Spectrochemical Analysis of Trace Elements in Fertilizers Boron, Manganese, and Copper EUGENE H. MELVIN' AND ROBERT T. O'CONNOR2 Bureau of Plant Industry, U. S . Department of Agrioulture, Washingtost, D. C .
A
S RESEARCH on the nutrition of plants and animals
hasis of relative consumption of each grade. The chemical methods therein involved for the determination of important secondary elements, whose concentrations range from only 0.0005 to 0.1 per cent, are both complicated and time-consuming. It is to this type of problem that spectrochemical analysis is particularly adapted. The spectrograph has the advantages of being direct, thereby avoiding error due t o chemical manipulations involved in separations of small quantities; of being rapid; and of achieving, when concentrations are particularly low, accuracy comparahle t o chemical methods. Accordingly, a study of the application of spectrochemical methods to the analysis of mixed fertilizers has been made (6) and a procedure devised which will permit the simultaneous determination of three important secondary elements, boron, manganese, and copper. This paper describes the application of this method to the spectrochemical analysis of the 44 representative mixed fertilizer samples collected and analyzed by Lundstrom and Mehring and compares the results and the time required with the chemical methods.
continues, the importance of the presence or absence in their food of the merest traces of certain elements has been more and more emphasized. Thus, for example, it has been shown that the presence of a few parts per million of boron in the soil will prevent the internal cork of apples or the heart and dry rot of sugar beets, but onehalf part per million in irrigation water was sufficient to damage some citrus crops in California; application of extremely small quantities of manganese will cure chlorokis of tomatoes or Pahala blight of sugar cane; while copper in only one-sixteenth to one-eighth part per million will prevent the permanent wilt of tobacco, amounts much in excess result in a stunting of plant growth. McMurtrey and Robinson (4) have compiled "A Reference List of the Secondary Elements and Their Relation t o Plant Development", in which they summarize the known effect of thirty elements on the growth of plant crops. Daniel ( 1 ) has similarly summarized the role of thirty-three trace elements in animal nutrition. With the importance of these secondary elements in t.he growth of plants and nutrition of animals established, knowledge as to their presence and concentration in available mixed fertilizers, where they are possible important sources, seems desirable. Fertilizer companies customarily guarantee only definite percentages of the primary plant nutrients, nitrogen (N), phosphoric acid (PzOa),and potash (K90).The total content of these plant nutrients in the average mixed fertilizer now amounts t o shout 19 per cent. The question of what constitutes the remaining 81 per cent was answered in this laboratory hy Lundstrom and Mehring (3) who determined the complete composition of 44 mixed fertilizers chosen on a
Apparatus and Method GENERAL METHOD OF PROCEDURE. As the concentrations of the secondary elements in the mixed fertilizer samples varied over appreciable ranges, the step-sector method of Scheibe (7, 8) was selected. A Hilger seven-step sector with a step ratio of 2 was used when photographing each sample, thus permitting plate calibration and wide selection of the intensity best suited for the photometric measurement of any. line. While this procedure limits to seven the number of samples which can be photographed on a single plate, it prevents any possibility of a selected line in the spectrum of any sample being too light or too dark t o permit an intensity measurement which will fall within the straight-line portion
I Present address, Northern Regional Laborstory. Bureau of Agrioulturhl Chemistry and Engineering, U. S. Department of Agriculture, Pearia, Ill. 3 Present address, Southern Regional Researoh Laboratory, New Orlesns,
LS.
344r I 1 313r K
CU
Be
cu 3248
FIGURE 1. SPECTROGR~MS OF MIXEDFERTILIZER SAMPLES 520
August 15, 1941
521
ANALYTICAL EDITION
of the calibration curve. Thus, by avoiding the necessity of rephotographing at another intensity, it undoubtedly effects a saving in both time and expense when a wide range of concentrations is encountered. The extreme difficulty of strict control of arcing conditions with the heterogeneous mixed fertilizer samples made advisable adoption of the internal standard method of Gerlach (a), where small variations are of no consequence. As it is believed that advantages of the spectrochemical method in both speed and accuracy lie in the directness of the method, a procedure was sought which would necessitate no chemical manipulations. Therefore no attempts were made to dissolve the difficultly soluble fertilizer samples nor to concentrate the small amounts of bhe elements to be determined by any extraction methods. This necessitated abandonment of the dternating current high-voltage arc, in spite of its other advantages, since studies of this method ( 5 ) have led to the conclusion that its applicability is limited to analysis of solutions or of powdered materials which are very homogeneous. Hence the low-voltage direct current arc, which permits complete volatilization of a more representative sample, was selected. Figure 1, Ireproduction of an entire plate, shows the spectra of seven typical fertilizer samples thus photographed. -4PP.4RATUS. The spectrograph is t,he large Hilger Littrow quartz instrument (No. E-492). The arc stand, patterned after the Sational Bureau of Standards design, with modifications to provide water cooling of the electrode holders and accurate settin of the electrode separation by a screw motion rather than the rac! and pinion, was built to fit the optical bench. The electrode holders are well insulated with soapstone to permit adjustment of elect'rode position during exposure. A constant electrode separation is maintained by aid of a tenfold enlarged image of the source projected, by means of an appropriately placed lens, between marks on the nearby wall. The seven-step sector is placed about 2 mm. in front of the slit and a lens in front of the sector imaging the source upon the collimator lens provides uniform intensity along the slit height. A variable sector, to control the over-all intensity, is placed in front of this lens. The arc is maintained by means of a 250-line voltage a t currents which can be varied by means of a water-cooled variable rheostat. The plates are measured in a Bausch & Lomb nonrecording densitometer. By means of an iris diaphragm described by Scribner (9) the full-scale deflection is held constant, so that galvanometer readings can be used directly for plate calibration without calculating opacities or densities. Another useful addition to the densit'ometer is a slow-motion screw, by means of which the galvanometer reading for various lines can be more conveniently obtained. EXPERIMENTAL PROCEDURE. The conditions chosen for the determination of boron, copper, and manganese in mixed fertilizers after detailed preliminary study were: a current of 15 amperes, a t 250-line voltage, an electrode separation of 4 mm., and a slit 7 mm. high and 0.06 mm. wide. The top electrodes were pointed graphite rods and the lower electrodes were made with a flat-bottomed cup 6 mm. deep, 4.3 mm. in diameter, and with a wall 0.5 mm. thick, shaped by a tool patterned after Myers and Brunstetter ( 6 ) which cuts the outside and inside of the rod at the same time. A 10-mg. sample was mixed with twice its weight of pure graphite, as preliminary study has shown that with such a mixture a steadier arc could be maintained. The mixture n-as placed in the graphite cup, which was made th,e anode. The lines !elected for the analysisowereboron 2497.7 A., manganese 2605.7 A., and copper 3247.5 A, 2nd the instrument was adjusted for the region 2450 to 3500 A. On the basis of studies made on the intensities of these lines, the variable sector was adjusted to eliminate one half of the emitted light. Moving plate pictures show that a 2.25-minute exposure time was required to volatilize the sample completely. As the complete composition of the fertilizer samples was known, a material to serve as a base for the spectrochemical analysis was prepared to correspond to the average amount of each of the inorganic constituents and 0.05 per cent beryllium was added as a reference element. All chemicals were checked spectrochemically for freedom from boron, manganese, copper, and beryllium impurities, Theoretically an element to serve as an internal standard should meet the qualifications (1) that it possess lines in the spectral region close to the lines selected for the analyses, (2) that
A o
401 -1.5
Mg 2779.8 Mg 2791 .O
I
I
I
0
I
2
Log R e l a ti ve Intensity
spectral lines of the reference element do not interfere with the lines selected for analyses, (3) that the lines of the reference element and the lines selected for the analyses represent similar energy conditions in the arc, and (4) that the reference element volatilize at approximately the same rate as the elements to be determined. A practical criterion of the suitability of an element as an internal standard is simply that it give constant intensity ratios with the lines selected for the analyses. The selection of an element to be added as an internal standard to a substance with a spectrum as complex as that produced by a mixed fertilizer, which will meet all these requirements, 's difficult. Beryllium, conforms to the first with lines a t 2494, 2650, 3130, and 3321 two conditions only. However, when constant ratios with the selected boron, manganese, and copper lines were obtained with use of the beryllium line a t 3130 A., this standard was selected. This use of a somewhat slowly volatilizing element as an internal standard made complete volatilization of the sample particularly important; otherwise low intensity ratios and consequent hi h values of the concentrations will result. %ach plate was calibrated by measuring the galvanometer deflection of the seven steps of two lines close together in the spectrum. The logarithm of the relative intensities of the seven steps of each of these lines is plotted against the galvanometer deflections. Figure 2 shows a typical calibration curve so obtained. Standards were prepared by adding boron, manganese, and copper in amounts varying from 0.0004 to 0.4 per cent to 1-gram samples of the base. Evaporation in a desiccator a t rather high vacuum over phosphorus pentoxide was found to be the most satisfactory method of preparing these standards. A solution containing appropriate amounts of the boron, manganese, and copper as well as the beryllium internal standard was added to the 1-gram portion of the base. The solution penetrates the entire sample and, when evaporated in this manner, the sample does not adhere to the evaporating dish as invariably happens when other methods of evaporating-i. e., over a hot plate or steam bath-are used.
8.,
The xvorking curves were obtained by plotting, after correction for step and background had been made, the logarithm of the ratio of the relative intensities as obtained from the densitometer readings and the calibration curves, against logarithm of the known concentrations. A background correction for any line is obtained by measuring the blackness on each side of the line and converting the average of these read-
Vol. 13, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
522
n
/
Y
A
Figure 3B Working Curve for Boron
ings to intensity by use of the calibration curve. This intensity value is then subtracted from the measured intensity of the line. Working curves for boron as first obtained (Figure 3-A) showed departure from proportionality a t the lower concentrations, indicating the presence of small additional amounts of the element. Further study showed that the boron was present in the graphite rods used, and when the purest grade of carbon was procured, the straight-line working curve (Figure 3-B) was obtained. Similar experience was encountered in the preparation of the working curves for copper and manganese. Only the purest grade rods are suitable for this work. Two sets of standards were prepared and separately photographed. The first set, represented by triangular symbols in Figure 3-B, established the working curves and the second set, represented by circular symbols, served as a check on the experimental accuracy in obtaining the working curves, and as a test of the accuracy of the procedure. Eastman “Process” plates were selected on the basis of previous tests made in this laboratory ( 5 ) . Eastman developer D-76-c, prepared as instructed, then diluted with 2 parts of developer to one part of water, was used as this gave a satisfactory value of gamma, although studies indicate that the choice of developer was not an important factor ( 5 ) . The plates were constantly brushed while being developed for 5.5 minutes at 18”C.
Results and Discussion -31
1
I
PRECIS~ON. Table I lists the results obtained from duplicate analyses of the 44 fertilizer samples for boron, manganese, and copper. The average per cent deviations of the du-
1
-2 Log Concentrotion B&
-3
-I
TABLE I. COMPARISON OF SPECTROCHEMIC i~ IKD CHEMICAI. IId. Va , Va
1929 1929 1929
51
s.c . c.
5.
La. Md.
x. T.
Va.
Ark. Va.
hl ab6 Ohio
N . Y.
Penna. Ind. >rain? Ga. Slinn. Mass. Va, Germany
1929
Ratio, chemical t o spectraC‘heniiralc cheniirxl Gc
0 0070 0 0047.5 0 00435
1.c.
Va. Sld. Va. Pe nn it Va. Va . Xd. Ind. >Id. Ga. Ga.
Copper
-
0,0056 0,0049 0.0097 0.017 0 010 0,0042 0,0028 0.030 0,0028 0.0097 0.010 0.013
0.57
0.48
0,0040 0.0014.; 0.015 0 ,0015 0.01035 0,0023 0.014 0.013; 0.021 0.042 0.00175 0.012 0.0037.5
0,0049 0.0042 0.0035 0.0056 0.0063 0,0056 0.0049 0 0049 0.0042 0.013 0.0049 0 0097 0.017 0,0049 0.014 0 0042 0.014 0.024 0.015 0.033 0,0056 0.021 0.0049
0.90 0.41 0.94 0.91 1.00 0.3‘3 2.88 0.60 5.00 0.90 0.75 1.33 0.90 1.26 0.93 0.68 1.23 0.67 1.14 :3.26 1.35 1.82 1.00 1.78 0.71 0.78 3.20 1.75 1.31
0.0025
0.0056
2.24
0.0084 0.0035 0.0035 0.0049 0.0021 0.021 0,0077
0.97 7.45 0.48 0.61 0.40 4.67 0.35
n 011
0.0056
1935 193.5 193,5 1935 1935 1936 1935 1935 1935 1935 1929 1935 1930 1934 1935 1927 1933 193.3 192h
0.80 1.03 2.23 1.75 0.74
0,0084
0.0042 0.0054R
0.0039 0,0045 0,019
X. J.
Ala. Tenn.
s.Y.
lfd. Ind. Ohio
Grade formulas stand for percentages of N,
P206,.
Spectlu-
uhemicalh .4 j . . cv; 0.027 0.018 0.0715 0.00745 0 0135 0,013 0 0085 0.0115 0,0085
0.016 0.062
0.0125 0.011 0.0185 0.014 0.098 0.0205 0.0245 0.016 0 011 0.0135 0.0195 0.038 0.013 0,0695 0,020.5 0.017 0 . 046,5 0.018 0,0148 0.01d 0.038 0.0135 0 0038 0.0081 3 0,00525 0.017 0.012 0,00245 0.125 0,019 0.023 0.025 0.1025
Ratio. chemical to spectroChemical( chemical
70 0.011 0.0099 0,0809 0.0050 0.0064 0.0092
0,0064 0.0057 0.0064
0.011 0.0035 0.0071 0.0071 0.0092 0.053 0.011 0.018 0.0071 0,0035 0.053 0.0057 0.011 0,028 0,0085 0,057 0.011 0.011 0.048 0.0078 0.011 0.040 0.0014 0.002s 0.0014 0.0014 0.0014 0.012 0.013 0.0014 0,059 0,0021 0.0050 0.012 0 056
0.41 0.62 1.09 0.67 0.47 0.70 0.75 0.50 0.75 0.69 0.56 0.57 0.65 0.50 3.79 0.11 0.85 0.29 0.22 4.82 0.42 0.56 0.74 0.65 0.82 0.54 0.65 1.04 0.44 0.71 2.67 0.04 0.21 0.37 0.17 0.2i 0.71 1.08 0.57 0.47 0.11 0.22 0.48 0 . 5.5
0.0020 0.0018 0.00515 0.0035 0,00655 0,0018 0,00235 0.00165 0.0016 O.OOI2 0.066
0.0013 0.0009s 0,0026 0.0019 0.00145 0.00295 0.0114 0.00695 0.0033 0.0021 0.0029
Ratio chemic;] t u spectroChemicalc chemical
% 0.012 0.014 0.0040 0.0024 0,0080 0,0072 0.0064
0.0032 0.0032 0.0095 0.0032 0,0072 0 0032 0.0048 0.0024 0,0040 0,0024 0,0072 0.0008 0.0032 0.0008 0.0032 0.0064 0,0008 0.0024 0,0095 0.0008 0.0048 0.0040 0.0032 0.0032 0.0008 0.051 0.0064 0.0024 0.037 0.0024 0.000s 0.0024 0.0032 0.0072 0,0032 0.0024 0.0008
1.12 0.93 2.29 0.31 1.15 2.57 1.25 3.20 1.78 1.56 3.64 1.92 0.72 2.18 0.67 1.45 1.26 3.69 0.55 2.46 0.90 1.16 3.20 0.44 0.47 2.71 0.12 2.67 1.70 1.94 2.00 0.67 0.91 4.92 2.45 1.42 1.26 0.55 0.80 0.28 1.04 0.97 1.14 0.28
and K20 in this order, except t h a t numbers indicated by a n asterisk stand f o r percentages of XHJ. Chemists.
b Values are expressed a8 element in accordance with recommendation adopted by Association of Official Agr!cultural c
Jpectrochemicalb A v . 70 0.0107 0.015 0 . 0 17 3 0,0077 0.00695 0,0028 0,0051 0,0010 0.0018 0.0061 0.00088 0.00375 0.00445 0,0022 0,0036 0,00278 0,0019 0.00195 0.00145 0.0013 0,00088R 0.00273
Chemical results from Lundstrom and hlehring are converted t o per cent of element t o facilitate cornpansons.
ANALYTICAL EDITION
August 15, 1941
TABLE 11. PRECISION OF SPECTROCHEMICAL ANALYSIR Boron Deviation from Concn. mean
L'rial
7 0.0083 0.0068 0.0070 0.0070 0.0070 0 0068 0 0062 Aa.
0 0068 0.0070
0
Nanpanese Deviation from Concn. mean
Copper Devia
tion
Concn.
frob mean
%
%
%
%
%
0.0013 0,0002
0,023 0.028
0.0036 0.0014 0.0014 0.0035 0.0035 0.0035 0.0014 0.0035 0.0027
0.0095 0.013
0.0095
0,0012 0.0020 0.0004 0.0004 0.0012 0.0012
0.0107
0.0004 0,0011
0,0000 0,0000
0.028 0.031
0 0002 0 0008 0 0002
0.023 0.028 0.031 0.027
o.oono
0.00034
0.023
0.011
0.010 0.0095
0.013 0.010
0,0020
charts the conclusion is gained that the precision of the results could be a t least doubled if a more accurate method of measuring background or a method of eliminating it were available. ACCURACY.TI7ith the working curves for each element established from a set of prepared standards, analysis of an additional set to obtain more points on these curves becomes a check upon the accuracy of the entire procedure. The results for boron from two independent sets of standards, shown on the working curve, Figure 3-R, were similar to those obtained for manganese and copper.
4v:;:;) ~,
F i g u r e 4 . Effect of Background on Precision
1
plicate spectrographic determinations from their means are B *3.58, M n *3.Gl, and Cu *4.24. The deviations from the average value of the duplicate determinations never exceed * 10 per cent and range from a minimum of 0.0 to maxima of *9.6 per cent for boron, *8.3 per cent for manganese. and * 10 per cent for copper. I n Table I1 are tabulated results obtained from reanalyzing the same fertilizer sample from time to time during the course of this work. The arithmetical mean and the average deviation from the mean are given a t the bottom of the columns, under each element. From the data listed the probable error of the mean, 0.6745
523
is computed to be
0.00014 for boron, 0.00073 for manganese, and 0.00024 for copper. The densitometer readings and intensity conversions from the calibration curve for background correction are often difficult to obtain with accuracy. The background intensity is due to compounds radiating band spectra as well as to solid particles emitting continuous light and hence is not uniform in even a given region of a spectrum and its exact value beneath a measured line can only be approximated. The high galvanometer readings of the comparatively weak background, furthermore, had to be read from the calibration curves a t low intensity value, often just off the straight-line portion, with consequent further sacrifice of accuracy, It is rignifica3t that the average deviation of copper, measured a t 3247.5 A. where appreciable background always necessitated a coriection, is greater than that of the boron determinations, measured a t 2497.7 8. where a weaker background made posrible many measurements entirely free from correction. The average deviation of manganese is between these two values. but closer to that of boron, just as its measured line, 2605.7 '1.. lies in the spectral region. The boron determinations offer an interesting opportunity to study the effect of background correction upon precision. Khen the intensity of the boron line is sufficiently strong, measurement can be made free from background, but when the intensity is weak, small background corrections are necessary. Examination shows that of the boron results listed in Table I, twenty were made without background corrections. while in the remainder, corrections were made in either one or both of the duplicate determinations. I n Figure 4 the duplicate analyses of boron are charted in five ranges of per cent deviations. Figure 4-8 shows the distribution of the total number of determinations throughout these ranges. The importance of background correction is strikingly shown when these duplicate determinations are recharted in two groups: those involving and those not involving background correction, 4-B and 4-C. The average deviation of the determinations made without background correction is * 1.4 per cent, while the determinations requiring background correction show an average deviation of *5.5 per cent. From these
0-2 2-4 4-6 6-6 8-10 0'0
Deviation
A- All B Determinations
0.2 2-4 4-6 %Deviation
0.2 2.4 4.6 6 8 6.10
8 - 8Determinations not requiring background correction
% Deviation C - B Determinotions requiring background correction
Such a check upon the accuracy of the determinations, however, ignores any errors due to differences in the constituents making up the base material and in the fertilizer samples themselves. This source of error can be best avoided, of course, by duplicating as closely as possible the material to be analyzed when preparing the standards, and for this reason the available chemical analyses of the mixed fertilizers were carefully followed in preparing the boron, manganese, and copper standards. However, an additional check on the ac-
TABLE111. ACCURACY OF SPECTROCHEMICAL ANALYSIS Fertilizer Grade0
Snux ce
Tear 4anipled
Iddd c
Original Analysis ID.
%
Subsequent Analysis
70
h r w
s . Y.
6*-8-6 12-24-12 0-14-6
Va. Ga. Mass. Germany Ohio
1929 1926 1935 1935 1930 1926
1*-9-4
s.Y.
\'a. Ga. Mass. Germany Ohio
1929 1926 1935 1935 1930 1926
1 *-9-4 4*-8-4
4*- 8-4
4*-8-4 4*-8-4 6*-8-6 12-24- 1 2 0-14-6
Manganese 0.014 0.014 0.0071 0.0071 0.021 0.021
s.Y.
lY%B
4'-8-4
Va. Ga.
6*-8-6 12-24-12 0-14-6
XIass.
1926 1935 1935 1930
1* 4 - 4
4*-s-4
Germany Ohio
0.027
0.027 0.014 0.014 0,042 0.042
1926
0.0lti 0.016 0.0080 0.0080 0.024 0.024
0,0070 0.034 0.0084 0.0015 0.0025 0.022
0.029 0.057 0,022 0.016 0.045 0.070
0.027 0.012 0.024
0.040 0.024 0.026
0.017
0.017 0.102 0.0107 0.0010 0,0019
0.0065 0.0019 0.0029
0,027 0,038 0.103
0.028 0.013 0,014 0.014 0.021 0.024
a Grade formulas stand for ercentages of N , PmOs,a n d Kz0 in this order; except t h a t numbers indicatecfby asterisk stand for percentages of " 8 .
524
INDUSTRIAL AND ENGINEERING CHEMISTRY
curacy which includes this factor was undertaken. To six of the fertilizer samples selected at random, definite amounts of boron, manganese, and copper were added and the samples were reanalyzed. The results and comparisons are shown in Table 111. COhlPARISOPi WITH CHEhfICAL RESULT^. A comparison Of the average of the spectrochemical determinations with the chemical results obtained by Lundstrom and Mehring (3) are included in Table I. Khile in a few cases the ratios of the chemical to the spectrochemical values vary appreciably from unity, in general these ratios are within a twofold variation. There is no significant trend in the comparison of the two methods, although the spectrochemical results for boron and manganese tend to be somewhat higher than the chemical, while the opposite is true of copper. Study to determine a possible cause for the discrepancy between the two methods has not been attempted. Khen the difficulties encountered in the separation of these small amounts from the complex mixed fertilizers for their chemical determination are considered, a twofold variation is, perhaps, not surprising. The spectrograph provides a method whereby the usual analyses for the important primary plant nutrients in the fertilizer can be supplemented by a ready procedure for the determination of the secondary elements important to plant nutrition. Once the method described is established as a routine procedure, an individual could maintain an average of complete analysis of two or three photographic plates a day. This would involve the determination of three elements in from 14 to 21 samples, or from 42 to 63 determinations a day. Working curves permitting determination from a few parts per million to a few per cent could be established to correspond to the composition of the average fertilizer with supplemental working curves for the unusual samples-i. e., a fertilizer containing practically no lime. Upon the completion of the usual chemical analyses for the major constituents the spectrochemical analyst would have sufficient information for selecting the most appropriate working curve, and the deter-
Vol. 13, No. 8
mination of the concentration of essential secondary elements and the absence (above guaranteed values) of those elements producing toxic effects could rapidly follow.
Summary Using the low-voltage direct current arc and a method involving a step sector and internal standard, a procedure for the simultaneous determination of three of the more important secondary elements in mixed fertilizers, boron, manganese, and copper, has been found to give satisfactory results with an accuracy of about * 5 per cent. Study has shown that unavoidable background is the greatest single source of inaccuracy, and that preparation of base material to correspond to the average composition of fertilizers is satisfactory to avoid any error due to the effect of one ion on the excitation of another. The spectrochemical method can be used to supplement the chemical analysis of fertilizers for primary nutrients by providing B rapid method for guaranteeing the concentrat’ion of the essential trace elements.
Literature Cited (1) (2)
(3)
Daniel, E. P., p. 213, Yearbook of E. S.Dept. Agr., 1939. Gerlach, W., and Schweitzer, E., “Foundation and Methods of Chemical Analysis by the Emission Spectrum” (authorized translation of “Die chemische Emissionspektralanalyse”, Vol. I, L. Voss), London, Adam Hilger, 1929. Lundstrom, F. O., and Mehring, A. L.. IND.E s c . CHEM., 31, 354 (1939).
McMurtrey, J. E., Jr., and Robinson, W. O., p. 807, Yearbook of U. S. Dept. Agr., 1938. (5) Melvin, E. H., and O’Connor, R. T., “Proceedings of Seventh Summer Conference on Spectroscopy and Its Application”, p. 42, New York, John W7iley& Sons, 1940. (6) Myers, A. T.,and Brunstetter, €3. c., IXD.ENG.CHmr., Anal. Ed., 11, 218 (1939). (7) Scheibe, G., Z.angeu. Chem., 42, 1017 (1929). (8) Scheibe, G., and Neuhausser, A,, I b i d . , 41, 1218 (1928). (9) Scribner, B. F., Proceedings of Fifth Summer Conference on Spectroscopy and Its Application”, p. 51, New York. John Wiley & Sons, 1938. (4)
Routine Determination of Phosphorus and Sulfur in Coke Catalytic Nitric-Perchloric Acid Digestion Method LOUIS SILVERRIAN, 5559 Hobart St., Pittsburgh, Penna.
W
ET oxidation of coal and coke to determine sulfur has been suggested (2, 11). This paper outlines a n analo-
gous procedure for phosphorus and presents a modification of the Smith (11) procedure for sulfur in coke. Recent investigations of the structure of coke have shown that nitric acid oxidation converts a large percentage of coke into mellitic and oxalic acids ( 3 ) . As graphitic carbon is easily oxidized by perchloric acid when chromium and manganese are present, no difficulties should be encountered in the rapid oxidation of coke. I n the procedures described below phosphorus is considered present wholly as phosphate, while sulfur may occur as ferrous sulfide, sulfate, free adsorbed sulfur, and sulfur-carbon solid solution (7, 8 ) . The hazards involved are no greater than those encountered in the digestion of rubber or of cast iron. It is best to add nitric acid to coke first, to take care of any volatile matter.
Coke and perchloric acid should not be heated unless nitric acid is present.
Reagents Csed Zinc oxidenitric acid solution, prepared by adding 200 grams of sulfur-free zinc oxide to 1 liter of concentrated nitric acid. Catalyst. Equal weights of potassium permanganate and potassium dichromate, ground separately, and then mixed. Use about 60 mg. for each determination.
Procedure for Phosphorus Keigh 1 gram of 60-mesh coke and about 60 mg. of catalyst. Transfer to a tall-form, narrow-mouthed 500-cc. Erlenmeyer flask, and cover with 20 cc. of fuming nitric acid (specilk gravity 1.5), 18 cc. of technical phosphorus-free perchloric acid (60 or 70 per cent grade), 1 drop of liquid bromine, and about 1 cc. of hydrofluoric acid. Place on a hot plate and boil gently for 10 minutes, then increase the heat to boil out the nitric acid and oxi-
