Page:The New International Encyclopædia 1st ed. v. 01.djvu/586

ANALYSIS. carefully weighed platinum terminals immersed in the iiijuid. 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 calcuhxtion of the weight of the lead.

Another metliod, involving the fusion of sub- stances by heat, and usually termed the "fire method," is applied chiellj' 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 phosphfite), 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 gravimetric 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 ever}' 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 iise of by the analyst in the following man- ner: He first determines the maximum volume of the given permanganate solution which can be completely deeoloiized by a known amount of iron. For this purpose, say, 300 milligiams 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, FeCL (and not as ferric chloride, FeClj). 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 ])lace. The burette then shows what vol- ume of the permanganate solution can be 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 cvidenc that one cubic centi- meter of the solution could be decolorized by 7. .5 milligrams of iron. A weighed portion of the ore to be 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 ferrous 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 41 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., 7.5X41) milli- grams of iron, and hence the ore is reported to contain 01.5 per cent, of iron.

. Any physical property' which depends on the amount of sub- stance present, and is capable of measurement, may be used for quantitative determinations. Tints, the specific gravity of liquids, which can be readily determined with great accuracy, is ex- tensively used to determine the amount of the dissolved substance in pure or nearly pure solu- tions. In this manner the amotmt of alcohol, potassium or sodium hydro-xide, common salt, and, indeed, of all the more familiar salts con- tained in aqueous soltitions may be determined more readily than in anj- other way. For determinations of this kind, when no high degree of accuracy is required, the hydrometer is ex- tensively used in chemical laboratories. (See Hydrometer, and Alcoholometry. ) Among other properties used maj- be mentioned the coefficient of refraction, the optical rotatory power — much used in determining the strength of sugar solu- tions (see SuG.R), the intensity of the color or the degree of ojiacity of solutions and of liquids containing solids in suspension, the electrical conductivity, the boiling point of solutions, the melting point of solids, etc.

Analysis of Gases. The analysis of gases differs from that of solids and liquids in that it is more easy to measure than to weigh gases, and hence the results are usually given in per- centages by volume. For many gases reagents are known which absorb the gas readily and com- pletely. Thus, a mixture of carbon dioxide, ethy- lene, oxygen, carbon monoxide, and nitrogen may be analyzed by bringing a measured volume into contact with caustic potash (which alisorbs the carbon dioxide), then with fuming sulphuric acid (which absorbs the ethylene), then with an al- kaline solution of pyrogallol (which absorbs the oxygen ), then w-ith a solution of cuprous chloride (which absorbs the carbon monoxide), and noting the contraction caused by each treat- ment. The nitrogen remains behind unabsorbed. Hydrogen and marsli-gas are usually determined by combustion with oxygen. Gases very soluble in water, such as sulpliur dioxide, are absorbed in that liquid, and then the amount dissolved is determined by a volumetric process. Carbon dioxide in air offers a special case. As in nor- mal air only 3 parts in 10.000 are present, the ordinary process of measuring the volume before and after treatment with caustic potash requires special apparatus and great care to get good re- sults. Usually a large volume is treated with a measiu-cd quantity of a solution of barium hy- droxide of kno«n strength, a portion of the barium hydroxide being thus converted into in- soluble barium carbonate, and the rest estimated voluinctrically.

Mien the highest degree of accuracy in gas analysis is required, the gases must be confined over mercury; further, only solid absorbents