X-Ray Absorption Measurements Comparative Method f o r Chemical Analysis Based on Measurements with Multiplier Phototube p. D. ZEW4N1, E. H. WINSLOW, G. S. POELL'WITZ, ~ N DH. 4. LIEHHiFSKY (;enera1 Electric Company, Schenectady, N . Y . A cornparatile method of measuring x-ra? absorption has been delised to reduce the errors of the direct method, used for exploratory anal?tical work previouslj reported. In the comparative method, there is rapid commutation in the x-raj beam from the unknown to a suitable standard. The method has been applied with satisfactory results to the identification of pure compounds, the determination of tetraeth3llead fluid in gasoline, and the determination of sulfur in crude oils. Deliations that can complicate the interpretation of results are discussed, and experimental data on the zirconia-hafnia s? stem are presented to illustrate one such tj pe of deliation.
I
S '~141.; tmlier work reported from this laboratory (8, 6), analytical rwults nTre obtained by the direct method of nieasuring x-ray absorption with a multiplier phototube. The iritimity (if thc x-ray beam was adjusted t o a standard initial value hy varying the x-ray tube voltage and current until the desired output current was obtained with a standard thickness of aluminum in the beam, when th(, voltage across the detertor and t,he am1)lificr setting were fised. Output currents obtained n-ith knonn weights of sample in tht. h a m were thcn u s t d t o givcb information ahout the composition of th(, sample. lo(
90
30 70
&o
50
40
5i w
a W
a 5
a30 K 0
2
v
.-
.\]though the direct method is suited t o esp1oratot.y \\.(~rl,,I \ does not yield the most rrliahle results attainabk with tht. lahoratory photometer (e, Figure 1); the two greatest souret's o f ( ' r t ' ~ ~ I are voltage fluct,uations and uncompensated changcss i r i t hc~ ixffective wave length of the polyehromatie x-ray hcxam. T I I ~ ~ comparative method described beloLv was designed t o reduw these errors. The seriousness of the voltage flurtuation problem is shovm for an extreme caw in E'igurt. 1. Because the data there plotttd prove that the output currents can vary as the 24th power of the primary voltage across the x-ray tuhe, it follows that this voltage should be maintained constant to about 0,,017cif- the decrease irr the intensity of thc x-ray beam due to absorption by the sample is t o be mcasured t o better than 1%. S o r is this the whole problem, for the data in Figure 1 \wre obtained with constant voltage across the detector. A similar curve for thcl detector Lvith constant voltage across the x-ray tube shows that the output current varies approximately as t h e 5th power of the voltage across the multiplier phototube. It is obvious t,hat the damaging effect of voltage fluctuations would be reduced if some way could be found of commuting suitably from unknown to standard in the x-ray bcam. If the standard is properly chos;c111, this coniniutatiori \ d l also reduce or eliminate uncrrtaintitrs due to changes in the rffrrtivt+ wave length. Thew changes have heen discussed t l , espcc-ially Figure 3). and the desirability of having s t m d a r d and unknown identical in mass and ultimate composition is clear from that discussion. Because this identity of standard and unknowr-1~ is not always convenicmt or nt'cwsary, another s u b s t a n w e.g., aluminum-may, on occasion, be used as the standard. The x-ray absorption of the sample can then be exprcsscd as tht. cquivalt~ntthickness of the standard. EXPERIMEST4 L METHOD
20
T o permit commutation between standard and unkno\vn, eacit in its cell, the stand of Figure 2 was mounted on the steel plate above the housing of the s-ray tuhe 1 hat served as source for the laboratory photometer ( 2 , Figure 1). T h e stand waspivoted so that a cell in a n y of the three positions could readily be swung into t h r x-ral- beam and stopprd at thP proper plaoe when a notch in the base of the stand engaged a spring-arrest on the plate. T h e rate of commutation between standard and unknown was generally the maximum a t which reliable output ourrent readings could be obtained-for example, all the readings in Tahle I were taken within approximately an 8-minut e period. 10
I
J
I
I
I
I
14.5
X RAY-TUBE
I
I
I
1
1
1
I50
In lieu of a detailed description of the comparative Illtathod, there are presented i n Tahle I the actual data for a n experiment that i.c one of a group describrd in the next section. Thew data
PRIMARY VOLTAGE
Figure 1. Variation of Output Current with Yrimar! Voltage across X-Ral- Tube
493
ANALYTICAL CHEMISTRY
494
Table I.
Detailed D a t a for Identification of lA2Triohloro-3,3,3-triflluoropropene
1. Auxiliary Data unknown. Solution containing 4.220 grams oi sample (presumably C8FDII)and 40.775 a h m s oi CrHs Standard. Solution containing 2.439 grams a i CCIt, 3.045 gram of CaHr C L and 39.505 grama of CrHa Cells. ' Nos. 5 and 6, matched, alur?inum. cvlindrioal (diapeter, 2.5 om.: height. 15.2 om.) oovered with aluminum foil 0.0025 om. thick Settings. X-ray tube ~ r i m a r yvoltage, 6 5 . 3 volts: filament ourrent. 10 ma.; detector voltage. 73 volts per stage, hmplifioation. Output ourrent rehd~ng X 10-1 = output current in milliamperes 2.
Detailed Oulmuf Current Readinsa
Determination of EBeotive Wave Second Comparison Length First Comparison Cell 5 Cell 6 Cell 5 Cell 5 Cell 5 Cell 6 (as (31.960-gram (as (AI foil (31.786-fram (31.807-gram unknown) standard) before) standard) before) added) 783 795 805 669O 762' 795" 786 793 797 797 775 660 785 733 793 794 755 658 778 783 785 778 755 665 775 802 774 774 763 673 795 812 778 676 733 775 803 767 784 795 765 683 786 808 802 756 669 788 793 792 783 785 772 652 785 793 793 786 76s 665 Means 786.9
787.5
786.5
764.3
795.7
tended t o he ident.ica1, the amount of unknown equivalent in x-ray absorption to 1 gram of the proper standard solution was det.ermined in the manncr of Table I. Table I1 gives the results. I n every case except the last, the departure from unity in the second part of Table I1 could have been due to the experimental error in the x-ray measurement; i t was concluded, therefore, t h a t the compounds in these cbses had the presumed composition and ncre of good purity. I n the last case, a chlorine titration after a peroxide fusion gave results in excess of the t,heoretical, which explains the low value 0.97. A somewhat simpler case was settled by using aluminum as the standard for the comparative method.
to be correct. Stsndsrdboiutio& of diehlaroethane in hensene were also prepared. Under identical conditions, the following equivalent bhicknesses of aluminum were obtained: solution of compound, 0.861 cm.; standard solution containing 3.62% chlorine, 0.859 cm. (Far a. standard solution containing 2.18% chlorine, 0.777 em. was calculated from experimental data.) The compound was eoneludedta he CL4HlnC14. Tetraethyllead Fluid in Gasoline. Because all atoms in a sample absorb x-rays, the m~asuremrntof this absorption is not
667.0
3. Relation between Standard and Unknown Cell Wt. of Solution. G. M e a n Readings Log Readings 5 31.786 786.9 2.89592 787.5 2.89626 6 31.807 764.7B 2.88349 6 31.960 Logarithmic averaging. 3311276 of 0.153 gram = 0,004 gram. Hence, 31.786-gram unknown is equivslent in x-ray absorption to 31.811-grhm standard. Conclusion. Aacording to x-ray absorption data. sample can be oonridered pure CsFzCla.
1.002 1.002 0.90
,.on:
0.974
4. Effective W e r e Length lnaerting hluminvm foil (0.0254 cm. thipk) reduoed transmittance to 53.8% of its value for cell 5 alone. With aid oi published dhth for &Iumi. num, one obtains 0.556 A. ior emotive -ave length of polyohromatio beam (a. p. 862. last phragrwh). a Readings in this column sandwiched between nearest readings on left. b Corrected reading obtained by adding 0 . 4 (change in mean reading for &I1 5 ) to 764.3.
show how the method n'B8 applied in a case where standard and unknown were pnsumahly identical in composition, and they emphasize the necessity for a statistical approach in all work of this kind. Except for obvious modifications, the procedure with duminum a8 standard vas identical t o t h a t of Table I. APPLICATIONS
Identification of Pure Compounds. Although ultimate analysis by recognized methods is still the court of last resort for establishing the composition of new compounds, i t is often possible to obtain sufficient evidence in less tedious ways. When all the elements in a compound are known, an x-ray absorption method can sometimes he used t o identify the compound and indicate its purity. Even when an ultimate analysis must eventually he undertaken, x-ray ahsorption measurements crrn furnish valuable preliminary information. Comparative x-ray absorption measurements were used here in the identification of various new compounds that could contain a t most the following elements: carbon, hydrogen, fluorine, and chlorine. The presumed composition of each compound, known in advance, was duplicated by properly blending carbon tetrachloride, benzotrifluoride, heptane, and hensene; the latter also waii used solvent far the unknorm. Under conditions in-
Figure 2.
S t a n d P e r m i t t i n g Comparison in Laboratory Photometer
At left, knob for m o v i n g stand, hole for beam, and notch to arrest afand i n correct position
495
V O L U M E 21, NO. 4, A P R I L 1 9 4 9 usually suited to a precise determination of constituents present in small amouiits. I n the case of tetraethyllead fluid in gasoline, how>ver,t h r absorption coefficient of the fluid sufficiently exceed:. that of the. h e stock (the unlraded gasoline) t o make the determination of thi:: fluid a highly promising application. This fact has long tw(3n rrcognized. Pome 20 yrars ago, Calingaert of t,h(B Ethyl ('oi~poratiorisuggestetl t h r application to .%born and Brown ( I ) , who used an ionization chamber as detect.or. Rccc~ntly. Sullimii and Friwiniari [.$it~mployc~d n ( k i g r r countvr for t,he -anit' purpose. T(>ti,atai hyllead fluid in lour gnsc~liiit~,. supplied by the ICthyl ( 'orpoi.ation was dctrrniined by the comparative niethod on \vc.ighcvl >amplw, aluniiriuin ierving a-: standard. These tetrac~thyll(~ati fluids could have. coiltailid ilihrnmo- and/or dichIorrJcstharw i n acidition to t h e Iiaacl compouiid. I
\\711e~ithe x-ray absorption nirasurenieiiili i v ~ i ' tniade, nothing was knoll-n itbout the samples esci~ptthat .IOT-l and B62AI-1 \yere the only two base stocks involved. T h e 16 values of equivalerit thicknrss obtained were transmitted to t h r Ethyl Corpoh ~isI ydone in ration, which ititt~rprrtrdthv cl;it:j ~ 1 1 1 ~ ~ t : i n t:I$ Tnhlr ITI.
Tahlr 111.
AOT-1 .\OT-2 AOT-3 .4OT-4
1)rtermination of Tetrarth?llrad Fluid i n Leaded Gasoline
0.3810.415?
0,445; 0,4867
0.3810 . 4 1d'?
Exprriiiimtal equatioii: AIj2AI-1 .4621\1-2 .\621\1-3
B6231-1 B623I-2 B62AI-3 86211-4
0.419 0,468: 0.5121
0,4707 0,5222
+ 0.03iO
1.72 2.84 L
0.8
1.8: 2.82
0.82
1.81 2.82
+ 0.0542 r
y = 0.3815 0,420 0,466: 0.~ 1 2 ~
Experiniental equation: y = 0 3815 0.383 0.383 0.429;
0.Y2
1.7; 2.84
0,4866
Exlirriiiiental equation: y = O.RS15 0,4257 0.4254 0.479 0 479 0,5345 0 :534;
0.00
0 0.99
0.445;
0.429: 0,4759 0.5216
0.80 1.7 2.7;
0.7 1.7;
2.72
+ 0.0481 r 0
0.00
I 01 2.01 a ,a0
1.90 3,01
1 .oo
Table IV. SslnlJlr 37807 37804 37805 Ilixture 1 37813 3Iiuture 2 LIixture 3 37814 87806
Sulfur in Crude Oils
Equiyalent Thickness, y (Cm. All Obsd. Calcd. 0.556 0,560 0.581 0.60 0.63: 0.680 0,693
n 7z5
0.729
I
