N
a
Quantitative Analysis by Spectroscopic Methods ‘0. S. DUFFENDACK, R. A. WOLFE,AND R. W. SMITH,University of Michigan, Ann Arbor, Mich. HE value of the spectro-
Spectroscopic methods for estimating the given frequency) with which we scope in identifying subamounts of various elements present in a mixare concerned and not the photographed image. The intensity stances and in detectture, alloy, or solution are reviewed and the of the light in the source may be ing the presence of small amounts continued advance in the precision of the dee x p e c t e d to depend upon the $of certain elements in mixtures terminations is noted. Further increase in amount of the e l e m e n t in the and compounds has long been source that is giving forth that recognized. The development of mecision has been attained bv the authors,. by“ lightwe have to m e t h o d s of e s t i m a t i n g t h e making use of methods recently developed for work with conveniently is the q u a n t i t y of a given element measuring the intensities of spectral lines, and photographed spectrum, and we present dates from 1874, when by a simple means of standardizing the arc must determine from it the relaL o c k y e r (10) n o t e d that the source used in the analysis of metallic alloys. tive intensities of the light of spectral lines of the more abundifferent wave lengths.. .Accu. dant elements present in the elecThe necessity for putting an intensity calibration detrodes of an arc can be observed pattern on each photographic plate in order to rate quantitative mands the use of the best availf a r t h e s t from the electrodes. insure uniformity and high precision of measureable of modern spectral The development of the method ment is emphasized. The procedure adopted photometry (17)and afullunderof perfiistent lines or “raies ulby the authors in making analyses of metallic standing of the photographic times” was begun by Hartley (8) processes (14). in 1882 and extended and peraUoys is described. I n order to provide a means fected by Pollock and Leonaxd of i n t e r p o l a t i n g b e t w e e n (13) and by de Gramont (6, 8). This method never proved completely successful on account of the fixed percentages in the method of Gerlach and Schweitzer the impossibility of determining just when a line disappears a t which certain pairs of lines are equally strong, Scheibe from the spectrum, especially since this depends as much on and Neuhausser (16) introduced the use of a rotating logathe characteristics of the photographic plate and apparatus rithmic sectored disk. The proper method for the use of this used and on the duration of exposure as upon the abundance disk was pointed out by Twyman and Simeon (91) and used of the test element. On account of this difficulty more recent by Twyman and Hitchen (19, 20) in the methods of quantitainvestigators (4,11, 18) turned to methods which involve tive analysis developed at the Hilger laboratories. The principal objections to the use of the logarithmic the comparison of the relative strengths of spectral lines, but these also proved to be of insufficient reliability, because disk are the assumption of the validity of the reciprocity of the impossibility of standardizing the conditions of ex- law, the introduction of the intermittency effect ( 2 ) , and the posure and development of the plate over the general range difficulty in determining the exact lengths of the photographed lines. The reciprocity law is known not to be valid, and of analysis. Perhaps the greatest errors in the older methods of analysis errors arising from its failure become very large in certain came from the comparison of lines in different spectrograms. spectral regions. Lines photographed with a sectored disk It is practically impossible to make equivalent exposures for have no definite length but gradually fade out a t one end. Like Twyman and Hitchen, Nitchie and Standen (12) the recording of spectra. Realizing this, Gerlach (3) developed a method of “internal control” in which the analysis make use of a working curve from which the amount of the was based upon finding pairs of selected lines of equal strength test element present can be read off directly. A micro-one of the element under test and one of the principal photometer is employed to measure the densities of the or basic element in the material being analyzed. These spectral lines, though, apparently, the blackenings were pairs of lines are examined in the spectrum of the unknown, actually used in the analysis. No means of calibrating the and analysis is made by comparison with their relative plate is mentioned, but the authors state that only those strengths in a prepared series of specimens. If no pairs have spectrograms should be used in which the relative densities equal strength in the spectrum of the unknown, an estimate fall upon the straight-line part of the characteristic curve of the concentration of the test element is made by an inter- for the plate. With this assumption, the difference in the blackening of two selected lines is taken as proportional polation between the steps in the standard series. to the logarithm of the ratio of the intensities of these lines PHOTOGRAPHIC SPECTRALPHOTOMETRY and the working curve is determined in a manner similar to In the foregoing discussion, the term “strength” of a that of Twyman and Hitchen. The authors have, for a number of years, been using specspectral line has been used, without definition, to mean the relative blackness or opaqueness of the line as it appears in trographic methods for estimating small amounts of metals in the photographed spectrum. The strengths of spectral lines various alloys, particularly nickel alloys, in connection with have usually been estimated merely by examining the the development of nickel alloys containing barium by the AC photographed spectrum, often during the time of measure- Spark Plug Company. Since early in 1931 this method has ment of the plate with a comparator. Purely arbitrary been used continuously for the quantitative determination of scales were chosen and the value given any line came from the barium in nickel alloys. During this time numerous checks by means of chemical analyses have shown that the spectroexercise of the best judgment of the observer. I n making use of spectral lines in quantitative analysis, scopic determinations are fully as consistent and accurate as it is the intensity of the light in a given spectral line (of a those made by chemical methods. This long-continued use 226
July 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
of a method of accurate quantitative analysis for a particular purpose clearly demonstrates the reliability and value of spectroscopic methods of quantitative analysis. The method has been used with equal success in determining silicon in steel, chromium in nickel, and magnesium, sodium, and potassium in various solutions, and seems capable of very general application.
227
quently show wide variations. These variations are not so great in an arc source, and it is sometimes possible to use the spark line of one element with the arc line of another, as was done in the case of the barium and nickel alloys used as an illustration in this report. Another troublesome matter is plate background. While corrections can be applied for background, none of them seems reliable and it is better to arrange the exposures so that the background is of negligible amount. The plates for Figure 3 and for all other measurements in this investigation had no appreciable background, the lines standing out on a clear plate. STANDARDIZATION OF ARC
FIGURE1. MICROPHOTOMETER RECORD OF A PORTION OF Two SPECTROGRAMS AND OF A STEP-DIAPHRAGM CALIBRATION PATTERN, PHOTOGRAPHED ON THE SAME PLATE
One satisfactory method of determining the relative intensities of spectral lines from their relative densities or blackenings is that of Hansen (1, 7, 9, 15). By means of a calibrated step diaphragm, a set of strips or bands is recorded on the photographic plate in which the relative intensity of the light is known. The measurement of the relative densities or blackenings of these bands gives a complete calibration curve for that particular plate, from which curve the relative intensities of any two spectral lines can be determined from a measurement of the relative densities or blackenings of the lines. Figure 1 is a reproduction of a microphotometer record of the relative blackenings of such a set of strips, together with those of a certain pair of barium and nickel lines taken from the same plate. It will be noted that the ratio of the blackenings of the barium and nickel lines changes markedly from one alloy t o the other. The exposures were so timed that the blackening of the more intense line, in this case the barium line, did not exceed the maximum blackening of the straight-line portion of the characteristic curve of the plate. It is the experience of the authors that loss in accuracy of analysis occurs if the blackening of one of the lines exceeds this amount. The blackening of the weaker line may be less than the minimum blackening of the straight-line portion without a serious loss of accuracy. From such a record as that of Figure 1 the curve of Figure 2 may be drawn. The slit width of the spectrograph should be sufficient so that the photographed lines are broad enough completely to intercept the beam of light in the microphotometer; otherwise the galvanometer deflections will not be proportional to the blackenings. Another convenient method of measuring the relative intensities of spectral lines is described by Thomson and Duffendack (17). The choice of lines for a given analysis is determined by a number of considerations. When possible, lines should be chosen which lie in a region of slowly varying contrast of the plate used. When an element occurs in fairly considerable amounts in a mixture, one must guard against reversal of the lines and so lines which are not readily absorbed should be chosen. Some lines of an element are very strongly affected by the presence of even small amounts of another element, while other lines of the same element are not. These effects are most pronounced where close resonance exists between excited states of two elements, and so, when a mixture of several elements is being examined, the lines chosen should be tested for such effects. With a spark source, the relative intensities of arc and spark lines fre-
It is possible to make accurate analyses by the use of an arc source as well as a spark source. Upon investigation it was found that the relative intensities of a selected pair of spectral lines vary with the arc current in a manner indicated by the curve in Figure 3, drawn for the lines barium 4554.0 and nickel 4546.9. A similar relationship was found for the lines chromium 4289.73 and nickel 4401.55. It may be noted from Figure 3 that after the current reaches a certain value, the ratio of the intensities of a pair of lines changes quite slowly with the arc current. Thus, for currents greater than a certain amount, the ratio of intensities of selected lines becomes quite insensitive to such fluctuations of the current as exist in a normal arc. By working with such arcs, errors due to variations in the arc current become insignificant, and accurate a n a l y s e s are possible. The arc has the advantage over the spark in that the spectral lines, especially arc lines, are much more uniform over the length of the arc. Furthermore, the arc consumes more material of the electrodes than does a spark and hence g i v e s a b e t t e r average c o m p o s i t i o n . A spark will seek out points on the cathode where materials of low work f u n c t i o n may exist in small i n c l u s i o n s , whereas a strong arc strikes to a considerable fraction of ~ A c K ~ M the surface of the ends of the VELOPED pHoToGRAPHrC electrodes. PLATEAND EXPOSURE OF PLATE METHODOF ANALYSIS The method of quantitative spectrographic analysis developed by the authors consists of the following steps: I. FIXINGOF EXCITATION CONDITIONS. An arc between electrodes consisting of or containing the material to be tested is used and the relation between the relative intensities of selected spectral lines and the arc current, as indicated in Figure 3, is determined. A value of arc current is chosen at which, for a small change in current, the change in relative intensity of the lines is small. A spark source may be used and standardized by the method of Gerlach and Schweitzer (4). 11. DETERMINING OF WORKING CURVE. A series of alloys, solutions, or mixtures is made in which the percentage composition of the material t o be estimated is varied over the range chosen for the analysis. The value of the logarithm of the relative intensities of the selected lines is determined for each alloy in the series. In every case the spectrum is excited in the manner indicated by the experiments under I. These results are plotted as in Figure 4. 111. ANALYSISOF AN UNKNOWNSPECIMEN. The spectrum of the unknown specimen is photographed in the manner determined under I and the logarithm of the relative intensities
~
ANALYTICAL EDITION
228
of the selected lines is determined. The logarithm of the relative intensities of the spectral lines is applied to the working curve, Figure 4,and the percentage composition is read on the scale.
determine whether the extent of variation is sufficient to affect the working curve of the other test element.
TABLE1.
DISCUSSION OF METHOD
Vol. 5 , No. 4
COMPARATIVE
ALLOYNUMBER
The method of analysis outlined above consists essentially in applying to this problem a correct method of photographic photometry. The calibration of each plate by putting upon it an intensity pattern is an essential feature of the method. No assumptions are made with respect to the reciprocity law or with respect to whether or not the line intensitie’s fall on the straight-line part of the characteristic curve. It was only after the introduction of this step that accurate, repeatable, consistent analyses could be made.
ANALYSESOF
BARIUM-NICKEL
ALLOYS
BARIUM
Chemical analysis Spectroscopicanalysis % %
716 637 821 682 667 669 736 662 692 665 678 420 179 178
0.016 0.021 0.022 0.048 0.052 0.058 0.060 0,064 0.078 0.095 0.130 0.142 0.37 0.52
0.015 0.028 0.023 0.051 0.055 0.057 0.059 0.064 0.075 0.091 0.130 0.145 0.38 0.49
I n order to indicate the degree of accuracy that may be attained in regular production practice, Table I gives some comparative analyses of the same specimens of bariumnickel alloys by spectroscopic and by chemical means. These analyses were chosen a t random from a list of approximately three hundred. The mean deviations in the determination of the percentage of barium in a given specimen by spectroscopic methods were approximately the same as by chemical
I1
1
8
\ Curry? 8
I
!
FIGURE3. RELATION BETWEEN ARC CURRENTAND RELATIVE INTENSITY OF A PAIR OF LINES IN SPECTRUM OF A BARIUM-NICKEL ALLOY
One working curve suffices for a considerable range of percentage compositions of a given kind of material. Steps I and I1 need be performed but once for a particular kind of alloy, solution, or mixture. Step I11 alone must be repeated for each analysis. In this step, only one calibration curve (Figure 2) is necessary for a plate which may bear the spectrograms of a number, ten or twelve, of specimens to be analyzed. I n the cases of barium-nickel and chromium-nickel alloys, spectrographic analyses have been found to be as accurate as chemical analyses and can be carried out in a small fraction of the time required for chemical analyses. If a certain material is analyzed for several different elements, it is likely that the lines which will be used for the several elements will lie in more or less separated spectral regions. On account of the change in the contrast of the plate with wave length, separate calibration curves like that of Figure 1 will be needed. I n such cases, it is desirable t o put on the plate an intensity pattern which extends over all wave lengths. This can be done by means of the Hansen method coupled with the spectrograph and more conveniently by the method of Thomson and Duffendack. With such a pattern, Calibration curves can be determined for any wave length desired. Figure 4 shows two working curves for the estimation of barium, one for a nickel alloy and one for a nickel-copper alloy. In both these curves, the same pair of barium and nickel lines is used, and it is obvious that the addition of copper to the alloy affects the variation of the relative intensities of these lines with the percentage of barium. This illustrates a situation which is frequently encountered, as was noted by Twyman and Hitchen (19, 20). Account must be taken of this effect and working curves are necessary for any given element in each type of alloy. If a second element also varies in amount in a given alloy, one must
./d.04
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.1,6 .20 .24 , FIGURE 4. RELATION BETWEEN PERCENTAGE OF BARIUMALLOYS AND LOGARITHM OF RELATIVEINTENSITY OF A PAIROF LINES IN SPECTRAOF
.Q8 .lZ
THE
ALLOYS
I, barium-nickel alloy. 11, barium-nickel alloy containing 20 per cent copper
methods and amount to *0.001 per cent in the range from 0.01 to 0.150 per cent of barium. Some of the larger discrepancies between the spectroscopic and chemical determinations may be due to slight nonhomogeneities in the alloy. ACKNOWLEDQMENT The alloys used for the analyses mentioned in this paper were prepared by the AC Spark Plug Company and the chemical analyses were made in the company’s laboratories. The authors wish t o express their appreciation of the friendly cooperation of D. W. Randolph, director of research a t the AC Spark Plug Company, and his assistants, and to thank the company for its financial support. LITERATURE CITED (1) (2) (3) (4) (5) (6)
(7) (8)
Cittert, P. H. van, 2. P h y s i k , 73, 249 (1931). Davis, R.,Bur. Standards, Sci. P a p e r 528 (1926). Gerlaoh, W., 2.anorg. allgem. Chem., 142,389 (1925). Gerlach and Schweitzer, “Chemische Emissions Spektralanalyse. Grundlagen und Methoden,” Leopold Voss, Leipzig, 1930. Gramont, A. de, Compt. rend., 144, 1101 (1906); 159, 6 (1914). Gramont, A. de, and Lecoq de Boisbandrau, F., “Analyse spectrale appliquee aux recherches de chimie minbrale,” J. Hermann, Paris, 1923. Hansen, G., 2. P h y s i k , 29, 356 (1924). Hartley, W. N., Chem. SOC.J., 33, 210 (1882); Trans. Roy. SOC.(London), 175, 11, 49, 325 (1884).
July 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hippel, A. von, Ann. Physilc, 80, 672 (1926). Lockyer, N., Trans. Roy. SOC.(London), 164,II, 479 (1874). Meggers, Kiess, and Stimson, Bur. Standards, Sei. Paper 444 (1922). Nitchie and Standen,IND. ENG.CHEM.,Anal. Ed., 4, 182 (1932). Pollock and Leonard, Proc. Roy. SOC.Dublin, (2) 11, 217, 229, 267 (1908). Ross, ‘F. E., ”Physics of Developed Photographic Image,” Van Nostrand, New York, 1924. Schachtschabel, Ann. Physik, 81,929 (1926). Soheibe and NeuhLusser, 2.angew. Chem., 41, 1218 (1928).
229
Thomson and Duffendack, J. OpticaE SOC.Am., 23, 101 (1933), and references quoted in this article. (18) Twvman. . F.. J . SOC.Chem. Ind.. 46. 284. 307 11927). ,--(19) Twyman, “Practice of Spectrum AGlysis with Hilger Instruments,” Adam Hilger, London, 1931. (20) Twyman and Hitchen, Proc. Roll. SOC.(London), A133, 72 (17)
I
(lQ31) ,- - - - ,. (21) Twyman and Simeon, Trans. Optical SOC.(London), 31, 169 (1930).
RECEIV~D November 28, 1932.
Determination of Equivalent Acidity and Basicity of Fertilizers W. H. PIERRE,West Virginia Agricultural Experiment Station, Morgantown, W. Va. URING the last few years there has been a general recognition of the acid-forming character of certain nitrogenous fertilizers. The amounts of limestone required to neutralize the acidity developed by these fertilizers are well established. The amount of acidity developed by mixed fertilizers, however, is not known; nor has it been possible to determine it satisfactorily. It is readily evident from a consideration of the common nitrogen carriers used that mixed fertilizers are usually acidforming. I n an attempt to correct this condition some progressive fertilizer manufacturers have been using ground limestone instead of sand as the filler. This practice is sound economically and should be encouraged. Recent work by MacIntire et al. (5, 6) shows that no loss of available phosphoric acid results from the use of the dolomitic limestone. Such a practice probably would not become general unless recognition were given to the value of the added limestone. Parker (7) has pointed out that if the use of limestone’ is to be encouraged, a method must be developed that can be used to determine the limestone in mixed fertilizers and thus make possible the guarantee and chemical control of this desirable constituent of fertilizers. With these considerations in mind this investigation was started with the purpose of developing a simple, quantitative laboratory method for determining the equivalent acidity or basicity of any fertilizer carrier or mixture. (Equivalent acidity may be defined as the acidity developed by the fertilizer, measured in terms of calcium carbonate required for its neutralization. Equivalent bssicity refers to the basic residue left in the soil by fertilizers, expressed as equivalent calcium carbonate.)
ACTIONOF FERTILIZERS ON SOIL ACIDITY Before discussing the method developed for determining the equivalent acidity or basicity of fertilizers, it seems desirable to review briefly the effect of fertilizers on soil acidity and the principles governing this action. A review of the investigations regarding the effect of fertilizers on soil reaction leads to the following conclusions: 1. The common potassium fertilizers such as kainit and muriate and sulfate of potash have no residual effect on soil reaction. 2. Phosphate fertilizers in which one of the three hydrogens of phosphoric acid is replaced by a base will in general have no permanent effect on soil acidity, whereas those in which more than one hydrogen is replaced by base will be basic in their 1 In this paper limestone includes both calcium limestone and dolomite. The latter is preferable in fertilizers, for it reacts less with superphosphate (MacIntire) and also serves as a source of magnesium.
effect on soil reaction. (For those phosphates in which the base is ammonia see next point.) 3. Fertilizers containin nitrogen in the form of ammonia or in a form subject to nitrifcation in the soil will produce acidity unless sufficient base is present to neutralize this acidity. 4. Nitrogenous fertilizers in which the nitrogen is in the nitrate form and combined with bases such as sodium or calcium will upon being utilized by plants result in decreased soil acidity. The fact that potassium fertilizers do not have a residual effect on soil reaction is generally recognized. With regard to phosphate fertilizers, however, there has been some controversy. Most of the results obtained in long-time field experiments show that superphosphate slightly decreases or has no effect on soil acidity. I n recent studies the writer has found that the action of phosphates on soil reaction will depend to some extent on the original acidity of the soil and that for cultivated soils of the humid region, the p H of which lie mostly between 5 and 6, superphosphate has no appreciable effect on soil reaction. Any method for determining the acidic or basic property of fertilizers should consequently consider monocalcium phosphate as neutral and the di- and tricalcium-phosphate as basic. Nitrogenous fertilizers containing ammonia have been shown to be acid-forming because of the process of nitrification which takes place when the fertilizer is added to the soil (8). A method for determining the acidity resulting from ammonium fertilizers should consider the acidity that is developed when complete nitrification of the ammonia takes place. Although in some cases there is likely to be some absorption of the ammonia by the plant before being nitrified, this amount is probably small under average conditions. The amount of acidity formed from ammonium sulfate was found (8)to be lower than that calculated on the assumption that all the nitrogen and sulfate would act as nitric and sulfuric acid, respectively. I n these experiments conditions were very favorable for nitrification and the low values obtained were believed to be due to the action of the plant. Apparently plants absorb from calcium or sodium nitrate more of the nitrate ion than of the calcium or sodium ion. The result of such an action is to cause sodium and calcium nitrate to leave a basic residue in the soil and to reduce the acidity resulting from the nitrification of ammonia present in acid-forming fertilizers. From a calculation of the basic action of calcium and sodium nitrate on soils (8) it was found that 51 per cent of the base had been effective in reducing soil acidity, indicating that approximately onehalf as much of the base had been absorbed by the plant as of the nitrate ion. It follows, therefore, that only one-half
*
