Ed it ion
Analytical Volume 1
JANUARY 15, 1929
Number 1
Quantitative Analysis with the Spectrograph' Charles C. Nitchie THENEWJSRSEY ZINC Co., PALMERTON, PA.
The superiority of spectrographic methods for the a l l o y s whose physical and estimation of small amounts of i m chemical properties, such as requirements of modern constituents in materials is PO hardness, ductility, and reindustrial operations, a advantages are (1) speed, (2) sistance to corrosion, are prolaboratory must be in a posicertainty in the identification of foundly modified by appartion to turn out analytical redetermined. Illustrative exam ently insignificant traces of sults in the shortest possible kinds of work for which they may be used. i m p u r i t i e s , In many cases time, consistent with reasonThe general principles of quantitative methods a t h e causes of variations in able accuracy. Too often the discussed, certain sources of error are noted and mea these properties have not been result of a p e r f e c t l y good suggested for overcoming them. recognized because of the analysis is of value only as an p r a c t i c a l impossibility of explanation of defects and I Methods which have been developed in the labor tory of The New Jersey Zinc Company are describe making the necessary analydifficulties, rather than as a These methods are used in carrying out several hundr ses. In such cases the specguide in the production or analyses each month, many of them being routine con- \ trograph provides the means selection of the material introl analyses. both for tracing these causes, volved. This is particularly and, when thev are known, true with analvses for those minute amounts of impurities, or of necessary constituents, for controlling the variations by proper selectibn of raw mawhich often modify to a remarkable extent the chemical or terials. In this way it has been possible to show that significant and reproducible effects are caused by the presence in a physical properties of materials. This being the case, it is surprising that so little attention metal of less than 0.001 per cent of some impurity whose has been paid t o the spectrograph as a means for carrying presence had not been suspected until it was revealed, and its out quantitative analyses. I t has long been recognized as amount estimated, by means of the spectrograph. an instrument for qualitative work, but even in this field its For such sinall amounts as these, ordinary chemical methusefulness has not met with the general recognition which it ods of analysis are often unsatisfactory. Large samples are deserves, particularly in industrial laboratories. Its remark- required, necessitating the handling of large volumes of soluable sensitiveness and the certainty and speed with which tions. Separations are incomplete, involving numerous remany of the elements niay be detected place it in a class by solutions and re-precipitations before it can be assumed that a itself, far superior, in many respects, to the ordinary methods precipitate is free from contamination- an assumption which all too often is not justified by the facts. With the large volumes of solutions used, the very slight solubility of nominally insoluble precipitates may cause significant errors. Finally Type of Work for Which Adapted the presence of unsuspected constituents, with analytical properties similar to those of an element which is being deSeveral types of analytical work fall particularly within its termined, may lead to totally erroneous results. Technical scope: methods of analysis ordinarily provide for the removal of only the most common interfering elements. As a result cases 1-Quantitative estimations of traces of impurities and minor constituents, when the amounts present are too small have been encountered in which a final precipitate, weighed and calculated as a compound of one element, has been found, for satisfactory chemical determination. 2-Rapid estimation of elements, present in small amounts, by spectral analysis, to contain none of that element whatever, which could be determined chemically, but only by complicated but to be really a compound of another element whose presand slow methods. ence was not anticipated when the method used was devised. 3-Determination of the approximate composition of materials RAPIDESTIBIATIONS-In the second class occur such dewhen the amounts available for analytical samples are too small for chemical analysis. terminations as that of cadmium in zinc (especially in highgrade zinc, containing less than 0.01 per cent of cadmium). DETERMIYATIOK OF IMPURITIES, ETc.-cnder the first To insure accurate results by chemical methods, a long and heading may be included the analysis of many metals and time-consuming series of separations is necessary and the analysis usually requires several days. I n contrast with 1 Received June 7, 1928.
I
I
' );.
--.
IK ORDER to satisfy the
2
ANALYTICAL EDITION
this, the spectrographic method requires but a small sample, it involves no chemical separations whatever, and yields just as accurate results in an hour’s time. Another typical example in this laboratory was the emergency analysis of five samples of zinc-base alloys for tin, magnesium, cadmium, and lead-all within 3 hours. At the same time it was definitely established that all other elements, which might have had a bearing on the particular problem then under investigation, were absent. At least 2 Qr 3 days would have been required had these been determined by ordinary methods. Similar savings in time are possible in the determination of many other elements. I n addition to those already mentioned, lithium, sodium, copper, silver, beryllium, aluminum, thallium, antimony, bismuth, manganese, and nickel have all been estimated-mostly in alloys and other zincbearing materials. Any element which yields a spectrum containing lines which permit its identification in the particular material involved can be determined in an hour or two. Most of them require as many days for estimation with equal or less accuracy by chemical methods, when present in the small amounts for which the spectrograph is particularly adapted. ANALYSIS OF MINUTE SAiwLEs-The third class, the analysis of minute amounts of materials to obtain a general idea of their composition, includes a large field of usefulness, and one in which the spectrograph is nearly indispensable. It is often necessary to know the principal constituents and their approximate ratios when only extremely small samples are available. Sometimes it may be a stain or a film of dust or discoloration on the surface of material. Again it may be a trace of precipitate, too small for chemical identification. At other times it is necessary to make analyses without defacing rare or valuable specimens. Many other instances could be given, but those described are sufficient to indicate some of the ways in which an instrument of this kind can furnish information which could only be obtained with difficulty, if a t all, by other means. Principles of Quantitative Spectral Analysis
The foundations of quantitative spectral analysis were laid by Hartley.2 His work with that of his successors, Pollok and L e ~ n a r djn , ~ Dublin, and of de G r a m ~ n t ,in~ ,Paris, ~ has been very fully reviewed by several recent writerse6to Briefly, the principle involved in the methods used by all these investigators is that of the gradual weakening and simplification of the spectra of all the elements, when the amounts present in the light source, whether flame, arc, or spark, are decreased. Thus if a pure element or a very, concentrated compound or mixture is used, the spectrum is made up of a considerable number of lines of different intensities. If, now, a second spectrum is observed, using a dilute mixture of that element with other materials, its spectrum lines will be found to have decreased, both in intensity and in number. I n general, those lines that were weakest in the first spectrum will have faded out completely. Lines of moderate in-
* Hartley, J . Chem. SOC.33, 210 (1882), Phzl. Trans. ROY. Soc. (London), 1884, pt. 1, p. 50; pt. 2, p. 327; Proc. R o y . Soc. (London), 69A, 283 (1902). 8 Pollok and Leonard, Sci Proc. Roy. Dublin Soc., 11 (N. S.),184, 217, 229 (1907); 257,270 (1908); 16 (N. S ), 273 (1918). 4 De Gramont, numerous papers in the Comgf. rend., 144 to 176 (1907 to 1922, incl ). 6 de Boisbaudran and de Grarnont, “Analyse Spectrale Appliquee aux Recherche de Chimie MinCrale,” Librairie Scientifique J. Hermann, Paris, 1923. 6 Twyman, “Wave-Length Tables for Spectrum Analysis,” A. Hilger, Ltd., London, 1923. 7 Twyman and Smith, Am. Inst. Min. Met. Eng., Tech. Pub. 79. 8 Meggers, Riess, and Stirnson, Bur. Standards, Sei. Paper 444. 9 Porlezza and Donati, Ann. chim. applicala, 16, 519, 622 (1926); 17, 3, 14 (1927).
Vol. 1, No. 1
tensity will have become weak, and strong lines will have decreased in intensity. Still greater dilutions result in the disappearance of other lines and the further weakening of those that remain, until, at the lowest concentration a t which the element can be detected at all, only one or two faint lines remain. These most persistent lines of all are called, in the terminology of de Gramont, the raies ultimes of that element. If the conditions of excitation and of observation are kept perfectly constant, and if the samples used are of similar character and size, these variations will always take place in the same way. I n other words, the same lines with the same intensities ...,alwapbe~foundin the spectrugof T i2 in-”*quest‘ion, w F i t isupresent a t the same concentration. A quantifatiz a n G i s can therefore K?made by comparing the number and intensities of the lines of the element to be determined, in the spectrum of the sample with those of a series of reference standards.
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Elimination of Variables
The most important factor in determining the precision which may be attained in such an analysis is, of course, the constancy with which all the variables involved may be controlled. Some may be kept within very close limits; others are subject to slight unavoidable variations. The most important point in devising the technic to be followed is, therefore, to minimize, as far as possible, the range of these variations, or else to conduct the analysis in such a way that the same variations occur in the comparison standards and in the spectrum of the sample to be analyzed. The relatively low degree of precision attained with spectrographic methods by some authors may be ascribed in large measure to a failure to realize that such compensation for unavoidable variations is possible. Two methods have been proposed for this purpose. Gerlachlo and Schweitzer,ll instead of comparing directly the intensities of the spectrum lines of the elements to be determined with those of the same lines in standard spectra, use, as reference standards, lines in the same spectrum, due to the principal constituent of the sample. This constituent is assumed to be constant in amount, or to vary only as a result of the variation of the element to be determined. Thus in analyzing for small amounts of tin in lead, the tin lines are compared with certain selected lines of the lead spectrum. From previous experiments it is determined, once for all, that under the conditions prevailing a certain line in the tin spectrum is equal in intensity to one of the neighboring lead lines at some one concentration, and to certain other lead lines a t other concentrations. Thus, under the conditions described by these authps, at 10 per cent tin the tin line of wave length 02572 A. is equal in intensity to the lead line a t A2629 A., while at 0.6 per cent it is reducedin intensity and is then equal t o the weaker lead line a t A2657 A. At the intermediate cgncentration, 5 per cent tin, another tin line, that a t A2422 A., has become equal to the lead line at A2412 8., while with 0.04 per cent tin the tin line at 42707 A. has the same intensity as the lead line of A2657 A. I n this way after a table has been prepared indicating the concentrations at which selected pairs of lines, one for each element, are of equal intensity, a simple inspection of the spectrum of the unknown will permit a fairly close estimate of the concentration, This method assumes that, inasmuch as the lines of the element to be determined and the reference lines are present in the same spectrum and photographed at the same time, any variations in the conditions of excitation and photography will affect all lines equally and thus eliminate any errors due to these variations. 10 11
Gerlach, Z. anot‘g. allgem. Chem , 142, 383 (1925). Schweitzer, Ibzd , 164, 127; 166, 364 (1927).
January 15, 1929
I N D UXTRIAL A N D ENGINEERING CHEMISTRY
3
and P b
and P b
and P b
and P b
vfg aind Pb
5
0 m N
Figure 1-Portion
z
U
N
c
N
of Preliminary Standard Plate for Estimating Magnesium a n d Lead in Metallic Zinc
This method offers distinct advantages in the analysis of simple mixtures of elements whose spectra consist of many lines with a. wide variety of intensities. On the other hand, where the principal constituent of the material to be analyzed yields a spectrum with relatively few lines, and these separated by considerable wave length intervals, it might be difficult to find enough lines to serve as standards in the near neighborhood of suitable lines of the elements which were to be determined. Furthermore, the presence of three or more variable constituents would be very likely to complicate the problem to such an extent that the accuracy of quantitative estimations in complex materials would be subject to considerable doubt. The second method, which has been used successfully in this laboratory for several years, is to compare the intensities of the lines of the elements to be determined in the spectrum of the unknown sample with those of the same lines in a graded series of standards of similar composition and photographed under similar conditions. A preliminary rough estimate is first made, in which a spectrum of the sample, on one plate, is compared with a series of standards, previously photographed on another. (Figure 1) On this preliminary standard plate the successive spectra are spaced so as to leave bettween them clear strips, slightly wider than the individual spectra, so that the sample spectrum may be placed between adjacent standards and a better comparison made than is possible when the spectra are a t some distance from each other. The plate with the sample spectrum is placed over the standard plate, and moved up or down until an approximate match with the standards is found, using a mediumpower magnifying glass for observing the lines. In this way the concentration is approximately determined by comparison of the lines of the constituent to be determined with those in the two adjacent standards.
Since no two plates are strictly comparable, because of variations in the emulsion as well as in the composition, age, and temperature of the developer, this comparison is not relied on for the final determination except in those cases where a rough estimation is sufficient. The final plate is usually taken with five spectra, close together, but not quite in contact with each other. (Figures 2 and 3) The first one is that of a standard with slightly higher concentration of the element to be determined than was indicated by the preliminary test. Next comes the spectrum of the sample, followed by that of a second standard, of about the estimated concentration. A second spectrum of the sample comes next, and finally a standard of slightly less than the expected value. In this way the intensities of the lines in the sample spectra are quite sure to fall somewhere between those of the high and low standards. It is then a simple matter to estimate the true concentration in the sample, free from errors due to variations in plates and development. The usual practice is to use a series of standards whose concentrations with respect to the element in question vary by multiples of 2 (0.1, -0.05, -0.025 per cent, etc.). These show differences between successive standards which permit the visual estimation of several intermediate degrees of intensity. Nothing is gained by using closer intervals between standards. It is helpful to use a rather wide slit in taking these spectra, as this gives an appreciable area to the lines and makes it much easier to judge intensities than when the usual narrow slit used in qualitative work is employed. Care must be taken not to use so wide a slit that the lines to be observed overlap other lines in the spectra. Attempts have been made to use a microphotometer for measuring line intensities, but no real advantage is gained. This is probably because, in spite of all the precautions that are taken to insure the constancy of conditions, there are
ANALYTICAL EDlTI0,V
4 Mg
n
Vol. 1, No. 1
lA41: IIIII
d-O.?% Mg
d-0.1%
d-0.05%
Figure 2-Final
Mg
Mg
Plate for E s t i m a t i n g M a g n e s i u m i n Zinc-Base Alloy (Mg 0.07 Per C e n t )
still uncontrolled fluctuations, especially in the light source, which produce variations of about the same relative magnitude as the errors in the visual estimation of intensities. Duplicate determinations by the method described ordinarily vary by less than 10 per cent of the amount being estimated from the mean, or from the results of careful chemical analyses and this is probably as good as can be expected from ordinary technical methods of chemical analysis on constituents present to the extent of less than 0.5 per cent, which is the range for which spectrographic methods are best fitted. Comparison of Chemical and Spectrographic Analyses
Table I gives some idea of the agreement that may be expected between chemical and spectrographic analyses. The figures were taken at random from records of routine work with samples on which both methods were used. In reading them it should be kept in mind that. except for the higher leads and cadmiums, the spectrographic method was adopted because chemical methods had proved unsatisfactory. For this reason differences cannot be considered as indicating which one is in error. A better basis for judgment is to be found in the consistency with which results can be duplicated on the same sample. This is shown in Table 11,which gives, for a single sample, two series of estimates made at different times and on different portions of the sample by two independent observers. Observer 1 prepared the samples and photographed the spectra, and knew what results had previously been obtained. Observer 2 knew nothing of the identity of the samples when grading the plates. They came to him simply as part of the daily routine, the only information given him being the element to be estimated and the concentrations of the standards used in preparing the plates. Choice of Lines to Be Compared
The choice of the lines to be used when making comparisons requires some consideration. The impression might be gained from a casual reading of the literature that certain lines have peculiar properties which make them more significant than others, or that they show more marked changes in intensity for a given change in concentration. Except for those extremely diffuse lines which are so ill-defined a t low intensities as to make them difficult to observe, this is not the case. All lines go through a sensitive range in that stage of dilution just before they completely fade out. In this range the line is of some shade of gray sharply defined and free from halation or "wings." Different lines, or groups of lines, will therefore serve for comparisons a t different concentrations. The raies ultirnes can only be used a t the very lowest. I n Figure 2, in which the magnesium is of the order of 0.1 per cent, the estimation is best made on the basis of the group of five weak lines between wave lengths 2783 and 2777
w.
Those of next higher intensity a t 2803 and 2796 far too strong to serve for comparison. Table I-Comparison
1.are
of Chemical a n d Spectrographic Analyses of Zinc a n d Zinc-Base Alloys
ELE-
ELE-
MENT CHEX- SPECTROMENT CHEXAXALY-DETER- KAI. GRAPHIC ANALY-DETER- ICAL SIS MINED ANALYSISANALYSIS SIS MINED ANALYSIS P e r cen2 P e r cent P e r cent 1 Pb 0 052 0,050 11 Cd 0.0078 2 Pb 0 054 0,060 12 Sn 0.0017 3 Pb 0,033 13 Sn 0 039 0.0005 4 Pb 14 0.040 0 038 0.25 Sn 5 Cd 15 0.022 Mn 0 023 0.0062 6 Cd 0 020 0.025 Mn 16 0.0083 Cd 7 0,026 0 021 17 Mn 0.0043 Cd 8 0 010 0.007 18 0.13 Mg 9 Cd 0 050 0.055 19 Cd 0.005 Cd 10 0 0017 0.0013 0.0026 repeat
Table 11-Repeated PLATE
Average
SPECTROGRAPHIC
ANALYSIS P e r cent 0.0070 0,0006 0.0006 0.26 0,0043 0,0050 0.0033 0.16 0.002
Analyses for C a d m i u m on S a m e Sample OBSERVER1 OBSERVER2 P e r cent P e r cent 0.035 0.03 0.04 0.04 0.04 0.04 0.04 0.039
0.03 0.04 0.036 0.04 0.04 0.036
At a concentration around 0.01 per cent (Figure 3) the group of five lines hasobecome too faint to serve, while the pair a t 2803 and 2796 A. is still too strong when the same exposure is given to the plate. As there are no suitable lines of intermediate intensity within this spectral range, recourse is had to a rotating sector wheel with but a 5 per cent opening between the arc and the slit. This gives about the right line intensity for the pair, with the same total time of exposure, which is necessary t o insure the complete volatilization of the sample. With this exposure the raie ultime of magnesium at wave length 2852 8. is also weak enough to serve. If the attempt is made to base the estimates on too strong lines, serious errors may be made. Many strong lines exhibit &versa1 and this sometimes appears as a weakening of the whole line, when the absorption is not strong enough to show complete reversal, with a clear centered line on the plate. It is possible, therefore, to find lines in the spectrum of an element, present a t high concentration,which appear weaker than in the spectrum of the same element a t considerably lower concentration. It may be that this effect is the real basis for the statement, originally made by Hartley2 and repeated by nearly all subsequent writers, that the strongest lines in the full spectrum of an element are not always the ones which persist to the lowest concentrations. This has been vigorously denied by Gerlach'o who maintains that the strongest lines are always the most persistent. A safe rule, therefore, is to base all estimates on the weakest lines that are clearly defined.
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I N D GSTRIRI, AND EiliGIiVEERI,VG CHEMISTRY
January 1.5, 1929
5
M g
Mg
Figure 3-Final
Plate for Estimating Magnesium i n Zinc-Base Alloy (Mg 0.009 Per Cent)
Limits of Concentration of Material
Means of Excitation
The question is frequently asked-how high concentrations can be determined spectrographically? The answer is that there is no limit, except that the errors of estimation will be proportionately large with high percentages. The relative precision is the same, whether the concentration is high or low. This means that spectrographic methods cannot comqete in precision with chemical in the determination of those elements which form the main bulk of a material. It is a t the other end of the scale, in the range of extremely small fractions of 1 per cent, that the spectrograph finds its greatest usefulness. Some elements, such as magnesium, bismuth, beryllium, or copper, can be determined with concentrations as low as a few parts in ten million. Others, however, are much less sensitive. Arsenic can just be detected by the method described a t one part in ten thousand. In the case of uranium the limit is higher still. The amount which can be detected also depends somewhat on the other elements present. A dilute solution of a lead salt in water, with no other metals present, yields a spectrum in which the lead lines are much weaker and fade out, on dilution, a t a higher concentration than one containing the same amount of lead and a considerable amount of zinc. Serious errors were made during one stage of the development of this method by failing to include aluminum in the standards used in analyzing, for lead, a zinc-base die-casting alloy containing 4 per cent of aluminum. The presence of the aluminum in the alloy intensified the lead lines so that they were nearly twice as strong as those of the equivalent standards. When the corresponding amount of aluminum was included in the standards, their lines also were intensified and correct rcsults were obtained. This simply emphasizes the necessity for using standards of very nearly the same composition as the materials to be analyzed.
Most of the workers in this field have made use of the hightension spark, exclusively, as the source of light.%3, 4, *, 10, 11 Lewis'* states, however, that the tendency in technological circles is to use the arc, as it is quicker and more convenient than the spark and gives a fair degree of accuracy. Porlezza and Donatig are also using the arc in their extensive study of quantitative spectrography. Twyman and Smith7 state that, in their opinion, the arc is the most generally useful form of excitation. The author prefers and uses the arc for most purposes, although some elements, such as selenium and tellurium in very small amounts and most of the non-metals and gases, can best be detected by means of the spark or vacuum tubes. No attempts have been reported, as far as the author is aware, t o use the spectrograph for quantitative gas analysis, although it appears to have some possibilities. It is wise, therefore, when equipping a laboratory, to make provision for both methods of excitation. The main advantages of the arc are its greater sensitiveness and greater speed, both being due to the extremely high temperature of the electrodes and the consequently greater density and concentration of the vapors evolved.
Limitation of Standards
The objection is sometimes raised that this method involves the preparation of a very large number of standards. While this is true, it is also possible, by limiting the field of activity to those analyses for which no satisfactory rapid chemical methods are available, to keep the number of standards within reasonable bounds. Many standards can be prepared containing more than one variable constituent, so that one analysis will serve for the determination of several elements at the same time. Most standards can be kept indefinitely, and as only minute amounts are used for each analysis, once prepared, they last a long time. Most important of all, the saving in time that results from the use of spectrographic methods is so great that it more than justifies the trouble involved in the preparation and care of the necessary standards.
Electrodes
Carbon electrodes are preferable, in most cases, to those composed of the material to be examined, even when that is a metal or other conducting substance. This is because the carbon arc is by far the hottest and will therefore reveal smaller traces of impurities than will the arc between metal electrodes. One American manufacturer is preparing specially regraphitized rods for this purpose, which are remarkably free from impurities, whose spectrum lines might interfere in the ordinary run of analysis. Traces of titanium, vanadium, silicon, magnesium, calcium, and boron are usually present, but not in sufficient amount to be troublesome, except when the very smallest traces of these elements are to be determined. A British optical firm furnishes a grade of carbon which is free from titanium and vanadium and is therefore useful when small amounts of those elements are to be detected or estimated. On the other hand, this carbon contains as much or more of the other impurities in the list given than does the graphite and, in addition, appreciable amounts of iron, copper, and manganese from which the graphite is free. The ordinary commercial arc-light carbons are of very limited usefulness, as all of these, together with other impurities, are present in larger and more variable amounts than in either of the other grades. In one respect the graphite is inferior to both the British and the common arc-light carbons. I t will not hold as steady 12
Lewis, Cantor Lectures, Royal Society of Arts (London), April, 1921.
A N A L Y T I C A L EDITION
6
an arc. With the graphite the arc has a tendency to wander from point to point on the surface of the electrode. This, of course, is highly undesirable, as it tends to give variable illumination on the slit and thus to yield erratic results. Two methods have been used to overcome this difficulty. Rapid rotation of the upper electrode helps considerably, but the best results are obtained when a stream of air is drawn in at the tip of the upper electrode through an axial hole. The electrode fits tightly in a special holder which is connected with a vacuum line, through a bottle arranged to insure constant suction. This device helps in two ways to improve the constancy and reproducibility of the light source. It not only reduces the tendency of the arc to wander, but i t also draws into the arc the vapors liberated from the sample material on the lower electrode, and makes it more certain that no vapor can escape without recording its presence by its radiation. The principal objection to the use of any form of carbon for electrodes is the presence, in its spectrum, of the very intense cyanogen and other bands in the visible and longer wavelength part of the ultra-violet regions. These make it very difficult to detect weak lines of other elements in the same regions or to compare intensities when they can be detected. Fortunately, however, there are very few elements for whose detection and estimation lines in this range have to be used. The ultra-violet, from about 2350 to 3400 A., contains lines of most of the elements, of sufficiently varied intensities to serve for estimating small amounts. Moreover, the higher dispersion and resolving power in this range, with a prism instrument, make it by far the more generally useful. The spectrum emitted by the carbon arc, between these limits, when photographed with an instrument of high dispersion and resolving power, is made up of an almost continuous band of faint, closely spaced lines. Fortunately, these are of very nearly equal intensity throughout, so that even when coincidences do occur with lines of other elements, there is no serious difficulty in detecting the latter by the increased intensity of the lines, in comparison with the neighboring electrode background, and with an adjacent “blank” spectrum from the bare electrodes. In exceptional cases it may be desirable to use electrodes of materials other than carbon. Copper is sometimes used, as its melting point is high enough to withstand the temperature developed by the arc without melting down. It is sufficiently free from impurities, even in the ordinary electrolytic grade, to serve for most purposes, and it does not oxidize so rapidly as some of the other metals that might be used. Also, its spectrum is composed of fewer lines and the chances are less that lines will be found, both from the electrodes and from the sample, so close together as to interfere. Many authorities use, as electrodes, rods of the material to be analyzed, when that is a metal or other easily conducting material, and cause the spark or arc t o play between them. This would be the ideal way were it not for two objections which are more or less serious, depending on the material involved. Many of the metals have such low melting points that they can only be used as arc electrodes if the current is very weak. This means low light intensity and long photographic exposures. This objection is not so serious as the other, which is that many metals are far from being homogeneous. Zinc, for example, on cooling from the melted condition is very liable to form large crystals, with most of the impurities segregated in the intercrystalline boundaries. It would thus be very improbable that the particular points between which the discharge played would be accurately representative of the average composition of the bulk of the material. The contention in a recent paper13that “on the other hand, 18
Twyman and Smith, Zoc. cit., p. 11
Vol. 1, No. 1.
a method such as this, which will detect segregation of impurities has very much to commend it” is perfectly true if that is the object of the analysis, but is certainly not valid in the case of the usual quantitative analysis, whose object is to ascertain the average composition of a larger quantity of material. Furthermore, the use of wedge-shaped electrodes, relying on the wandering of the discharge along the edge to insure a true average, is by no means certain to achieve that result. Cast metals frequently contain single crystals an inch or more in diameter; therefore, electrodes prepared from them could not be relied on to yield an average spectrum. Preparation of Sample Several years’ experience in the analysis of this kind of material has served to emphasize the necessity of choosing a sampling method that will insure that every portion of material whose spectrum is registered on the plate shall be as nearly as possible truly representative of the main bulk of the material. The amount of sample necessary for a single determination is ordinarily from 20 to 50 mg. This is much too small an amount to obtain by cutting down the solid material, even in the form of an extremely fine powder. The safest method is to use solutions. If these are properly preparedand mixed, there can be no question of uniformity. Hartley, Pollok and Leonard, de Gramont, and others employed solutions in much of their work. I n their method one of the spark electrodes (a platinum or gold wire) is immersed in the solution, the other being placed directly above it. A bit of capillary tubing, of glass or silica, surrounds the lower electrode and projects slightly above its tip and above the level of the solution in the containing vessel. Capillary attraction then raises the solution in the tube above the surrounding level, so that the tip of the electrode is submerged. I n this way the spark passes between the upper electrode and the surface of the solution in the capillary and is high enough above the surrounding solution so that the path of the light, from the spark to the condensing lens, is not obstructed by the solution. A modification of this, using graphite rods in place of platinum wires for electrodes, has also been suggested. A somewhat similar arrangement has been used, a t times, with the arc, but is not nearly so satisfactory as the method which has been developed in this laboratory for a great variety of routine determinations. This method is to dissolve the sample (which should be of such size, and obtained in such a may that it conforms to the usual laws of sampling) in appropriate acids or other solvents and to adjust its volume in some simple ratio to the weight of sample used. When the concentration of the constituent to be determined is small, the volume should be small, too, so that the solution may be as concentrated as possible. Procedure for Analysis A measured amount, usually 0.1 cc. of this solution, is dropped from a capillary pipet into a hole in the upper end of a graphite electrode. A convenient size for the electrodes is 8 mm. in diameter and 50 mm. long. Prior to adding the solution, the electrodes are prepared by burning them for a minute or two in a 6- or 8-ampere arc. This develops a certain amount of porosity and at the same time helps to expel traces of impurities from the graphite. The electrodes are then cooled before adding the solution. After the solution has been added, the electrode is dried in an oven. The impregnated graphite is used as the lower electrode in the arc, the upper one being also of graphite, arranged as previously described, to permit air to be drawn through it.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
January 15, 1929
A quartz condensing lens serves to concentrate the light from the arc on the slit and to exclude that from the incandescent carbons, whose strong continuous spectrum would be objectionable, Suitable adjustments are provided, which permit the arc to be moved both horizontally and vertically so as to keep it properly centered during the exposure. Constant arc length is insured by keeping the tips of the electrode images always a t the same points above and below the aperture in the slit diaphragm. With a reasonably constant line voltage in the power supply, this control of the arc length is all that is necessary to maintain constant current and voltage drop in the arc. I n order that the operator's attention may be concentrated on keeping the arc properly centered, an automatic timing device has been installed. This consists of an electric clock so connected through a system of relays that it operates only while the arc is burning, and extinguishes the arc as soon as a predetermined exposure time has elapsed. This is particularly valuable in photographing the spectra of very volatile materials, which often yield almost explosive arcs, difficult to keep burning continuously. If it were necessary for the operator to keep track of the intermittent exposures that result under such conditions, he could not give proper attention to the centering of the arc, which is one of the most important factors in insuring that all spectra shall be accurately comparable. Using a large Littrow-type quartz spectrograph, with an objective of 70 mm. aperture ando170 cm. focal length, the spectrum from A2350 to A3400 A. is photographed on a plate 10 inches (25.4 cm.) long. This is the spectral range that has been found the most generally useful for much of the work that is encountered in the average metallurgical laboratory. Other ranges may be utilized, if desired, by a simple change in the adjustment of the instrument, but experience has shown that a t least 95 per cent of the work can be done within these limits. Persistent lines of most of the elements are found there, and the dispersion is great enough to insure accurate identification of lines. The most complete and accurate lists of lines are available for this region, and ordinary plates are sensitive to these wave lengths without special treatment, such as bathing in sensitizing dyes or fluorescent oils. Smaller instruments can be used for a great deal of work. Several makers put out quartz spectrographs in which the full length of the photographable spectrum is included within the length of a single 10-inch (25.4-cm.) plate, and excellent results can be obtained with them on many materials. Occasions do arise, however, when only the larger instrument
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can resolve closely coincident lines of two elements, which must be separated to permit the certain identification of one constituent and its estimation, free from the possibility of error due to an overlapping line of another. I n general, therefore, any laboratory which has a large amount of work adapted to the spectrographic method will find that the additional cost of the larger instruments is fully justified by the greater certainty of the results which may be obtained and by the wider range of possible applications. Three-minute exposures are given with most materials, using an arc carrying 10 amperes with a 60-volt drop across the arc terminals. This length of time is sufficient to insure the complete evaporation of the sample, or at least the expulsion of all of the constituents to be determined, It is quite necessary that none be left in the electrode, otherwise erratic results will be obtained. Slow photographic plates give the best results. Their fine grain makes it easy to compare line intensities, and their freedom from fog is an important consideration, particularly when comparing extremely weak lines. Conclusion
No hard and fast rules can be given for methods applicable to the determination of all elements in all kinds of materials. Some of the general methods have been sketched, which have been found useful in a laboratory devoted mainly to the study of problems connected with the production and use of zinc and zinc-bearing materials. Each type of analysis requires individual study to find the most suitable details of technic. The main purpose of this paper is to call to the attention of the chemists of this country the apparently littleappreciated fact that in the spectrograph they have a powerful tool, capable of yielding information of the greatest value, with a speed and certainty that cannot be matched by any other means. A few laboratories have installed the necessary equipment and are finding new uses for it almost daily. In that of The New Jersey Zinc Company several hundred spectrographic analyses are made each month, mostly quantitative estimations, and many of them are daily routine and control analyses. Acknowledgment
Grateful acknowledgment is made to George W. Standen, whose painstaking care and attention to details have been important factors in the progress of this work.
Simple Pressure Regulator for Vacuum Distillations' Henry L. Cox* MELLONI N S T I T U T E
O F I N D U S T R I A L R E S E A R C H , U N I V E R S I T Y OF P I T T S B U R G H , P I T T S B U R G H ,
T
HE chemical literature of the past two decades contains numerous descriptions of devices designed to maintain a constant pressure in a vacuum distillation apparatus. Most of these devices are not entirely satisfactory in that they are not sufficiently accurate for the intended purpose or the construction is so complicated as to discourage use in all except the most, exacting investigations. Several of them employ mechanically or electrically operated valves requiring a high precision in their manufacture and subject to the 1
Received August 3, 1928.
* Senior Industrial Fellow, Mellon Institute of Industrial Research.
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disadvantages inherent in such devices-for instance, corrosion and sticking or breakage. The apparatus described in this paper is to a large measure free from most of these disadvantages. It contains no valves or moving parts other than an efficient motor-driven pump and a relay, the armature contacts of which are of sufficient size to carry the electric current required to operate the motor. The apparatus is entirely automatic, requires little attention, and maintains any desired pressure constantly within *O.l mm. of mercury. A closed-arm mercury manometer containing a sealed-in
