IDEOLOGICAL UNCERTAINTIES in TITRIMETRY M. G. MELLON Purdue University, Lafayette, Indiana
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MONG the analyst's wide variety of methods of measurement, titrimetry has attained an almost dominant position, a t least in industrial practice, in the century and a third since the proposal of the first procedure by Descroizilles (1) in 1806. 1 This may be attributed to certain unique advantages of titrimetric technic and to the range of applicability of the method. One might reasonably expect that more than a century of general use would have brought definiteness and consistency in, and adequate justification for, the concepts and terms employed. An inspection of recent analytical publications and of examinations submitted by graduate students from nearly every state in the Union has convinced the author, however, that certain aspects of the subject are still confused in the minds of many. For this state of uncertainty writers must take the most blame, as they are the principal originators and transmitters of recorded ideas. Administrators come next in line for selecting and tolerating uninformed, uncritical, and mentally lazy instructors. Teachers who have little knowledge of, or interest in, analytical chemistry can hardly be expected to make much contribution to the subject, or even to be accurate interpreters and transmitters of the work of others. Finally, there are the non-skeptical students who accept everything on authority. The purpose of this article is to direct attention once more to certain ideas or concepts on which agreement in usage seems both feasible and desirable. If the chemists concerned will cooperate, most of the present confusion and uncertainty need not persist.
phere. Obviously, l/lm of a liter is a milliliter and not a cubic centimeter. Although analytical work could be done in terms of cubic decimeters and cubic centimeters, the National Bureau of Standards has specified the use of liter and milliliter, and manufacturers of the best American apparatus mark their products in these units. It seems unfortunate, therefore, for any individual writer, or committees editing official methods, to use the term liter and then designate the subdivision as a cubic centimeter. Such carelessness or inconsistency is often defended on the ground that it makes no practical differencesince cubic centimeter and milliliter differ by only 27 parts per million. Even this amount, however, would be an intolerable error in weighing 100 g. of solution to four decimal places, as in precise density determinations. 4 In addition, disregard of such items is scientifically unsound. Another definition still often inadequately specified is that for equivalent weight (2). The statement should indicate that the values, as ordinarily used, refer to the naturally occurring isotopic mixtures of hydrogen or oxygen. Furthermore, the significant figures given should be adequate for analytical work, as 1.0080 for hydrogen and 8.000 for oxygen. A third point concerns standard solutions. Perhaps everyone agrees that such a solution is one of known value, and that this value may be expressed in terms of the concentration of the solute, or of some constituent with which the solute will react, or to which i t is chemically equivalent. What is often forgotten, or unrealized, is that the concentration may be expressed either on a volume or mass basis, the former being applicable to volume burets and the latter to weight Definitions In the establishment of standards for the metric sys- burets. When the volume basis is used, usually one tem an error in measurement resulted in our now having employs the normal or molar system, or expresses the two units of volume. The first, the cubic meter, is concentration in terms of grams per milliliter. For the related to, or derived from, the standard meter bar. mass basis the molal system is a recognized favorite One cubic centimeter is '/l,aoo.ow of the cubic meter. among physical chemists, but analysts prefer simply The second, the liter,=is related to the standard of mass, grams of solute per gram of s o l u t i ~ n . ~ the kilogram, in being defined as the volume occupied by a kilogram of "pure" watera a t the temperature of its Terminology maximum density and under a pressure of one atmosMany years ago Winkler (6) recognized the merits of
' Information supplied by Professor W. T . Hall.
a The liter is often designated as a unit of capacity. See BRIGGS, Rev. Modern Phys.. 11,111 (1939). 'This is water consisting of the naturally occurring isotopic mixtures of hydrogen and oxygen.
If such densities are to carry six or seven significant figures, one must designate whether the unit is g . / ~ m . ~ o r g . / m l . 6 The percentage system, correctly applied, is sufficiently definite but not very useful.
