ANALYTICAL CHEMISTRY AND THE PHASE R U L E CLASSIFICATION1 BY WILDER D. BANCROFT
We define physical chemistry as the study of the formation, separation and identification of phases and the examination of the various relations between the factors governing change and equilibrium. This is practically equivalent to saying that physical chemistry aims to present the science of chemistry as a complete and systematic whole. I t has been found that the best method of presenting the subject as a whole is to classify our material first according to the number of components, distinguishing one-component, two-component, three-component systems and so on, while, for the sake of brevity, we often call all systems containing more than two components multi-component systems. Having made our first great separation into groups, we subdivide each group according to the number of phases. This classification according to components and phases is known as the phase rule classification. It is unquestionably the best classification for presenting what is ordinarily included under physical chemistry, and some of us hold the belief that it will one day be possible to treat such a subject as organic chemistry profitably along these same lines. That, however, is a dream of the future and only the first steps in 'this direction have as yet been taken. If the aim of physical chemistry is to present the science of chemistry as a complete and systematic whole, it is obviously impossible to omit so important a branch of chemistry as analytical chemistry, and it is therefore necessary to consider this subject with reference to the phase rule classification. I t is my object to show that the scientific significance of analytical I Read before the New York Section of the Americati Chemical Society, December 6 , 1901.
?
Analytical Chemistry
107
chemistry can be brought out clearly and naturally in a course of lectures on elementary physical chemistry. This does not mean that analytical chemistry should be taught as laboratory work in connection with such a course. T h e actual drill in a series of specific methods is one thing and can best be acquired in a distinct course arranged for that end. T h e relation of these methods to the science of chemistry is another matter and one which I think is not very generally appreciated. In the elemectary course to which I have referred we begin with the simplest case, that of a one-component system and study the physical states of the substance, the thermal and optical relations, density, viscosity, surface-tension, conductivity, etc., and the way in which all these vary with varying conditions of pressure, temperature, wave-length, intensity of magnetic field, electromotive force, etc. This is also our first introduction to analysis. Any property or combination of properties can be used as a meansof identification. In organic chemistry, for instance, the melting-point or boiling-point, is one of the most frequently used standards, while in inorganic chemistry we rely more on other tests. In gas analysis, we make use of the pressure-volume-temperature relations in order to tell us the mass of the gas, after we have determined its nature in other ways. It was a density determination, however, that led to the discovery of argon and the other inert gases. In microchemistry we depend largely on the crystalline form as a means of identification, while the spcctroscope gives us a very sensitive method of recognizing certain substances by means of the rays emitted or absorbed at various temperatures. Our knowledge of these properties and consequently our identification by means of them presupposes that we have pure substances to study, a condition which can be approximated very closely in a number of instances, but which very possibly can not be realized actually in any one. We must therefore have some criterion as to the purity of any given substance, and. since a pure substance is one in which the presence of the other component or components cannot be detected by any analytical
I08
Wilder D.Bammft
means a t our disposal, it will obviously be in order to take up next the characteristics of two-component systems, including so far as may be -all special properties. These properties will be the same that we studied in the case of the one-component systems with the added complication of the variation of these properties with the relative masses of the two components. A pure substance is then to be considered as the limiting case of a two-component system, the case in which the mass of the second Component approaches zero. T h e delicacy of our tests will depend on the amount of variation of some property with the concentration when the concentration is very small. Having determined that a given substance is not pure, it is often of interest to know what the impurity is. I n some cases this can be done directly by volatilizing or precipitating one component more or less completely and identifying it. Instances of this are the separation of two liquids by fractional distillation, the sublimation of ammonium chloride from a mixture of ammonium and potassium chlorides, the driving off of occluded gases, the evaporation to dryness of a solution, or the freezing-out of either solvent or solute. These particular types are but a relatively small proportion of those to be considered. In the remaining cases, we either start with two or more impurities, in other words with a multi-component system, or we add one or more components in order to get a phase or a characteristic which we can identify. A general system of qualitative analysis is not possible until we have studied the properties of multi-component systems. In the study of multi-component systems, special stress is laid on the variation of solubility with the nature and concentration of the other components, and also on the formation of compounds by metathetical reactions, both points of utmost importance in qualitative and quantitative analysis. I t is not too much to say that the bulk of what is included in a qualitative test for bases and acids is based on a study of relative solubilities and is therefore physical chemistry pure and simple. In the qualitative analysis of the laboratory course we are dealing,
A n a ly tica I Chemistry
109
so far as possible, with a carefully selected number of facts in regard to the solubilities of a limited number of substances, the basis of selection being the desire to obtain a limited number of substances successively as solid phase of approximate purity. T o illustrate this point, let me run over the regular scheme for the detection of the bases as given by Dennis and Whittlesey in their little manual. Silver, mercury as mercurous salt, and lead are distingnished’by the insolubility of the chlorides. T h e chloride of silver is soluble in ammonia, while the mercurous chloride blackens. Lead is recognized here by the solubility of the chloride in hot water and the sparing solubility of the chromate or iodide. Mercury as mercuric salt, lead, copper, cadmium, bismuth, antimony, tin and arsenic are distinguished by the formation of sparingly soluble sulphides in hydrochloric acid solution and the last three, antimony, tin and arsenic, are grouped together owing to the solubility of their sulphides in animonium sulphide. Mercury is identified by the insolubility of the sulphate in nitric acid, lead by the insolubility of the sulphate in sulphuric acid, cadmium by the relative solubility of the sulphide in sulphuric acid as compared with the sulphide of copper. Bismuth is recognized by the insolubility of the basic chloride and copper by the color in ammoniacal solution or the insolubility of the ferrocyanide. Arsenic and antimony are distinguished from tin by the volatility of the hydride and from each other by the insolubility of the antimony-silver compound, by the orange color of the insoluble antimony sulphide and by the insoluble ammonium arsenomolybdate. T h e final test for tin is the reduction of mercuric chloride by stannous chloride. I n the next group, nickel and cobalt are recognized by the fact that the sulphides, when formed, are insoluble in dilute hydrochloric acid. They are distinguished by the insolubility of cobaltic hydroxide in boiling animonium hydroxide solution, while the solubility of nickel sulphide in solutions containing ammonium salts is worth referring to. Iron gives a black sulphide soluble in hydrochloric acid ; but the color reaction of the sulphocyanate is the real test. Chromium is recognized by ,
I IO
Wilder D.Bazmoft
conversion into insoluble lead chromate, aluminum by the precipitation of the hydroxide from boiling aluminate solutions on addition of ammonium chloride. Manganese is identified by the color of the permanganate solution and zinc by the insolubility of the sulphide in acetic acid. T h e test for barium is the insoluble chromate or sulphate ; for strontium the precipitation of the insoluble sulphate by calcium sulphate ; for calcium the insoluble oxalate, and for magnesium the insoluble phosphate. Out of this whole list, the only bases not finally identified by the production of an insoluble substance are mercury as mercurous salt, copper, iron and manganese, and the test for copper with ferrocyanide is much more sensitive than the color test, while with mercury we have the change of one insoluble precipitate into another. One great advantage of looking at the matter from the standpoint of relative solubility is that neither teacher nor student can fail to recognize that the test will succeed only in case the amount of substance formed is more than is necessary to make a saturated solution. Looking at the matter in this way we see that the text-books on qualitative analysis should lay emphasis on the approximate limits of sensitiveness in each particular precipitation. Though this point has been urged for years by Ostwald, it is still ignored very completely in most laboratories. Where a graded scale of solubilities is impracticable, other methods of identification must be resorted to, and these are just as much to be classified under qualitative analysis as those methods which ordinarily pass under that name. Thus a separation by fractional distillation, if carried out for the purposes of identification, is qualitative analysis quite as much as the orthodox precipitations of the inorganic chemist or the more recent “group reactions ” of the organic chemist, and it would give the student a clearer idea of the interrelation of the different branches of chemistry if this point were lnsisted upon more often. Some of these methods are so recognized. In the qualitative tests for sulphur and iodine, we sublime these substances
Analytical Chenzistry
I11
without change. I n blowpipe work on charcoal we leave behind the substance we are to identify. In the case of the borax bead we depend on color changes. T h e absorption spectrum is of great importance as a qualitative test in many rare earth separations. After we have determined what the constituents are in a given mixture, it is only natural to ask how much of each constituent is present. From a scientific point of view quantitative gravimetric analysis is a matter of separation and estimation of a phase, and a course in quantitative gravimetric analysis is very largely a course to develop manipulative skill. I quote Fresenius on this point. ‘( T h e most extensive and solid theoretical acquirements will not enable us, for instance, to determine the amount of common salt present in the solution, if we are without the requisite dexterity to transfer a fluid from one vessel to another without the smallest loss by spurting, running down the sides, etc. T h e various operations of quantitative analysis require great aptitude and manual skill which can be acquired only by practice.” T h e choice of phase for determination in gravimetric analysis depends primarily on the solubility relations as in qualitative analysis, but is subject to the further condition that the new phase must be one which can be weighed accurately, or which can be converted into one which can be so weighed. I n electrolytic work the precipitation is effected by the current instead of by an addition of a reagent. While quantitative gravimetric analysis is a matter of phase separation and estimation, other methods of quantitative analysis are based on relations between some property of a phase and the relative masses of the constituqnts. If we tabulate the way in which any given property varies with the relative mass of any one of the components, we can then use the data thus obtained as a means of determining the relative mass of that constituent provided the property in question varies only with the relative mass of that constituent. T h e density, color, boiling-point, freezing-point, conductivity, index of refraction, temperature of clouding, electromotive force, rotation of the plane of polarized
,
light are a few of the properties that have thus been used. Since many of these properties, such as the density, boilingpoint, etc., vary with the relative masses of all the constituents, these properties can be used only for analysis in the case of twocomponent systems. Thus we can determine the amount of alcohol in aqueous alcohol from the density, but the test fails completely if the alcohol contains acetone, fusel oil, or any other impurity. We can determine the strength of acetic acid by its freezing-point, but the test fails if ammonia has been absorbed from the air. We can determine the amount of phenol in a hot aqueous solution by observing the temperature at which the solution clouds, but the presence of anything else changes the temperature of clouding and makes the method worthless. Conductivity determinations are of no value when two or more salts are present in relatively varying amounts. In the cases where these methods are serviceable, it is to be noticed that the property in question usually varies with the temperatiire and pressure as well as with the relative masses and that each measurement must therefore be made at a definite temperature and pressure to be of any value. T h e flash test for kerosene is one in which so many factors enter that the conditions have to be and are specified in great detail. While many of the properties of systems are useful for analytical purposes practically only in two-component systems, this is by no means necessarily the case. Within certain limits the rotation of the plane of polarized light by an aqueous sugar solution is independent of the nature and relative masses of the other components and consequently this property can be and is used, within these same limits, as a nieans of quantitative analysis. Siniilarly the difference of electrical potential between a metal and an electrolyte is assumed to be a function of the concentration of the same metal as ion in the solution and not to be primarily a question of the concentration of any other substance as ion. S o long as this condition is satisfied, the measurement of an electromotive force gives us the means of analyzing for that metal as ion, a method which has been used very mucli in physical chemistry.
t
r
Aitalydical Chemistry
113
When studying the various properties of multi-component systems, we should naturally include color phenomena in presence of so-called indicators. After we have settled, once for all, that a given color change takes place when the ratio of the masses of an acid and a base exceeds a certain value, we can then m e this knowledge to enable us to determine the amount of an acid in a solution by finding the amount of the base necessary to produce the color change. This is the general principle underlying all quantitative volumetric analysis. I t is to be noticed that this is entirely independent of any hypothesis that we map make to account for the color change occurring at all or occprring at that definite ratio. Indicators had been used for years before the present explanation of their action was advanced and methyl orange is one of the standard indicators, though we are by no means certain what the significance of the color change actually is. A theory as to the cause of the particular ratio is equally superfluous, though possibly very desirable. We find experimentally that the color change occurs for instance when the ratio of sodium hydroxide to hydrochloric acid exceeds 40:36.5 in round numbers, and that is all that is necessary for the volumetric determination of sodium hydroxide by means of hydrochloric acid or vice-versa. If we choose to account for this particular ratio in terms of combining weights and compounds, we may do so ; but our analytical methods rest on the experimental data and not on the hypothesis. This will be seen if we reflect that the facts on which we base the atomic hypothesis were obtained by quantitative analysis. As a matter of fact, we do not always get exact ratios in volumetric analysis. In titrating silver nitrate with sodium chloride we have two distinct end-points, one where further addition of sodium chloride produces no precipitation and one where further addition of silver nitrate produces no precipitation. These two end-points do not coincide and neither corresponds to the atomic ratio between silver and chlorine. Yet we can take either one and get results which are as accurate as the det.errnination of the end-point. When we titrate a fer-
*-
rous solution with permanganate, i t is not necessary that we know the reaction. We standardize against a known solution of ferrous iron. As a matter of fact, we don't use the atomic ratio, because we apply a correction for the amount of permanganate necessary to produce a standard tint. When the iron solution contains hydrochloric acid, other precautions are necessary. I quote from Fresenius : " Make the iron solution to be tested up to 1/4 liter, add 50 cc to a large quantity of water acidified with sulphuric acid (about one liter), titrate with permanganate, then again add 50 cc of the iron solution, and titrate again, etc., etc. T h e numbers obtained at the third or fourth time are taken. These are constant, while the number obtained at the first time, and sometimes also the second time, differs. T h e result multiplied by 5 gives exactly the quantity of permanganate proportional to the amount of ferrous iron present." We shall also see that the analytical results come first and the theory afterwards if we consider the neutralization of phosphoric acid in presence of methyl orange or phenolphthalein, T h e ratios are different with the two indicators, while the nature of the indicator does not affect the ratio when we neutralize sulphuric acid. We re-state the result in terms of the electrolytic dissociation theory, but this is not essential for analytical purposes and our analyses will remain accurate even if the electrolytic dissociation theory be superseded. When i t comes to titrating weak acids in presence of methyl orange, for instance, we find either that the end reaction is not sharp or that i t occurs at a slightly different ratio from that predicted by our theory. I n the first case we discard the indicator; in the second case, we actually adopt the experimental ratio, though we usually disguise this by taking the so-called theoretical ratio' and applying a '( correction.,' Titrating chlorides in slightly acid solutions with silver nitrate when using sulphocyanate as an indicator would be a case in point. I have taken up this question of volumetric analysis at Cf. Ridenour. Jour. Franklin Inst.
,
",.
152, 119 (1901).
Analytical Chemistry
11.5
some length because I wished to bring out that there is no really fundamental difference between analysis by means of density determinations and analysis by titrations. In both cases we construct a table empirically and refer back to that. Inmany cases of titration we can predict our table with a somewhat greater degree of accuracy than we can predict the densities of mixtures from a knowledge of the percentage composition ; but that is all. Our methods of titration or of precipitation are better adapted to in111ti-component systems than many of the so-called physical methods because they are, as a rule, methods for one component or constituent, This, however, is subject to limitations. Precipitation with silver nitrate and weighing as silver chloride is not accurate when bromides or iodides are present. Precipitation with sulphuric acid and weighing as barium sulphate is inaccurate when lead salts are present. Of course no one would make this latter inistake ; but I doubt whether most analysts appreciate the close analogy between removing leid salts before making quantitative determinations for barium and removing all except two components before making a quantitative analysis by density determinations. In the first case the process is a simple one and they have done it often. Ir, the second case the process is not always feasible and it is a problem which rarely occurs. Consequently i t seems to involve entirely new principles. As a matter of fact, there are special advantages and special limitations in each analytical method; but it is a complete mistake to try to draw a sharp line between analysis by comparison of so-called physical properties and analysis by comparison of so-called chemical properties. T h e two pass one into the other and it is utterly impossible for us to say, for instance, whether the density of a given solution is due to the partial formation of a compound or not. I have tried to show the theoretical aspect of analytical chemistry when considered from the standpoint of physical chemistry and this brings us face to face with the problem of how far the teaching of analytical chemistry and of physical chemistry should be modified in consequence of such a point of
view. I can see no reason at present for making any very radical changes, and yet a general broadening would be profitable in both cases. T h e physical chemist should know more analytical chemistry than he usually does. Our mathematical relations are based on results obtained by quantitative determinations and are to be confirmed by other results obtained by quantitative determinations. T h e more accurate our mathematical relations become, the more accurately we must make our determinations if we are to reach any definite conclusion as to the accuracy of our hypotheses. Of course if we are dealing with approximation theory and regard a difference of ten percent as a pretty good agreement, we can afford to be slovenly and inaccurate in our analytical work. In the theory of solutions we have been going through a period of that sort with its attendant results ; but I hope that a new era is dawning when we shall develop exact theory, and when we shall study all solutions and not merely infinitely dilute ones. T h e advantages to the analytical chemist froin the point of view which I have outlined are that he will perceive that there is much more to analytical chemistry than he had perhaps realized, that there are many cases where other methods besides those of gravimetric and volumetric analysis are important, and that he should be familiar with those methods. This has alreadv been recognized by Kriiss in his little book, Spezielle Methoden der Analyse,” published in 1892. I am not suggesting anything really new, but I can carry it further now than Kruss could do nine years ago. I t is essential to know the analytical methods of the physical chemist. I t is quite as important to know them and the subject from the phase rule point of view. One illustration will suffice. An analysis of a piece of steel will show the percentage composition. I t may show a little more, the amount of free” and ((combined” carbon ; but the result of the analysis tells relatively little about the physical properties, which however are the essential things from a technical point of view. A microscopical analysis will give a great ((
((
.
Analytical Chemistry
117
deal of information and is rapidly coming more and more into use. It is quite as important therefore for an analytical chemist in a steel works to understand microchemical analysis as to be familiar with the more usual methods of analysis. Having got the facts, i t is necessary to interpret them, and this cannot be done satisfactorily without a knowledge of the phase rule. T h e microscope may show the existence of a number of compounds ; but it tells nothing as to the relative stability or as to the conditions of formation. Of course these can be worked out empirically and have been so done to a certain extent, but this means an immense waste of time. T h e recent work of Roozeboom in applying the phase rule to iron and steel shows clearly the difference between having a theory to guide one and groping more or less blindly. This has been fully recognized in England at any rate. T h e way in which the properties of a metal or alloy vary with the thermal history is quite as important as the way in which these properties vary with the percentage composition, and the two should be studied together. Is the analytical chemist going to qualify himself for such work or not ? Comell University
