Table II. Visible Absorption Characteristics of Some tmb-Metal Complexes in Chloroform Solution
Compound A=, mp e FeClztmb 417, 445 67, 71 CoClztmb 573, 653 254, 486 SiClztmb 489, 610, 830, 144, 12, 32, 975
75
form covalent bonds with Cu+. If this is so, the normal B values observed for solutions of [Cu(tmb)a]NOa and [Cu(tmb)z]C1O, are to be expected, since the equilibrium between the complex and its dissociation products must lie predominantly in favor of the bis complex. I n these cases the positions of the equilibria are little changed by the mass action effect of excess tmb (see Table I). Tmb shows a negative ferroin reaction (9, 13) and is reported not to complex with iron(I1). Our experiments indicate that complexes with iron(I1) are
formed. As for the dmp complexes of iron(II), these are pale yellow solids and their solutions are noticeably colored only when concentrated. It appears that tmb is able to complex with the transition metals generally. Table I1 lists some of the compounds that have been isolated and their spectral characteristics in CHCb solution. The suitability of tmb for the determination of copper in the presence of other transition metals will depend upon the Amax values of their complexes and the intensities of the solutions. Results indicate that other metals will not interfere because of either small e values for the complexes or remoteness of regions of maximum absorption from 454 mp of the [ C ~ ( t m b ) ~ion, ] + just as in the case of dmp. LITERATURE CITED
(1) Brandt, W. W., Dwyer, F. P., Gyarfas, E. C., Chem. Revs. 54,959 (1954). (2) Case, F. H., J . Am. Chem. Soc. 70, 3994 (1948).
(3) Fox, D. B., Ph.D. thesis, University of Queensland, 1963. (4) Fox, D. B., Hall, J. R., Plowman, R. A,, Australian J. Chem. 15, 235 (1962). (5) Gahler, A. R., ANAL.CHEM.26, 577 (1954). (6) Hall, J. R., Marchant, N. K., Plowman, R. A,, Australian J . Chem. 15, 480 (1962). ’ ( 7 ) Irving, H. M., Cabell, M. J., Mellor, D. H., J. Chem. SOC.1953,341i. (8) Jones, P. D., Newman, E. J., Analyst 87,637 (1962). (9) . , Linnell. R. H., J . Ow. Chem 22, 1691 (1957). ‘ (10) Linnell, R. H., Manfredi, D., J . Phys. Chem. 64,497 (1960 ). (11) Natl. Bur. Standards, Letter Circ. LC-1017 (1955). (12) Kewman, E. J., Peters, G., “2,9Dimethyl-1,lO-phenanthroline: Reagent for Copper,’’ Monograph 40,
Hopkin and Williams. Chadwell Heath, Essex. 1960. (13) Schilt, A. A., Smith, G. F., Anal. Chim.Acta 16, 401 (1957). (14) Smith, G. F., McCurdy, W. H., Jr., ANAL.CHEU. 24,371 (1%i2).
RECEIVEDfor review March 29, 1963. Accepted August 26, 1963.
Purity Examination of Silicon and Germanium Halides by Long-Path Infrared Spectrophotometry MYRON J. RAND Bell Telephone Laboratories, Inc., Allenfown, Pa.
b The infrared absorption spectrum of 10-cm. liquid layers of silicon and germanium tetrahalides is capable of detecting many impurities in the 1 - to 200-p.p.m. range. Dissolved gases and volatile compounds, and especially organic contaminants, are seen readily. The technique is also useful for following the progress of purification procedures. Typical long-path spectra of liquid SiC14, GeC14, SiBr4, and GeBrc from 2 to 15 microns are shown, and methods are given for distinguishing impurity absorptions from weak bands of the matrix material. A compilation of common impurities, with absorption maxima, approximate absorpiivities, and detection limits, is also presented. Batches of supposedly hyper pure “semiconductor grade” halides from several suppliers show significant differences. Atmospheric moisture introduces hydroxyl groups and such compounds as Si2OC16 so that exposure is a source of oxygen contamination, even if all the absorbed water reacts with the halide.
T
of the Group IV metals are now used extensively for t h e production of the pure elements by various reduction processes and for the HE HALIDES
2126
ANALYTICAL CHEMISTRY
preparation of pure oxides and pigments by hydrolysis or burning. SiCl,, GeCl,, and T i c & are particularly important, and the Si and Ge chlorides and bromides are of interest in semiconductor technology not only for production of the bulk metals but for epitaxial deposition from the gas phase. For all these uses the purity of the halides is of paramount importance, but these compounds are good solvents for many gases and organic materials, and they are powerful dehydrating agents, Hydrolysis by atmospheric moisture contaminates them with hydrogen halides and oxygen-bearing compounds; the latter are particularly unwelcome, since small amounts of oxygen may seriously degrade the physical and electrical properties of Group IV metals. Little has been published on the detection of impurities in the halides, aside from conventional spectrochemical techniques for foreign metals. It is the purpose of the study reported here to point out the wealth of information obtainable from the infrared absorption spectrum of deep liquid layers of Si and Ge halides. Liquid sample thicknesses of more than 1 mm. are rarely used in infrared work, since most materials would be virtually opaque over most of the 2- to 25-micron analytical
range. The Group IV liquid halides, however, are tetrahedral molecules with quite simple spectra; eren more important, they contain no light atoms, so that all the fundamental absorptions lie beyond 15 microns. Even the first overtones and simple combinations usually lie beyond 10 microns. Consequently these compounds are quite transparent in the 2- to 10-micron range even in path lengths of many centimeters. Organic compounds and other volatile impurities which do have strong absorptions in this range may then be detected in concentrations as low as 1 p.p.m. Halides from various sources, all represented to be of the highest purity-e.g., “semiconductor grade”-have given spectra showing significant differences. Often the exact impurities may be identified with fair confidence. The success of attempted purification procedures may be followed readily. Only one compound, titanium tetrachloride, has been examined extensively by the long-liquid-path technique (4, 15,,24,.%%%9. It is evident from these results, as well as from the present work, that absorption maxima of contaminants ordinarily shift very little upon solution in the halide, generally 0 to 0.1 micron; the same now seems true of the variation
Table 1. Fundamental Vibrations of Some Group IV Metal Halides
(Cm. -1)
Tic, (8, Sic4 SiBr. GeCl, GeBr, IS, (7)
( 7 ) (7)
150 90 v4(.f2) 221 137 424 249 Vl(U) vnifz) 610 487 v3,-observedn 621 498 a For the gas phase. for the liquid. vz(e)
( 7 ) 21)
132 80 120 172 112 140 396 235 388 453 327 485 460 330 . .. Other data are
in different halide solvents. Thus, many of the existing d,.tta on impurity absorptions in T i c 4 a-e useful for Si and Ge halides. It is the intention of this report to outline the method of examination and to show typical results, rather than to present complete qualitative and quantitative characterizaticn of impurity spectra. Calibration may be done readily for any impurity considered important, but there are many likely contaminants, usually varying with the history of the halide, the manufacturer, and even the lot number. Furthermore, absorptivities should bc determined on the same spectrophotorieter t o be used in the analysis. Findly, for many purposes almost any impurity is undesirable, and exact identification of an unknown band (which niay be laborious) is of only secondary interest. INTERPRETATION
OF SPECTRA
In interpreting long-liquid-path spectra it is considered tllat the general trancmittance level, as well as drifts and minor fluctuations (of this level with wavelength are all without analytical significance. There is a long and uncompensated path of high-refractiveindex liquid. bounded by windows of high index and high rdectivity which may not be exactly parrtllel; some deviation of the beam, and some scattering as well, are likely. A well defined absorption band departing from the neighboring transmittance h e 1 is certainly significant, however. The baseline level becomes uncertain as one approacher the region where the Si or (>e halides themselves absorb, since with 10-cm. samples there is strong absorption from the wings of these bandr. The positive identification of an unexpected impurity ,olely from the infrared spectrum is usually not possible. With some experience and some knowledge of the history of the sample, however, certain recurring pbsorptions soon take on identity. Two simple techniques which have been very helpful are distillation and deliberate contamination. In the latter thl: sample, in the cell, is “doped” with a contaminant to observe the exact wavelength of the
absorption. A microsyringe is convenient for liquids, and gases are simply bubbled slowly through the cell, in one filler tube and out the other. Minimal exposure to the atmosphere is essential. With distillation, an absorption band which is more prominent in the first fraction evidently belongs to a more volatile or gaseous impurity; a less volatile impurity concentrates in the boiler residue. A band which is unchanged by distillation probably belongs to the halide itself, although there is always the chance of an impurity with a boiling point too close for significant fractionation. EXPERIMENTAL
Most of the spectra were obtained on a Beckman IK-4 spectrophotometer; a few were observed also on a PerkinElmer 421. Instrument settings and operation were routine. For comparability of spectra the reference beam was attenuated to adjust the transmittance to 9&95y0 in a region without bands, usually 4.5 to 5 microns. -4 compensating cell was not required. The sample path length was 10 cm. throughout. Cells were cut from 30mm. 0.d. borosilicate tubing and provided near each end with a filler tube with overflow bulb. The windows were 1-mm. sheet silver chloride, cemented on with Araldite epoxy resin (Ciba Co.). The usual KaCl or KBr windows contain far too much water for service with the Group IV halides and are quickly coated with insoluble hydrolysis products. Silver chloride was always satisfactory except where a sample contained some strong reducing agent which stained the windows with a silver film. In this case windows of polished high-resistivity germanium nere used. Irtran-2 (Eastman Kodak Co.) also serves well. The still was designed to avoid any joint grease or rubber gaskets, since these are attacked or dissolved by the hot halide, with results evident in the spectrum. The still was flushed continuously with dry nitrogen. The receiver was the cell itself, and provision was made for rejection of a fore-run, Where as-received electronic grade Sic14 was further purified the method
was essentially that of Clabaugh et al. (4)followed by the silica gel treatment of Theuerer (SO). This latter was omitted when the SiC14 was distilled directly into the absorption cell, since the spectrum showed that silica gel may contribute impurities as well as remove them. In the figure captions, different code letters indicate different manufacturers. SPECTRA OF Si AND Ge HALIDES
The infrared spectra of S i c & vapor (68) and GeC14 vapor (17) have been
published. There are no reports on the liquid phase, and no reports of any kind on either SiBr4 or GeBr4. The strongest vapor absorptions of all four compounds have been located in earlier work in these laboratories, and are given in Table I, along with the fundamental vibrations known from Raman spectra ( 7 ) . From the studies cited, it is evident that the v3(f2) vibration produces by far the strongest infrared absorption, and most of the other strong bands of the spectrum are overtones or combinations of us. The highest frequency absorption ordinarily visible in the vapor is 2u3, lyhich lies a t these wavelengths: SiC14, 8.2 niicrons; SiBrc, 10.3; GeC14, 11.0; GeBrr, 15.2. With a 10-cm. liquid path this absorption governs the useful working limit, since from this region out through lower frequencies the sample is opaque. Some of the frequencies higher than 2 u 3 observed in the long-liquid-path spectra are due not to impurities but to the matrix material itself; these are combination and overtone frequencies too feebly absorbing to be seen under the usual conditions of measurement. The origin of such bands is indicated by two pieces of evidence: their intensity does not change when the sample is distilled; and a single pattern of frequenry assignment accounts for them in all of the Si and GI. halides studied, and in Tic14 aka. Table 11 lists certain combinations. all involving u3, calculated from the data of Tahle I. As shown, in almost’eI.pry case a persiitent absorption band is observed very cnlose to t’he calculated frequency. Intensities vary, but u1 2u3 is generally the stronged. I n the spectra shown in this report’, absorption bands of the matrix inaterial arc marked by a heavy dot.
+
Table II. Absorptions in Long Liquid Paths
(Cm. -1) 2vi pa VI 2vs SiC14,calcd. 1441 1458 1644 obs. 1452 1643-56 GeC14, calcd. 1078 1245 1302 obs. 1084 1242 1305 SiBr,, calcd. 1111 1223 9f5 obs. 1224 GeBr4, calcd. 766 797 889 obs. i66 790 8781 TiCl,, calcd. 1110 1274 1384 obs.c 1110 1258 1359 Masked by strong foreign absorption. * The strong 2v3 absorption interferes. Refs. (16,84). v4
+ 2va
+
+
3 v3 1830 1825 1359 1360 1461 1463 98 1 9iO 1495 1478
2 va 1220
< 1310 906 Opaque < 980 9 74 Opaque < 1140 654 Opaque < 700 998 Opaque < 1050
Opaque
VOL. 35, NO. 13, DECEMBER 1963
2127
,'t * T
II
P
3
4
5 MICRONS
Figure 1 .
---
. .. . --
Sic14 liquid,
6
7
8
RESULTS
-4bsorption maxima (microns) and their assignments follow: 2.22, unknown, but associated with 2.70; 2.70 hydroxyl group, nonhydrogen-bondedmost probably Si(OH)Cla from hydrolysis ( 2 ) ; 3.39, 3.50, HCI; 4.25, COz; 4.35, unknown, has been observed in several halides-strongest in distillation forerun, probably a gas; 4.43, SiHC13; 6.44, CCL; 6.48, Si20C16-see below; 7.09, unknown. Bands assigned t o Sic& itself are
Figure 1 presents typical long-liquidpath spectra for SiCl,, and is an example of n-hat can be learned about competing samples, all supposedly "pure." Impurities observed, with an order-ofmagnitude estimate of concentration in p.p.m., are: HCI, 50-260; COS, 5-50; SiHC13, 1000; CC14, 100; Si, OCls, 100-500: and either lvater or some hydroxyl-containing nonorganic com-
-
8
MICRONS
10 cm.
Sample M, as received Sample S, as received Sample M, purified and distilled into cell
'I
6
Figure 2.
--.. .. - .- .- .
SiCI4, liquid,
10 cm.
Sample M, as received Sample M, purified Sample M, purified and distilled into cell Sample M plus 0.39% SinOCla
included in Table 11. It is suspected that the 5.5 and 6.1 micron S i c 4 bands are partially masking the two strong bands of phosgene (5.51, 6.05), a common impurity in samples made by chlorination of the oxide. The 5.5 micron SiC1, band also is almost certainly masking the 5.43 band of SisOCls,
-
il .I1
:I :I
:I :I
:I :I
4
3
I I t I
1
I
r
P
3
4
5
6
P
Figure 3.
--.. .. 2128
ANALYTICAL CHEMISTRY
SKI4, liquid,
4
3
MICRONS
5
MICRONS
10 cm.
Sample M, as received Sample M, purified
Figure 4.
.. . . -
Sicla, liquid,
10 cm.
Sample M, purified Same, after several weeks' storage
P
3
4
6
5
1
10
9
8
14
13
15
MICRONS
GeCI4, liquid, 10 cm.
Figure 5.
- Sample E,
. ... --which is stronger tkan 6.48. This compound arises from limited hydrolysis :
+
+
Sic14 H20 -+ Si(OH)Cls HC1 SiC1, + Si,OCls HCl Si(OH)Cl,
+
+
Hexachlorodisiloxane, SiPOCl6,is volatile (b p. 137OC.) and is a source of oxygen contamination of silicon. To confirm its presence the experiment shown in Figure 2 was curied out: pure Si20C16was prepared (27) and added to SiC1,. The result confirms the assignment of 6.48-micron absorption to Si20Cl6. The purification procedure evidently removes mosi, of it, probably solely becauqe of the distillation step. The sample with no atmospheric exposure had no 6.48-micron absorption. Figure 3 demonstrate? the monitoring of the 8iC1, purification irocedure by the infrared niethotl I t iq seen that the hydroxyl content and the unknonn 4.35micron absorption are little affected; HCl is much diminished; and virtually complete removal of CO,, SiHC13, SinOCls, and several minor absorptions has occurred. Figure 4 s h o w the changes occurring as SiCL stand> in a closed (but not sealed-off) borosilicate glass container. Hydroxyl, HCl, and (:On have all increased. Some 3.0-micron absorption is attributed to hydrcgen-bonded hpdroxyl; this is been ir, all the halides after prolonged exposure, and is characteristically broad and shallow. Control tests in which traces of liquid and gaseous water were added confirmed the -OH 2nd HCl assignments. Even the minute amount of CO? added with the liquid water was visible. S o additional absorptions appeared in the 5- to 6-micron range.
as received Sample S, as received Sample S, distilled into cell
Germanium Tetrachloride. Figure 5 shows long-path spectra of “semiconductor grade” GeC14’s. Again the sharp hydroxyl (2.75 microns), HC1, and CO, absorptions are seen. Sample E probably contains phosgene (5.51 microns). There is some structure around 6 microns which could be a trace of water, or some organic compound. Both as-received samples contain some unknown impurity Fith absorptions at 6.93, 7.07 microns; GelOC& is a possibility. The sharp band at 8.22 microns is rreaker in
the distilled sample, and is assigned t o SiCl, (or, less probable, CHC13). The 9.22-micron absorption in sample E is due mostly to an all-but-invisible trace of amorphous silica on the cell windows, since this cell had been used for SiCl,. Windows are easily cleaned by lapping with Linde A or other fine abrasive, however. Silicon Tetrabromide. Spectra before and after distillation of one sample of SiBra are shown in Figure 6. -4gain the hydroxyl absorption (2.74 microns) is present; 3.81, 3.98 microns
\. !
$ I
i ~
iII
I
\\)\
II
I l
Ii \
0
1 ’ P
3
4
5 MICRONS
Figure 6.
---
-
6
1
I I
8
9
SiSr4, liquid, 10 Em.
Sample A, as received Same, distilled into cell
VOL.
35, NO. 13, DECEMBER 1963
2129
P
3
4
6
5
8
1
9 MICR 0NS
....
__
are HBr. The sample is evidently contaminated by some organic material with t h e C-H bond (3.37 microns) which is difficult t o separate by distillation. Since its spectrum is obviously unusually simple it is sugTable 111.
Impurity
HBr CO, P-H SiHCla Si-H Ge-H
cos
COCl? COBrz CClaCOCl CCl4
c.
- 67 - 78
..
33
...
... - 48 8
CSClS CHzClz POC13
65 118 77 46 69 325 40 105
SOClZ
79
cs, so;c1z
57 61 137
Si02
...
4
't 3 (4 (4 4 5 6 (5
~~
SiCl4 CHCla Si20Cl6
voc1,
gested that the impurity is bromoform (b.p. CHBr3, 149°C; SiBr4, 153'c.). The absorption a t 5.61 microns may be COBrz (2s). Germanium Tetrabromide. GeBrr spectra are shown in Figure 7. The
(Ten-em. liquid path) Absorpiibsprption tivity, Detection limit, maximum,a wt. 7C-l Refs. p.p.m. crn.-' I.1 Tw ... 2 70-2 80c ... ... 3 28-2 3jd ... (25) 2 Tw; (4, 15, 24, 32-35) 15 3 53 3 41 3 99 4 27"
5
6 6 7
7 7 7 8 8 4 8 8 8 5 6 9 9 4
6) 8) 89 51 05 48) 54 44 57 04 69 93 95 21 08 06 18 24 98 43 49 2 66 84
10
3
...
...
80
1
... ... ... ...
200 50 5 ... 40
0.5 500 75 10 20 80 45
100 0.5 0.5 30 60 0.8 0.2 ...
... ... ... ... 0 2
1
Tw
Tw; ( 4 , 16, 84, 52-35) (1, 18, 19, 22, 26)
Tw; ( I O ) (S, 6, 12, 14, 16, 20, S1) ( 6 , 9, 11, 29) (14, 38-36) ( 4 , 16, 34, 32-36)
10 ... 1
100 0.05 2 B
5 2
x
0.5 25 100 3 1 40 150
...
(34,32-36)
Tw
0.5 ( 4 , 16, 24, 32-36) 50 Values in uarentheses are for the uure materials: it is not known whether any shift
127
100 0.5
occurs in soluiion. Tw = this work (absorptions established by addition of known materia1:toPhe halides). c The addition of water gives a broad, ahallow band near 3p, attributed to hydrogenbonded hydroxyl. For most organic compounds the C H absorption is usually given as 3.3-3.4~. e Doublet under high resolution. Boiling points: SiCL, 57" C.; GeC14,83" C.; SiBr,, 153" C.; GeBr4, 186" C. 2130
ANALYTICAL CHEMISTRY
12
13
14
15
Sample E, as received Sample K, as received Sample E, distilled into cell
Impurity Absorption Bands in Group IV Metal Halides
13. P.,
11
GeBr4, liquid, 10 cm.
Figure 7.
---
10
pure compound has no absorption in the 2- to 10-micron range and deserves consideration as a solvent for infrared work. It is not so sensitive t o hydrolysis as the other halides and i t is suspected t h a t small amounts of water can exist in equilibrium with it. The broad, shallow 5- to 7micron absorption in as-received sample E, Kith a sharp peak at 6.26, iq believed to be nater. The addition of a drop or two of BBr3, a potent witer-wavenger, removes this absorption, as ivell as the 2.77-micron hydroxyl band. There is little HBr. The persistent band a t 7.28 microns is presently assigned to CBr4(2v3), nhich boils only three degrees higher than GeBr4. The infrared method re1 eals that sample K, represented by the manufacturer to be of high purity, is in actuality hopelessly contaminated with organics. There is a heavy 3.35-micron C-H band, many other high-frequency bands, and almost total absorption beyond 5.6 microns. Such a sample would be a poor choice for growing epitaxial germanium. Addenda. For the convenience of those employing the long-liquid-path infrared absorption technique, a summary of what is presently known about impurity absorptions, qualitatively and quantitatively, is appended in Table 111. h complete literature search for the spectra of t h e compounds listed is incorporated in this compilation. Some of the data are from the TiCl, studies, believed valid for Si and Ge halides also. Absorptivities should be con4dered only approximate. References are given throughout. It will be obvious that as much art as science must go into trace analysis by infrared; but even if all the impurities cannot be identified much will be learned about a sample which mould not
be revealed easily by any other technique. ACKNOWLEDGMENT
The author acknowledges with gratitude the patient help of L. P. Adda in obtaining the infrared spectra. The Sic14 purification was done by C. E. Shoemaker. REFERENCES
( 1 ) Beachell, H. C., liatlafsky, B., J . Chem. Phys. 27, 182 (1957). (2) Beattie, I. R., hlcQuillan, G., J . Chem. SOC.1962, 2072. (3) Bethke, G. W., M'ilson, M. K., J . Chem. Phys. 26, 1107 (1957). (4) Clabaugh, W. S., e ! al., J . Res. Nat2. Bur. Standards 55, 261 (1955). ( 5 ) Crawford, Y.A., et al., J . Chem. Phys. 37, 2377 (1962). (6) Curl, R. F., Jr., Pitzer, K. S., J . Am. Chem. SOC.80. 2371 11958). (7) Delwaulle, I f . L.; et bl., J . Phys. Radium 15, 206 (1954).
(8) Dove, M. F. A., et al., Spectrochim. Acta 1962,267. (9) Drake, J. E., Jolly, W. L., J . Chem. Soc. 1962. 2807. (10) Gibian, T. G., McKinney, D. S., J . Am. Chem. SOC.73,1431 (1951). (11) Griffiths, J. E., J . Chem. Phys. 38, 2879 (1963). (12) Hawkins, J. A , , et al., Ibid., 21, 1122 (1953). 113) Hawkins. IT. J.. Camenter. D. R.. ' J. Chem. Phis. 23, i700 (i955). ' (14) Jam, G. J., Mikawa, Y., Bull. Chem. SOC.Japan 34,1495 (1961). (15) Johannesen, R. B., et al., J . Res. Natl. Bur. Standards 53, 197 (1954). (16) Kriegsmann, H., et al., 2. Chem. 1, 346 (1961). (17) Lindeman, L. P., Wilson, &I. K., Spectrochim. Acta 1957, 47. (18) SfcConaghie, V. M., Nielsen, H. H., J . Chem. Phus. 21.1836 119531. (19) McKean,"D. C., Schatz, P: N., Ibid., 24,316 (1956). (20) Meal, J., Wilson, M. K., Ibid., p. 385. (21) Moszynska, B., Bull. Acad. Polon. Sci. 1111) 5. 819 (1957) fC.A.. 52. 3515~1. (22) Kixon, E. R., J . Phys. Chem. 60, 1054 (1956). I
.
~
I
.
I
I
(23) Overend, J., Scherer, J. R., J . Chem. Phys. 32, 1297 (1960). (24) Reimert, L. J., Rand, PIT. J., J . Electrochem. Soc., in press. (25) Robinson, C. C., et al., Proc. Roy. Soc. (London)A269,492 (1962). ( 2 6 ) Schindlbauer, H., Steminger, E., Monatsch. 92,868 (1962). (27) Schumb, W. C., Stevens, A. J., J . Am. Chem. SOC.72, 3178 (1950). (28) Smith, A. L., J . Chem. Phys. 21,1997 (19.53). (29) Straley, J. W., et al., Phys. Rev. 62, 161 (1942). (30) Theuerer, H. C., J . Electrochem. SOC. 107,29 (1960). 1311 Tindal. C. H.. et al.., Phus. " Rev. 62. ' i b i (1942). (32) Tsekhovol'skaya, D. I., et al., Zavodsk. Lab. 25,300 (1959). (33) Tsekhovol'skaya, D. I., Zavaritskaya, T. A., Zh. Analit. Khim. 16, 623 (1961). (34) Zavaritskaya, T. A . , Titan i Ego Splavy, Akad. A'auk S S S R , Inst. illel. 1961, 195. (35) Zavaritskaya, T. A., Zevakin, I. &4., Zhur. Prikl. Khim. 34, 2783 (1961). \ - - - - I -
RECEIVEDfor review July 18, 1963. Accepted September 10, 1963.
Sensitive land Selective Spectrophotometric Reaction for Determination of Trace Amounts of Calcium MANOLITA HERRERO-LANCINA' and T. S. WEST2 Chemistry Department,, The University of Birmingham, Birmingham 7 5, England
b The reagent Calcichrome, cyclotris-7-( 1 -azo-8-hydro.~ynaphthalene-3, 6-disulfonic acid), provides a method for the spectrophotometry of calcium down to the 0.1-p1.p.m. level. At p H 12 and 61 5 mp the procedure has a molar absorptivity of 7600. The method may b e applied in the presence of the other alkaline earths-e.g., 5000 bg. of Ba+-with some reduction in sensitivity. Several hundred micrograms of metal!; such as AI, Pb, Zn, Co, Hg, and Cd do not interfere under specified conalitions, but M g +* and interfere. The color system develops within I5 to 20 minutes and maintains an unchanged absorbance for more than 24 hours. The reagent solution is also stable.
A
many rrethods have been proposed for the spectrophotometric determination of cdcium, in general the majority are subject to interference from other ions, and are rather unstable or not very sensitive vhen compared to spectrophotometric pi*ocedures for the determination of othei. metals. Sandell ( 7 ) has recently reviewed and discussed these methods. The oxalate procedure is a very indirect one and is subject to rather restrictive conditions. It is dependent on initial complete separation LTHOUGH
of calcium oxalate with minimum coprecipitation of oxalate ion and i t involves a filtration or centrifugation step. The chloroanilate method is less restrictive, but also involves filtration. The most advantageous procedures so far devised appear to be those based on murexide (ammonium purpurate), phthalein complexan (2,6-xylenolphthalein-a, a'-bisiminodiacetic acid), and glyoxal bis(2-hydroxyanil). The murexide method appears to be applicable in the range 1 to 3 p.p.m. of calcium, but a high concentration of reagent must be used to ensure quantitative formation of the calcium complex. Unfortunately, the reagent is unstable and ea. 50% decomposition occurs over 4 hours a t room temperature a t the p H of determination. Practically all heavy metals interfere and the tolerance for strontium and barium is 1 and 5 p.p.m., respectively. Magnesium also forms a color and >400 p.p.m. of sulfate interfere. The results obtained by the method appear to be somewhat lacking in reproducibility, though a more detailed and reliable method has been reported (3, 5 ) . The phthalein complexan method is also nonselective and the color system is unstable, so that the absorption must be measured immediately under carefully prescribed conditions.
Undoubtedly the best reagent to date appears to be glyoxal bis(2-hydroxyanil). According to Williams and VVilson (9), the method based on this reagent operates in the range 0.5 t o 10 pg. per ml. The color system also lacks stability, but when extracted into chloroform it remains unchanged for 15 minutes. Centrifuging of precipitate is required, but removal of liquid is avoided by extraction with chloroform and clarification of the extract by centrifuging. The color of the extract obeys Beer's law in the range up to 10 pg. (0.5 to 10 p.p.m.) and the calibration curve appears to be fairly reproducible, though it must be checked whenever used. The information about interferences is limited. Ten times the concentration of magnesium, or one tenth the amount of strontium or of iron, does not interfere with the determination of 210 pg. of calcium. Strontium gives a color with the reagent, but the addition of carbonate is apparently successful for amounts I: 1 to 10 ratio. Earlier papers on the use of the reagent (4) state that in the presence of carbonate ion, cobalt and nickel give red precipitates with the reagent, and On study leave from the University of Zaragosa, Zaragoza, Spain. 2 Present address, Imperial College, Universit of London, South Kensington, London, g.W.7. VOL. 35, NO. 13, DECEMBER 1963
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