Precise Measurement of Volume in Titrimetric A na Iys is WILLIAM M. THORNTON, JR., Loyola College, Baltimore, Md. A review of precise measurement of volume in titrimetric analysis is presented. Detailed descriptions of the burets and special techniques used b y the author are given. Representative standardizations indicate the reliability to b e expected under the prescribed conditions.
expansiveness is a matter of some importance; hence its f u ~ h e r study would seem to be justified. THERMAL EXPANSION
Thiessen and his collaborators, and also Chappuis, have determined the density of water (lLordinarywater-substance”) at various temperatures with great exactness. From their data the expansivity may be calculated. Accordingly, Circular 19 of the Bureau of Standards (6) enables one to corrert the observed volume to what it would be a t 20” C.the standard temperature for volumetric analysis throughout the United States (2.4). The small numbers that are to be added or subtracted have been computed on the assumption that the glass forming the measuring utensils has a coefficient of cubical expansion of 0.000025 per degree Centigrade. (Certain borosilicate glasses, 47, such a3 Pyrex, exhibit thermal expansions much smaller than the foregoing.) These corrertions apply not only to water but also, practically speaking, to sufficiently dilute aqueous solutions. Furthermore, a supplementary table ( 5 , below Table 38) gives the percentage inrrease in the corrections for water to be applied when standard solution. of four common acids and bases-namely, nitric and sulfuric acids and sodium and potassium hydroxides--are under consideration. This increase is abuut 5 per cent for the before-named reagents when of 0.1N concentration. In like manner, it is but 3 per cent foi 0 . I S hydrochloric acid (44)-an almost negligible increment. Yet the higher the normality of th? solution the greater must be the augmentation of the temperature corrections for pure water As early as 1869, Gerlach (13) studied the expansion of aqueous solutions of acids and salts; and in 1877 Casamajor (6),utilizing Matthiessen’s data for water. attemDted to correct the volume of his standard soluiion for changes of temperature. Some 5 years later, Schulee (39) determined the rate of expansion for a good many of the better known volumetric solutions, and these apparently reliable values have been used to a considerable extent, Schloesser (30, 31) and several other authorities (7.9,10, 20, 25, 38) have contributed to the subject in one way or another. Finally, Osaka (22) hns made available his extensive investigations.
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SIDE from the choice of a suitable chemical reaction, or the resultant of a series of successive reactions, on which to base a dependable titrimetric process, one is confronted with the responsibility of performing the physical measurements with oufficient nicety. More specifically, if the highest accuracy is to be attained, the following points must be given serious consideration: (1) influence of temperature change, (2) position of the meniscus, (3) drainage or afterflow, (4) errors of graduation, ( 5 ) evaporation, and (6) point of complete reaction. Except for the last two, these sources of uncertainty can be removed by weighing, instead of measuring by volume, the standard solutions. For this purpose, various forms of “weight” or “weighing burets” have been invented; and a few investigators (1, 2, 16) have seen fit to take account of the solution of known concentration by weighing the reaction vessel both before and after the titration. If the loss of water (or other solvent) from standard solutions during long periods of storage be left out of account, it is safe to assume that the evaporation taking place within the time required to effect a volumetric determination is of no significance (19). On the contrary, rendering a decision as to the equivalence point looms up as an unquestionably difficult objective in this kind of analysis, and one in which the weight buret, as compared with the volume buret, offers no advantage. If the volume burets are properly designed, and used with certain precautions, a much higher dcgree of accuracy may be realized in dealing with a number of the well known volumetric solutions than is commonly supposed. The very fine work of Ponndorf ($6) may be cited in support of the above contention, and this constitutes by no means the sum total of the evidence (cf. 11, 14, 21, 48). Admittedly, when many titrations have to be made, it is expedient to feed the evaluated solution from a large reservoir directly into the buret. Moreover, in the case of solutions that are sensitive to the oxygen of the air and have to be kept under a nearly constant pressure of an indifferent gas (4, l 7 ) , the transfer to a short buret for weighing would probably lead to a lowering of the titer (28). Weighing the titration flask is not always feasible: in some experiments the reaction mixture must be heated; it is often desirable to bubble an inactive gas (carbon dioxide, for example) through the test solution (43); it is sometimes necessary to introduce an indicator during the latter stages of the procedure. However, Lee (18) has developed an ingenibus weighing buret, wherein a known solution of titanous sulfate may be obtained by shaking acidulated titanic sulfate with zinc amalgam. But such a device, owing to the high de,psity of the amalgam (about 4 per cent zinc, presumably), would seem to be unduly heavy, besides being restricted to the preparation of only a small quantity of the reducing agent a t a time. The chief limitation to measurement by volume in precise analysis is the lack of experimentally established data regarding the thermal expansion of numerous very useful solutions. This.
In this connection, it may be desirable to call attention to a point that is apt to be overlooked. When an auxiliary reagent is added to a titrimetric solution, even though it does not enter into the stoichiometric relations, it must needs alter the thermal expansion of the liquid. Many such cases might be cited, but a
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