Fourteen 25.00-ml samples were titrated under optimum conditions of tip placement and stirring rate. The automatic titrator failed to reach an end point for the 14th sample because an air bubble became trapped over the porous membrane at the electrode tip (see Figure 1). The recessed membrane of the electrode has a tendency to trap an air bubble which prevents the electrode from contacting the solution. Care must always be taken to remove the air bubble when the electrode is first placed in any solution. The first 13 samples titrated in this series gave an average end-point volume of 25.22 ml, and an average deviation of k0.014 ml. This gives a relative precision of =tO.O5z. Furthermore, 8 of the 13 values show a deviation of *0.01 ml or less. For comparison purposes, 3 samples were titrated manually with a conventional 50-ml buret and Eriochrome Black-T indicator. These end-point volumes were 25.25, 25.26, and 25.25 ml. Thus, it appears that the automatic end point occurred slightly before the traditional colored end point. A precision of =t0.05 is better than should be expected for this titration, especially when the titration curve is considered.
The instrument manufacturer reports a sensitivity of 0.5 mV. This was checked in the following manner. After the buret had stopped for the final time and the meter indicated 20 mV, changing the end-point setting to 19.5 mV always caused the buret to deliver another increment of titrant. The sensitivity of the instrument compensated in part for the small break in the titration curve. The calcium-selective electrode has permitted precise endpoint locations in the automatic titration of calcium with EDTA using a commercially available automatic titrator. Any slowness in response to changing Ca2+by the electrode was more than compensated for by the amount of chemical anticipation achieved in the titration procedure. ACKNOWLEDGMENT
The author expresses his gratitude to the Fisher Scientific Company for making the instrumentation available, and thanks Dr. Sidney Soloway for his useful suggestions.
RECEIVED for review May 15, 1969. Accepted July 11, 1969.
Rapid Mass Spectrometric Determination of Chromium as Chromium(III) Hexafluoroacetylacetonate James L. Booker and T. L. Isenhour Department of Chemistry, University of Washington, Seattle, Wash. 98105
R. E. Sievers Aerospace Research Laboratories, ARC, Wright Patterson Air Force Base, Ohio 45433
DETERMINATION of chromium by classical methods is, in general, difficult because the predominantly trivalent chromium undergoes reactions similar to those of other transition metals with which it frequently occurs. The only determination of chromium for which interferences are of minor importance is that based upon the formation and reduction of the dichromate ion ( I ) . For this method, however, a sample containing several milligrams of chromium must be oxidized vigorously, a rather slow process requiring constant attention and involving several quantitative manipulations ( 2 , 3 ) . Techniques for the determination of trace amounts of chromium are also quite limited, and only in certain cases can an accurate analysis be obtained. The standard colorimetric methods are interference sensitive ( 4 ) ; atomic absorption spectrometry is limited by the formation of the refractory chromium sesquioxide (5); for emission spectrometry, the sample must often be preconcentrated (6); spark source mass spectrometry requires special physical preparation of the (1) A. I. Vogel, “A Text-book of Quantitative Inorganic Analysis,”
John Wiley, New York, 1963. (2) N. Lounamaa, Ana/. Chim. Acra, 31, 213 (1964). (3) C. S. Richards and E. C . Boyman, ANAL.CHEM.,36, 1790
(1964). (4) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd Ed, Interscience, New York, 1959. ( 5 ) W. Slavin, “Atomic Absorption Spectroscopy,” Interscience, New York, 1965. (6) J. H. Yoe and H. J. Koch, “Trace Analysis,” John Wiley, New York, 1957.
sample, and is limited in accuracy (7); and activation analysis is limited by the low isotopic abundance of W r and the long half-life and low gamma-ray yield of S1Cr (8). This paper describes the quantitative mass spectrometric analysis of chromium as chromium(II1) hexafluoroacetylacetonate [Cr(hfa)& This determination can either be directly applied to nanogram-sized samples or to larger samples by appropriate aliquoting, and from 20 to 30 samples, including difficult to dissolve materials such as stainless steels, may be processed in one day. EXPERIMENTAL
Mass spectra were obtained using an Associated Electronic Industries MS-9 double-focusing mass spectrometer modified by removing the gas inlet capillary and replacing it with a flange having a T 12/18 inner joint. The standard operating conditions to provide a resolution of 1000 are: source slit, 0.100 in.; analyzer slit, 0.040 in.; ionizing voltage, 70 V; and accelerating potential, 8 kV. The recorder was set to run at 0.05 inch per second. Other high resolution mass spectrometers are suitable for this analysis providing there is a means of locating the mje of interest without using the sample as its own mass marker; both stabilizing the magnet current and peak matching from a lower mass standard have proved satisfactory. (7) G. H. Morrison, “Trace Analysis,” Interscience, New York, 1965.
(8)DIStrorninger, J. M. Hollander, and G. Seaborg, Reo. Mod. Phys., 30, 626 (1958). VOL. 41,NO. 12, OCTOBER 1969
1705
100 90 80
70 -0
\
60
50 40
30 20 IO
500
700
600
m/a
Figure 1. Mass spectrum of Cr(hfaIaabove mass 200 700r
I
,
I
,
I
I
, ,
I~
6 00
500
.-e i 400
u)
4:
#
300
U
200
100 0
0 20 40 60 80 100 120 140 160 180200 ng Cr
the remaining volume of the tube with H(hfa), reseal, and heat for 4 hours at 130 "C. The tube, now containing Cr(hfa)s, is opened into a volumetric flask so that a solution can be made from the contents. To make the solution, shake the contents to one end of the tube, freeze them in liquid nitrogen, and break off the other end of the tube. Immerse the open end in a suitable solvent (ether or benzene) in a 10-ml volumetric flask; because the thawing solution will effervesce violently, this must be done quickly. Break off the other end and wash the tube thoroughly with solvent. Dilute the solution to volume. Cool the solution to -78 "C before withdrawing an appropriate aliquot, usually 5 or 10 pl, Cool a test tube having a T 12/18 outer joint to -78 "C and inject the aliquot into it. Attach this tube to the modified fitting on the inlet block of the mass spectrometer and evacuate with the rough pump until the solvent evaporates or sublimes completely. For ether, this requires about 10-15 sec/pl; for benzene, 10-15 minutes. Open the tube directly to the source, then warm the contents to 100 "C by immersing the tube in hot water
Figure 2. Integrated area of W r (hfa)s+peak us. weight of chromium Cr(hfa)3 was prepared by the reaction of chromium nitrate with hexafluoroacetylacetone [H(hfa)] (9); after filtration the crude product was purified by sublimation in uacuo at 100 "C rather than be recrystallization. H(hfa) was obtained from Columbia Organic Chemical Company and redistilled prior to use. All other reagents were of reagent grade, and were used without further purification. Stainless steel samples were obtained from the National Bureau of Standards. Standard solutions for calibration containing 1,000, 10.00, and 100.0 ng of chromium per pl were prepared from purified Cr(hfa)3. Procedure. The chromium-containing materials were quantitatively oxidized to hexavalent chromium by reaction with perchloric acid in a sealed tube and then reacted with H(hfa). Place about 0.25 mg of the chromium-containing compound into a borosilicate glass melting point capillary along with 1 pl of 70z perchloric acid. Seal the tube, and'place it into a short piece of metal tubing open at both ends as a protective shield. Heat at 180 "C for 4 hours. After shaking the contents to one end and freezing in liquid nitrogen, open the tube by breaking off the other end. Minimize effervescence by carefully thawing the solution. Fill half (9) R. E. Sievers, R. Moshier, and M. Morris, Znorg. Chem., 1, 966 (1962). 1706
0
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION
This method requires that all the chromium in the sample be converted to chromium(II1) hexafluoroacetylacetonate. Corresponding complexes of acetylacetone and trifluoroacetylacetone may be prepared; however, these have much lower vapor pressures and are, therefore, inferior for mass spectrometric methods. In some of its chemical forms, chromium will react directly and completely with H(hfa). Chromium metal and sodium dichromate, for instance, react 2Cr
+ 6H(hfa)
+
2Cr(hfa)r
+ 3H2
and
+
Na2Cr207 6H(hfa) -+ 2Cr(hfa)a
+ oxidation products
Several steels and ores, however, are passive to H(hfa), so it is necessary to uniformly oxidize them to hexavalent chromium with perchloric acid, then quantitatively reduce and complex them with the ligand. The mass spectrum of Cr(hfa)3 showing all detected peaks above mass 200 is given in Figure 1 ; the relative intensities and mass assignments for the major peaks are given in Table I. This spectrum is characteristic of Cr(hfa), for ionizing voltages greater than 25 V. The most prominent peak is that assigned to Cr(hfa)2+(Z/Zo = loo), and this peak was chosen for all analyses, although depending upon the experiment undertaken, another peak may be more desirable. Peaks
Table I. Relative Intensities and Mass Assignments for the Mass Spectrum of Crhfa3 mte
Calcd
Found
PFTBA Peak
Ratio
672.9046 653.9062 603.9094 465.9169 415.9188 396.9214
672,8906 653.8982 603.8868 465.9685 415,9187 396.9157 301.9537 277.9238 267.4465 258.9218 227.9281 208.9252 189.9259
613 ,9647 613.9647 501.9711 463.9743 413.9775 375.9807 263.9871 263.9871 218.9856 218.9856 218.9856 180.9888 180.9888
1.095976 1 ,065042 1.203031 1,004298 1,004689 1.055681 1.143820 1,052793 1.221297 1.182369 1.040836 1.154354 1.049379
...
277.9269 258.9285 227.9301 208.9317 189.9333
involving chemical rearrangements, Cr(hfa)F+, Cr(hfa)gF-CF3+, Cr(hfa)Fz-CF3+, and Cr(hfa)F-CF3+ are unpredictably strong, and, conversely, with the exception of the Cr(hfa)z+ peak, those involving only fragmentation, Cr(hfa),F+,c ~ - ( h f a ) ~ - C FCr(hfa)z-CF3+, ~+, Cr(hfa)+, and Cr(hfa)-CF3+, are surprisingly weak. Using the peak switching feature of the MS-9, one locates the position of the most intense peak of the Cr(hfa), spectrum, the 466 peak of 52Cr (hfa)z+, by first locating the PFTBA standard's 464 peak on the low mass range, setting the mass ratio at 1.004353, and then scanning the high mass range at fast sweep. This gives two scans over a narrow mass range (including the 466 peak) per second. By using this feature of the MS-9, two of the peaks may be monitored; the Tr(hfa)z+ and the 52Cr(hfa)2+peaks, for example, may be recorded simultaneously, allowing greater flexibility in sample size, and permitting an easy check for interferences from other sample constituents. In Figure 2, the area of the recorded graph is shown as a function of the sample size from 0.5-200 ng of Cr from standard Cr(hfa)3. The area was determined by cutting out the curve and weighing after it was determined that photographic chart paper is of uniform density; several swatches of equal size being found to have a standard deviation of less than 1 %. Because the signal decays as the sample is exhausted, the reading was terminated, after approximately five minutes, when the height of each spike was twice that of the background noise. The slope of the line is the average of the ratios of the area of the curve to the sample size. At the low end of this analysis, the precision was determined by measuring nine identical samples of 2.00 nanograms each. The measured values were 1.85, 1.98, 1.97, 2.12, 2.02, 1.79, 2.18, 2.14, and 1.89 for an average of 2.00 with a standard deviation of 0.13. At the high end of the calibration (100 ng), the result of seven determinations were 96.5, 100.0, 102.1, 98.2, 102.3, 98.5, and 101.4 ng for an average of 100.0 with a standard deviation of 2.2.
Ill0 45 10 5 100 15 (5
