Flame Photometric Study of Boron JOHN A. D E A N
and C L A R I C E
THOMPSON
Department of Chemistry, University o f Tennessee, Knoxville, Tenn.
Demineralized water, used exclusively in preparing all solutions and samples, was prepared by passing ordinary distilled water through a bed of Amberlite RTB-3 resin.
This study was undertaken to develop a flame photometric method for boron and, in particular, to adapt it to the Beckman DU spectrophotometer with the Model 9220 flame attachment and photomultiplier unit. The effects of acid and methanol concentration and of various anions and cations commonly associated with boron upon the flame emission of boron in 1 to 1 methanolwater solution were studied for the three prominent oxide band systems: 492, 518, and 546 mp. The interference of many elements was of sufficient magnitude to necessitate making a pseudo-background correction by measuring the luminosity at the minima or troughs between the overlapping band systems of boron. Compensation by this means rendered interference effects by many elements negligible. The flame photometric method is more rapid than existing chemical methods and is comparable in accuracy and precision to them. Optimum range of applicability is 50 to 200 p.p.m. of boron. Sensitivity is within 1 to 3 p.p.m., depending upon particular phototube response.
Spectrophotometer Settings. The instrument settings used for measuring the boron flame emission were as follows: Sensitivity control Selector snitch Phototube resistor S!it Acetylene Oxygen
5 t o 6 turns from clockwise limit 0.1 22 megohms 0.030 mm. 5 pounds per square inch 8 t o 16 pounds per square inch depending upon burner used
Individual operators should be aware that different burners, even though of similar construction, will not necessarily reproduce the tabulated luminosities for the operating conditions used in this work. I n particular, obstructions in or around the oxygen orifice, often due to accumulating carbon deposits, will affect not only the flow of oxygen but also the rate of aspiration of the solution under examination. These factors will alter flame temperature and therefore will affect both the flame background and the boron luminescence. Consequently, the burner should be cleaned frequently.
T
HIS investigation describes the application of the flame photometer to the rapid, routine determination of boron. The method should be of particular interest to those laboratories that are concerned m ith borohydrides and other boron-containing compounds. The flame photometric procedure offers a rapid instrumental means of determining boron compared to conventional distillation methods and is particularly applicable to the processing of a large number of samples. The boron-containing compound can be dissolved in an appropriate solvent and the resulting solution aspirated directly into the flame. Thus, all preliminary decompositions, either by n e t or dry methods, are no longer necessary. A considerable saving in time results and there is no longer any danger that part of the sample will be lost, through either incomplete digestion or volatilization during the preliminary decomposition step. Determinations of boron by means of the green flame coloration are discussed by Stahl ( I S ) and applied to agricultural materials by McHargue (9), McHargue and Calfee ( I O ) , Calfee and McHargue (S), and Weber and Jacobson ( 1 4 ) . These workers used either visual methods or photographic recording in conjunction with a spectrograph. It would be desirable to have a method available which would permit the use of modern flame photometers.
Table I.
Oxide Band Systems of Boron in Flames
W a r e Length of Band Maxima, MP
Rel. Intensitv of Band Svstem >fin Quantity Easily Detectable, P.P M of Borona 200 30
345 452/454 10 471/473 492 5 3 518 3 545/548 10 577/580 603 . . .b . . .b 620 ..,b 639 a For slit w.idth of 0.03 mm. b Relative intensities not reported by Singh (fd), b u t found to be very low in this work.
Table 11. Relative Emissivity of Roron from &lethano]Water Solutions" Methanol present. d./lOO ml. total soln. 0 20 50 Emission intensity, transmittance scale units 5 12 34 a Boron concentration 100 p.p.m. in all solutions.
GENERAL EXPERIMENTAL WORK
Apparatus. A Beckman Model DU spectrophotometer with Model 9220 flame a t t a h m e n t and photomultiplier unit was used. The spectrophotometer has been described (4). An all-metal atomizer-burner unit, supplied Kith the flame attachment, was used as the excitation source. The gases chosen were oxygen and acetylene, largely because of availability. However, the higher excitation energy available from the oxyacetylene flame, a s compared with an oxygen-hydrogen flame, was a prominent consideration. Reagents. A standard solution of boron, 1.00 ml. equivalent to 1.00 p.p.m. as boron, was prepared by dissolving 5.715 grams of fresh crystals of reagent grade boric acid in demineralized ~ a t e r and diluting to 1 liter. A typical flame photometric standard solution, containing 100 p.p.m. of boron, was prepared by pipetting out 10.0 ml. of the first solution, adding (by pipet) 50.0 ml. of drum grade methanol, and diluting to volume in a 100-ml. volumetric flask with demineralized water. For storage, all solutions should be transferred to polyethylene containers. 42
75
RO
53
55
Characteristics of Boron in the Flame. Certain molecules which can exist in the oxyacetylene flame may be excited to emit band spectra, also known as molecular spectra. Such a molecule is B,O,. The general characteristics of band systems are that they arise from transitions between a few of the lowest electronic levels of the molecule concerned. With each electronic level is associated a suite of vibrational levels, and with each vibrational level is associated a suite of rotational levels. These latter t n o transitions cause the emitted radiation to be spread over a portion of the spectrum rather than being concentrated in a discrete line. The radiation is centered about the wave length associated with the electronic transition, with the energy of the bands degraded either toward the red or the blue portion of the spectrum. Cor,sequently, the bands are not symmetrical about any center. The fine structure is not observable with the dispersion obtained from the optics of the Beckman spectrophotometer; rather only the envelope is observed.
V O L U M E 2 7 , N O . 1, J A N U A R Y 1 9 5 5 Because the excitation potential of B,O, falls within the exitation range of the oxyacetylene flame, the emission of narroL? bands attributable t o the B,O, molecule is observed when boron is introduced into the flame. These boron flame bands belong to molecules which are electrically neutral but are not stable in the chemical sense (If ). Gilbert and associates ( 7 )reported six oxide bands, and from photographic studies Singh (12’) has reported four additional band systems. The wave length of the band head of these oxide bands together with the relative intensities of each are listed in Table I. Strangely, Lundeghrdh (8) reported no flame spectra for boron in his pioneering investigations.
100
80
2:
Gi
p
60
I3
-I
40
PO
480
520
560
43
The recent paper by Caton and Bremner ( 5 )should be consulted Curtis et al. (6) found that greater intensity and sensitivity may be obtained by atomization from certain hydrocarbon and nonhydrocarbon solvents than by solubilizing and atomizing from aqueous solutions. All of the data reported subsequently in this paper x e r e obtained from solutions and standards which were cornposited from aqueous solutions and an added amount of methanol equal to 50% of container volume employed for the final dilution. T o ensure uniformity, the methanol was added before diluting t o volume, so that additional demineralized water could be added t o care for the volume contraction which occurs upon mixing aqueous solutions with methanol. Calibration Curve. The overlapping band systems of boron presented a problem not often encountered in flame analyses: how to choose a general background reference wave length. Usually one refers to the general background reading in the vicinity of the band head or emission line in order to determine the correction to be applied to the observed emission a t the wave length of the band head or emission line. Because the band systems of boron overlap in the useful region, this procedure is not possible. Fortunately, the minimum intensity in the troughs between the band heads will serve the same purpose as a normal background reading. The calibration curve for boron is strictly linear up to a t least 300 p.p.m. of boron. The luminosity reading, given by the minimum in an adjacent trough, subtracted from the luminosity reading of the neighboring band head, gave a net relative luminosity which was plotted against the concentration of boron present in the respective standard solution. Calibration curves were constructed from a series of standards each time a set of samples was analyzed.
Table 111. Consistency of Proposed Method of Calculation
WAVE LENGTH
Figure 1.
Emission Spectrum of Boron in 1 to 1 Methanol-Water Solution Present. 200 p,p.m. of boron Slit width, 0.030 mm. Lower line is background of solvent alone
Most of the boron band systems overlap each other. I n Figure 1 is given the flame emission spectrum of boron in the region suitable for the flame photometric determination of boron. The more intense flame emissions occur from the band groupings centered around 492,518, and 546 mp. The intensity of the boron emission is influenced strongly by the nature of the solvent. Introduction of methanol causes a marked increase in the intensity of the boron bands. A continual increase is eyperienced as the methanol concentration is increased. Table I1 gives the relative emissivities for a series of methanolwater solutions. The effect may be due in part to the formation of methyl borate, but more probably is caused by the lowered surface tension of the solution undergoing aspiration into the flame. Other alcohols also enhance the emission characteristics of the boron oxide bands but to a lesser degree. Ethanol and 2-propanol, for example, are roughly 75% as effective as methanol, volume for volume. The effect of acetone and dioxane on the relative emission has been reported ( 1 ) . Unfortunately, this report was called to the authors’ attention toward the conclusion of this work. Apparently a slight advantage would be gained from the use of either of these two solvents in place of methanol. It is further apparent that any advantage accruing from the use of any of the aforementioned solvents must be due to injection of larger amounts of sample into the flame because of lowered surface tension of the aspirated solution, rather than the formation of a more volatile boron compound involving the solvent.
Fuel Pressure Lb./Sq. Inch’ AcetOxyylene gen
5
16
Slit Width, Mm.
0.06
Emission in Scale Divisions Boron DifWave Wave ference, Preslength, length, 518 mp - ent. 518 mp 505 m p 505 m p P.P.M.
10
24 32 42 50
11 14
18.5 22 24.5
-1 10 13.5 18 25.5
0 25 50 75 100 Av.
Ratio of Emission at 518 m p - Background to Difference
...
1.27 1.51 1.58 1,5l 1.58 =t 0.04
Table I11 illustrates the consistency of this method of calculation for different operating conditions. The data were obtained from measurement of the emission intensities a t the boron oxide band system maximum located a t 518 mp and from the minimum point in the valley or trough at 505 mp, which lies between the band heads a t 492 and 518 mp. The other band heads and troughs could also have been chosen and would further verify the conclusions. I n columns 4 and 5 of Table I11 are listed the emission intensities observed as instrument scale divisions for the respective concentrations of boron given in column 7 . Normally a calibration curve is graphed simply by taking the observed emission reading (column 4), subtracting from it the background reading observed for the solvent blank alone (the first line across
ANALYTICAL CHEMISTRY
44 for no boron present), and plotting these difference values against the respective concentrations (column 7). Instead, the emission reading obtained a t 505 mp (column 5), the minimum in the trough between the 492 and 518 band heads, ia subtracted from the emission readings at 518 mp (column 4), and these differences are plotted against the respective boron concentrations (column 7 ) . If this alternative method of graphing the data is valid, the ratio of the differences obtained in each case should be constant. That these are constant within experimental uncertainties is shown by the agreement of the values in column 8. The background emission is not the same a t the two wave lengths and this necessitates a corrective term to be added to the difference values: 518 mp 505 mp. Other pairs of wave lengths equally applicable are the 492-mp band head and the 482-mp trough minimum and the 546-mp band head and the 536-mp trough minimum. Influence of Various Elements. EFFECTOF CATIONS. A major part of the experimental work in this research R L ~ S con-
7
loo
Table IV.
I
MG
-
I
I I 500 520 540 WAVE LENGTH
I
I
560
Figure 2. Emission Spectrum of Potassium and Magnesium Effect of Cations in Determination of Boron by Flame Photometry
(100 p.p.m. of boron present in all cases) Boron Found, P.P.M. a t Cation Concn., 492 518 546 Tested P.P.ILI. mp mlr mp 0 3000 95 Aluminum ... 2000 98 ... ... 130 1000 98 133 500 113 100 112 3000 Ammonium 100 100 100 1000 100 100 100 Cadmium 3000 96 ... 2000 ... ... 94 1000 ... 94 500 ... ... 96 Calcium 2000 . . .n 94 127 1000 96 107 ... 97 102 500 100 100 100 ... Chromium 3000 97 136 2000 106 ... ... 136 1000 99 500 100 118 100 100 99 ... 400 90 Cobalt ... 110 ... 300 ... 106 95 200 ... 103
...
...
.
I
.
.
.
I
... ... .
Copper
Iron
Lead Lithium
Magnesium
1000 500 500 300 100 1000 400 3000 2000 1000 500 3000 2000 1000 400 100
I
.
102
100 100 127 118 104
100 100 78 97 100 105
103 100 84 95 95 100
...
...
142 109
142 108
152 126
iii
.. .. ..
... ...
80 90
...
... ...
100 100 90 96 96 94 106 109
9H 102 102
... 200 127 ... ... 100 ... 101 500 111 92 Nickel .., 94 100 103 3000 Potassium 110 88 37 100 58 2000 103 1000 100 79 103 500 100 89 100 100 100 98 3000 Silver 100 ... 3000 Sodium ... 103 , . 2000 103 130 103 103 112 103 1000 500 100 108 100 Strontium 2000 100 103 103 100 1000 3000 Zinc 98 ... 2000 ... 98 1000 100 A strong band or line is emitted b y the test cation which coincides with or overlaps the boron band head or adjacent trough minimum. Manganese
...
... ...
... ... ... ... ...
I..
Present. 2000 p.p.m. of potassium (upper curve) a n d 1000 p.p.m. of magnesium (lower curve), each in 1 t o 1 methanol-water solution Slit width, 0.030 mm. Base line is background of solvent alone
cerned with determining the radiation interference, if any, caused by the various cations generally associated with boron. For each substance tested for interference, a series of methanol-water solutions was prepared containing several known concentrations of the test substance and generally 100 p.p.m. of boron. Table I V shows the effect of various cations on the determination of boron. For much of the earlier work the boron band head at 518 mp wm used with the background taken at 505 mp. However, because of serious interference from certain cations a t this pair of wave lengths, the study was extended to the 492- and 546-mp band heads, with background readings taken a t 482 and 536 mp, respectively. As a further check on interference effects, the emission characteristics of each cation individually were determined in the methanol-water solution in the absence of boron. These results for the more important interfering cations are shown as Figures 2, 3, and 4. On these plots the little arrows indicate the pairs of wave lengths used when measuring the boron flame emissions: the 492-mp peak and 482-mp background, 518-mp peak and 505-mp background, and the 546-mp peak and 535-mp background. From these studies the cations investigated can be grouped roughly into four categories: 1. Elements which offer no interference and whose emission spectrum in methanol-water is indistinguishable from that of the solvent blank. These are ammonium, cadmium, copper, lead, silver, and zinc ions. 2. Elements exhibiting general background radiation. This occurs with most of the cations when Dresent in relativelv high concentrations. S o t e the emission spectrum of potasscum i n Figure 2. This type of radiation, to a great extent, is compensated by the method of measurement employed. Very large changes in general background radiation were without effect upon the recovery of boron. 3. Coincidences or near coincidences, such as the enhance ment of the boron 518-mp band head by magnesium 518 or the overlap of a weak potassium band with the 505-mp reference trough (Figure 2). Sometimes the latter type of interference can be mitigated by the use of narron- slits, as with potassium, but with a corresponding loss in sensitivity for boron. Enhancement from calcium will be significant unless small slit widthq 0.03 mm. or less, are used, since the boron 518-mp band ia adjacent to the short wave-length side of the prominent, calcium 553-mp band (Figure 3). 4. General interference a t all of the prominent boron band systems and their intervening trough minimums is observed for chromium, manganese, iron, cobalt, and nickel. ~~
Effect of Anions. The effect of various anions was nest de-
45
V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5 ternlined, chiefly those which \\-ere used for the solution of inorganic samples or which might be associated with boron in its coordination compounds. A series of several concentrations of each of the anions as their sodium salts was prepared with 100 p.p,m. of boron. The results obtained are given in Table V. Acetate ion caused a small error. Nitrate and chloride ions are recommended when acid anions must be introduced. I00
80
I:
I
Boron Found.a P. P AM. 100 100 100
100 91
0.50
1.0 2.5
100 p.p.m. of boron present in each case. 518- to 505-mp pair of wave lengths.
83 74 Measurements taken a t
Ca absence of absolute standards, the accuracy of the flame photometric method can only be discussed in relative terms and compared with values obtained by other chemical methods. The volumetric results were obtained by the procedure described by Brunisholz and Bonnet ( 2 ) . These authors removed all metallic ions from the sample solution, adjusted t o pH 1 to 5, by passage through Amberlite IR-120, a strong cation exchange resin in the hydrogen form. Boric acid is displaced and is eluted with 200
Z
3 aR
Influence of Hydrogen Ion Concentration
HC1 Present, Moles/Liter 0.01 0.05 0.10
4.0
I
I
Table VI.
40
PO
100
500
520
540
560
WAVE LENGTH
80
Figure 3. Emission Spectrum of Calcium and Aluminum
I:
8 60
Present. 2000 p.p.m. each of calcium (upper curve) a n d aluminum (lower curve) Base line is background of solvent alone Slit widtli. 0.030 mm.
zc
3
Tahle
\-.
Effect of .inions on Boron
(Added as their sodium salts) Boron Present P.P.M. 100
Anion .4dded Acetate
-\nion Present, P P 31 3000
2000
1000 500 100
40
ep
PO
Boron Found, P P 31 a t 492 518 546 *Mp
Llp
M p
106 106 104 103 100
111
116 110 103 108 (10
108 106 104 98
500
520
540
560
WAVE LENGTH
Figure 4.
100
Emission Spectrum of Iron
Present. 3000 p.p.m. of iron in 1 to 1 methanol-water solution Slit width, 0.030 min. 100
100
Chloride
3000 2000 1000
95 100 97 97
97
98 102 104 102 102
118 103 100 107 105
103 103
103 102 100
130 112 I08
100
100
Bromide
1000
102
100
Iodide
1000
102
100
Fluoride
1000
102
...
...
Influence of Hydiogen Ion Concentration. The influence of hydrogen ion concentration on the flame emission of boron is of interest t o those dealing 11ith the analysis of gas streams containing high concentrations of hydrochloric acid and hydrolyzable boron halides Further, acids are used for the qolution of inorganic residue?. Variation in the concentration of hydrochloric acid, 0.5L1fand less, had no effect on the determination of boron. Higher concentrations of acid exerted a depressing effect on the boron emiqqioii The results obtained are shown in Tahle VI DISCUSS103 OF RESULTS
Taljlr \'I1 lists the flame photometric results obtained on mineral * w n i p l ~ supplied ~ by the Pacific Coast Borax Co In the
Table VII. Comparison of Flame Photometric and Chemical Results on 3Iineral Samples 1
&lea l G 2
c;
.4r. value and associated std. dev.
2C
Boron, Yo, Volumetric 11.30 11.30 11.28 11 28
Boron, %,
by Flame Photometric Analysis .4t 492 mp At 518 mp
11.45 11.30 11 50 11 15 11 30 11 15
11.40 11.65 11.25 11.30 11.30 11.15
11.29 + 0 . 0 1
11 :31 5 0 . 1 5
1 1 . 3 1 =t0.Ii
12.38 12 29
12 50 12.00
12.60
12.28 12.25 12.30
Av value and associated 12.46 +0.20 std. dev. 12.32 f O . 0 5 12 21 + 0.22 16% CaO, 5% NaaO, 1 a Samples G , Gerstley borate, contained: A h O ~ ,4% MgO, a n d 10% S O * : samples C , colemanite, contamed 28% CaO, 2% &Os, 1% MgO, and 5% SiO2.
ANALYTICAL CHEMISTRY
46
ml. of distilled water. The eluate was then titrated with boronand carbonate-free sodium hydroxide, using bromocresyl purple indicator; then mannitol was added and the titration continued until the pink color of phenolphthalein indicator was observed. Theamount of base consumed between the bromocresyl purple and phenolphthalein indicator changes is equivalent to the boric acid present in the aliquot sample. This ion exchange and subsequent volumetric titration method requires approximately 1 hour’s time in comparison with the rapid method of analysis employing the flame photometer, without considering any extra time which would be consumed in decomposing certain boroncontaining compounds prior to their titrimetric determination. The reproducibility of the flame photometric determinations was very good. The standard deviation from the mean of replicate samples was approximately 2.0’%,. The difference between the flame photometric and chemical results was within 1.0%. This accuracy and precision are probably somewhat fortuitous as flame photometric methods generally are not credited with accuracies exceeding +3-5% of the amount present. The sensitivity of the flame photometer used in this work limited the determination of boron to the nearest 1 to 3 p.p,m,, depending upon the particular sensitivity of the phototube employed with the photomultiplier attachment The concentration of boron in the aliquot taken for flame photometric determination vas adjusted to lie in the range between 50 and 200 p.p.m. The usc of a photomultiplier attachment ITith the flame photometer is imperative. Without it slit widths larger than 0.03 mm. must be employed to achieve any reasonable sensitivity. With such large slit widths. serious interference will be encountered from many elements. More important is the excessive overlapping of the boron band systems which precludes the use of the intervening troughs between the band heads as a pseudobackground reference point. Methods for Circumventing Radiation Interferences. Several elements frequently present in materials containing boron offer serious interference in the flame photometric method for boron. I n ore samples, iron, aluminum, calcium, and magnesium cause results to be high when they are present in amounts exceeding 100 p.p.m. in most cases. Even the relatively innocuous elements, when they are present in very large amounts, will cause the results for boron to be high. Methods have been suggested for overcoming the interference of one element upon the flame emission of another. Disturbing effects often can be eliminated by xorking a t sufficiently high dilutions. Hon ever, this step is undesirable, since the boron band intensities are not particularly high and sensitivity then suffers. Self-compensating standards can be prepared-that is, standard solutions Containing all the important constituents of the unknonn in approximately the correct concentrations can be used. The approach is particularly feasible when a large number of samples of similar constitution are undergoing analysis. Compensation for disturbing factors and elements usually is much more complete than with any other correction method. Rather than prepare self-compensating standards, one could simply determine the concentration of an interfering element by another method or a t another wave length and its luminosity at
I-
the boron wave lengths. Difficulties with magnesium and aluminum at both the 492- and the 518-mp band regions can be resolved in this manner. When working with samples that are not of a routine nature and when interference effects are suspected or feared, a simple and practical procedure has been suggested by Gilbert et al. ( 7 ) . The apparent concentration of the element in question in the undiluted sample is compared with that in a portion diluted to half its original concentration, using a suitable pair of standards having a concentration ratio of 2 to 1. I n the absence of interference, the second value will be exactly half the first. If this is not the case, a first approximation to the corrected reading on the second sample will be twice the second reading minus half the first. This method of correction is useful when it is necessary t o operate with strong acid solution or with the hydrolyzates of nonmetallic halides. As shown in Table VI, a hydrogen ion concentration in excess of 0.5X causes a marked depression in the flame emission of boron. However, this depressant effect is roughly proportional to the acid concentration, when it exceeds 0 . 5 M J which permits this type of correction to be employed. When applied to known samples, the results found for boron are in fair agreement with the amounts present, even for solutions containing 5M acid. ACKNOWLEDGMENT
The effort of Morton Salkind on the early phases of this investigation is greatly appreciated. Thanks are due the Pacific Coast Borax Co. for kindly supplying samples of colemanite and Gerstley borate. LITERATURE CITED
Bricker, C. E., Dippel, 1%’. A . , and Furnian, S . H., U. S. Atomic Energy Commission, R e p t . NYO 794 (Dec. 31, 1951). Brunisholz. G., and Bonnet, J., Helt. C h m . Acta, 34, 2074 (1951). ~I
Calfee, R. K., and AIcHargue, J . S.. ISD. ESG. CHEM.,Aivar.. ED.,9 , 288 (1937). Cary, H. H., and Beckman. A. O., J . Opt. S O C .A m e r . , 31, 682 (1941).
Caton, R. D., and Bremner, R. W., d s n ~ CHEM., . 26, SO5 (1954).
Curtis, G. W.,Knauer, H. E., and Hunter, L. E., Am. SOC. Testing Materials, Tech. publ. 116, 67 (1952). Gilbert, P. T., Hawes, R. C., and Beckman, -4.O., AN*L. CHEM.,22, 772 (1950).
Lundegirdh, H., “Die Quantitative Spektralanalyse der Elemente.” Jena, G. Fischer, Part I, 1929; Part 11, 1934. NcHargue, J. S., J . Assoc. Ofic. Agr. Chemists, 1 6 , 465 (1933). IIcHargue, J. S., and Calfee, R. K., ISD. EKG.CHEM.,ANAL. ED.,4 , 385 (1932). Pearse, R. W. B., and Gaydon, A . G I “Identification of hIolecular Spectra,” 2nd ed. rev., Wiley, New York, 1950. Singh, S . L., Current Sci. ( I n d i a ) , 11, 276 (1942). Stahl, W., 2. anal. Chem., 83, 268, 340 (1931). Weber, H. C., and Jacobson, R. D., I N D . ESG. CHEM.,A N A L . ED.,1 0 , 273 (1938). RECEIYED for review April 30, 1954. Accepted October 20, 1954. Presented before the Southeastern Regional Meeting of the AMERICAN CHmfIC A L SOCIETY, Birmingham, 41a., 1954. Contribution 136 from the Department of Chemistry, University of Tennessee.
5
