of tantalum pentoxide up to a maximum of 30 nig., three precipitations are indicated. A small amount of titanium and niobium still coprecipitates with the tantalum but corrections can be made by determining the coprecipitated elements spectrophotometrically. Results of duplicate tests on mixtures of all three elements are given in Table I. The filtrates from one set of duplicate determinations were used to test for the presence of dissolved tantalum. The elements were recovered by cupferron precipitation and after ignition, the oxides were fused with pyrosulfate. After leaching with tartaric acid, an attempt was made to precipitate any tantalum present with selenous acid according to the prescribed procedure. S o tantalum was found. Mixtures of niobium and tantalum, and titanium and tantalum were not particularly different in extent of coprecipitation of either niobium or titanium. Behavior of Other Elements. X h e n prcsent alone, 100 mg. of the oxides of scandium, yttrium, cerium(III), zirconium, vanadium(V). molybdenu m (VI), tungsten( VI), uranium (VI), iron (111),aluminum, gallium, tin( IV) , lead, antimony(III), bismuth, and lrss than 35 mg. of thorium dioxide gave no precipitation. Of these, tungsten slowly hydrolyzed after 2.5 hours of heating. Table I1 lists the results obtained with one precipitation
Table II. Behavior of Mixtures of Some Elements with Single Precipitation of Tantalum
(Ce, Zr, V, Mo, W, Fe, Sn, Sb, and Bi. Values in mg.) Amt. of TazOs Ta206 Oxide Each Total Found, Taken Element Oxides Fraction 2.4 2.4 2.4 12.0 12.0 12.0 30.0 30.0
30.0
2 5 10 2 5 10 2 5 10
18 45 90 18 45 90 18 45 90
2.4 2.5 2.6 12.0 12.3 i3.i 30.3 31.2 32.5
in the precipitation medium. At the same time, a few milligrams of selenium are precipitated in the elemental state. Determination of Tantalum in Tantaloniobate Ores. This procedure was tested on two tantaloniobate ores for which tantalum determinations had been made gravimetrically by eight cooperating laboratories (3), and colorimetrically by Dinnin ( 5 ) . The results obtained on duplicate determinations are 51.7 and 51.8% tantalum pentoxide on the poor ore, and 70.3 and 70.5% on the rich ore. Following Dinnin, the results are compared on a frequency diagram with those cited earlier (Figure 1). LITERATURE CITED
on mixtures containing zirconium, antimony, tin, vanadium, bismuth, iron, cerium, tungsten, molybdenum, and various amounts of tantalum. I n one set, 2 mg. of the oxides of each element was taken, in another 5 mg., and in the last set, 10 mg. The data indicate that double precipitation of tantalum should yield a pure tantalum product for all test samples. The last sample shown in the table with a tantalum fraction weighing 32.5 mg. was spectrographed. Tungsten was the only element found as an impurity and caution should be used with samples containing large amounts of tungsten. Vanadium tends to be reduced to the quadrivalent state
(1) Alimarin, I. P., Burova, T. A., Zhur. Przklad. Khim. 18, 289 (1945). (2) Alimarin, I. P., Stepanyuk, E. J., Zauodskaya Lab. 22 (lo), 1149 (1956). (3) Atkinson, R. H., Steigman, J., Hiskey, C. F., ANAL.CHEX 24, 477 (1952). (4) Belekar, G. K., Athavale, V. T., Analyst 82, 630 (1957). (5) Dinnin, J. I., ANAL. CHEX 2 5 , 1803 (1953). (6) Freund, H., Levitt, A. E., Ibid., 23, 1813 11951). (7) Moshier,’R. W., Schwarberg, J. E., Ibid., 29, 947 (1957). (8) Sankar Das, M., Venkateswarlu, C., .4thavale, V. T., Analyst 81, 239 (1956).
RECEIVED for review May 8, 1958. Accepted July 23, 1958. Publication authorized by the Director, U. s. Geological Survey.
Stable Nitrogen Isotope Analysis by OpticaI Spectroscopy H.
P. BROIDA and M. W. CHAPMAN
National Bureau of Standards, Washington, D. C. Optical spectroscopy with photoelectric detection provides a rapid and precise micromethod for determining the nitrogen isotopic concentration of nitrogen gas and of nitric oxide gas. This paper describes the effect of operating conditions on the precision and the accuracy of such measurements. A determination can be made on a sample of 20 cc. at a pressure of 1.5 mm. of mercury (0.04 cc.-atm.) in approximately 10 minutes, to an accuracy of a few per cent.
0
spectroscopic isotope analysis is a rapid and precise method for the measurement of isotopic concentrations. It uses the wave length separation of the emission bands (or PTICAL
lines) of the isotopes due to the isotopic shift. The method has been used successfully for isotopic determinations in hydrogen (7, 22), water (6, 11), carbon (15),nitrogen (8, 9, 12, 16), oxygen (12), lithium (1-3, 13, 21, 24), lead and uranium (2, 4, as), and polonium (17). -4review of isotopic measurements has been given by Striganov (20). KO published work on nitrogen isotopic determinations by optical spectroscopy, using photoelectric detection, has been located. A method for the measurement of relative concentrations of nitrogen isotopes nitrogen-15 and nitrogen-14 is presented. The sample is nitrogen-15enriched nitrogen or nitric oxide gas and both give strong emission of the 2nd positive system of Nz (C3ny+B3n,). The method also should be adaptable
for use with other gaseous compounds of nitrogen-15 enriched samples. The sample, contained in a discharge tube, is excited by a high frequency electrodeless discharge. The light emitted from the discharge is dispersed by a monochromator and the intensities of lines in the 1,0 or 0 , l band head of the second positive system of nitrogen are recorded and measured (Figures 1 and 2). Ratio as used in this paper is computed from one of the formulas
?2 --
x 1 4
.\I
N16N16 N14N16
2N16N16
__ N14Nl4, ___ 2N14N14’ Or
where W 4 W 1 4 , N14N16, and N16XI6 represent intensities observed in the band head. The derivation of these formulas is given by Hoch and Weisser (16). The 1,O band head is used for VOL. 30, NO. 12, DECEMBER 1958
2049
The optical-spectroscopic recording system was a high resolution, scanning monochromator with photomultiplier detection and pen recorder (IO, 14). (This instrument was constructed by the Research Department of Leeds &Northrup Co. and loaned to the National Bureau of Standards on a field trial arrangement.) A scan speed of 5 A. per minute TTas used and all measurements were made using the second order of the grating. Resolution of lines separated by 0.1 A. is readily achieved.
with 125-watt maximum) through nitrogen or nitric oxide gas operated a t pressures near 1 mm. of mercury was used as the light source for these studies. Figure 3 shows the wave guide cavity which provides a satisfactory coupling to a large variety of gases in the range from 0.1 to 100 mm. of mercury [for method of coupling to a system see Figure 3 or ( I S ) ] . The brightness of the radiation from nitrogen discharges reaches a maximum near 1.5 mm. of mercury and is very stable. Xitrogen (or nitric oxide) was put into Vycor tubes of 13-nim. outside diameter and 25 mm. long, nith larger borosilicate glass bulbs added to increase the volume when it !vas desired to reduce the possibility of nall effects (16). Right-angle vacuum stopcocks were attached to provide for changing samples. When these stopcocks were used, the samples held their pressures well without leaks for a t least 2 months.
low ratios and the 0 , l band head for high ratios thus obtaining better intensity above background. I n the low concentration range it was necessary to increase the amplification of the S14S15 peak electronically which did not add any error for the precision obtained. The ratio, computed from intensities in the band head is a monotonic function of the true ratio of the nitrogen-15 and nitrogen-14 isotopes. For precise work the measurement is a relative rather than an absolute one, in that the relationship between intensity ratio and isotope ratio is obtained from calibration curves using mass spectroscopic data for comparison. APPARATUS
An electrodeless discharge (2450XIc. from continuous wave magnetron
c 'n. - 8 +lo2nd
EXPERIMENTAL
Many of the samples were prepared and measured as follows. Isotopic samples of nitrogen gas were prepared from ammonium nitrate (only the nitrogen of the ammonium radical enriched) using the method described by Sprinson
Positive
12
I
I
100-
I
3536.7
3576.9
3500.5 A
0.01
50-
0-
1
3371.3
5339.0
.
I
3309.0
I 3285.3
I
3268.1 A
eo-
40-
0-
1
2976.8
d2.0
29512 P
Figure 1. Comparison of four bands of second positive system of nitrogen, using tank nitrogen (ratio = 0.00365) W o v e lengths are shown.
2050
ANALYTICAL CHEMISTRY
Intensities not corrected for differences in photomultiplier response a t different wave lengths
least 20 cc., was filled to a pressure of about 1.5 mm. of mercury. The Vycor discharge tube containing the sample was placed through the center of the wave guide slot and the high-frequency energy applied. The discharge was ignited with a Tesla coil. The 1,0 or 0,l band head n-as scanned manually to determine which amplification scales
and Rittenberg (19). Air was removed very gradually to prevent excessive bubbling and premature mixing in the Y-tube; this evacuation was aided by freezing the solutions, The technique used for measurement was as follow: The sample tube, with a volume of a t
Figure 2. Isotopic separation at various ratios
for two bands used for
0.11
measurements Symbols above the N l 5 N I 4 = 1. Traces point to lines used for intensity readings 8 , N14N14; 0, N14N16 and QN"N16
0.43
e
1.0
I
L 9.0
to use. Then the band head was scanned twice automatically if only an approximate determination rT-as desired, and the intensities were recorded. Less than 10 minutes was required for the determination, including the time to place the sample in the wave guide, make traces and measurement of the band head, and compute the ratio. For precision measurement the band head was scanned 10 times, always in the same direction. X precison measurement and data calculation required less than an hour. The method used for the calibration samples is described later. Operating Characteristics. Under these experimental conditions, several factors affected the measured ratio. These were studied to determine their relative iiiiportance and to choose points for obtaining optimum reproducibility and accuracy. The considerations necessary in choosing which band head to scan, which points to use for background measurements and which lines to use for intensity measurements are illustrated in Figures 1, 2, 4, and 5 . Figure 1 shows a comparison of the ATr = 0. =tl, + 2 bands a t a normal abundance ratio. Figure 4 shows a comparison of their band heads a t ratios of 0.11, 1.0 and 9.0, with symbols indicating lines n-hich could easily be used for intensity measurements. The isotopic shift is to the left(increasing nave lengths) for the 1,0 and the 2,O band heads, then it is to the right (decreasing wave lengths) for the 0 , l band head, and there is no shift for the 0,O band head. Figure 2 shows the 1,0 and 0 , l band heads a t ratios over most of the range measured. The lowest ratio a t which the K15516 peak was seen clearly in this study was approximately 0.01. A measurement made a t the normal abundance ratio of 0.004 used the S l 4 K 1 S and the K14Y4 band heads. It was arbitrarily chosen to measure backgrounds a t some of the lowest points to the left of each intensity line to obtain nearly the same ratio from each of the three formulas given. The calibration curves in Figure 6 show that there was no great attempt to obtain identical values. Figure 2 shows that, because of a difference in background lines, the X 1 4 5 1 4 and X1W5 peaks are a t equal levels for a ratio of 0.43 in the case of the 0,l band head and for a ratio greater than 1 in the case of the 1,0 band head. The rotational lines used for intensity measurements, as indicated by symbol in Figures 2, 4, and 5 , were chosen to achieve good intensity above background, as well as small effect on ratio with change in resolution. Variation in resolution resulting from variation in slit width of the monoVOL. 30, NO. 12, DECEMBER 1958
* 2051
chromator is shown in Figure 5. The amplification was adjusted for each trace to bring the peak line to full scale on the recorder. The great variation in intensity of the band head in relation to the adjacent lines with change in resolution, requires that the slit setting remain constant for any set of measurements that are to be intercompared. The IV4?;14 band head of the 0 , l band was used for Figure 5 . Both flow and nonflow systems were tried. A flow system has fewer traces of previous sample remaining on the walls of the sample tube. A nonflow system is more practical in that it allows the use of small samples as compared with the amounts necessary for the mass spectrometer, but special care must be used in filling tubes for the nonflow measurements. Two high frequency sources ( 7 ) , 150 and 2450 Mc., were tried and gave somewhat different operating characteristics. Both power supplies are rated a t 125 watts. The brightness of the radiation induced by the 2450-hfc. source was roughly four times that of the 150-Mc. source. The 150-?vIc. source allowed adjustment of the tuning and a wider pressure range over which the discharge was lighted. However, the tuning needed to be adjusted with each sample, resulting in longer operating time. The length of the discharge using the 150-Mc. source is spread over a greater length of the sample tube. The higher frequency source was used for all measurements reported because i t did not need to be tuned and gave greater brightness. Effects on the ratio and intensity of changes in power in the 2450-Mc. source are shown in Figure 7 . Over the entire range from 30 to 1OOyopower the maximum changes found were 2% in the ratio and 30% in the intensity. (The power is expressed in per cent because the watts are not known.) The operating power was chosen as 90% to give minimum changes in ratio and intensity with variation in power. A change in power of =kl’% from the operating point produces a maximum change of 0.2% in intensity and 0.05% in ratio and the power must be held to *2% to achieve a precision of 0.1%. The effects of pressure changes on intensity and on ratio are illustrated in Figure 8 for a ratio of 1.2. An operating pressure of 1.5 mm. of mercury was chosen to use as much intensity as possible and still produce a minimum change in ratio with fluctuations in the pressure. Use with small samples was also a consideration. A change in pressure by A 1 mm. from the operating point produces a maximum change of 13y0in intensity and 4% in ratio; the pressure must be held to *25 microns to achieve a precision of 0.1%. Calibration Curves. Break-seal 2052
ANALYTICAL CHEMISTRY
I-’
I
L-
I
I-. /
a&---
*OTL ITA*O.RO T * o RO0 *IN
sccnw #A UTLRIAL-MAII IlNIU
~
MOI c
m
COIIIAL L I T
Figure 3.
Table 1.
Power of HF source Pressure
Wave guide for 2450-Mc. source
Operating Conditions Change in Ratio Caused by Change in Selected Operating Condition Conditions 90% 0.05% per 1%
1.5mm.Hg
47, per mm. Hg
Spectrum 1,O band head for ratios less than 1; 0,l band head for ratios of 1 or greater Slit width Constant, t o Kot measured give resolution of 0.1 A.
tubes of nitrogen gas a t pressures of 22 t o 39 cm. of mercury were used for t h e calibration curves in Figure 6. Each tube was sealed t o a flask with a discharge tube and a stopcock attached. T h e flask volume was chosen t o result in a pressure of 1.5 mm. of mercury when t h e break-seal wag broken. Xitric oxide samples also were received in break-seal tubes and ratios were obtained in the same manner as with nitrogen gas. No calibration curve was obtained because of difficulties in measurement by the mass spectrometer, which occurred after measurement by optical spectroscopy and the accompanying dissociation in the high frequency discharge. However, the results by optical spectroscopy corresponded closely to those of the nitro-
gen gas samples and there was no difficulty in taking the measurement. The curves in Figure 6 were drawn from points measured at the 0 , l and 1,O bands and computed, using different band heads. The divergence of the curves appears due to the choice of background. The accuracy varies between 0.1 and 3% (depending upon ratio, being less accurate at the extremes). The displacement of the normal abundance ratio from the straight line s h o m the need for more calibration points in the range below 0.01. A calibration curve also was determined from weighed samples of between 20 and 50 cc. which were made as follows : Normal and enriched ammonium nitrates were pulverized separately and dried in a n oven a t 100” F. to reduce the hygroscopic error. Then, while being weighed, the nitrates were combined in such amounts as to give a variety of ratios. This calibration curve fell slightly below and almost parallel to those on Figure 6. The scatter of these points was considerably greater than those obtained by mass spectroscopic calibration. This increased scatter presumably mas due to inaccuracies in weighing and preparation. A measure of the precision of this method is obtained from the standard deriation of the mean of the ratios obtained from a series of successive measurements. Using sets of 10 successive measurements (or using a set of n SUCcessive measurements and multiplying by dmn),taken under the operating conditions set forth in Table I, the standard deviation of the mean (16)was found to vary between 0.1 and 3%. Precision as good as 0.1% can be ob-
I
?
2 .o
0.0
0.1
0.11
Figure 4.
I .o
9.0
Isotopic separation at each of four band heads for three ratios
Symbols above traces indicate lines used for intensity readings. In 0,l band, secand major line was chosen instead of the third because of proximity of the third to the fourth, with the result that resolution becomes critical (see Figure 51. 9 , N"N"J ?N"N16 and 8 , N16N15. No isotopic shift In 0,O band head
VOL. 30, NO.
12, DECEMBER 1958
2053
Y 80
t >
6o
t
40
2o
t nij
O L
Figure
5. Variation in intensity of 0,l band head in relation to adjacent lines as resolution changes
Resolution was controlled by varying slit width, and amplification was adjusted for each trace. Comparison of trace heights without amplification adjustment made b y multiplying intensity scale at left b y factor adjacent to each trace. Arrow i s over trace showing resolution used for this study and at the line used for intensity measurements
Figure 6. Calibration curve using nitrogen- 15 enriched samples analyzed by mass spectrorneter
tained with sets of a t least 10 readings on samples of a t least 100 cc., requiring up to 60 minutes per sample. For samples having only a small concentranormal abuntion of one isotope-e.g., dance-the precision drops to 37,. For routine measurements needing an accuracy to only 5% or better, two measurements are sufficient and can be taken in less than 10 minutes. A sample remeasured after 7 days reproduced the original value within 1%.
/
3c -
Upper set of lines determlned from measurement in the 0,l bands and lower set from measurements in the 1,0 bands. Dotted lines drawn from points computed using N"N" and N16N" band heads, Solid lines drawn from points computed using N14N16band head and band heod to right of it on Figure 2
EVALUATION
-.CCI
-_~_
_I--
1
1
001
A summary of operating variables is shown in Table I. The main advantages of the optical spectroscopic method for nitrogen analysis as presented are short measurements and calculation time (10 minutes), small amount of sample needed (20 cc. a t a
01
I00
IO
I
1000
Nt5/NH. W S S SPECTROSCOPY
b
2 5:ICIGE
-
I
GOES CL-
OPTIMUM OPERPTING POINT
1
I
i
\I
. $ 1
049c
1
1
I
I
1 , 04EL I
0
0
2C
I:
40 POWER
5t
60
I
80
90
1
j
I
I ,
I
'0
'..I
Z
0
30
I 2
I
I
3
4
I 5
I 6
1 7
PRESSURE. MM OF HG.
PER CEQT
Figure 7. Ratio and intensity vs. power at pressure of 1.5 mm. of mercury
Figure 8.
Intensity is sum o f NJ4NI4,Nl4NI5, and NI5Nl6band head intensities as measured on Figure 2
Intensity i s sum of NI4N1', Nl4Nl6,and N15N16bond head intensities as measured on Figure 2
2054
ANALYTICAL CHEMISTRY
Ratio and intensity vs. pressure at a ratio of
1 Q
I .L
pressure of 1.5 nini. of mercury), ability to recover sample (although the chemical form may be altered), and easy detection of errors due to impurities (other than isotopic nitrogen.) Most of the difficulties with the optical spectroscopic method are not fundamental but may be overcome by precautions or further development. The sample tubes should be kept as free from water, ammonia, organic solvents, and stopcock grease as possible to reduce the background to a mininium. ACKNOWLEDGMENT
The authors nish to thank T. I. Taylor, Columbia Vnirersity, for providing the nitrogen and nitric oxide gas calibration samplcs. They also are indebted to the nicmbers of the ?\lass Spectronwtrg Section :it the Yational Bureau of Standards for measurement of the calibration samples. Demonstration of technique by Sinioii Rothberg, discussion and advice from R. E. Ferguson and technical assistance froin ,Johan
deGroot are also gratefully acknowledged. LITERATURE CITED
(1) Artaud, J., Blaise, J., Gerstenkorn, S., Spectrochim. Acta 10, 110-18 (1957). (2) Brody, J. K., J . Opt. SOC.Am. 4 2 , 408-15 (1952). (3) Brody, 5. K., Fred, &I.,Tomkins, F. S.,Spectrochim. Acta 6, 383-412 (1954). (4) Brody, J. K., Tomkins, F. S., Fred, M., Ibid., 8 , 329-47 (1957). (5) Broida, H. P., Morgan, G. H., d s . 4 ~ . CHEM.2 4 . 799-804 11952). (6) Broida, H. P., Morbwitz,’H. J., Selgin, M., J . Research Natl. Bur. Standards 5 2 , 293-301 (1954). ( 7 ) Broida, H. P., Moyer, J. W.,J . Opt. SOC.Am. 4 2 , 37-41 (1952). (8) Clusius, K., Angew. Chem. 66,497-506 11954). ( 9 j Clusius, K., Becker, E. R., 2. ~Vaturforsch. 2 a , 154-9 (1947). (10) Crossvhite, H. AI., Fastie, W.G., J. Opt. SOC.,4nz. 46, 110-15 (1956). (11) Dentsov, Y. P., Striganov, A. R., Zhur. Anal. Khim 12, 5-9 (1957). (12) Dicke, G. H., U. S. Patent 2 , 5 8 5 , 9 0 1 (Feh. 19, 1952). (13) Fassel, V. A, Hettel, H. J., Spectrochive. Acta 7, 175-8 (1955).
(14) Fastie, W.G., J . Opt. SOC.-4m. 4 2 , 641-7 (1952). (15) Ferguson, R. E., Broida, H . P., ANAL.CHEM.2 8 , 1436-8 (1956). (16) Hoch, IM., Weisser, H. R., H c l ~ . Chim. Acta 33, 2128-34 (1950). (17) Hunt, D. J., Pish, G., J . Opt. SOC. Am. 46, 87-91 (1956). (18) Kostkowski, H. J., Broida, H. P., Ibid., 4 6 , 246-54 (1956). (19) Sprinson, D. B., Rittenberg, D., J . B i d . Chem. 180, 707-14 (1949). (20) Striganov, .4. R., Uspekhi F i z . S a u k 5 8 , 365-414 (1956). (21) Stukenbroeker, G. L., Smith, D. D , Werner, G. K., McNally, J. R., Jr., J . Opt. SOC.-4m. 4 2 , 383-6 (1952). (22) VeInberg, G. U., ZaIdel, A. N., Petrov, -4.A,, Optika i Spektroskopiya l , 972-82 119.56) (23) \T’alc‘her,-’W., Nucleonics 6 , 28-36 (June 1950). (24) Werner, G. K., Smith, D. D. Ovenshine, S. J., Rudolph, 0. B., RIcNally, J. R., Jr., J . Opt. SOC.Am. 4 5 , 202-5 11955). (25) Youden, ;:1 J., “Statistical Methods for Chemists, p. 17, Wiley, Kea, Tork, 1951.
RECEIVEDfor review March 5, 1958. Accepted July 17, 1958. Work supported in.part by the U. S. htomic Energy Commission.
Spectrophotometric Determination of Microgram Quantities of Indium E. JOHNSON, M. C. LAVINE,
J.
and A. J. ROSENBERG
Lincoln laboratory, Massachusetts Institute of Technology, lexingfon, Mass.
b 5,7-Dibromo-8-quinolinol has been utilized in a colorimetric determination of indium in the microgram region. An extraordinarily linear color response with an average deviation of 0.12 y was obtained with samples containing up to 100 y of indium.
I
of studying the dissolution of indium antimonide i t became necessary t o measure 1 to 100 y of dissolved indium with a sensitivity of 0.1 y. A colorimetric method for the estimation of small amounts of indium, given by Rloeller (Z), utilizes the yellow color of the chelate compound of indium and 8-quinolinol, and yields accurate and reproducible results for 15 to 1000 y of indium. =Ittempts t o adapt AIoeller‘s proccdure to the above requirements through the use of microtechniques were unsuccessful. The reagent blank was high and unpredictable. This fact, coupled n ith a nonlinear absorbanceconcentration dependence a t low concentrations, invalidated all determinations below 2 y. The blank was reduced by preparing the indium quino5 THE COCRHE
linate as a n aqueous suspension, drying the solution in a desiccator, and leaching the product into d r y chloroform. The reproducibility of the procedure was still inadequate. I n an attempt to obtain a more reproducible system, the 5,7-dibromo derivative of 8-quinolinol was tested. In view of recent work (1, 3, 6) with 5,7dihalo-8-quinolinols , an increase in color intensity was also t o be expected. Actually, the use of 5,7-dibromo-8-quinolinol shifts the absorption maximum t o a higher wave length and the absorbance is diminished (Figure 1). Furthermore, the absorbance is a strict linear function of concentration over t h e entire range from 0 t o 100 y. The analytical procedure is simple, rapid, and relatively specific. Neutral salts inhibit the rate of color development but do not otherwise interfere. PROCEDURE
Standard indium solutions were prepared by dissolving weighed amounts of the pure metal in concentrated hydrochloric acid. A convenient concentration of stock solution is 1 mg. of indium per ml. T o obtain the calibration curve,
aliquot portions of indium solution are pipetted into 15-ml. graduated, glassstoppered centrifuge tubes, and 2.5 ml. of 0.2M potassium acid phthalate are added. The p H is adjusted to within the range 3.5 t o 4.5 using a p H meter with a suitable probe electrode, and the solution is diluted to 10 ml. with distilled water. Five milliliters of a solution containing 0.1 gram of 5,7-dibromo-8-quinolinol (recrystallized seyera1 times from absolute alcohol) in 100 ml. of reagent grade chloroform are added t o the centrifuge tube which is stoppered and shaken vigorously a t intervals over a &minute period. After brief centrifugation, samples of the chloroform layer are transferred by pipet to a 1-cm. cuvette, and the absorption is measured at 415 mp, using pure chloroform as the reference solution. The same procedure is utilized in sample analysis. With the aid of the calibration curve the weight of indium present in the sample and reagent blank may be determined. Complexing of indium by various anions, reported by Sunden ( 8 ) , may inhibit the rate of color development, in which case a more prolonged shaking time may be required. The recornVOL. 30,
NO. 12,
DECEMBER 1958
0
2055
