r
0.20
The precision and accuracy data for the methyl ester of dimer (Table 111) are typical results obtained, using this method. The precision of this method is slightly better than the 2 x value reported for the carbazole method ( I ) . The determined molarity of the dimer methyl ester agrees with the added molarity within 2 %. The gas chromatographic method requires considerably less time than the combination of methods used previously. The utility of the method was demonstrated by the facile analysis of the enzymic breakdown products of trimer. The trimer breakdown yielded monomer and unsaturated dimer in a 1:1 ratio corresponding to the results of Nagel and Anderson (18)who demonstrated that the Bacillus polymyxa endopectic acid transeliminase attacks trimer at the glycosidic bond farthest from the reducing end of the molecule.
0.16 I
%
= z_
0.12
32 0.08 0 0
J c
0.04I 0
0
2
4
6 Time (Hr 1
8
10
Figure 3. Rates of esterification of trimer using O.12N HCl and 0.12N Dowex 50 (H+) as the esterification catalysts DISCUSSION
The decay data demonstrate the instability of the trimethylsilyloligouronide derivatives. The manner of decay is not certain but a failure to obtain any increase in peak height upon resilylation indicated that the decay was not simply due to Si-0 bond cleavage. The fact that the trimethylsilyl ether of glucose was stable coupled with the fact that esterification considerably reduced the rate of decay indicated that the carboxyl groups were involved in the decay mechanism.
ACKNOWLEDGMENT
The authors are indebted to D. Smittle and K. McMichael for their helpful suggestions, and to D. Coahran for his valuable technical advice.
RECEIVED for review April 28, 1969. Accepted August 6, 1969. Scientific paper No. 3270, College of Agriculture, Washington State University, Pullman, Project 1620. This investigation was sL,iported in part by funds provided for biological and medical research by State of Washington Initiative Measure No. 171, and by an N.D.E.A. Title IV Fellowship to W. R. R. (18) C. W. Nagel and M. M. Anderson, Arch. Biochem. Biophys., 112, 322 (1965).
Automatic Titration of Calcium with EDTA Using a Calcium-Selective Electrode Stanford L. Tackett Indiana University of Pennsylvania, Indiana, Pa. 15701
IONSELECTIVE ELECTRODES are convenient and desirable for quantitatively determining the ionic composition of a variety of solutions. A feature article by Rechnitz gives an excellent description of the electrodes ( I ) . The uses of the electrodes for general analysis have been reviewed by Toren (2), and their application to water analysis has been considered by Andelman (3). Lingane reported the use of the fluoride-selective electrode as the indicator electrode in the titration of fluoride with calcium, lanthanum, and thorium (4). Light and Mannion have also used the fluoride-selective electrode for titrations (5). Rechnitz and Lin studied the response of the calciumselective electrode in the presence of magnesium in the titration of calcium by EDTA (6). The calcium-selective electrode is less precise than the fluoride-selective electrode, since it has (1) G.A. Rechnitz, Chem. Eng. News, 45, (25), 146,(1967). (2) C. E.Toren, Jr., ANAL.CHEM., 40,402R (1968). (3) J. B. Andelman, J. Warer Pollution Control Federation, part 1, November 1968. (4) J. J. Lingane, ANAL.CHEM.,39,881 (1967). (5) T.S. Light and R. F. Mannion, ibid., 41, 107 (1969). (6) G.A. Rechnitz and Z . F. Lin, ibid., 40, 696 (1968).
a Nernst slope of about 30 mV/log a 7s. 60 mV/log a for the fluoride-selective electrode. Rechnitz and Lin reported that the calcium-selective electrode responded fairly fast to a change in calcium activity in the absence of magnesium, but slowed considerably in the presence of magnesium (6). This research was undertaken to determine if the calciumselective electrode could be used for precise end-point location in the titration of calcium by EDTA with currently available automatic titrators. Potentiometric titrations are inherently more precise than direct potentiometric determinations, and the time required for an automatic titration is about the same as that for a manual direct potentiometric analysis. Gardels and Cornwell titrated 11 metal ions with EDTA by an automatic potentiometric method using an amalgamated gold thimble electrode (7). They resorted to an electronic relay to intermittently start and stop the buret near the equivalence point to allow equilibration. In the present study a commercial titrator will be used without modification, as it is currently supplied by the manufacturer. (7) M. C. Gardels and J. C.Cornwell, ANAL.CHEM., 38,774 (1966). VOL. 41, NO. 12, OCTOBER 1969
0
1703
TOP VIEW STIRRER
SCE
C o e SELECTIVE ELECTRODE
BURET TIP
I
, ,,
q
I
I
[
0
I
2
3 4
I
I
,
[
[
8
9 IO II
I
[
0
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SELECTIVE
(I
6
7
I2 13
Volume of EDTA (ml)
Figure 2. Titration of 10 ml of Ca2+solution with 0.1MEDTA
MEMBRANE
I=
-_--
_--I
SIDE VIEW
Figure 1. Titration cell EXPERIMENTAL
Apparatus. The automatic titrator was the Fisher titralyzer, Catalog No. 9-319-1OOV2, manufactured by Fisher Scientific Company. The calcium-selective liquid membrane ion-exchange electrode was manufactured by Orion Research and marketed by Fisher Scientific Company (Fisher Catalog No. 13-641-800). The electrode was put into operation according to the manufacturer’s instructions. Titrations were performed in 200-ml Berzelius beakers. The electrodes, buret tip, and stirrer were arranged as shown in Figure 1. A metal paddle stirrer was substituted for the glass spiral stirrer furnished with the instrument. Solutions. Reagent grade calcium chloride dihydrate was weighed, dissolved in distilled water, and diluted to the mark in a 1 1. volumetric flask. The pH 10 buffer was prepared from reagent grade ammonium chloride and ammonium hydroxide, dissolved in distilled water. The 0.1M EDTA solution was supplied prestandardized by the Fisher Scientific Company, Fisher Catalog No. SoS-412. Procedure. To a 200-ml tall form beaker, 25.00 rnl of 0.1M calcium solution, 10.0 ml of pH 10 buffer, and 35 ml of distilled water were added. Sixteen such samples could be loaded into the titrator turntable. The metal paddle stirrer was adjusted to a moderate speed, and the preselected end-point potential was set on the instrument. The titrator switch was turned to “automatic” according to instructions supplied by the manufacturer, and the instrument successively titrated each sample automatically and printed each end-point volume on a paper tape. RESULTS AND DISCUSSION
A 10.00-ml sample of the calcium solution (the beaker also contained 10 ml of the pH 10 buffer and 50 ml of distilled water) was titrated manually with 0.1M EDTA. The resulting titration curve is shown in Figure 2. A small end-point 1704
ANALYTICAL CHEMISTRY
break was obtained. The total break from the start to the conclusion of the titration was 32 mV, and the break in the endpoint region (9-11 ml) was only 17 mV. From the graph, 20 mV was selected as the end-point potential. The observed end-point break was less than that shown in the Orion Manual. This was attributed to the dilution of the sample prior to the titration, and to the presence of ammonium ion in the buffer. Whitfield and Leyendekkers have recently shown that the presence of ammonium ion decreases the calcium electrode response (8). Results of this research indicate that precise end-points can be obtained with the observed break. The first 5 titrations of 25.00-ml samples of calcium solution gave an average end-point volume of 25.28 ml, and an average deviation of i 0 . 0 3 ml. While this precision is fairly good, the response of the instrument for chemical anticipation was unsatisfactory. A new buret tip was fabricated (shown in Figure 1) which would allow the titrant to be delivered below and to the side of the porous membrane of the indicator electrode. Stirring was adjusted so that the titrant solution would continually bathe the calcium-selective electrode tip. When the buret tip and stirring rate were adjusted to the optimum conditions, a good chemical anticipation was obtained. As the end point was approached, the solution in the vicinity of the indicator electrode was depleted in Ca*+as long as the buret was delivering titrant. When the CaZ+ depletion matched the end-point condition, the buret automatically stopped. After a few seconds of mixing, the potential rose above the 20-mV end-point potential, and the buret automatically started again. Many such cycles occurred successively until the potential remained at 20 mV. The print delay timer was set at 40 seconds to prevent a premature stopping of the titration. Adjustment of the buret tip and stirring rate allowed as many anticipation cycles as desired. The buret delivery rate is fast, 8 ml per minute, and the titration precision is quite poor if no anticipation is obtained. A total of 18-24 cycles were sufficient to give the best precision for this titration. Each off cycle lasted from a fraction of a second to a few seconds, so the total titration time was not excessively long. (8) M. Whitfield and J. V. Leyendekkers, Anal. Chim.Acta, 45, 383 (1969).
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
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