CLASSIFICATION OF METHODS IN QUANTITATIVE CHEMICAL ANALYSIS GORDON D. PATTERSON, JR. and M. G. MELLON Purdue University, Lafayette, Indiana
ANALYTICAL chemistry, like other ever-changing, constantly expanding branches of knowledge and science, suffersfrom the lack of any universally accepted and used outline of the areas of its content. The attendant variations in nomenclature and differences in opinion regarding the subject matter to be covered complicate intelligent discussion by workers in the field and unnecessarily hinder productive study by students attempting to familiarize themselves with its phenomena. In the belief that the need is great for continually examining current concepts and devising better organized classifications of the methods of quantitative chemical analysis whose sections will be based on consistency, analogy,, clarity, and continuity, the following outline and discussion is presented as a suggested step in the right direction. Any outline must start with a consideration of what the field to be covered has in common in all its parts. As has been stated previously,' every analysis can he regarded as consisting of operations on two general materials, (a) the sample and ( b ) the desired constituent. Therefore, the outline to be proposed is first divided into these two sections. THE SAMPLE
In connection with the sample, it may be agreed that, in general, samples must not only be (a) selected and prepared, but (b) measured and (c) given preliminary treatment. Therefore, these three are placed as the primary divisions under "The Sample." The factors to be considered in each op&ation have been discussed elsewhere.'. It should be remembered, of course, that operations on samples in various kinds of analyses do not always include all three steps. In efforts toward simplification, consistent with desired accuracy and precision, the first two steps are often combined, and the third may be eliminated altogether. Such simplification is especially desirable in the automatization either of sampling operations or of complete procedures which will be discussed in another paper. THE DESIRED CONSTITUENT
The desired constituent may have to be separated from interfering substances before it can undergo measurement, the ouerational goal of everv analvsis.
' MELWN,M. G., J. CHEM.EDUC., 25,610 (1948). W E L W N ,M. G., "Methods of Quantitative Chemical Andysis," Macrnillrtn Ca., New York, 1937.
The two divisions under "The Desired Constituent," then, are (a) separation and (b) measurement. Again, these steps in an actual analysis are simplified as much as possible, separation processes being avoided whenever feasible. Separation. The operation of dividing a system into two or more phases may be called separation. Whenever the system under consideration is to he measured by a physical property whose magnitude is dependent on foreign constituents as -.ell as the desired constituent, some kind of separation is necessary. In general, this means that the system must be altered chemically in some way so as to cause the desired physical or mechanical severance; in fact, the separation step is frequently the only chemical part of a "chemical analysis." Separation techniques which are most commonly used may be divided into five major groups: (a) precipitation, (b) electrodeposition, (c) volatilization, (d) extraction, and (e) magnetic separation (magnetization). Precipitation and filtration of a substance of known and constant composition (either the desired constituent itself or a substance accurately,related to it), followed by gravimetric measurement of the product, has long been fundamental in classical methods of analysis. The change of phase from liquid to solid for separation from a liquid medium is accomplished usually by means of chemical reaction or by a change in the solvent. Separation by electrodeposition consists essentially in combining or separating elections and ions. The resultant plating of anions or cations on suitable electrodes accomplishes a phase transformation analogous to that in the precipitation process. Volatilization involves a phase transformation from a solid or liquid to a gas or vapor. This type of separation is particularly useful i n work with petroleum constituents. For example, the spread between the boiling points of gaseous hydrocarbons is large enough to warrant use of fractional distillation as a quantitative separation. Fifteen theoretical plates are necessary for the quantitative separation of low molecular weight gaseous hydrocarbons. Due to the failure of combustion or explosion classical methods to give sufficient information, the fractional distillation technique is a t the present time a basic analytical method. Extraction as a method of separation is a term used to include such operations as dissolution, absorption, adsorption, ion exchange, dialysis, and partition.
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These all have in common the "leaching out" of a constituent by the direct action of some solid or liquid reagent on the sample (solid, liquid, or gas) in a process which may be either physical or chemical or both. Use of magnetism as a separative force is made mainly in the mass spectra procedures to accomplish the separation of charged ions according to their masses. Measurement. The measurement operation must always provide information as to how the substance being studied may be expected to behave under various conditions and may involve not only determination of its chemical composition, but also actual performance tests under artificially produced conditions. The dividing lmes between the methods in analytical chemistry, pure physical measurements, and performance tests are hazy at best, and become even more so when one considers the trend toward automatic methods. Inasmuch as the measurement operation is the climax of all quantitative analyses, the naming of the various classes of methods should be based on the kind of measurement made. Individual methods, of course, are named as indicated in the 1947 report of the Committee on Naming Analytical method^,^ and it is believed that a class name which best indioatea the fundamental operation involved in measuring the desired constituent is an essential part of a logical outline. It is the measurement of the actual quantity of the desired constituent present which has received the most attention in recent developments of analytical methods, especially in efforts toward automatization. Primarily, of course, the measurement operation, in itself, is always one of the magnitude of some physical property, and this fact presents a reasonable opportunity for devising a systematic basic nomenclature. Physical Properties. Any property which describes a substance, either qualitatively or quantitatively, is a physical property, and any such property whose magnitude can be measured accurately can serve as the basis of a quantitative analytical method. Many of these have been so used, and new possibilities are being studied with increasing intensity. The more k i d s of unique properties which a substance is kn8wn to have, the more possible will be the choice of some one whose measurement will provide the analyst with a relatively specific method giving the desired information quickly, accurately, and inexpensively. Hydrogen, for example, differs widely from oxygen in thermal conductivity, density, refractive index, viscosity, and specific heat. There are many ways of determining these properties, each of which is theoretically useful for quantitative analysis. Automatic devices not only may enable the study of such a property, but frequently, themselves, are horn in the discovery of the dependence of some physical magnitude on the concentration of a desired constituent. Such properties may be known for many years without being so utilized until an urgent need arises. Whenever possible, the physical property related to a
MELION,M. G., ETAL., Anal. Chem., 19,931 (1947).
the desired constituent is measured directly on the sample system, as was emphasized in the discussion of simplifying the sampling steps. This search for properties more or less specifically related to the desired constituent, but which may be measured on the system on a whole, has resulted in further diversifying the strictly physical measurements included under the title of "analytical chemistry." These have become essential to the analytical chemist, as indeed physical measurements have always been. It is clear that the only chemistry involved in a quantitative analysis scheme is the preparation of the sample and the separation of the desired constituent preparatory to the inherent physical measurement. Furthermore, the trend in all methods, automatic or otherwise, is away from the necessity of such preparation and separation whenever possible. Thus the artificial boundaries between "analytical chemistry" and the other fields of science become less and less distinct, as more and more physical properties are adapted for quantitative analytical methods. CLASSIFICATION OF PHYSICAL PROPERTIES
In considering the problem of classifying and presenting physical properties, the measurement of which serves the purpose a t hand, whatever it may be, it seems reasonable to divide all physical properties into two groups, static and dynamic. Static properties of any system would be those which are characteristic of the system as such and which do not fundamentally involve external measurement of energy (or particles) passing from or through the system. Such physical properties. as mass, volume, density, pressure, viscosity, temperature, magnetic and chemical reactive properties may be considered to be inherent or "contained" in the system and do not fundamentally require the application or emission of external energy waves or particle waves for their measurement. Dynamic properties, on the other hand, are those whose measurement depends on the motion of energy waves or particles passing through, or originating in, the system. Such phenomena exhibit many characteristic behaviors and numerous techniques have been devised for handling them. All optical properties belong here, as do those for-the entire electromagnetic spectrum. Here may be included measurements of heat conduction, electrical phenomena, sound, and radioactivity. Fundamentally then, these are the properties which control the transport of waves, whether their nature be electromagnetic, electric, sonic, thermal, subatomic, atomic, or molecular. The properties thus classed as dynamic have been further divided into those due to (a) energy wave motion, and (b) matter or particle wave motion. A strict division of properties into those related to (a) energy and (b) particles is complicated by the fact that many particles (e. g., the electron) exhibit wavelie properties, while on the other hand such things as light, odmarily de-
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scribed in terms of waves, must be considered in terms of photon particles in order to explain certain phenomena. According to Gla~stone,~ however, matter waves differ fundamentally from the electromagnetic waves constituting ordinary radiant energy, due to the anomaly to which the folloming equations lead: From de Broglie's theory, A = h/mu
(1)
where X is the wave length, h is Planck's constant, m is the mass, v is the "group velocity" of the matter waves which differs from the velocity u of the waves themselves (their "phase velocity"), and u is the frequency. For matter waves, Thus, according to the quantum theory, Energy of the particle = hv = hu/X
(3)
By relativity, Energy = me2 = hulk
(4)
And thus from (1) and (4), Z'2i
= ca
Since v must always be iess than c, it would follow that the phase velocity, u, of the material waves must always be greater than that of light. Partially from this unexplained anomaly the above conclusion -concerning the fundamental difference between energy waves and matter waves was made. Further subdivision of the general classes is based on the nature of the energy or particles concerned, generally in the order of increasing energy or mass (decreasing wave length). It will be noted that the letters "imetry" are consistently used for the endings of all class names. It is felt that this involves least disturbance of the present nomenclature and forms a basis for a reasonably self-descriptive consistat terminology which permits classifying all methods and yet is flexible enough to permit additions or minor rearrangements as such changes becomer desirable. A few remarks concerning several of the measurement classes seem in order. Under "Static Properties" the heading L'Temperature-thermimetry" is meant to include the measurement of temperature for such properties as colligative properties (e. g., change of freezing or boiling point or vapor pressure) and dew point. Two possibilities, heat of reaction and thermal conductivity, are excluded from this class, only due to their inherent importance and logical fit under other categories of this classificat,ion. Under "Dynamic Properties" the phenomena listed under wave motion are electromagnetic in nature except sound. They are listed in order of decreasing wave lengths, and each range is selected and named best to include a group of measurement techniques GLASSMNE, S., "Textbook of Physical Chemistry," Van Nostrand, Inc., New York, 1946, p. 14.
which are as nearly fundamentally autonomous or intrinsic as possible. Actually, of course, the spectrum is continuous and has no sharply defined general boundaries to facilitate such a classification. The longest waves made use of are those known as sound waves. Here, the term sonimetry is used in a sense to include both frequencies which can be heard by the human ear plus those in the supersonic ranges. Analytical applications are limited as yet, but basic studies are being made as the necessary equipment becomes available, and it seems not impossible that velocity and resonant-frequency measurements of sound will find usefulness in giving quantitative information in various applications. The longest electromagnetic waves are those in the radio range of the spectrum. Thus far, the so-called "microwaves," those so useful in the science of radar developed during the last mar, have been found sensitive to water (and other molecules such as ammonia) in the atmosphere at a wave length of 1.33 cm., although there is little definite indication as yet that actual quantitative analytical determinations can be based on these waves. The four wave-length regions considered under the term "radiant energy," i. e., infrared, visible, ultraviolet, and X-ray, involve fundamentally similar principles and so are considered together. This is not to say arbitrarily that waves in the other spectral regions should not be classed as forms of radiant energy. Doubtless, measurements in such regions will become as thoroughly studied and understood as are those in the four ordinarily considered to compose the radiant energy spectrum. However, at the present stage of usefulness of the spectrum as a whole to quantitative analysis, it seems logical to include only the four mentioned. The phenomena connected with radiant energy may be classed under three headings. The measured quantity depends on (a) an emission, (b) a transmission, or ( c ) a reflection of radiant energy. Thus the main subheadings are termed emissimetrp, transmissimetry, and reflectimetry. The various types of emissimetry include: flame photimetry (derived from the familiar "flame photometer"); emission spectrimetry (used in place of "emission spectrometry" and including line spectra due to electrical excitation); ramanimetry, for the Raman effect; fluorescimetry io include measurement of radiant energy emitted due to the process of fluorescence; and pbosphorimetry similarly to include measurement of phosphorescence. The phenomena accompanying transmission of radiant energy include absorption, thermal conduction, turbidity, optical rotation, refraction, and diffraction. Methods depending on measurements of these phenomena have been named absorptimetry, thermoconductimetry, turbidimetry, optirotimetry, refractimetry, and diffractimetry, respectively. Two types of reflection measurements are included: reflectimetry where radiant energy is reflected from a plane surface, and nephelimetry where the radiant
SEPTEMBER. 1949
energy is reflected by suspended particles in a medium due to the Tyndall effect. The two wave-length areas of still higher energies include the penetrating rays of the gamma and cosmic regions. Analytical uses of gamma rays are at present limited but may potentially find wider application. Cosmic rays are of course still out of the question in so far as the current status of quantitative analysis is concerned. The second general type of dynamic properties is connected with particle motion. Here the subdivisions are based on the nature of the particles in question, atoms and atom groups, electrons, and nuclear particles. Mass spectra are obtained as a result of the first of these and the term masspectrimetry covers measurements in this field. Numerous methods depend on the transport of electrons through a system. These are divided into the following five groups: conductimetry, dependent on the conductivity or resistance of the sample; potentiimetry, which involves comparison of the electrical potential of two points in a system; currentimetry, where the current flow is measured; dielectimetry, dependent on the capacitance or dielectric constant of the substance being investigated; and electron diffractimetry, where electron diffraction patterns are measured. A sub-group of sub-atomic particles such as beta and alpha particles and neutrons and is called radioactimetry. OUTLINE OF QUANTITATIVE CHEMICAL ANALYSIS THESAMPLE I. Selection and preparation 11. Measurement 111. Preliminary treatment I. Separation A. Precipitation. . B. Electrodeposlt~on
C. Volatilization D. Extraction E. Magnetization Measurement IIa. Static Prooerties
1. Combining poGer-titrimetry 2. Hest of reaction-calorimetry 3. Speed of reachn-hronimetry IIb. Dynamic properties A. Wave motion 1. Sound-sonimetry 2. Radio (microwaves)-radiimetry 3. Radiant energy (infrared, visible, ultraviolet, X-ray) a. Emission+missimetry a'. Flame photimetry b'. Emission speotrimetry o'. Ramsneffcct-ramrtnimetrv
e'.
Phosphorescence-phosphorhetry
t,. l ' r n n i t n i ~ i i u n - t r u n s ~ ~ ~ i i i i ~ n t ~ ~ r ~ a'. Srlcctivr st>sorl,tion-absorptimetry
b'.
l'lstual cor~cl~~ctioa-~lt~;r~~~oo~onduo....> ".J Turbidity-turbidimetry
"
c'. d'.
Opticalrobtion-optirotimetry Refraction-refractimetrv f'. Diffraction-diffraetimetb c. Reflection a'. Reflectimetry b'. Tvndall effeet-neohelimetrv 4. Gamma rays-gammimeh 5. Cosmic rays-asmimetry B. Particle motion 1. Atoms and atom groups a. Mass spectra-masspectrimetry 2. Subatomic particles e'.
e'.
Current-krentimetryf
a'. ~ e tparticles i h'. Alpha particles o'. Neutrons
