YSIS; and HYDROGEN IONS, DETERMINATION OF.) Neutralization of Acids by Bases.—In the neutralization of an acid by the gradual addition of a base, the essential reaction consists in the combination of hydrogen ions with hydroxyl ions until the concentration of both reaches the value for pure water, viz., gram-equivalents per litre (25°), which condition is shown by suitable indicators (q.v.). The constancy of the heat of neutralization of the strong acids by the strong bases is ex plained in this way (see THERMOCHEMISTRY). If one equivalent of a base BOH is added to a solution which contains one equivalent of each of two different acids and the rela tive proportions of the two acids which are neutralized afford a measure of the so-called relative affinities of the acids for the base. Because of such differences stronger acids tend to displace weaker acids from the corresponding salts. Such displacement effects, however, are not always the result of a difference in the affinities of the acids, for complications may occur in consequence of the removal of one of the resultant substances from the sphere of action in the form of an insoluble solid or a volatile gas; e.g., the displacement of nitric acid by the relatively very weak acid hydrogen sulphide in accordance with is directly due to the insolubility of silver sulphide. The basic affinities of a pair of acids can in fact only be derived from a determination of the relative amounts which are neutralized by a base when the system is homogeneous. If x is the fraction of the acid HA, which is neutralized by the base, BOH, in the above solution, the resulting equilibrium can be represented by and the evaluation of x, which can be effected by a variety of physical methods, gives us the ratio x/(I —x) which expresses the affinity of relatively to When such affinity values, re ferred to an acid chosen as standard, are compared, it is found that the acids form a series which is essentially the same as the series given by the relative conductivities and catalytic activities. It can be shown that this experimental result is generally consis tent with the hydrion theory of acids for the application of the mass law to the equilibria which are involved in the above men tioned competition of two acids for a base shows that the affinity ratio x/(i —x) is the ratio of the degrees of ionization of the two acids. The fact that x/(i —x) is independent of the nature of the base is also explained at the same time. Similar considerations apply to the relative strengths of bases. At a given concentration their characteristic properties can be explained in terms of the hydroxyl-ion concentration. Since degrees of ionization vary with the concentration, it is convenient to eliminate the latter. This can be done by comparing the strengths of acids and bases in terms of the respective ionization constants.
Many means are employed. The chemist may work under high pressures or in almost complete vacua, at temperatures approach ing absolute zero and those of melting tungsten. It is obvious then, that the apparatus employed by the chemist will vary with the work in hand. He needs porcelain, which will stand sudden and extreme changes of temperature; glass that is insoluble in the majority of the reagents he uses, which will stand great temperatures, and possesses high mechanical strength ; and fash ioned crucibles, dishes and other devices from platinum, fused quartz, and special alloys. The balances he employs are of the highest precision. He utilizes the wave length of light in measure ment. The spectrograph, the microscope, the polariscope are familiar instruments. Burettes enable him to measure by the drop, while sensitive electric instruments have become essential in determining the concentration of hydrogen ions, the modern method of determining acidity. He uses the burning flame to judge contents by colour or to identify the lines in the spectro scope; electric conductivity becomes the measure of pureness of the water employed in analysis ; and the complexity of the com pounds and mixtures with which the chemist has to deal, fre quently introduces in the perfection of apparatus problems well nigh as difficult as the one with which he began. (H. E. H.) The forces which operate in chemical change are effective only through extremely short ranges as measured on the everyday scale of length. Accordingly one of the first tasks of the practical chem ist is to subdivide his reagents (see REAGENT) so finely that they can be brought into very intimate contact. This intimate mixing is most effectually promoted by dissolving these reagents in suit able media. Hence a considerable proportion of laboratory appa ratus is devoted to the processes of solution, extraction, filtration, precipitation, evaporation, distillation and desiccation. All of these have to do with the treatment of chemicals either in or separating from the dissolved condition. For these purposes reagent bottles, filter pumps, water baths, funnels, evaporating basins, filter stands, flasks and stills are employed. Representative types of such ap paratus are here illustrated.
Certain important chemical reagents are gaseous under the ordinary laboratory conditions. Special apparatus has been de vised to deal with such highly volatile fluids. Gases are con veniently manipulated in drying towers, aspirators, Kipp's gas generators, wash-bottles and glass tubing connections including three-way taps.
Chemical analysis (see CHEMISTRY : Analytical) is the touch stone by which all chemical theories must ultimately be tested. Hypotheses which fail to pass through this ordeal are regarded as no longer valid or useful. A considerable proportion of laboratory equipment subserves the essential needs of the analyst. The ac curate chemical balance is described elsewhere (see BALANCE), but among the typical requirements of analytical chemistry figured below are the following : weighing bottles, crucibles, potash bulbs, measuring vessels (burettes, pipettes, graduated flasks and cyl inders), and special apparatus, for example, for the quantitative evaluation of arsenic and carbon dioxide and for the determination of vapour density.
The following list refers to the accompanying illustrations of apparatus commonly used in a chemical laboratory: General Apparatus.
3. (a) Spring clip (Mohr's pattern). (b) Screw clip.
4. Reagent bottle.
5. Filter pump, for use with filter flask and either Buchner funnel (fig. 7) or Gooch crucible (fig. 31) . The stream of water is broken up at the jet A, and the resulting drops tend to carry air down the tube B. This is capable of reducing the pressure as low as the vapour pressure of water.
6. Water-bath and rings on tripod. The bath is nearly filled with water which is heated by a burner underneath. Removal of one or more of the rings enables any circular vessel (e.g., an evaporating basin) to rest in the hole of appropriate size, and its contents are slowly heated or evaporated. A constant-level device may be fitted (see fig. 13) to maintain the level of the water in the bath and obviate refilling.
7. Buchner funnel fitted in filter flask, which is attached to a suction pump (see fig. 5) . A circle of filter paper is placed on the perforated bottom, and the material retained by it can be sucked dry, washed, and pressed.
8. Test-tube stand and test tubes.
9. Evaporating basin (porcelain) .
50. Filter stand, funnel, and beaker. A filter paper is folded so as to fit in the funnel as shown ; it retains solids, while liquids filter into the beaker.