An analysis can thus be completed m 15 minutes that would formerly have occu pied a day or more. Independently of its ap plication to pure chemistry, it facilitates to a great extent the chemical analysis of wine, of waters, of manures, soils, etc. Consult Mohr, (Lehrbuch der Chemischen-Titriennethode); Rieth, (Volurnetrische Analysis' and other handbooks of analytical chenustry.