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SOME DEFINITIONS IN QUANTITATIVE ANALYSIS RONALD P. GRAHAM McMaster University, Hamilton, Ontario
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Wami I use a word,'' Humpty Dumpty said, in a rather scornful tone, "it means just what I choose it to mean-neither more nor less." The situation would be chaotic if everyone adopted the rather egoistical attitude of Humpty Dumpty, but it is nevertheless true that usage is more important than etymology in giving meaning to a word. If, however, a technical word (or any word, for that matter) is used in many different ways, it may cease to have any clear meaning. For example, as Hammett has pointed out (9),"valence" has been used in so many differentsenses, some of them contradictory, that the term has become one with a very vague meaning. In the present paper attention will b e drawn to a number of wbrds, commonly encountered in quantitative analysis, which now have confused, ambiguous, or contradictory meanings, or else are in danger of becoming less meaningful than they should be. And further--in an effort to encourage thought and discussion-suggestions are advanced regarding "proper" usage. Clarity of expression cannot be achieved unless terms have a precise meaning ("A name need not be etymologically sound but it should certainly be d e h i tive" (S)), and it needs to be emphasized that "the scientist, of all people, is under obligation to write, not only so that he may be understood, but so that he cannot be misunderstood" (17). Technique (Technic). Most of the textbooks concerned with the practical aspects of quantitative analysis offersuggestions and directions regarding the technique of the subject (and thereby imply what is meant by technique) but all too few books attempt to define the word. The etymology of technique helps to explain the meaning usually attached to it by chemists: it is de, or pertaining to art, artistic, rived from T ~ X U L K ~ Sof skillful. A person who has a good command of the technique of quantitative analysis is skilled in the manipulations and operations of analytical chemistry; he is practiced and expert in the handling of chemical apparatus, and he is adroit &ndingenious in using scientific equipment for the carrying out of weighings, titrations, precipitations, filtrations, evaporations, ignitions, etc. The possessor of technique is a careful practitioner, his work is organized, and it is carried out in a sure, deft, and facile manner. We may be reasonably certain that Mr. Sherlock Holmes (who was an analytical chemist of some accomplishment (6, 20) was familiar with tcchnique when we read from the record of Dr. Watson: ". . he was possessed af extraordinary delicacy of touch, as I frequently had occasion to observe when I watched him manipulating his fragile philosophical 596
[i.e., scientific] instruments." I think that all of these ideas we or should be involved in the meaning of technique as applied to analytical chemistry. Technique has been defined (7) as the details of a method of procedure, but such a definition is to be deplored. Analytical Chemist and Chemical Analyst. A chemical analyst has been defined (19) as "one who has a good command of the quantitative technique, with or without a thorough understanding of the chemistry of the analytical processes involved." A chemical analyst in this sense of the expression (cf. the very different definition of Lundell(l4)) need not be a chemist as d e h e d by the Council of the American Chemical Society (2). An analytical chemist may be defined as a properly qualified chemist who has made a special study of both the theory and practice of analytical chemistry: he has a working knowledge of the specialized literature of analytical chemistry, he is aware of the sources of error in analytical work, he is quamed to assess the applicability of a given method of analysis to a particular case, he has had training and experience in the solving of analytical problems, i. e., the devising of new analytical procedures or the adaptation of old ones to new uses. Precision and Accuracy. It is to be regretted that these terms are frequently used interchangeably or should be used with otherwise misused. Precision reference to the reproducibility of a determined value, whereas accuracy is a measure of the correctness of the determined value. There is ample authority for the use of these terms in the senses given (e. g., 12, 15, 21, 95). Speaking strictly, the accuracy of a determination is never assessed, because the degree of correctness can be ascertained only if the exact (true) value is known, and the latter is unknowable. In the best practice, the accuracy of a determination is measured by the divergence of the value obtained from what is considered to be the most probable value of the quantity on the basis of analyses carried out by skilled investigators who have critically examined their methods for sources of error. Coprecipitation, Occlusion,. Postprecipitation, ete. In probably no branch of analytical chemistry is the terminology more varied than in that dealing with the contamination of precipitates. Before illustrating the confusion it will be well to point out that there appears to be, general agreement regarding what, in general, is to be understood by two of the terms commonly used, adsorption and postprecipitation. Contamination of a precipitate by adsorption means that the foreign substance is on the surface and not inside the precipitate.
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Postprecipitation refers to the phenomenon whereby a second (and usually undesired) precipitate slowly precipitates after the precipitation of the desired substance, or primary precipitation, is "complete" or virtually so (this phenomenon is not to be confused with simultaneous precipitation, by which is meant the precipitation of two or more substances a t the same time). Now the divergent. usages merit attention. The term occlusion has been used to refer t o contamination of a precipitate by the incorporation into the body of it during its formation of foreign substances whether or not the latter give rise to mixed crystal formation (solid solution) (e. g., 18, 81, 85). It has been used to include both solid solution phenomena and contamination of the precipitate by adsorption (9); it has been used to indi$ate contamination by basic salts and other substances not adsorbed and not in solid solution (4); and I have heard a noted analytical chemist reserve the term to indicate the contamination of a precipitate by m e chanically trapped solute or solvent ions or molecules. No doubt one could locate other senses in which occlusion is or has been used; the term is almost omnivorous. Coprecipitation is used to describe the contamination of a precipitate by a substance or substances normally soluble under the conditions of the precipitation or the term may include contamination caused by postprecipitation. The term inclusion has been used in one or other of these senses. And, it may be added, coprecipitation hap been used with the meaning of simultaneous precipitation. I would suggest that the term coprecipitation be reserved to describe the contamination of a precipitate by soluble impurities regardless of whether the impurity is on the surface or within the precipitate. Postprecipitation refers to the relatively slow precipitation of a second substance not soluble, or at least not completely soluble, under theconditions employed to encourage the development of the primary precipitate. This phenomenon is, therefore, on the basis of the definition recommended for coprecipitation, a distinct but related one. It. is urged that coprecipitation phenomena he subdivided into two cla~ses:adsorption, or surface contamination, and occlusion, in which case the impurities are located in, rather than on, the precipitate. The usages so far recommended are those of Kolthoff and Stenger (IS). When the mechanism of the occlusion is known or suspected, I suggest that this be indicated using terms such as those given in the classification below:
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Contamination of Precipitates '.". (1) Postprecipitation (2) Coprecipitation A. Adsorption B. Occlusion . (a) Solid dlution . ,~ (b) .Chemical reaction (c) Mechanical '(d) Other (if any) . . .... If t h e ' c ~ n t i m i n a t i nions ~ bui1t"into the crystd lattice of the precipitate one may speak of occlusion by
solid solution or occlusion of the solid solution type. When base is added to a solution of certain salts (e. g., those of aluminum) the ratio of hydroxyl ions to other anionsin the gelatinous precipitate of hydrous oxide may vary rather considerably (12). There is reason to believe that the extraneous anions are coordinately bound to the metal ion (84, and in such a case it would be reasonable to refer to the contamination as occlusion due to chemical reaction. When solute ions or solvent molecules are lodged in submicroscopic cracks and pores of a crystal lattice (caused by distortion of the lattice as a result of the presence of extraneous ions in solid solution) or carried down in an amorphous precipitate by enmeshment and trapping, the expression mechanical occlusion may he applicable. It is to be noted that occlusion may be preceded by adsorption; this is true of occlusion of the solid solution type. Whereas coprecipitation is wholly objectionable in ordinary gravimetric work, the phenomenon is used very advantageously (e. g., in radiochemistry (8)) for the carrying down of relatively small quantities of highly desired substances. An interesting and important r e cent application is the separation of plutonium from uranium by coprecipitation of the former with an unnamed "carrier" precipitate (83). The specificity of coprecipitation phenomena is well illustrated by the fact that Pu++++ is coprecipitated with the "carrier," hut Pu++++++is not. Adsorption and Absorption. As has just been noted, coprecipitation may be occasioned by adsorption*, a surface phenomenon. One speaks, however, of the absorption of a gas by a liquid and of the absorption of light by a solution. Absorption is not a surface phenomenon; rather, itjnvolves the permeation of a liquid or a solid (palladium may absorb hydrogen). Multiplieate and Replicate. The word replicate has been used in the chemical literature in the sense of repeated (e. g., ". . .the variation between replicate determinations" (82)). As an adjective, replicate usually refers to something folded over or folded back upon itself, according to the New English Dictionary on Historical Principles, Funk and Wagnalls New Standard Dictionary, and Wehster's New Internationd Dictionary (2nd edition, unabridged). Of these authorites only Webster gives a second--meaningsuch as multifold, manifold, or repeated. It should be said that the use of replicate to mean repeated has little to recommend itself from an etymological standpoint. Surely a more appropriate word to convey the desired meaning is multiplicate. There is authority in all the aforementioned dictionaries (and in others) for the use of multiplicate in this sense. Its etymology is sound, and its relationship to duplicate, triplieate, etc., is clear. AEidimety and Alkalin&y. .Acidimetry has been defmed (4, 7, 81,25) as the determination. of an acid by titration with. a standard alkali solution; it has also * In view of the esf$dished use in analytical chemistry of such &xpressions as adimptton, adsorption indicator, and chromatogi'Zphie adsorptia, it.:%eemsfutile tolqkraw attention.to the ~dvantagesofthe term,i'sorption" ( 1 6 ) . . ~ ,,
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been defined (12) as the titration of a base with a standard acid solution. If argentimetry (or argentometry) is used, as it commonly is, to denote or imply the use of a standard silver solution in quantitative analysis, then it would be consistent to reserve the term aeidimetry for titrations in which a standard acid is employed, and to employ alkalimetry for titrations in which a standard base is involved. It should be noted that alkalometry is a term to be used only in connection with alkaloids, and tbat an alkalimeter has nothing directly to do with alkalies but is an apparatus for the determination of carbon dioxide (an acidimeter, on the other hand, is an obsolete term (7) for a hydrometer). lodimetry and lodometry. These terms have been used interchangeably (7) to refer to volumetric methods which involve a titration using either a standard iodine solution or a standard thiosulfate solution. The general usage, however, is to reserve the expression iodzmetry for application to those titrations involving the use of a standard iodine solution. The use of a standard solution of thiosulfate (to titrate liberated iodine) has been denoted as iodometry (e. g., ld,15), thiosulfatimetry (18), and indirect iodimetry (21). The objection to iodometry is that i t is very easily confused with iodimetry; the other expressions are somewhat cumbersome. The use of thiometry could be recommended if thio salts other than thiosulfate were not used in analytical work, but the importance of thiocyanate precludes such a term. Certainly some standardization bf terminology is highly desirable. Balance and Scales. Chemicals are weighed in the analytical laboratory using a balance; potatoes are weighed in the market place on scales. Equivalence Point and End Point. When the amount of a standard solution containing x equivalents of the reagent has been added to a solution originally containing x equivalents of the substance being determined, the equivalence point of the titration has been reached, whether or not the analyst is aware that this correspondence has been attained. This stage a t which cbemically equivalent quantities of the reactants have been brought together is sometimes called the theoretical end point (a poor expression) or stoichimetric point. In certain textbooks of quantitative analysis one finds end point defined in a manner that is either wrong, meaningless, or inadequate. It is hoped that if criticism is directed a t the following definition, it will be only on the grounds of inadequacy: the end point of a titration has been reached when sufficient reagent has been added to the solution being titrated to cause the auxiliary substance known as the indieator to exhibit its characteristic color change; or when the reagent brings about some other pronounced change in the physical properties of the solution (e. g., appearance or disappearance of fluorescence or turbidity, of colored precipitate or colored adsorption complex, of color due to excess of one of the reactants) which investigation has shown to occur a t or close to the equivalence point. One endeavors to arrange matters so that the diierence between the end point and the equivalence point is as
JOURNAL OF CHEMICAL EDUCATION
small as possible. This difference is known as the titration error (15). Cubic Centimeter and Milliliter. These terms refer to different quantities, although many students consider the expressions to be synonymous. The cubic centimeter is directly related to the meter; on the other hand, the milliliter is derived from the liter, which is the volume occupied by a mass of exactly one kilogram of water a t the terhperature of its maximum density (3.98%) and under normal atmospheric pressure. Originally it was intended tbat the mass of the kilogram should be equal to that of one thousand cubic centimeters of water a t the temperature of its maximum density. This conveniently simple relation was not realized, for it'has been shown that one liter is equivalent to 1000.028 cubic centimeters: For very ipany purposes the difference-between a cubic centimeter and a milliliter (amounting to one part in approxiniately thirty-five thousand) is of no consequence, but this is no excuse for referring to the thousandth part of a liter, the generally accepted unit of volume, as a cubic centimeter. It needs to be added that the use of the oral abbreviation mil for milliliter is unnecessary and unfortunate; as the term mil is used to designate a wire measurement (l/lwoinch), its use in the sense of a volume is to be deprecated. Titrimetric and Volumetric.. The writer supports Mellon's plea (18) that the termtitrimetric be used to designate procedures in which there is measured the amount of a standard solution chemically equivalent to the substance being determined. The term volumetric should be reserved for analyses in which the volume of the substance being determined is ascertained. Concentrated and Strong (Also Dilute and Weak). It is conceded generally that a strong acid is one which in solution is highly or virtually completely i0nized.t A concentrated acid, on the other hand, is one in which the molarity is relatively high. The distinction between a concentrated solution and a dilute solution is a relative and arbitrary one, but a 10 M solution of sulfuric acid is certainly a concentrated one and a 0.01 M is a dilute one. In view of the fact that the adjectives strong and weak as applied to electrolytes have an established and recognized relevance t o the degree of ionization, it is a pity that the terms are so often used with the sense of concentration. . For example, the expression "strong acid solution" has been used (11) to. denote an aqueous solution in which the pH is less than one, which is, of course, a very d i e r e n t thing from a l'solution of a strong acid" which might have any pH value below seven. In important and valuable works on ohernical analysis (11,15) one 6nds "weak acid" used to mean a
t The strength of an acid depends, among other factors, on the proton-accepting tendency or protophilic nature of the solvent. Hydrochloric acid remains a strong acid in methanol and etlianol, whereas nitric acid is a weak acid in these solvents (10). In an acidic solvent possessing no basic properties, such as hydrogen chloride, even a substance which is normally s,strong acid is unable to exhibit its acidic properties because there are no proton-accepting molecules present (6).
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solution in which the pH is greater than one (and, presumably, less than seven), "very weak acid" used in reference to a solution with a pH of five to six, and "dilute acid" solution denoting one with the pH of two to three. There is no need to employ weak and dilute as synonymous terms, and, a t least for students, the usage is a confusing one. It is' not expected that readers will agree wholeheartedly with the viewpoints expressed above and with all the usages recommended. But it is hoped that this paper may encourage those who are particularly interested in the terminology of quantitative analysis to further the establishment of a consistent nomenclature in this field. Students of analytical chemistry are a t present to some extent justified in referring their professors to Alice's Adventures in Wonderland: "Speak English!" said the Eaglet. "I don't know the meaning of half those long words, and, what's more,,I don't b e lieve you do either!" LITERATURE CITED (1) BOTTGEB, W. C. (ed.), (trans. by R. E. OESPER),"Newer Methods of Volumetric Chemical Analysis," D. Van Nostrand Company, Inc., New York, 1938, pp. 19-20. (2) Chem. &. Eng. N m , 22,613 (1944). (3) C ~ n a oL. , H., J. Colloid Sn'., 1, 261 (1946). (4) FALES,H. A,, AND F. KENNY,"Inorganic Quantitative Andysis," D. Appleton-Century Company, New York, 1939. S., "Textbook of Physical Chemistry," D. Van (5) GLASSTONE, Nostrand Company, Ino., New York, 1946, p. 976. (6) GMAM, R. P., J. CHEM.EDUC.,22, 508 (1945). (7) GUNT, J. (ed.),. "Hackh's Chemicd Dictionary," The Blakiston Company, Philsdelphia, 3rd ed., 1944. (8) HAHN,O., "Applied Wioohemistry," Cornell University Press, Ithaca, 1936.
(9) HAMMETI-,L. P., "Solutions of Electrolytes," McGraw-Hill Book Company, Inc., New York, 1936. (10) HAMMETI.,L. P., "Physical Organic Chemistry," McGrawHill Book Company, Ino., New York, 1940, p. 261: W. F., AND G. E. F. LUNDELL,"Applied (11) HILLERRAND, Inorganic Analysis," John Wiley & Sons, Inc., New York, 1929. I. M., AND E. B. SANDELL, "Textbook of Quan(12) KOLTHOFF, titative Inorganic Analysis," The Maemillan Company, .New York, Revised Edition, 1943. I. M., AND V. A. STENGER, "Volumetric Analy(13) KOLTHOPP, sis," Interscience Publishers, Inc., New York, Val. I, 2nd ed., 1942. (14) LUNDEL~, G. E. F., I d . Eng. Chem., Anal. Ed., 5, 221 flQ??> ,A"-",. (15) LUNDELL,G. E. F., AND J. I. HOFFMAN,"Outlines of Methods of Chemical Analysis," John'Wiley & Sons, Inc., New York, 1938. (16) MCBAIN,J. W., Phil. Mag., 18 (6), 916 (1909); T ~ a w . Fa?. Soe., 14, 202 (1919). (17) MCCLELLIWD, E. H., J. CHEM.EDUC.,20. 546 (1943). (18) MELLON,M. G., "Methods of Quantitative Chemical Analvsis." The Macmillan Comvanv. New York. 1937. j.CHEM.E&., 14, (19) MELLON,M. G., AND D. R. MEAN; M. G., ibid., 17,422 (1940). 365 (1937). MELLON, (20) MICHELLJ. H., AND H. MICHELL,Bake? Street J., 1, 245 (1946). (21) RIEMAN,W., J. D. NEUSS,AND B. NNMAN,"Quantitative Analysis," McGraw-Hill Book Company, New York, 2nd ed., 1942. (22) SMITH,F. H., Id. Eng. Chem., Anal. Ed., 18, 658 (1946). (23) SMYTH, H. DEW., "Atomic Energy for Military Purposes," Princeton University Press, Princeton, 1945, pp. 13748. A. W., Tolloid Chemistry," McGraw-Hill Book (24) THOMAS, Company, Inc., New York, 1934, and later work by AND CO-WORKERS in J. Am. Chem. Soc. THOMAS (25) WILLARD, H. H., AND N. H. FURMAN, "Elementary Q u a titative Analysis," D. Van Nostrand Company, Inc., New York, 3rd ed., 1940.
