A knowledge of the spectra of these solids as well as their spectra in solution is of considerable use in understanding the origin of these spectra, because their similarity is such that the possibility that the absorption is due to charge transfer to solvent can be ruled out (20). Sodium Dithionite, Metabisulfite, and Dithionate. Spectra of single
crystals do not appear t o have been recorded. Aqueous solutions of dithionite are unstable, b u t are reported t o ha7.e intense bands at 313 and 250 mp (‘7, 13). The spectrum recorded [Figure 3,a(I)] s h o m no resolved band in the 313-mp rtlgion. Various explanations for this (’an be proposed, one possibility being that the surface of the solid dithionite has undergone extpnsive decomposition into nietabisulfite ( p y r o d f i t e ) and thiosulfate (15). Because metabisulfite has a peak a t 250 nip [Figure 3,a(II)1, a superimposition of the true ditliionite spectrum and that for metabisulfite could give the recorded spectrum, the 313-mp band being hidden under the tail of the metabisulfite band (thiosulfate has no absorption in this region). CONCLUSIONS
Band maxima can be reproduced with
fair accuracy down to 220 mp, and relative heights of different bands are measured reliably. Surface changes are readily detected and often give rise to spectra which are not characteristic of the bulk solid. This is a severe analytical limitation, but it could be of great use in studying surface phenomena. Because the method is very simple and reproducible results are readily obtained, it is strongly recommended as an analytical tool.
(8) Hiickel, TV., “Structural ,,Chemistry
of Inorganic Compounds, p. 592, Elsevier, New York, 1951. (9) Jahoda, F. C., Phys. Rev. 107, 1261
i 1957). (10) Kortum, G., Spectrochim. Acta, 1957 Supplement, 534. (11) Kortum, G., Kortum-Seiler, \I., 2. n’aturforsch. Za, 652 (1947). (12) Kortum, G., Schlotter, H., 2. Elektrochem. 57, 353 (1953). (13) Koson-er, E. M., Bauer, S. W.,
ACKNOWLEDGMENT
IT’th Intern. Congress of Biochemistry, Vienna, Austria, September 1958. (14) McLachlnn, A. I)., Symons, M. C. R., Tomnsend, 31. G., J . Chem. Soc., in press. (15) Meyer, J., 2. anorg. Chem. 34, 43
The authors thank C. A. Parker, who supplied the specially purified sample of sodium dithionite.
(1903). (16) Schaumann, H., Z. Physilz. 76, 106 (1932). (17) Seitz, F., Revs. M o d . Physics 26, 7 i -1R.54) ”--
(2) Bennett, J. E., Ingram, D. J. E.,
(18) Sh&ata, S., J . Biocheni. (Tokyo) 45, 599 (1958). (19) Smith, hI., Ph.D. thesis, Southampton University, 1958. (20) Smith, RI., Symons, 91. C. R., Trans. Faraday SOC.54, 338 (1958).
82 (1951). (5) navies, W. G., Prue, J. E., Trans. Faraday SOC.51, 1045 (1955). (6) Fesefeldt, H., 2.Physik. 64,623 (1930). (7) Hellstrom, H., 2. physiol. Chenz. 246, 155 (1937).
RECEIVED for review November 28, 1058. Accepted April 27, 1959. Research scholarships from the University of Southnmpton to T. R . G. nnd K. -1.K . I,.
LITERATURE CITED (1) Bailey, N., Lott, K. A. K., Symons,
11.C. R., J . Chern. SOC., to be published.
Symons, >I. C. R., George, P., Griffith, J., Phil. Mag. 46, 443 (1955). (3) Butkov, K., 2. Physik. 71,6i8 (1931). (4) Crouthamel, C. E., Hayes, A. >I., Martin, D. S., J . Am. Chem. SOC.73,
(21) Thompson, J. K., Kleinberg, J., J . Am. Chent. SOC.73, 1243 (1951). (22) Whaley, T. P., Kleinberg, J., Ihid., 73, 79 (1951). 123) Zeitlin. H.. Siimoto., A,., .Tuture 181, 1616(19%).
Determination of Manganese in Gasoline by X-Ray Emission Spectrography RICHARD A.
JONES
Research laboratories, Ethyl Corp., Detroit, Mich.
b A method has been developed for determining manganese in gasoline by x-ray emission spectrography. It was necessary to compensate for the interferences caused b y variations in gasoline base stock and in the concentration of certain additives. This method uses a compensative reference, which consists of an iron rod positioned in the liquid sample at a fixed distance from the specimen-holder window. The intensities of selected manganese and iron lines are measured, and the concentration of manganese i s calculated by comparing the manganese-iron intensity ratio for the sample to ratios obtained with known standards. Because the manganese-iron intensity ratio i s reasonably free from interferences due to the base stock or additives, a single calibration curve suffices for all gasoline samples. The time required for a determination i s approximately 15 minutes. Over
the range of 0.1 to 1.0 gram of manganese per gallon, the average standard deviation varies from 0.003 to 0.007 gram per gallon.
R
(methylcyclopentadieny1)manganese tricarbonj-1 has been developed b y Ethyl Corp. as a n e x antiknock additive for gasoline. I t s use will require analytical methods for the determination of manganese in gasoline. This TTork was undertaken to devise a satisfactory x-ray emission method. X-ray emission spectrography has been very useful in the petroleum industry (2-7) and in the analysis of gasolines has been used for the determination of lead (2j 7 ) and of bromine (2, 6). The procedures developed for these determinations were complicated b y absorption effects due t o variations in the composition of the matrix. ComECENTLY
pensation n‘as made for these absorption effects by devices such as a series of calibration curves, a correction based on density, or a n added internal standard. This paper describes a successful technique for dealing with absorption effects in gasolines. All necessary measurements are made with the x-ray instrument, and a single calibration curve applies to all gasoline samples normally encountered. d b o u t 15 minutes are required for a single determination. INSTRUMENTATION
The instrument used was a Norelco (Philips Electronics, Inc.) invertedsample, three-position x-ray spectrograph equipped with a Norelco FA-60 x-ray tube. A schematic diagram of the experimental setup is shown in Figure 1. The x-ray tube was operated at 45 ma. and 55 kv. t o excite to a suitable intensity the K , line of manganese at 2.103 A . and the K , line of iron a t 1.937 A. The liquid specimen holder supplied VOL. 3 1, NO. 8, AUGUST 1959
* 1341
rMYLAR
6000,
1
, /
0 ISOOCTANE
LOCK NUT IRON ROD
DETECTOR
..
SOURCE
Qb
2 - oo MI
Figure 1.
Arrangement of equipment in x-ray unit
with the instrument has a n easily replaceable Mylar bottom. The cap for the holder was adapted so that an iron rod could be attached to it. The screw in the center of the cap was replaced with a special one 7 / * inch long. inch One end of a pure iron rod-'/( in diameter and 5,'s inch long-was tapped to fit the special screw. One locking nut was used to hold the screw in place and another to hold the iron rod in the desired position on the screw. The distance between the Mylar bottom and the end of the iron rod could be varied by screwing the iron rod u p or down on the center screw. The correct distance was determined experimentally, and then the iron rod was maintained in the same spatial arrangement with regard to the specimen holder. However, in the spectrograph used, the specimen holder is free to rotate in the horizontal plane, making it difficult to maintain the same orientation in space between the iron rod and the x-ray optics. This conceivably could cause differences in intensity. To average out such differences, the specimen spinner attachment was used to rotate the specimen holder during intensity measurements. A flat crystal of lithium fluoride was used to diffract the desired wave lengths of fluorescent radiation, which were then collimated with a flat-plate collimator having 0.020-inch spacings. The detector was a scintillation counter. An Atomic pulse height analyzer was used to discriminate against higherorder radiation and to reduce the background count. For the pulse height analyzer, base line and window width settings suitable for the determination of manganese K , radiation were found experimentally according to procedures recommended b y Philips Electronics, Inc. The same settings were also suitable for the iron K , radiation. These settings must be determined for each individual instrument. The puke height analyzer had been modified by Philips Electronics, Inc., to have window widths ranging from 0 to 21 volts. Window widths as high as 21 volts are often required when a scintillation counter is used. For the lithium fluoride crystal used, the goniometer setting for maximum intensity was 62.95" for the manganese K , line and 57.49" for the iron K , line. 1342
ANALYTICAL CHEMISTRY
1/04.
32
3'
06
08
0
2
Mn IG4L
Figure 2. Use of compensative reference to minimize base stock interference
Four layers of nickel foil of 0.00035inch thickness each were placed between the collimator and the detector whenever intensity measurements were made a t the iron K , line. The nickel foil attenuated the intensity to less than 5000 counts per second, which is about the maximum input counting rate that the pulse height analyzer can handle satisfactorily.
256,000 are used. For samples with low intensities, the fixed count of 32,000 is used. For those of higher intensities, the fixed count is chosen so that the time required will fall between 20 and 45 seconds. S o background correction is applied. The manganese-iron intensity ratio is calculated by the equation:
CALM RATION
where 1 is the average time in seconds required for a fixed count, C, at the iron K , line, and D is the average time in seconds required for a fixed count, B, a t the manganese K , line. The manganese concentration is obtained by referring this ratio to the calibration curve.
Calibration data are obtained d d y from a series of standards covering the concentration range of 0 to 1.0 gram of manganese per gallon. A master standard is prepared by blending a knonn concentration of manganese as manganese naphthenate with an additive-free gasoline. The other standards are prepared by accurate successive dilutions of the master standard. After blending. the standards are stored in amber qcren--top bottles of appropriate capacity. The standards are stable for seyeral weeks but should be prepared fresh every 2 weeks as an extra precaution against error. The manganeseiron intensity ratios of the standards are determined, and a calibration curve is drawn by plotting each ratio IX.the ( orresponding concentration. PROCEDURE
The instrument is allowed to warm u p for 30 minutes before any quantitatire measurements are made. For each determination, a new Mylar bottom is fitted on the specimen holder according to the directions of the manufacturer. The holder is filled approximately half full with the sample, and the cap fitted with the iron rod is placed in position on the holder. The holder is then placed in the instrument and moved to the test position, where i t is rotated by the specimen spinner. An intensity measurement is taken at the manganese K , line. Four layers of nickel foil are placed between the collimator and the detector, and an intensity measurement is taken a t the iron K , line. For each intensity measurement, three to five time-readings for a given fixed count are averaged. Fixed counts ranging from 32,000 t o
Mn-Fe intensity ratio
=
AB/CD
DISCUSSION
A method for the determination of manganese in gasoline should be simple, reasonably rapid, and applicable to a wide variety of base stocks containing a n y combination of several additive elements, I n choosing x-ray emission, problems such as interference from a higher-order line and deviations from proportionality due to the base stock and additive elements were expected. Sufficient intensity could be obtained at the first-order manganese K , line with the concentrations of manganese in question by using a scintillation counter and a collimator with 0.020inch spacings. However, with this detector and collimator, the background count was undesirably high and the resolution was poor. The resolving power of the collimator was not sufficient to separate the second-order K , line of bromine from the firsborder K , line of manganese. Bromine was expected to be a n additive element in most of the gasolines to be analyzed. Line interference from bromine Iyas eliminated and the background count \\.as materially reduced through the use of the pulse-height analyzer without any significant loss of net intensity. It was expected that the manganese intensity would vary with the composition of the matrix. Comparison of the linear absorption coefficients of different
matrices for 2.10A. radiation ( l I n K,) gives t'he best indication of the interference to be expected from this source. The higher the value for the linear absorption coefficient, the lower will be the observed manganese intensity. The linear absorpt,ion coefficient of :i substance is obtained b y multiplying its mass absorption coefficient by its density. The mass absorption coefficient, gem, of a mixture of elements -4 and B can he obtained as follows: (rm)ae =
+
~ V A ( P ~W A B(P~~)E
where W is the m i g h t fraction. Using the data and equations given by I'ict,oreen ( I I ) , the mass absorption coefficients of carbon and hydrogen for 2.10 A. ( A h K,) radiation were calculated to be 11.3and 0.51, respectively. The greater t'he carbon content of a liquid hydrocarbon, the greater nil1 be its mass absorption coefficient a t the wave length under consideration. Because the density of a liquid hydrocarbon increases 1%-ithincreasing carbon content, two hydrocarbons differing by a small amount in their mass absorption coefficients will differ by a greater amount in their linear absorption coefficient's. Table I gives the calculated linear absorption coefficients for isooctane and toluene at 2.10 A. Because these coefficients differ markedly, the base stock effect for the manganese determination was expected to be large. Elements other than carbon and hydrogen can also affect the manganese int.ensity; t8hose normally encountered in gasolines are lead, bromine, chlorine, sulfur, phosphorus, oxygen, and nitrogen. The mass absorption coefficient for lead a t 2.10 A. was found to be 2 2 by a n extrapolation of a plot of niass :ibsorption coefficients as a function of thc cube of their wave lengths ( I O ) . Using data and equations given bp S'ictoreen ( 1 2 1 , the mass absorption coefficients for 2.10 A. radiation of bromine, chlorine, sulfur, phosphorus. oxygen, and nitrogen were calculated t'o be 213, 250, 220, 177, 28, and 19: respectively. Lead as tetraethyllead is added to commercial gasolines as part of a n antiknock fluid mix whicli also contains a bromine compound and? generally, a chlorine compound. Table I gives the linear absorption coefficieiit for iso-octane containing representative amounts of lead, bromine, and chlorine. Because it differs considerably from that for iso-octane without additives. the combination of lead, bromine, and chlorine was expected to interfere seriously with t'he determination of manganese. Table 1 also list's the linear absorption coefficients for iso-octane containing representative amounts of phosphorus and of sulfur. Because neither of theee coefficients differs greatly from the linear absorption coefficient for pure iso-octane, phos-
phorus and sulfur nere not expected to interfere to a n y great extent. The lon concentrations of oxygen and nitrogen present were ignored. A t least partial compensation for matrix absorption effects can be obtained by the use of a n added internal standard (8). Such standards also compensate in some degree for enhancement effects and for some instrument variables (1). I n general, one of the two elements adjacent in the periodic table to the element to be determined is chosen as the internal standard element. These two elements will most closely approximate the sought element in fluorescent yield and absorption properties. Because the intensities ot the sought element and the internal standard element will vary in nearly the same way, the ratio of the intensitie. of the two elements will be affected much less by changes in the matrix than will the individual intensities. The main disadvantage of the use of a n added internal standard is the additional time required to mix the internal standard thoroughly with the sample. The compensative reference technique used in this method uses an added internal standard but no niising is required, because the reference element is present in the form of :t solid rod. EXPERIMENTAL
Iron n-as chosen as the compensatir-e reference in preference to chromium because pure iron is easily obtained in rod form. The distance between the iron rod and the specimen holder windo\v was fixed experimentally to give the correct degree of compensation This was done using t n o standardthat ddered greatly in density but \vhich had the same manganese concentration on a 1Teight per unit volume basis. The standards were blenderi using iso-octane and toluene whicli have densities of 0.692 and 0.86i, respectively. The concentration of manganese was 1.0 gram per gallon. The distance was varied until both standardnould give, within experimental error. Table IV.
Base Stock Iso-octane
Gasoline 9
Gasoline B
a
Added 0 989 0 494 0 247 0 124 1.026 0.513 0.256 0.128 1.04i 0 524 0 . "6" 0 131
Table I. Linear Absorption Coefficients x = 2.10 A.
Sulistsnce Toluene Iso-octane Iso-octane 0 0048 wt. Pa Iso-octane 0 050 wt. % S b Iso-octane 0 121 wt. C, Pb 0 0166 wt. 7 Br 0 5413 n t . 7 c1c
+ + +
Pl
8 6 6 6
+
+
98 64 65 72
7 23
Phosphorus present as trimethvl phos. .
phate. b Sulfur present as thiophene. c Lead, bromine, and chlorine present as tetraethvllead. ethvlene dibromide. and ethylene dichloride,"respectively . Table (I. Effects of Other Elements
Other Elements, wt. yo Ph, 0.121: Br, 0.0466 C1, 0,0413 Pb, 0.121; Br, 0.0559
ManganeEe Concn., Gram/Gallon Input Founda 0.923
0.918
C1: 0.0413
0.952
0.952
Pb, 0 . 1 2 1 ; B r , 0.0931 Pb, 0.121; Br, 0.0466 C1, 0.0413; P, 0.0048
0.931
0.929
0.955
0.945
Pb, 0.121; Br, 0.0466 Cl, 0.0413; S, 0.050 0.932
0.929
a
riv. of three determinations.
Table 111. Comparative Determination of Manganese in Gasoline
Sample 1 2 3 4
5
z k
9 10 11 ~~
12 13 14
Manganese Concn., Gram/Gallon Flame X-ray photometer 0.51 0.26 0.23 0.29 0.25 0.23 0.49 0.25 0.25 0.49 0.99 0.12 0.25 0.51
0.52 0.25 0.23 0.29 0.26
0.25 0.50 0.25 0.26 0.51 1.00 0.13 0.26 0.50
Precision and Accuracy
Manganese Concn., Gram/Gallon Found5 Std. dev. from mean 0 992 0 494 0 247 0 124 1.031 0.513 0.256 0.131 1.049 0.528 0.263 0.136
0 007 0 007 0 003 0 002 0,005 0.006 0.003 0.002
0.008 0.006 0.003 0.003
Average of 6 determinations
VOL. 31, NO. 8, AUGUST 1959
1343
the same manganese-iron intensity ratio. At distances shorter than the optimum, the absorbing path for the radiation directed at the window was too short to provide sufficient compensation. I n these cases, the iso-octane standard gave a higher value for the manganeseiron intensity ratio than did the toluenc standard. When the distance m-as too large, overcompensation resulted, and the toluene standard gave higher values than did the iso-octane standard. The optimum distance was 0.023 inch. This distance would be expected to be different for other instruments. It is probable that the distance can be in error by =tO.OOl inch without noticeablj affecting the accuracr, Figure 2, A , shorn calibration curve3 obtained when the reference was not in the samples. The samples used mere two base stocks, iso-octane and Gasoline A. Gasoline A had a density of 0.758. At the 1.0 gram of manganese per gallon level, the manganese intensities for the two base stocks differed by about 10%. Figure 2, B , shows data obtained for the same standards when the compensative reference was used. It illustrates the effectiveness of the compensative reference technique in eliininating the effects of base stocks. With the reference in position, manganese intensities decreased by about 67,, and background intensities increased on the order of 50%. The effect of lead, bromine, chlorine, phosphorus, and sulfur on the accuracy of the method was studied. Five samples tyae prepared that contained various combinations of these elements together with manganese naphthenate, Iso-octane n n s the base stock used for the samples. Calibration standards
Table V.
Precision of X-Ray Method
J1angane.c
Concn., Gram/Gnllon 1.0 0.5 0.26
J7stimated Error,
:c (965-; Confidence Limits) 1 6
2 6
2 3 4 3
0.13
Ivere blended using manganese naphthenate and additive-free iso-octane. Analysis of the five samples by the x-ray method gave the results shown in Table 11. It was concluded that, a t normally encountered concentrations, none of the elements tested would significantly affect the accuracy of the x-ray method. The effect of using tn-o different manganese compounds for calibration standards v a s studied. It would be predicted that even nithout compensation, the manganese compound used should have almost no effect, if the compound consisted mainly of manganese and light elements such as carbon, hydrogen, and oxygen. Calibration data obtained using manganese naphthenate and (methylcyclopentadieny1)nianganese tricarbonyl indicated that these two compounds gave equivalent calibration curves. RESULTS
The results for typical gasoline samples containing manganese that were analyzed by the x-ray method and a flame phot,ometric method (9) are given in Table 111. These samples included a variety of base stocks and coi-ered a uide range of phosphorus and sulfur concentrations. A. the results show.
there was good agreement between the two methods. To obtain data on precision and accuracy, 12 samples containing knon-ii amount3 of manganese naphthenate were analyzed six times each. The samples were blended using three different base stocks. All samples contained lead, bromine, and chlorine in the amounts usually found in commercial gasolines. The T2 determinations were made over a period of several days (Table IS-). The data n ere treated by accepted statistical procedures to obtain pooled estimates of the precision of the method a t different concentration levels (Table V). LITERATURE CITED
(1) .hdermann, Georgc, Iiemp, J. ~ A L CaEaf. . 30, 1306 (1968).
,
(2) Birks, L. S., Brooke, E. J., Friedman, Herbert. Roe. R. 11..Zbad.. 22. 1258
(1950). ’
(3) Davis, E. I., Hoeck, B. C., Zbid., 27, 1880 (1965). (4) Davis, E. K., Van Sordstrand, R. A . ,
Zbid.,26,973 (1954). (5) Dyroff, Q. V., Skiha, Paul, Zbicl., 26, 1774 (1954). 16) Xokotailo. G. T.. Damon. G. F.. Ihid..25. l l k 5 119535. \
,
(9) Smith, 0. R., Palr
~,
Photometric Determination of Lead and Manganese in Gasoline,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsbur h, Pa., March 2, 1950. (10) ffP roull, W. T., “X-Raya in Practice,” McGrax-Hill, Iiew Torli, 1946. (11) T‘ictoreen, J. A., J . B p p 2 . Phys. 20, 1141 (1949). RECEIVEDfor rovien- January 26, 1959. iicccptcd April 23, 1959.
Spectrophotometric Determination of Vanadium in Plant Materials G. B. JONES Division of Biochemistry and General Nutrition, C.S.I.R.O., University of Adelaide, Adelaide, South Australia
J. H. WATKINSON Rukuhia Soil Research Station, Department o f Agriculture, Hamilton, New Zealand
b A spectrophotometric method has been developed for determining as little as 0.2 y of vanadium in plant materials. With minor modifications, it can b e used to determine vanadium in soils.
F
of the several methods for the determination of microgram quantities of vanadium are specific. The EW
1344
ANALYTICAL CHEMISTRY
most marked interference comes from iron and titanium, except in the method of Cozzi and Raspi (S), published after most of this work was completed, and in .cvhich cobalt, molybdenum, and tungsten interfere. Because, in plant analysis, interfering elements including iron are always present, the final choice of method depends largely on the ease with which these interferences may be avoided.
The polarographic method of Jones ( 6 ) , although very sensitive, is lengthy,
and not so readily adaptable as the proposed spectrophotonietric method for handling a large number of samples. Of the reagents used in spectrophotometric analysis, benzohydroxamic acid was selected for study because of its sensitivity, specificity, and tolerance of a wide range of conditions in the formation of the vanadium complex.
