Rayleigh Interferometer for the Analysis of Liquids ERNEST GRUNWALO and B. J. BERKOWITZ Chemistry Departmenf, Floridu State Universify, Tallahassee, Fla.
A, method i s described for the unambiguous identification of the zeroorder fringe. This method requires no modification of the commercially available interferometer, but involves the use of a tungsten light source and of two auxiliary monochromatic sources. It i s also suitable for the measurement of the relative dispersion of liquids; its use i s illustrated for aqueous solutions of sodium chloride. An interferometer cell for volatile liquids was used in the analysis o f dioxanewater mixtures, and an accuracy within 0.006 weight % was achieved.
T
Rayleigh interferometer measures the interference pattern produced by two coherent light beams, one passing through a chosen length of a reference liquid and a glass plate of fixed thickness, the other through the same length of a n unknown liquid and a glass compensator plate of variable thickness (6, I S ) . By adjustment of the latter, the two optical paths can be made equal for a n y wave length, and the refractive index difference between the two liquids can be deduced. Because the interferometer is sensitive to refractive index differences less than 1 X 10-6, i t is ideally suited for the precise measurement of the composition of two-component liquid mixtures, and of solute concentration in dilute solutions. Unfortunately, two problems have so far hindered the extensive use of this instrument in chemical analysis. One of these is the very great difficulty in locating the zero-order fringe without elaborate prior calibration (4, 9, 11); the other is the difficulty of measuring and handling volatile liquids ( 5 ) . These problems are re-examined and workable solutions are presented. HE
ZERO-ORDER FRINGE
Jl7hen a monochromatic source is used for the production of the interference pattern, the fringes are indistinguishable from one another. Hence, with such a source, it is impossible t o identify the zero-order fringe when the unknown liquid has been placed in one of the light paths, this being necessarily a discontinuous operation. (By contrast, with gaseous unknowns one can gradually increase the pressure and follow the displacement of the zero-
124
ANALYTICAL CHEMISTRY
order fringe.) When a white tungsten source is used, only a limited number of interference fringes is formed. The central three or four of these have sharp maxima and minima, but on both sides the intensity differences become gradually less pronounced and eventually disappear. Since the energy distribution of the tungsten source, multiplied b y the relatire brightness sensitivity of the human eye, is a nearly bellshaped function, with maximum close t o 5700 A. and half-width close to 1000 A. (7), the interpretation of the fringe system is as follows: near the center, the phase difference. p, betn-een the two interfering beams is nearly equal, a t least for the wave lengths in the neighborhood of 5700 A. which are perceived as relatively bright. Towards the edge, these wave lengths get gradually out of phase, and the fringes disappear. Near the center of the fringe system one may therefore expect to find a n achromatic point, defined ( 2 ) as the point where, for A = 5700 A.,
The achromatic point has played a central part in previous discussions of interferometry with a tungsten light source (2, 8 ) . For liquids with normal dispersive properties, the scale reading a t which the achromatic point is brought to the center of the field of view (as indicated by the fiduciary fringe system) is greater than that of the zero-order fringe, but, after a small correction, is very nearly proportional to it, the proportionality constant being a function of the dispersion of the two liquids and hence characteristic for each solvent-solute system. Because of this proportionality, there would be no difficulty in locating the zero-order fringe if the precise position of the achromatic point could be recognized. Unfortunately, this is in general not the case. The best that can be done is to locate the fringe within which the achromatic point lies, by means of the coloration near the intensity minima. Here, the “bright” yellow and green spectral components, of wave lengths near 5700 4.,interfere destructively and, if the phase angles are favorable, the red and/or blue components can be seen. A t the achromatic point, 9 is b y definition equal for all wave lengths near 5700 A., and does not change much with wave length, even a t other wave lengths in the perceived range of the
tungsten source. Thus, near the minima immediately surrounding the achromatic point, the red and blue intensities are also low, so that these minima tend to look colorless, whereas further away the minima are more definitely colored. On this basis, the achromatic point is assumed to lie between the two most nearly achromatic minima. This theory has the advantage that i t introduces a n identifiable fringe, which we shall call the end point fringe, and the center of which can be located precisely. According t o the theory, as the concentration of the unknown solution becomes increasingly different from that of the reference solution, the interferometer scale reading of the end point fringe also increases consistently, but jumps ahead by one fringe every time the achromatic point passes through a n interference minimum. The theoretical working curve of scale reading us. concentration, therefore, exhibits a numbrr of discontinuities ( 5 ) . I n practice, this behavior would be merely inconvenient if there were not additional complications. There is the trivial complication that, in the neighborhood of the points of discontinuity, there is no unique “achromatic” fringe, but the color patterns of two adjacent fringes are roughly symmetrical about a black central minimum. This is consistent with the theory and indicates that the achromatic point is near the black central minimum. There are, however, real complications when, because of the approximations of the theory or subject i r e factors of color vision, a fringe is selected unambiguously as end point fringe which does not contain the achromatic point, or when two adjacent fringes are comparably achromatic without a black central minimum. Complications of either type may arise, especially at the higher scale readings. One saving grace in these cases of error or uncrrtainty appears to be t h a t the error does not usually exceed one fringe, at least i t has not so far in the authors’ experience. Measurement of Achromatic Point with Auxiliary Monochromatic Sources, The achromatic point can be
located unambiguously if, after determination of the end point fringe with a tungsten source, two auxiliary monochromatic sources are used. For convenience in practical application, the relevant equations are written in terms of corrected interferometer drum readings,
>I
- mi
?&
FRINGE ORDER
= y(6~-' -
ai-*)
(8J
it is possible to choose XI and h p qo that p o is made to coincide with raciir. On equating the expressions 6 for p o and for r a c h r . and using 7 and 8, the dispersion constant, y,is found t o be a conimon factor which can be eliminated. and the equation is obtained, A I SINGLE WAVELENGTH 61-2
kDISTANCE
B E T W E E N EPUIPHASE
POINTS4
p- I
PC
-I
0
1X*-$!
B l T W O WAVELEYGTHS
Figure 1 . Variation of phase angle, rected drum reading, r
r (see Equation 21), and interfringe spacings, 6. The r value when the zero-order fringe for the wave length is a t the center of the field of view is termed mx. :iiid is giTen b y the equation,
4,
with cor-
order number 2 , though in principle a function of nave length, is equal for adjacent waxre lengths, Hence n e 01,tain for the scale reading of the achromatic point, Tachr
=
+2~/6~,
mzo
(5)
Xow rschrmay be located by means of
nhere An is the difference in refractive index betiveen the unknown and the reference liquid, and L is the length of t h e light path through these two liquids. At this point, the phase angle, 9 , is of course equal t o zero. As r is changed, p at the center of the field of view varies according to the sawtooth diagram shown in Figure l , a , the analytical e\pression for nhich is p = 2H(T
- mx
- x6)/6
(3)
here I is the integral order niimber. We n-kh now t o find the achromatic point-that is, the 1 alue of r nhere, for the effective wave length A, of the tungsten soiirce. dp/dA = dp,'d6 = 0. Strictly $peaking, n e \\ish to find the uncorrected drum reading y, where dp,'dh = 0. However, tlie correction term, as well as its derixatke with respect to n a i e length, is small, and the present approximation is adequate (Equation 2 3 ) . For this purpose, mi is expressed as a function of n a v e length hy the equation,
mx
= Y*
+ ?/a2
(4)
Equation 4 is a modified Cauchy equation; y* and y are dispersion constants characteristic of the two liquids being measured and of the glass of the compensator plate ( 2 ) ,and 6~ is used in place of A, since these two quantities are nearly proportional (see Equation 19). The
two suitably chosen monochromatic sources of wave lengths and A?, whose interfringe spacings, and 62, have been measured independently. If, after the end point fringe has been obtained with a tungsten source, first one and then the other of these source; are switched on and the r value of an interference mavinium near the end point fringe is measured, a sawtooth diagram may be constructed, similar to the one shown in Figure 1,b. The interesting feature of this diagram is that there is a series of points where the two sawtooth patterns cross, and where the phase angles are therefore equal. The r values of these equiphase pointe are therefore equal. The r values of these equiphase points are found, by solving Equations 3 for X I ant1 A?, to lie a t the points p
= rill
+ &(m2 - nL1)/(61(22
-
~
62) 1
i &82/(61 )
- 81'
(6)
where (I?- xi) may be 0, & I , i 2 , etc. The distance between eyuiphase points is therefore equal to 616~,/(61 - &), and normally amounts to sex era1 times the interfringe spacing ai, or 6, as shown schematically in Figure 1. The equiphase point where (12 - xc?)= 0, denoted b y po, is of special interest. Since, from Equation 4, ntp - m i =
and
?(&-2
- &-2)
(7)
+
62-2
+
61-162-1
= 36,-2
(9)
Because Equation 9 does not involve a n y terms or factors characteristic of the particular measurement, the same tn-o monochromatic sources, XI arid X2, will bring about coincidence of p o TT ith Tachr in any measurement. The r value of the end point fringe is nithin 0.5 6, (or, in case of singlefringe error, within 1.5 6,) of raehr. Hence the equiphase point, po, which coincides with raOhr,may be located uniquelybyitsproxiniityto the end point fringe, provided only that 6162/(61 - 6 2 ) > 36,. Beginning with the experimental values r1 and r2 which have been obtained near the end point fringe, the corresponding values of p are calciilated from the equation p
= 71
f
&(n - n ) / ( 6 1
- 62) =I=
- 8 2 ) (10) 15-herethe integer i = 0, 1,2,. . , In this ZSIS2/(S,
set there is a unique member nhose r T aliie is n ithin 1.5 6, of the end point fringe, and this member is identified :Is
71,.
Table 1. Wave Length Pairs Satisfying Equation 9 for 150 C. P. Pointolite Source, A, = 5680 A""
5680
4 5680
5920 6360
5150
xi,
a
to
x
xi,
5460
Xlh>/(X,
-
X.'X
12 4 4 8 3 4
6700 4960 CJlculated by assuming 6 pioportional
A number of wave length pairs satisfying Equation 9 are shown in Table I. Since the accuracy of the experimental x-aliie of po improves with increasing AI - h2. the two values should be as far apart as possible, provided only that 6 1 6 ~ / ( 6~ 62) > 36,. Zero-Order Fringe. After locating raChr, the zero-order fringe can be identified with the aid of Equation 5 if a n estimate of y is available. This estimate need not be highly accurate, since the value of ( r a c h r - 2y/P,'i is used only t o indicate the zero-order fringe and can therefore be in error by as much as 0.3 6,. An a c c u r a t e d u e of m, is obtained b y actual measurement of the fringe which is now known to be the zero-order fringe. VOL. 29, NO. 1 , JANUARY 1 9 5 7
125
An adequate estimate is obtained on the following basis. It is assumed that I / L is proportional to the difference in concentration, (e - eo), between the unknown and the reference liquid a t all u-ave lengths-namely, mx = k x
(C
Yk*
+ Yk/6'
(12)
Hence the relationship between and m, is found to be, rachr
= ?nw(l
po/n21 = 1
+ 23k/k&w2)
(13)
The term 2yk 'k,6w2 is independent of ( e - eo), but is still characteristic of the solute-solvent system and of the measuring instrument and must be evaluated. For this purpose, i t is convenient to measure several different solutions of known concentration against the common reference liquid. T h r values of ( e - co) are kept sufficiently small so that, for the smallest, one may assume that Tachr differs b y less than one fringe from mw. On the basis of this theory one can then compute a provisional value of 27k/kw6w2 = (yaehr m,)/m, from the data for this solution. This provisional value is used to identify the zero-order fringe for the other solutions; its correctness is verified by the constancy both of (Taohr - m,)/rn,, and of m, 'ic - eo) for all solution.. If these quantities are constant, a n ai-erage d u e of 2y&d,2 is computed. if they are not constant, a different initial assumption must be tried, and it nil1 be lirlpful to obtain additional data for still smaller values of (e - ro). Because the present method already involves the uqe of nionocliromatic
+
-,'61/(61
-
62)
' , e
=
(1712
-
??21)/?n1
?nl* = 616d61 =
(k,
- k*)/kl
Tachr
=
+ 62)/
-,' [612 + 6162 f 622 - 3(612622/6,2)] (17)
(15)
and is a constant for any particular coiiibinatioii of solvent, solute, and instrument. The method of measuring y' is entirely analogous to the one just described for 2yk/k,6,2. The use of ml and m2instead of m, for the analysis of unknowns has the additional advantage that the correctness of the identification of the zero-order fringes can be verified in each esperiment. Equation 14 and its analog for m2 merely indicate the zero-order fringes, the actual values of ml and m 2 being based on direct measurement, Thus each experiment leads to a new, independent value of y' which must be consistent with the previous average. If the identification of ml and m2 is incorrect, the computed value of y' d l be seriously in error. Any Two Monochromatic Sources. For practical reasons it may not be possible to choose XI and h2 $0 that po coincides with Taehr. Identification of the zero-order fringes, m l and m2, is then still possible. The general expreqqion for p o - rachr is, Po -
end point fringe may be p - I rather than PO. The scale reading ml* of the zero-order fringe a t which po - Tachr is exactly equal to 6162/(61 - 62) is given b y the expression,
114)
\There
- eo) (11) - CO) may be es-
I n Equation 11, (e pressed in any convenient unit, such as moles per liter or weight fraction. Equation 11 is consistent with 4 only if both y and y* are also proportional to (e - co). Defining y k * = y*/ (c - co) and yk = y/(c - co), one obtains for the dispersion of IC, k =
light sources, and thc monochromatic fringe settings are more precise-see, for example, the values of in Table 11-4 is foiind from Equation 6 that
11213'
Again, y' = irnz - vzl)/ml. K h e n the coefficient of mlis positive, p , is always greater than Taehr. Hence po [ d l tend to lie ab0T.e the tungsten end point fringe a t small values of inl, and will do so consistentlyat higher values. I n fact, as ?nl becomes large, p o - Tachr may exceed the distance, & 6 2 / ( 6 1 - &), between equiphase points, and the equiphase point which lies just above the tungsten
If ml* is greater than the total scale of the micrometer scren-, no ambiguity will ever arise, and the zero-order fringe is found by the method described in connection n i t h Equation 14, except that a t the higher scale readings the equiphase point above the end point fringe is used, even when the one below it lies closer. If ml* is less than the total scale of the micrometer screw, the choice of po requires a certain amount of judgmcnt, but can still be made objectively. The choice is guided by the criterion that, for zero-order fringe positions near ml*, the distance from the end point fringe to po must be of the order of b 1 8 2 j ( 6 1 - 62). Moreover, if p o n-ere chosen incorrectly, the dispersion ( m 2 m l ) / m l of the (incorrectly) calculated zero-order fringes would usually not approximate the known value of y ' within reasonable limits. =Inalogous considerations apply rn lien PO - Tachr is negative. It may therefor be concluded that a tungsten source, followed by any two Convenient inonochromatic sources, can uniquely define the zero-order fringes, proridcd only that a moderately accurate estimate of y ' has first been obtained. This is done without auxiliary instrumentation by means of several solutionr with .mall valueq of ( e - eo)< as described in the preceding section. APPARATUS
The Rayleigli interferometer is manufactured by Hilger & Watts, Model 11 7 5 , The glass of which the fixed plate and the compensator plate are made had the refractive index, ?IC = 1.51655; = 1.51906; and ?IF = 1.52515 ( 1 4 ) . These n value. were fitted to the equation, n = 1 50610
Table II. Data for Light Sources Corning Filter 0.13 0.12
A. 6361 5784
105 -4 6 2.220 2.217
12.473
0.09
(5461)
2.216
9 869
0 18
4355
2 206
13 499 12 98
0 11 0 22
,5898 5677
2 218 2 182c
A,
Designation
S O .
6
UQ
Hg-R Hg-Y
2412, 3 . 0 min. 3480, 2 . 0 mm. 4303, 3 . 9 mm. 3484, 3,O mm. 4303, 3 . 9 mm. 5120, 4 . 8 mm. 3389, 2 04 mm. 5113, 3 76 mm.
14.58 13.231
Hg-G Hg-B Na
IT a
For BJB. Readings of EG were not eo precise. Equation 22; g = 2000; t = 20.32 cm. -4 - 6, (dA/d6),.
126
ANALYTICAL CHEMISTRY
+ 0 00450/A2
(18)
The follon ing light sources were used. The tungsten source was a 150-c.p. Pointolite lamp, Model F2.10, made by Hilger & K a t t s ; the sodium vapor lamp was Model SLA-5C, of George TV. Gates and Co.; and the mercury source was a 100-natt arc, Type H-100 -44, made by General Electric Co. The latter was used n ith Corning glass filters, as described in Table 11, to isolate the red, yellon-, green, and blue-violet regions of the spectrum. The effectiveness of these filter combinations was checked by means of a small spectroscope, with the following results. For H g R , Hg-G, and Hg-Y, the filters effectively isolated the lines a t 4358 9. (as Tell as less intense lines at 4348 4.,
'
and 4339 A ) , 5461 A, and the doublet a t 5770 and 5791 A., respectively. For Hg-R, there were a large number of closely spaced red lines in the range 6000 to 7000 A., with the light transmitted by the filter being biiglitest in the range 6200 to 6500 A. T h e intensity of the sodium arc mas unfortunately not bufficient to permit the use of isolation filters for tlie Ka-D line. The interfringe spacings, 6, and the standard deviations of single settings, us for thebe light sources are shown in Table 11. The effective wa\-e lengths of the qoiirces were calculated from the interfiiiige ~pacings,using Equation 19, X ' 6 = ', 1 - [2no(n - no)/
Ho(1
-
Ho")] JXo/Go
(19)
I n Equation 19, 10 is the reference n a v e length, 1 is the unknonn n a v e length, a0 and 6 are the corresponding interfringe spaciiip, no and 71 are the corresponding refractile index values of the glass plates 0-it follows from Equation 16 that p o is greater than Taohr. Hence the following procedure. First, the position of the Hg-R fringe nearest the previously measured tungsten end VOL. 29, NO. 1 , JANUARY 1957
127
Table 111.
Sample Calculations for Aqueous Sodium Chloride Solutions at 25.00’
(Reference liquid, water. L 10%
rma
TR
TB
0.6154 0.8994 1.179 2.083 3.077 5.013
58.8 85.2 112.0 210.2 315,4 511.2
57.83 84.53 111.38 210.3
59.81 87.84 115.93 214.05 317.7 511.6
315.9
511.4
59 81 3 42 3 91 87 84 115 93 4 08 205 2 4 34 297 9 3 76 486 0 3 93 I‘alue of end point fringe
57 83 84 53 111 38 195 7 287 1 467 6 a
I?,
=
TR
+
BR(TB
- rR)/(6R
-
BE)
4.001 cni.)
(21)
All y values are the averages of four to six readings. I n order to avoid backlash of the micrometer screw, all settings are made to a dead stop, always approaching from the same direction. -4fter measurement of r B and r R , the r values for other sources may be obtained in the same range.
lnR’ ... 86.3 112.4 197.7 284.8 467.6
Po 63,9
95.4 125.4 221. 8 321.5 524.4 9 9 9 9 9 9
3 76 3 94 4 I9 3 94 3 94
point fringe is measured. S e x t , the Hg-B fringe just above the previous Hg-R scale reading is located. The values are then converted to the r scale, using Equation 21, and the next higher equiphase point is computed from the formula. p =
=
C.
40
40 45 40 33 33
nioiioclironilitic sources, one-fringe error might have been made in the identification of m w if classical procedures had been followed (1 5 ) . Ry the present method no such error \vas made. ~
??lB’
... 89.2 116.2 204.9 295.6 484.6 0 0 0 0 -0 0
26
59 79
45 18 03
The present procedure is very sensitive to error in the handling of the most dilute solution. For example, if an error of one fringe were made, the correct values Tvould be for the 0.006154M solution, niR = 43.25, mB = 49.94, y’ = 0.155, and k R = 7.03 X lo3. For the 0.008994M solution, i i i R ’ would be 64.5, and ?nB’ 72.1. The nearest actual fringes would be, m R = 69.95 and m~ = 67.10. These assignments are evidently untenable, as they would lead to a negative value of y’. The last column of Table I11 lists values of ( r d - rw)j6m, rA being computed by means of Equation 16. It is seen that in two cases out of the siu listed, T A is not within half a fringe of the end point fringe. Kithout the
RESULTS
Sodium Chloride in Water. Using the data in Table I11 and additional data for Hg-G and Hg-Y, values of An n-ere computed by means of Equation 2 . The required values of A and 6 viere taken from Table 11. The results for Hg-G are listed in Table IV and are compared with previous precision data. The agreement is reasonably good, the average discrepancy being 0.37,. Although values of K in t h e equation An = Kc
(25)
decrease slightly with increasing concentration, arerage values were coniputed for each light source for a rootmean-square concentration of 0.0181 and are compared in Table IT’ with previous values at 0.0177M. The agreement is satisfactory. Dioxane-Water Mixtures. The data in Table V- illustrate the accuracy with which the composition of volatile binary liquid mixtures can be measured by interferometry with the special cell shown in Figure 2 . Over this n a r r o r range of composition, K is satisfactorily constant, and Ann, divided by the arerage
Calculation of m. T h e procedure is illustrated for sodium chloride in n-ater a t 25.00’ C. T h e relevant d a t a are summarized in Table 111. T h e first step is t o measure a solution of such high dilution t h a t and T B may Table IV. Data for Sodium Chloride in Water, 25.00’ C. be presumed equal t o VZR and ~ i g , respectively, and to obtain a first. though 103 ~n (5461 a.) 1O2K -~ not very precise, estimate of 7’. The This This Kruis 103~ rrork Phisa x work (10) correctness of this value is then verified, and its accuracy is improved, by 6 154 0 0643 0 06394 6361 1 0206 1 020i 8 994 0 0936 0 0934 5784 1 0301 1 0306 measuring several solutions of somewhat 11 79 0 1230 0 1223 5461 1 0364 1 0369 higher concentration. For example, for 20 s3 0 2166 0 2158 4355 1 0733 1 0752 the 0.008994M solution, V L R ‘ and m ~ ’ 30 77 0 3179 0 3182 are, according to Equation 14, equal 50 13 0 5173 0 5169 104 04 1 069 1 066 to po/(l 3.097’) and p o / ( l 2.097’), respectively, where y’ is the value obCalculated from An = Kc. Values of K were obtained by interpolation or, a t lonest tained for the most dilute solution. concentrations, by a small extrapolation of data of Kruis (IO). If the estimate is correct, mR’ and mg’ will lie xithin one or two scale divisions of actual maxima, which are thereby identified as m R and mB. This is inTable V. Data for Dioxane-Water Mixtures, 25.00’ C. deed the case, and a second value of y ‘ , and an average value T’,can be comjO(.lSo)a lo*(.\- - A\70)a loz-!’ 1 0 3 . 1 ~ ~ ~ ~ o ~ K R 1 0 y 5 - ATO). (calcd.)* puted. T o eliminate the possibility of accidental coincidence, a few additional 7.02 1.117 84.159 1.112 2.06 0.7808 0.357 7.01 1.92 0.2498 83.870 0,366 solutions are measured, and in each 7.00 0.502 2.04 0.3812 83.798 0.502 case the computed values V L R ’ and 6.93 0.211 83.669 0.213 2.06 0.1478 V L B ’ (based always on the average of 6.91 1.055 S3 069 1.068 2.37 0.7373 the preceding measurements) are within 7.09 2.16 0.892 E3.056 0.880 0.6238 two or three scale divisions of actual Mean 2 10 rt 0.11 6 99 ct 0 05 maxima, even a t the higher concentrations. The validity of the calculations a 5-and 5-0are weight fractions of dioxane. is checked by the satisfactory constancy * lo‘ An~/6.99. of the values of k R .
+
+
+
r’,
128
ANALYTICAL CHEMISTRY
x i h e of K R , fits the synthetic values of 11- - ATon i t h a mean deviation of only 0.0067,. Actually, the precision of thc interferometer readings is so good t h a t the factor which limits the accuracy in these measurements is the q n t h e t i c value of N - IV,.
LITERATURE CITED
( 1 1 ..\dams. L. H.. J . Ani. Chem. Sac. 37. 1181’ ( 1915j.
(2) Adams, L. H., J . Wash. i l c a d . Sci. 5 , 267 (1915). (3) Bacarella, 8. L., Finch, d., Grun-
Kald, E., J . Phys. Chevz. 60, 573 (1956). (4) Barth, W.,2. wiss. Phot. 24, 145 (1926). ( 5 ) Bauer, K., “Physical,,llethods of Organic Chemistry, A. Keissberger, 2nd ed., p. 253, Interscience, Kew Tork, 1949. (6) Candler, C., “Modern Interferometers,” Hilger & Watts, London, 1951. ( 7 ) Evans, R. M., “Introduction to Color,” Wiley, Sew York, 1948. (8) Faust, R C., Marrinan, H. J., B?zt. J . A p p l . Phys. 6 , 351 (1955). (9) Karagunis, G., Hawkinson, A,, Danikohler, G., Z . p h y s i k . Chem. A151, 433 (1930). (10) Kruis, -1., Zbid., B34, 13 (1936 ,
(11) Macy, R., J . Srn. Chem. SOC.49,30TO (\ -1927). - - . I
(12) Marshall, H. P., Grunwald, E., Ibid., 76, 2000 (1954). (13) Williams, W. E., “Applications of
Interferometry,” 4th ed., AIethuen, London. 1950. (14) Tates, H.‘ W., -Optical Works RIanager, Hilger & Watts, Ltd., London, private communication.
RECEIVED for reviex May 5, 1956. .4ccepted September 25,1956. Work carried out under contract between the Office of Naval Research and Florida State University. Reproduction in whole or in part is permitted for any purpose of the L-nited Ststes Government.
Separation and Determination of Microgram Amounts of Molybdenum GLENN R. WATERBURY and CLARK E. BRICKERI University of California, Los Alarnos Scientific Laboratory, los Alarnos,
,The separation and determination of microgram amounts of molybdenum in plutonium alloys and other samples in the presence of iron are accomplished b y extracting the molybdenum into hexone (4-methyl-2-pentanone) from a 6M hydrochloric acid-0.4M hydrofluoric acid medium. This i s followed b y back-extraction into water, removal of iron b y precipitation from the aqueous extract as hydrated iron(ll1) oxide, and colorimetric estimation of the molybdenum in the aqueous extract using chloranilic acid for the color reagent. For 32 determinations o f 19 to 96 y of molybdenum in solutions containing various foreign ions, an average value for the molybdenum found of 99.870, with a standard deviation of 1.670, was obtained. Of 29 foreign metals investigated, only tin, tungsten, and bismuth interfered seriously with t h e determination.
B
*E of the need foi an analytical method for the determination of 0.01 to lYG molybdenum in plutonium and plutonium alloys, possible methods foi the determination of molybdenum were imestigated. For these loir concentiations especially TT here only a srn:ill aniount of sample is uvnilnble, :t quantitative separation and a sensitive nietliod for the deterniination are necessary. Although the precipitation E C it
Present address, Chemistry Department, Princeton University, Princeton, N . J.
N.M.
and recovery of microgiam amounts of molybdenum using a-benzoinoxime have been reported (16), extraction separations are usually more readily adaptable t o microgram amounts, and in these cases colorimetric methods offer the sensitivity required. Consequently, the present investigition has been limited to extiaction methods for separation couplrd n-ith coloriniptric methods for the estimation of molybdenum. Since the first mention of the ether extraction of molybdenum from a hydrochloric acid solution by Pechard in 1892 (16), methods for the separation of molybdenum by extraction have been discussed by many authors. In general, these methods involve the extraction of molybdenum cupfeirate by chloroform (1. 7 , 20) or ethyl nitrate ($I), the extraction of molybdenuni thiocyanate by various organic solvents (8,14, 19. %), the extraction of molybdenum dithiol by h t y l or amyl acetate (9, 28). and the extraction of molybdenum from mineral acids into organic media (3, 15, 19, 24). Of these methods, the evtraction of molybdenum from mineral acids n i t h nn organic solvent utilizes the most simple system, and the investigation perfoimed 11)Seliclou- and Diamond (15) indicates that this separation can lie highlv efficient, depending on the solvent and acidities used. From their study, the extraction of molybdenum from G to 7 M hydrochloric acid by hexone (-1methyl-2-pentanone) was shon n to be about 96% efficient; the efficiency of the L I S U ~ Idiethyl ctlier extrartion iq les..
Tiibutyl phosphate extracts niolylxlenum more efficiently than does hexone. but the number of other elements coextracted is larger. The colorimetric determination of molybdenum hss been reported using thiosulfate (5), tannic acid (W3), hydrogen peroxide (279, hydrogen sulfide (1, d), potassium ethyl xanthate (I?), phenylhydrazine ( I O ) , dithiol ( 2 ) , and thiocyanate-tiniII) chloride (11, 12). Of these methods, the thiocyanate-tin(11) chloride color procedure is the most widely used. Although molybdate is f i b t e r l as an interference in the d r t w
lii
STIRRER CHUCK
DIA’rlETEi HOLE
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Figure 1. Hollow stirrer extractor with test tube VOL. 2 9 , NO. I , JANUARY 1957
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