QUANTITATIVE ANALYSIS. Before beginning a quantitative analysis the chemist must know, in part at least, the qualitative composition of the substance to be analyzed. This knowledge may be obtained by a special qualitative analysis, or, more frequently. from the results of numerous analyses of similar substances.
Methods of quantitative analysis which involve weighing (see BALANCE) are termed grarinictric. Methods that involve measuring the volumes of solutions arc termed volumetric. Finally, meth ods involving the decomposition of substances by means of an electric current are termed dee fro/nth..
As an illustration of the methods of gravi metric analysis, we may take the analysis of an alloy of silver and copper, such as is need for silver coins in the United States. If high-class weights and a balance are at the disposal of the analyst, not more than half a gram (less than one-fourth of a dime) is the most suitable weight to be taken of the alloy. if the weights or the balance is inferior, a larger amount must he taken, so that the errors of weighing may remain proportionately small. The alloy is dissolved in nitric acid, the insoluble residue (carbon and tin oxide) filtered off and weighed, and the fil trate is treated with hydrochloric acid to pre cipitate silver chloride, just as in qualitative work. In quantitative work, however, certain precautions must be taken in carrying out this simple operation. Thus, only a slight excess of hydrochloric acid must be added, since silver chloride is somewhat soluble in a large excess of that acid; the liquid must be vigorously stirred and warmed to cause the precipitate to assume a form in which it can be easily filtered and washed, etc. The silver chloride is then filtered off, dried, and weighed, proper corrections being made for the weight of the ash of the filter. The amount of silver in the alloy is then readily cal culated from the weight of silver chloride yielded. The filtrate from the silver chloride contains copper and usually a small amount of lead. The exact amount of copper contained in this filtrate may he best determined by electrolysis. For this purpose the filtrate is first evaporated to dryness, in order to get rid of the hydrochloric acid; the residue is taken up with dilute nitric acid, and the solution thus obtained is subjected to the action of an electric current passing between two carefully weighed platinum terminals immersed in the liquid. The copper is thus deposited in the metallic state on the electro-negative termi nal, while the lead is deposited in the form of lead dioxide on the terminal connected with the positive pole. The gain in weight of the ter minals gives directly the weight of copper and permits the calculation of the weight of the lead.
Another method, involving the fusion of sub stances by heat, and usually termed the "fire method," is applied chiefly to the determination of metals in ores, and is especially useful in the case of gold and silver ores. Thus, the amount of silver in an ore free from gold may be easily and quickly found by heating a weighed portion of the ore with metallic lead and a little fused borax in an oxidizing atmosphere. The lead melts, the ore floats on the surface, sulphur and arsenic are volatilized as oxides, the lead is partly oxidized, and the oxide of lead forms a liquid slag with most of the constituents of the ore. At the end of the operation a lead button
is obtained, containing the silver. This button is placed on a porous support made of bone-dust (calcium phosphate), and again heated in an oxidizing atmosphere. The lead melts and oxi dizes, part of the oxide passes off as gas and part sinks into the porous support, while the sil ver remains behind as a metallic button, which can be weighed. If gold is present, it is found and weighed with the silver, and then separated by a wet process.
Although gravimetrie methods are the more generally applicable, volumetric methods are much more commonly used in the everyday work of the technical analytical chemist. Hundreds of volumetric determinations are made daily in all great manufacturing centres for every one gravi metric determination. As an illustration of volu metric analysis, we may take a method used for the determination of iron in iron ores, and ap plicable to all iron ores found in the United States, except those containing titanium. The process depends on the fact that when a solution of potassium permanganate is added to an acid solution of iron in the ferrous state, the iron is changed into the ferric state, while the strongly colored permanganate is transformed into an al most colorless manganous salt, the volume of potassium permanganate solution thus decolor ized being proportional to the amount of ferrous iron present in the acid solution. This fact is made use of by the analyst in the follow-big man ner: He first determines the maximum volume of the given permanganate solution which can be completely decolorized by a known amount of iron. For this purpose, say, 300 milligrams of pure iron are dissolved in hydrochloric acid and some metallic zinc is added in order to make certain that all the iron is present as ferrous chloride, Feel, (and not as ferric chloride, The given permanganate solution is then slowly added from a burette to the solution of iron until the disappearance of the color has ceased to take place. The burette then shows what vol ume of the permanganate solution can he decol orized by 300 milligrams of iron dissolved as a ferrous salt. Suppose the volume of permanga nate solution thus measured is 40 cubic centi meters. Then it is evident that one cubic centi meter of the solution could he decolorized by 7.5 milligrams of iron. A weighed portion of the ore to he examined, say, 500 milligrams of it. is now treated in exactly the same manner as were the 300 milligrams of iron: i.e., the ore is dis solved in hydrochloric acid, its iron is carefully reduced to the fen ous state, and the perman ganate solution is slowly added from the burette until no more can be decolorized. Suppose the volume of the permanganate solution decolorized this time is 4l cubic centimeters. Then, since 7.5 milligrams of iron are required to decolorize every cubic centimeter of the permanganate solu tion, it is evident that the 500 milligrams of the ore must contain 307.5 (i.e., 7.5 X 41) milli grams of iron, and hence the ore is reported to contain 61.5 per cent. of iron.