Modern Physico-Chemical Methods.
Owing to the growing importance of physics in chemical analysis there has been a good deal written under such titles as “Physico-analytical Techniques” and “Instrumentation in Analysis,” and it is not pretended that anything original can be added to the subject. Perhaps most chemists will have seen an article on “Physics and the Analyst,” by R. C. Chirnside, that appeared in the Analyst in April, 1945. It is proposed to take this as a guide, following much the same lines, but substituting anything possible from personal experience, and in Iieu of his conclusion on the social implications of the new developments, to consider the proper application of the new methods in New Zealand, and to make some observations on the bearing of the new trends on industry and research here. In the classical gravimetric analysis, the analysis of last century, and the kind that many chemists may sub-consciously regard as the only reliable kind for any important work, the aim is to separate the constituents in the form of pure compounds, and weigh them. To this method was added volumetric analysis, which depends on measuring the amount of reagent to completely precipitate a constituent, neutralise it, or oxidise it. To determine the end of the reaction we require a knowledge of chemical equilibria, solubility products, dissociation constants, introducing, one sees, physical chemistry. The new physical methods introduce radically different procedures. One no longer starts by dissolving the sample and carrying out chemical separations. In spectrochemical analysis a minute portion is raised to a high temperature, and the light emitted is separated into characteristic wavelengths; in mass spectrography the material is broken into ion fragments and these are separated by magnetic and electric fields; in polarography (although one commences with chemical solution), separation potentials and currents are recorded with infinitesimal material separation.
It is this use of physical methods that marks the new development in analysis. Physical instruments have long been used by the chemist. One has only to think of the refractometer, the viscometer, the calorimeter. But these instruments were used in general to determine the properties of materials, not to ascertain their composition. Now it has been pointed out that analysis is the examination of a material to ascertain its composition, its properties and its qualities, so that the earlier use was definitely a part of analysis, but now the use of physical methods has extended to cover the whole analysis.
Reasons for the New Development of Physical Methods.
The first reason to be discussed is the realisation by both industry and research of the importance of minor constituents formerly neglected. It is only necessary here to mention the effect of lead in zinc-base die-casting alloy, the low cobalt content of bush-sick soil, and ascorbic acid in fruit and vegetables. The need for these determinations has called forth methods particularly adapted to them. Some of these are not dependent on the recent advances in physics, and there does not seem to be any reason why the use of the spectrograph could not have developed long before it did, likewise visual absorptiometry.
A point that may be of interest is that some of the new techniques estimate quantities on a logarithmic scale that corresponds better to actual facts than does the usual scale. Chemists have become familiar with a logarithmic scale in the pH scale. This is perhaps not a good example, as what one is really interested in pH is acid-base equilibria and not the actual number of hydrogen ions. Now a characteristic of the spectrographic method is that amounts are recorded on the spectrogram on a log scale, for the density of the line is roughly proportional to the logarithm of the concentration. This means that one determination can cover an enormous range of possible concentrations, e.g., for chromium from 10−8 to 1, or 0.0001 per cent, to 100 per cent. Now it will often be quite sufficient to say that a content is between 10−8 and 10−5 whereas it is not accurate enough to state the content as between 10−2 and 10−2, i.e., between 10 and 1 per cent., just as we cannot use the pH meter to determine the acid in a nearly normal solution.
The second reason put forward for the recent advances in analytical methods is that the potential demand in industry for rapid control methods could only become effective when the new revolutionary techniques in physics became available. It is particularly the advances in electronics that have made possible direct instrumental methods of analysis, methods in which the analysis may be read off on a dial or chart. Such methods have existed previously in special cases, as. for example, the carbon-dioxide recorder for flue gases, but now there will be a great extension of the use of highly complex instruments which require-little operator skill because they are semi-automatic. The production of photo-multiplier cells which are sensitive to minute amounts of radiation have made possible the direct measurement of the intensity of spectrum lines, and hence the direct - reading spectograph, the Quantometer.
Advances in the detection of infra-red radiation have lead to the development of recording infra-red spectrometers, applicable to automatic analysis of organic liquids in plant control, as in petroleum refineries.
The technique of mass spectrometric analysis has developed from the part played by the mass spectrograph in the study of the isotopic constitution of the elements. In the mass spectrometer, molecules bombarded by electrons produce positively ionised fragments, the relative abundance of which are characteristic of the molecule. Computation of the results is complex, but electrical instruments to solve many simultaneous equations are available to give results directly. This instrument, besides being of application in industry as in synthetic rubber plants, is required in research for isotopic tracer work using the stable isotopes of carbon and nitrogen.
Another technique on the point of making great advances is the use of the radioactive isotopes becoming available from atomic nuclear fission in the atomic pile. The use of radioactive isotopes is similar to putting a spy in an enemy country to send information, but the sensitivity of detection of radioactivity is so great that it is equivalent to having one spy in a population several times that of the earth. It can also be pointed out that by spectrographic methods 10 atoms may be detected, whereas 5.106 radioactive atoms may be detected. In a gram of matter, taking the universal presence of all elements, there will be in the case of one of the rarest, rhenium, about 1012 rhenium atoms, thus this rare element could probably be estimated anywhere in a small sample. Moreover, methods based on the use of radioactive isotopes would lend themselves readily to direct instrumental measurement.
Methods Based on the Use of Radiation.
But it is desired to confine this discussion mainly to the use of physical methods in analysis, and there is only space here to discuss those methods based on radiation, either its emission or its absorption.
The first to be considered is emission spectrography. In principle it depends on the analysis of radiation emitted by atoms of the sample heated to high temperature. This means that it can only give information about the elements present with no reference as to how they are combined. Most of the non-metals cannot be detected, so that its field is confined to inorganic chemistry, it is most useful for metals, with salts it can tell us only about the cationic constituents, not the anionic. Nevertheless, emission spectrography is the best means yet available for detecting certainly a large number of elements at once, and its greatest use is still for rapid qualitative analysis. However, it is, even for
metals, not a universal method, for many elements are of low sensitivity. Development in spectrography has by no means comes to an end, and progress is being made in the examination of refractory material, in the detection of non-metals, and in the combined uses of spectrographic and chemical methods, such as the use of the new organic reagents.
Emission spectrography developed on the basis of spectrographs in which there has been little change, progress being mainly in accessories and methods. Absorptiometry has developed from the simple instruments used for colorimetry, through colorimeters, filter absorptiometers, spectrophotometers, and photoelectric spectrophotmeters. Absorptiometry depends on the absorption of light by the molecule, and even in vapours, absorption by the molecule is accompanied by electronic, vibrational and rotational changes, so the absorption spectrum is a complicated one of bands of fine lines, not single lines. Moreover, in liquids interference causes a blurring of even these bands, and absorption is in the broad bands. Hence anything like the separation of atomic emission lines is impossible, and only a few absorbing constituents can be estimated together. Absorptiometry has the advantage of giving us a means of estimating the actual compounds present, and in the ultra-violet it is particularly useful for organic compounds, as, for example, in the determination of Vitamin A in fish oils or linoleic acid in drying oils. Owing to the overlapping of bands, in general, a preliminary chemical separation is required or the production of an absorptive compound by a selective reagent. For the latter the new organic reagents have opened a wide field for absorptiometry in the visible.
Another way out of the difficulty is found if one uses rays of much longer wave-length. In the infra-red, absorption is due only to vibrational and rotational quanta. In liquids the rotational fine structure is obliterated, and one is concerned only with vibrational frequencies. As the magnitude of the vibrational quanta depends not only on the masses of the components of the molecule, but also on the configuration, every molecular species has a characteristic infra-red spectrum. Present day prism instruments, using artificial crystals, can pass rays to 30μ and have sufficient resolution to make them powerful analytical tools. Speed of analysis is being made sufficient for industrial use by the development of sensitive detectors. Progress is still being made, for instance, recording by the oscilloscope, and the new German lead sulphide photo-cell sensitive to infrared promises much. The field of the method is the analysis of organic liquid mixtures and gaseous mixtures.
Liquids are sufficiently analysed if the molecule species present are determined, but as regards solids neither the elementary nor the molecular composition is sufficient. The properties will also depend on the actual crystalline structure. Another form of radiation, X-rays, can assist here. The scattering of monocromatic X-ray radiation gives a pattern of lines that can inform us of the crystalline structure. The most generally applicable method is the powder method, and recorded data for about 3,000 substances are now available to enable a rapid identification of most common inorganic crystalline materials from the X-ray diffraction data.
To complete the story, it must be pointed out that even if the crystalline structure is known, we do not know all the properties of a sample. It is necessary to know the size and shape of the aggregates of which the sample is built up. If these were below the range of the microscope, there were formerly various methods of determining the size of particle in the colloidal range, but up till recently there was no method of determining the shape of the particles. A beam of electrons has an equivalent wave-length much shorter even than X-rays, and the electron microscope, using an electron beam instead of light, enables one to see particles far smaller than are visible in the microscope, and for the first time is is possible to see the shapes of sub-microscopic particles such as constitute carbon black, zinc dust, and other fumes, pigments and emulsions.
Rome Examples of the Uses of Spectrography.
In order to illustrate the utility and versatility of a physical instrument, it is proposed to discuss some of the methods that have been applied with the small wave-length spectrometer, with camera attachment, which can be transported for demonstration, whereas the large quartz spectrograph is hardly portable.
(1) The simplest and most rapid use of a spectrographic method is the spark examination of a metal and is is particularly favourable for zinc-base die-casting
alloy. Suitable electrodes are easily cut from the runners to the castings, and the ends are soon filed. The spark between two electrodes placed 7 cm. from the slit gives a spectrogram which is compared with that from a standard alloy with the impurities allowed by the specification. Fortunately, this standard is easily prepared by melting pure zinc with the addition of the required constituents. It is possible to determine lead, iron, copper, magnesium and cadmium. In this routine examination, it is best to collaborate with the chemist by having any rejected samples analysed by a chemical method, instead of attempting to get the exact amount by more work at the spectrographic method.
(2) Qualitative or semi-quantitative examination of steels: A great variety of alloys is covered by the term steel, and it is a great help to the analyst if he knows what to determine and the approximate amount. It is not so easy in this case to find a method of spectrographic analysis. The samples are in many forms, and it is very difficult to make uniform electrodes from them. It is not possible to make synthetic standards unless solutions are used, and for speed chemical treatment is to be avoided. The method finally adopted is to use 50 mg. chips or turnings. This weight is placed on the flat end of an 11 mm. diam. iron electrode and arced with D.C. current of 5 amps., with an upper electrode of 5 mm. diam. Owing to the intensity of the iron are it is necessary to reduce the illumination to permit the burning time to be enough to consume the sample. This is done by projecting an intermediate image on a hole in a screen. After the lens a slotted diaphragm can be placed to reduce the illumination to any extent desired. Standards samples are obtained first from pieces cut from analysed electrodes obtained from Hilger's and from the U.S. National Bureau of Standards, secondly from portions of filings from standard samples, either British Chemical Standards or N.B.S., thirdly from samples analysed in the Dominion Laboratory. To permit rapid examination for each constituent, standard plates are prepared, each containing about ten spectrograms of standards with one element arranged in ascending order, say, chromium from 0.02 per cent, to 18 per cent. To allow for variations in exposure a rotating step sector is used before the slit, so that each spectrogram is in four steps of different exposure. With the use of the Judd Lewis comparator, to compare the two plates, it is possible to pick a step on the sample spectrogram to match one on any standard in regard to iron lines. With this method it is possible to get the approximate composition to within about 50 per cent. Not only is this estimate useful to the analyst, but if some constituents are not significant this estimate is sufficient, or if several samples all appear alike, one only need be analysed.
(3) Corrosion problems: The method used here illustrates the use of spectrographic analysis as a micro-method, and also the need for combined chemical and physical work. The corrosion product is dissolved in acid, and the metallic constituent removed by precipitation. The evaporated residue is weighed, dissolved in a definite volume, and 0.02 ml. evaporated on a copper electrode. The residue is arced with an interrupted direct arc, and in order to improve the uniformity, the lower electrode is oscillated by a double pendulum device.
On the spectrogram, sodium, potassium, calcium, magnesium, strontium and barium can be detected, and the relative proportions of these is often a good indication of the cause of the corrosion, especially if due to sea-water.
(4) Quantitative estimation of lithium: Lithium commonly occurs in mineral waters, and although only in small amount, it should not be neglected, especially as, owing to its small equivalent weight, in balancing cations and anions it counts for four times as much as the equivalent of the sodium it is generally included with.
In this method the solution from which all but alkalies are removed is evaporated with sulphuric acid and the residue made into pellets. These are arced on graphite electrodes. In this case the bands so troublesome with graphite electrodes do not interfere. Plates sensitive to the infra-red for lines up to 9,000 A. are used. Lithium, rubidium and caesium can be estimated by comparing the spectrograms with those of standards containing sodium sulphate with known amounts of potassium, lithium, rubidium and caesium. The agreement between the chemical and spectrographic results for potassium as shown in the table indicates that a fair accuracy can be obtained.
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(5) Quantitative estimation of strontium: This is an example of the use of spectrographic analysis for the estimation of a single element. In general, a spectrochemical method is economical only for a number of elements together, or else for a large number of routine analyses. In this case, however, there is no simple chemical or colorimetric method for strontium, so that work on a spectrographic method for strontium in rock analysis seemed justified. After a method for determining it in the calcium oxide separated in analysis had been used, it seemed desirable to determine it directly from the rock sample. The method is to remove silica with hydrofluoric and nitric acids. The analysis of the rock must be known for to the sample is added enough aluminium nitrate. ferric nitrate, magnesium, sodium and potassium nitrates to give a uniform composition for all samples. A portion, 0.02 ml., of the solution containing also a spectrographic buffer is evaporated on a copper electrode and sparked on the oscillating pendulum stand, mentioned before. Standards with known amounts of strontium are also sparked. In this case the spectrograms are evaluated with the microphotometer, by means of which the density of the lines is determined. This determination has been mentioned because it brings up another problem in connection with spectrographic analysis. The difficulty is that the density of the lines on the plate is not simply related to the intensity of the illumination. Now it is the light from the spark which is proportional to the number of atoms of the element to be determined, so that to determine from the plate the light emitted, it is necessary to calibrate the plate. There have been several methods of doing this. One is to use a rotating step sector. Another is to put a grey filter over half the slit so that there is a definite difference. which does not need to be known, between the two halves of all the lines in one spectrogram. By measuring both portions of a number of lines with the microphotometer, it is possible to construct the calibration curve.
During the war, one of the projects of the War Metallurgy Committee of the U.S. War Production Board was to have determinations made of the relative intensities of suitable lines in the iron arc. To calibrate a plate it is only necessary to make a spectrogram of the iron arc, and measure the density on the plate of a number of iron lines. So far, only a little has been done in this field, and the second method has been tried, but the third seems promising.
The Rational Employment of Physical Methods.
These few illustrations should enable us to give consideration to the suitable application of physico-chemical methods. Different factors must be weighed according as the location is a small industrial laboratory, a general analytical laboratory, or a specialised research laboratory. For either of the first two, if there are definite routine determinations that are known to be suitable applications for a physical instrument, it is easily judged that it should be obtained, it may be for only one application as, for example, Vitamin A by the quartz spectrograph, or zinc by the polarograph. In such a case the method can be learned by any competent analyst. However, if the apparatus is to be got for general purposes, the working out of methods for a few determinations may take more time than is justified, and in any case it will be necessary to have someone available to specialise in the use of the instrument. As an illustration, it may be pointed out that of the five examples in spectrographic analysis given, only the first is described previously, and even that not for this particular type of instrument. On the other hand, the specialised research
laboratory may need physical instruments for a certain project, but there it will be possible to consider improvisations and adaptations, and the generally adaptable basic instruments will meet its needs without the more expensive accessories or refinements for rapid working that a routine laboratory would not forego.
Some of the newer instrumental methods of analysis are going to be very expensive. In this respect New Zealand industries may be at a disadvantage in routine control of their products. However, it cannot be said that the question yet arises, for the field of these instruments is as yet in industries such as petroleum refining, synthetic rubber, and polymers for plastics, that do not exist here. Even in regard to metals the rapid instruments are required for primary production of metals and alloys, whereas here there are likely to be only secondary metal products.
Similarly in regard to the use of some of these instruments in research, they are going to be very expensive, too expensive for any but very large laboratories. In this respect, a small country like New Zealand is going to be at a disadvantage. It is very desirable, however, that such instruments should be available when needed in an investigation. Hence it will be a good policy even if there is not much demand for some of this equipment, to see that at least one is obtained by some institution in New Zealand prepared to carry out work with it for other investigators.
Another deduction that can legitimately be made from the new developments is that consideration will have to be given to the part this country can take in pure research. Most of the problems in fundamental investigation will tend more and more to be solved with the aid of the expensive equipment available in well-endowed institutions of the major industrial countries. It has seemed to many of us workers in applied chemistry that, particularly in physics and chemistry, the University Colleges have not given sufficient attention to the problems for which New Zealand conditions offer opportunities, facilities or inspiration. It would seem that with our limited means, it would be better for institutions here with opportunities to do pure research, not to attempt to play a very minor part in general world research, but to concentrate on the investigations suggested by local problems.
Broader Implications of the New Methods.
The new instrumental methods are going to make greater demands on the training of the chemist. It seems that in future there will be increasing demands on the qualifications of a chemist, he will have to be something of a physicist (including an electronic expert), a metallurgist, an engineer and a geologist. In passing one may wonder whether he can continue here to get that knowledge in a four-year course, and at the same time produce a thesis for an M.Sc. degree, whereas in other countries a four-year course is needed for a B.Sc. Honours degree.
However, it is becoming more and more difficult for one man to master the whole field of chemistry, and it would appear that analysis will have to become more and more the co-operative effort of a team of experts. The attainment of this co-operation, however, will present problems in laboratory organisation. It is hardly possible to organise a laboratory on the basis of the instruments used, and even in a large laboratory, the chemist on a problem tends to carry through the whole investigation himself even by conservative methods, rather than share the investigation with a specialist or, on the other hand, he wants to make the physical chemist a hack worker, providing only routine results, without sharing in the solution of the problem. Indeed Chirnside rather welcomes this tendency, and looks forward to the time when more and more of the routine work can be put off on the physicist with his instrumental methods, leaving the chemist free to develop the chemical problems of analysis. In any case spectrographic analysis remains the main example of a method that needs such expensive apparatus and special experience that a separately organised laboratory may be required for it. The tendency will be to simplify the design and operation of physical instruments, so that the metallurgist, the organic chemist, the food chemist, each can have his own.
Finally it should be stressed (and this may be a comfort to the older type of chemist skilled in inorganic or organic methods, and suspicious of the new developments), that the main problems in analysis remain chemical. From
personal experience it can be said that the difficult problems in spectrographic analysis have been the chemical ones. The physical problems can be mastered with improvements in apparatus and technique, but the chemical ones are the difficult ones, the preparation of the sample in the form suitable for the method, the interpretation of the significance of the results. The spectrographic method is by no means yet a universal one, there are many elements not detected and great differences in sensitivity, and refractory materials offer difficulty. The shortcomings may be overcome by a combination of the chemical and physical methods, but the chemical problems are not those to which there are answers at hand, for instance selective precipitants are familiar, but in spectrochemical work we seek precipitants for groups of elements together.
In a new field opened by physics, radioactive tracer work, recent inquiries show that inorganic chemistry has an important part to play. New techniques will have to be developed to carry out operations with dangerous materials behind screens with a minimum of handling, and methods will have to be devised for separations with the stress on simplicity and speed.
To conclude, it seems that physical methods will offer to chemistry analytical tools of increasing precision and rapidity of application. Chemical analysis will become less and less of a laboriously learned art, the chemist will be more and more relieved, by automatic instruments, of the drudgery of routine analysis and he will be freer to use his training and experience on more fundamental chemical problems.