MODEL OF A MASS SPECTOGRAPH JOSE FERNANDEZ and SAMUEL H. LEBOWITZ Straubenmuller Textile High School, New York City, New York
Tm Aston Mass Spectrograph and its later modifica-
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tions have played so important a part in atomic studies that an understanding of the instrument is of significant value to students of even elementary courses in chemistry and physics. A recent article in THISJOURNAL' described an analog of the Aston instrument which used falling steel balls and an electromagnet to simulate the atoms and the fields in the actual instrument. I t was felt by one of the authom that this arrangement had certain undesirable features; principally, that the balls falling freely in the gravitational field moved too rapidly to be followed readily by the average observer. Several years ago, an attempt had been made to roll steel balls past a magnet with a view to deflecting them. This did not result successfullybecause it seemed impossible to obtain constant deflections. It was then decided to substitute a mechanical field for the magnetic one, the final choice being an air stream. The presently described apparatus, using the principles indicated-slow speed balls to represent atoms, and an air stream for the field-was designed for public exhibition a t a science fair. As will be seen from the 'NORTON, F. J., THISJOURNAL, 25, 677 (1948). accompanying photographs there is an elevating and feeding device which feeds balls of identical size but varioui weights a t the rate of one every ten seconds. These roll down a directing trough and directly into a n opening which returns them to the elevating mechanism. With the field disconnected, the balls roll in a straight line regardless of their weight. When the field, i. e., air stream, produced by an old vacuum cleaner is turned on, the heaviest balls are but slightly deflected. Those of intermediate weight go to a second receiving point, while the lightest ones, made of wood, are driven off almost a t right angles to their original direction of motion. All return to the elevator to be used again. To permit operation by the observer, a set 07 pushbutton switches was incorporated; one to control the feeder mechanism, the other, to turn the field on and off. In ~arallelwith these. and a t a ~ o i n not t accessible to the-public when the machine is on exhibit, are two other switches which permit constant operation when this is desired.
ON THE NATURE OF CHEMICAL PURITY HAROLD G. CASSIDY Yale University, New Haven, Connecticut
Tm
nature of chemical purity is a subject of great chemical interest, and I would like to discuss it on a rather broad basis, as an exercise in understanding scientific language, and as a sort of fitted scaffoldnrithin which to erect the more interesting edifice. The world "pure" has many meanings, the specific meaning depending on the particular context within which the word is used. This is well understood in ordinary matters. For example, no adult would confuse the meaning of "pure" as used in the advertisements of a well-known soap with the use of the word by the Watch and Ward Society of Boston. But the student can become very confused when the word is used with different meanings by chemists, for often it is not pointed out that differentmeanings are involved. The problem of understanding the use of a word like this is centered around the problem of relating the word as a symbol to the fact which it is supposed to represent. This, of course, is at the root of all difficulties with language, and so it is evident that if we can clarify the issue in the case of this one word "pure" we have served to clarify the issue for the language. The problem of what is a pure substance can be examined from a number of points of view, and I have selected three as being of particular interest in conuection with our larger objective. I would like to deal with parts of the historical aspect, parts of the language, or semantical aspect, and parts of the experimental aspect of the problem of chemical purity. In dealing with these more or less separately I do not wish to imply that they are not connected. Only poets and other literary and artistic men can deal with a multitude of aspects of reality simultaneously, and they are accused of all sorts of faults of which vagueness is one of the least. The scientist usually has to deal, if possible, with one variable a t a time. This practice has its advantages but also its faults. It is interesting to observe new developments which permit dealing with many simultaneously changing variables. The problem of what is a pure substance must have a very long history. Certainly it must always have been a
problem of the greatest importance to commerce. Imagine the questions which might have arisen in olden days in using coins to pay for a length of dyed Egyptian cotton. On the one hand, the seller must be sure that the coins he receives really are gold, and not some cunning but worthless alloy of baser metals. On the other hand, the buyer must ascertain that the cloth really ismade of the long-fiber Egyptian cot.ton, andnot some less durable material, perhaps improperly dyed. The problem then usually was one of identity rather than of actual purity: did the coins correspond in color, ring, hardness, and weight to the standard set up by some prince or state. A similar problem was investigated by Archimedes when he determined the density of gold and its alloys with silver in order to test the cronll of Hiero. The early modern chemists had a sufficiently difficult time with problems of this sort. A classical example can be found in the controversy between Claude Louis Berthollet and Joseph Louis Proust. Berthollet had found that the courses of reactions could be influencedby the quantities of reagents. His ideas were close to some which we have now. For when two salts are mixed in solution the components of each are in equilibrium in solution, and the course of any reaction which may occur is influenced by the solubility, volatility, etc., of the products. However, he drew the conclusion that, within limits, elements should unite in all proportions. His rather imperfect analyses tended to confirm this conclusion. Proust was able to show, however, that the series of substances which Berthollet claimed to represent a number of composition ratios were really not single substances but mixtures. Proust established the law of definite proportions. The problem here was obviously the distinction between pure substances and mixtures. The controversy which was carried out over a period of some seven or eight years-and always, it is said, with the greatest courtesy on both s i d e s a i d a great deal to advance the state of chemistry and clarify the concept of a pure compound. It is interesting to see in the development of chemical
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science a sequence of overlapping stages. This sequence seems to be characteristic of the history of any science. First comes the data-gathering, or analytical stage, which itself shows an internal structure. The observations which sum up to the data of the science are first qualitative in nature and deal with size, shape, color, etc. Then almost imperceptibly there is a transition toward more quantitative observations, which supplement the qualitative by making them more precise. Qualitative analysis thus leads into quantitative analysis, and this, after a time, into arrangement analysis, or structure analysis. This stage may he followed in the more advanced sciences by abstract theory and philosophy. It is apparent, therefore, that as the science goes through this development many words will undergo concomitant changes in meaning. Many chemical words have undergone such change. For example, the concept of "element" to Empedocles was certainly very different from that which Boyle must have had, and Boyle's concept, though rather advanced, from the "element" of the atomic scientist. The same word,"element," isused for each concept, but almost unconsciously in our minds we recognize the different meanings by some such qualification as might be symbolized thus: "element" B.C.w; "element" m a ; " element" 1949. Once the student understands this difference many of his problems disappear, and his interest is greatly stimulated. We can now write down definitions: "eIement"~.c.ao "element"law "element"1840
Empedocles' air, earth, fire water Boyle's substance which has not been decomposed Substance which cannot be dcoomposed by chemical means
It thus becomes obvious that the word "element" is definable only in its context. I believe that small problems like this can be extraordinarily confusing to students, who may thereby be discouraged from continuing our really fascinating subject. This brings us to the language aspect of our problem. Students often do not realize that the language of a science is as much a part of the science as observation and experimentation. Science is quite inseparable from its language, and it would be difficult to find a clearer statement of this fact than that made by Lavoisier: The impossibility of separating the nomenclature of a science from the science itself is owing to this, that every branch of physical soience must consist of three things: the series of facts which are the objects of the science, the ideas which represent these facts, and the words by which these ideas are expressed. Like three impressions of the same seal, the word ought to pr* duce the idea, and the idea to be a picture of the fact. And, as ideas are preserved and communicated by means of words, it necessarily fallows that we cannot improve the language of any science, without at the same time improving the science itself; neither can we, on the other hand, improve a science without improving the language or nomenclature which belongs to it. However certain the facts of any science may be, and however just the ideas we may have formed of these facts, we can only communicate false impressions to others, while we want words by which these may he properly expressed (1790).
It is the new-language aspect of chemistry which often
is claimed by students to be their nemesis, and yet I have observed these same students to revel in the descriptive language of comparative anatomy which seems far more difficult and disorderly in its structure. The problem may go back to some very fundamental issue which we have not yet touched on. The problem of understanding the use of such words as "pure" is largely the problem of relating the words as symbols to the facts which they are meant to represent. Here the devices for distinguishing between the various uses of the word "element" might again serve in good stead. Indeed, these qualifying devices are often required by law in connection with statements of purity because the property of purity as applied to medicines or soaps has a commercial value and it is necessary to prevent abuse of it. We therefore see on labels these qualifying statements: "U. S. P.," "C. P.," "Tech. Pure," "99 and 44/100% pure." In such cases as these the purity is defined to some extent. But in the mind of the student of chemistry certain confusious may arise about the matter of purity which have to do with the use of the word in a more abstract way. Would one speak of a pure element, or a pure substance, or a pure compound, or a pure molecule? I asked some graduate students this question, and this started a one-hour discussion. Elements are substances which cannot be decomposed by ordinary chemical means into two or more substances. Elements are therefore ipso facto pure. By definition a compound is a substance which can be decomposed by ordinary chemical means into two or more simpler substances, thus, I suppose, a compound may or may not be pure. A material may certainly be pure or not pure. But can the word be applied to a molecule? Obviously not, for purity is a property which comes into existence only with assemblages of molecules. Another way of saying this is that pure can be used to qualify only those chemical terms which have a certain generality. We could, perhaps, set up a rough chart of ~vordsof different generahty : Greatest enerality
f
Least eenerditv.
Marroscovic meaning Substance Compound Element
Microscovic meanino Molecule of a compound Molecule of an element
The concept of purity is applicable only a t the macrosco~iclevel to the words of greater ~eneralitv. word needs to be said at this point abbout macroscopic and microscopic meanings. Suppose we consider water as a convenient subject. Water is such an important and commonly observed substance that the language is full of words for describing it as it appears in different states and under different conditions. These distinctions are most important from a practical standpoint. For example, water runs down hillsides in tiny rivulets after a rain. These combine into a rill, or streamlet, or brooklet. Brooklets run together into a branch, or creek, which may run into a river. Present tendencies seem in some ways to be toward destroying
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many of the fine distinctions between words. The reasons for this would make an interesting study. In chemistry the ignoring of these distinctions can cause a great deal of trouble to students. My teacher of analytical chemistry a t Oberlin College, Professor Chapin, used to say at one of the first class meetings of the year, that we were not to use H20 when we meant "water." Water is a substance, he would say, H 2 0is a molecule. In equations, H 2 0 refers t o a molecule of water, though it may also mean a mole of water. HzO therefore really does not represent "water." We can show this in a striking way by asking if water is HzO. It used to he thought to be so, but now we know that water is a mixture in dynamic equilibrium described by the following equilibria: H10?=H+ f OH-
Hf H20
+ H*O = HaO+ + H20.= (HnO)*,etc.
But these equations are not very detailed as they stand. We might ask some questions about structure, and when we do this me start writing Hz0 as H-0 . En\
H0 quiring further we see that by taking account of elec.. trouic structures me can write H :O : where H and O do H not mean quite the same as they do when found in a table of atomic weights. Now we can understand a .. little better some of the equations: H : 0: .. $ H + H [:o:H-I, etc. HIO therefore does not represent "water." Water is a macroscopic phenomenon, something we can handle, and wring out of a wet dishrag. The "form" of water, so to speak, is produced by the functioning of HzO and its derivatives, and Hz0 is produced by the functioning of electrons and protons. What we have here are various levels of abstraction. They are called levels of abstraction because the process of abstraction is a process of leaving out details, and in moving from one level to the next "higher" we leave out quantities of details which describe the phenomenon a t the level below. These levels might be described as follows:
+
First-order, observable facts
Leuel of ahstmelion Ezamule Macroscopic water-
1 Molecular
I
Inferential rlnta Subatomic
H*O,etc. H:O:H
Characterization Less detailed, qualitative More detailed, qualitative, quintit* tive Still more detailed, qualitative, quantitative, structural
We can see now that whereas it is quite proper to speak of "pure water," it is certainly incorrect ..to say "pure H20," and meaningless t o say "pure H :0: .. ." H
It will he noticed that so far no definition of purity has been given. This has been done intentionally so as to help indicate some of the difficultieswhich may be encountered. It is now necessary to define what we mean in chemistry by a pure substance. This brings us to the experimental aspect of our problem: how do we determine purity in a substance? Upon close examination of what we mean by the determination of purity, we discover that what we really mean is the detection of impurity. There is no sure way of proving a substance t o be pure. Consider, for example, some definition such as "a pure substance is one in which all molecules are identical." Assuming that we know what we mean by identical how can me show experimentally that all the molecules are identical? It is experimentally impossible because to make such a measurement would require infallible instruments of absolute accuracy. We can only determine for a substance its degree of purity as judged by tests for impurity. We can only show with any confidence that if the substance contains impurities of thus and such a kind they cannot exceed thus and such an amount. If we use crude methods of finding impurities then the degree of purity can be stated only to a crude approximation. With progressively more acute examination we become correspondingly better able to detect impurity. I t is evident, therefore, that the definition of a degree of purity must he made on the basis of the operations used to detect impurity. We do not define "purity," we define a degree of freedom from impurity as disclosed by certain given measurements. This is what is known as the operational definition of purity. It emphasizes that purity is a relative matter and depends on the operations and instruments used to detect impurity. Since purity is a relative matter there is consequently no such thing as absolute purity, and any statement such as the one rejected above which purports to define an absolute purity is operationally meaningless. We can now return to some of the problems which caused us concern. The element air of Empedoeles' time could certainly have been called pure by him in the sense in which the word would have been used in his time. We can still talk of air as pure, but usually we mean free from dust, as me measure it with our eyes, or free from odors, as we operate with our noses. We might use chemical instruments to examine the purity of air, and say it is pure if freed from moisture, but if we look for a single gas, then air is not pure, but turns out to he a mixture. Suppose we isolated the oxygen from the air. The oxygen so obtained has a purity which can be judged only on the basis of tests for other elements and compounds. If these are absent to our best tests we may still raise the question of isotopic purity. But now we come to a possible dilemma which can only be resolved by bold action. The problem can be stated, returning to water as our subject. Let us say that a sample of purified water shows no amounts of metals, salts, or organic compounds which can be found by the most sensitive tests. Yet certain physical tests show the presence of hydrogen ions, hydroxyl ions,
JOURNAL OF CHEMICAL EDUCATION
dimeric and polymeric molecules. "Can such water be present in a certain ratio fixed by the equilibria whose pure?" asks the puzzled student, who keeps thinking functioning a t the molecular level produces "water" that water is HzO. He knows, of course, that hydrogen (macroscopic). Pure water is pure because we cannot ions are not water. Therefore, if water contains hydro- separate it into distinct, different fractions by ordinary gen ions then it must not be pure. This must he emi- chemical or physical methods. nently reasonable. I am indebted to Professors Stuart Brinkley, Harold From what has been said, it must be apparent where Dietrich, and Erwin Kelsey, who kindly read this the root of this problem lies. It lies in the fact that the manuscript and discussed some points with me. student has not learned to distinguish between levels of REFERENCES abstraction. He doesn't realize that what he says ( 1 ) BORN,MAX, "Experiment and Theory in Physics," The about "water" is not necessarily directly transferable to Macmillan Co., New York, 1944. "hydrogen ions." Once he realizes that the word (2) BRIDGMAN, P. W., "The Logic of Modern Physics," The Macmillan Co., N e w York, 1938. purity is applicable only at the macrosocpic level, and H. G., PTOC. Am. Fed. B i d , 7, 464 (1948). furthermore that "purity" is relative, depending on the (3) CASSIDY, (4)HAYAKAWA, S. I., "Language in Action," IIarcourt, Brace $ instruments or methods used to measure it, he sees that Comoanv. Inc.. New York. 1941. pure water is water in which one can find no impurities. (5) J O E N S ~ N , it;., ''~eoplein ~uahdaries,"Harper and Brothers, New Yorlt, 1946. And he also sees that hydrogen ions, hydroxyl ion, J., "Chemical Species," Chemical Publishing hydronium ion, etc., are not impurities if they are (6) TIMMERXANS, Co., Inc., New York, 1940.
