by the appropriate variation of parameters, for example, by changing flow rates, solvent, raising the temperature, adjusting the pH, manipulating the periodic acid concentration, or by incorporating salt into the aqueous phase. An interesting observation made during this study was the complete absence of formaldehyde in the products of oxidized substrates in which formaldehyde is predicted. No trace of the 2,4-dinitrophenylhydrazoneof formaldehyde was evident on the thin layer plates. Even when 100 mg of l-monopalmitin or glycerin-1-octadecyletherwere oxidized formaldehyde was not detected by odor or by analysis of the 2,4-dinitrophenylhydrazones. The fate of the formaldehyde was not determined. Periodic acid-impregnated Celite and fine glass beads were unsatisfactory supports for the oxidation. The most obvious advantage of employing the periodic acid column for the oxidation of susceptible structures in lipids is that the need for finding a suitable mutual solvent is circumvented. Maerker and Haeberer (7)point out that the ratio of water to dioxane which they used as a solvent in their studies
was fairly critical as far as yields and solubility of the substrate were concerned. The periodic acid column procedure would seem to be more amenable than a one-phase system for structural studies on microgram quantities of an organic compound. The columns can be reduced in size which accordingly will decrease the volumes involved. Thus analysis of the effluent by gasliquid chromatography-mass spectrometry, for example, should be possible without further manipulations such as extraction or evaporation. ACKNOWLEDGMENT
We thank G. Maerker and his colleagues, EURDD, Philadelphia, Pennsylvania, for generous samples of many epoxides used in this study. RECEIVED for review November 26, 1968. Accepted March 25, 1969. Mention of brand or firm names does not constitute an endorsement by the Department of Agriculture over others of a similar nature not mentioned.
AIDS FOR ANALYTICAL CHEMISTS High-Stability LowNoise Precision Spectrometer Using Optical Feedback Harry L. Pardue and Stanley N. Deming Department of Chemistry, Purdue University, Lafayette Ind. 475'07
INrecent years there has been a rapid growth of interest in analyses based upon measured initial rates of chemical reactions (1-3). The resultant need to detect very small concentration changes in reactants or products places stringent requirements on the stability of the detection system used. For highest precision and accuracy in these measurements, it is important that any observed change of signal not be caused by the measuring instrument itself. This is of special consideration if the kinetic run is made over a long period of time, if the observed change in signal is a small part of the total signal, or especially if both situations exist. Futhermore, the ultimate sensitivity, precision, and accuracy of equilibrium methods based upon photometric techniques are dependent upon the stability of the photometer used. Recent reports have demonstrated that the use of optical feedback to stabilize photometric sources can lead to improved stability (4, 5). The instrument reported by Loach and Loyd (4) utilized a high quality monochromator and yielded excellent short term stability. However, no information on the long term stability was given. The instrument reported by Pardue and Rodriguez (5) yielded high long term stability but utilized interference filters to isolate desired wavelength regions. To date, no optical feedback system employing a grating or prism monochromator and having the (1) E. M. Cordos, S. R. Crouch, and H. V. Malmstadt, ANAL. CHEM., 40,1812 (1968). (2) G. E. James and H. L. Pardue, ibid., 40,796 (1968). (3) H.L. Pardue, Rec. Chem. Progr., 27, 151 (1966). (4) P. A. Loach and R. J. Loyd, ANAL.CHEM.,38, 1709 (1966). (5) H.L. Pardue and P. A. Rodriguez, ibid., 39,901 (1967). 986
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ultra-high stability required for the indicated kinetic studies has been reported. This report describes a high-stability, low-noise precision spectrometer utilizing optical feedback and including a monochromator for continuous wavelength variation between 400 and 625 nm. The working stability of the spectrometer system was evaluated on NiEDTA solutions at 590 nm. Results demonstrate that over the absorbance region studied (04.007 absorbance unit), the standard error of estimate is 0.000056 absorbance unit. These data are compared with results obtained using the 0 4 . 1 absorbance unit scale of a Cary Model 14 recording spectrophotometer. Consecutive 1-hour observations over periods greater than 10 hours demonstrated minimal drifts in the modified instrument of *0.007% T per hour with average drifts of about &0.02% T per hour. INSTRUMENT DESIGN
A Bausch and Lomb Spectronic 20 spectrometer was modified for optical feedback stabilization. The existing electronics in the spectrometer are disconnected. Modifications in the optics and control and measurement circuitry are described below. Spectrometer Modifications. A rigid bulb mount is constructed from a block of bakelite plastic. The bulb occupies the same position it does in the unmodified instrument but is rotated 60' about its symmetrical axis from its normal orientation to increase the light intensity and to minimize effects caused by filament motion. Leads from the programmable power supply are soldered to the bulb for external control of intensity. A 0.25- X 0.50-inch* rectangular hole is cut in the base of the instrument directly under the light path between the
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Figure 1. Spectral response of spectrometer Bulb voltage: 6.2 V Sample amplifier feedback: 20M, 0.005 mF Phototubes: 1P39 attenuating slit and the wavelength-defining slit. The hole is centered approximately 0.375 inch away from the pedestal supporting the attenuating slit mechanism; the longer dimension of the hole is parallel to the light path. A beam splitter is supported above the hole to direct a portion of the light beam downward while allowing the remainder of the beam to pass through the sample cell. A phototube housing (McKee Pedersen Instruments, Danville, Calif.) is attached to the spectrometer directly beneath the rectangular hole by means of light-tight Cannon connectors. The reference phototube is rigidly held in a Plexiglas mount which is fastened directly to the phototube housing. A slit of the same dimensions as that in front of the cell in the spectrometer is placed within the Cannon connectors to define the light beam and to allow approximately the same portion of the spectrum to fall on the reference and sample phototubes. (No attempt was made to optimize the phototubes in this respect.) Care is taken to assure that the reference beam does not strike the anode of the reference phototube and cause shadowing. The cell is a fully-thermostat4 1- X 1-cm2 cell similar in design to that described by Pardue and Rodriguez (5)but the jacket is cylindrical and made of copper. The opening in the jacket through which the light beam passes is approximately 0.375 inch in diameter. The sample phototube is disconnected from the existing electronics and external leads are provided. Sample and reference beams are detected by 1P39 phototubes (S-4 response). All internal surfaces of the modified spectrometer are sprayed with ultra-flat black‘paint to minimize stray light. To minimize thermally-generated mechanical stresses and movement, the base of the spectrometer was placed in a box and surrounded with insulating material to increase the thermal time constant of the spectrometer. Instrument warmup is approximately 4 hours for maximum stability; thus the instrument is turned off only for bulb replacement or other minor repairs. Control Circuit. The control circuit is essentially that shown in Figure 1 of Pardue and Rodriguez (5)with the following changes. The stabilized operational amplifiers are powered from a Philbrick/Nexus 2101 voltage regulator. The -1.35 ‘V in the reference current circuit is dropped through a 20K resistor to a 5K potentiometer and then to ground. The 1M resistor at the input of OAl in their Figure 1 is replaced with a switch which can be used to select any one of 12 resistance values from 250K to 20M. The feedback of OAl is capacitive only (0.001 pF). The feedback
Figure 2. Short-term stability and noise Time increases to the left Wavelength: 522 nm Reference amplifier feedback: 0.001 mF Sample amplieer feedback: 20M,0.005 mF Reference current: 0.05 PA loop of OA2 contains switches to select resistance values from 1M to 55M and capacitance values from 0.4 pF to 0.002 pF. A l00K potentiometer is in series with the selected resistor and both resistances are in parallel with the selected capacitor. These modifications allow greater versatility in obtaining the desired output voltage and time constant over the useful spectral region of the spectrometer. EXPERIMENTAL The spectrometer was evaluated by making transmittance measurements on NiEDTA solutions. Preparation of NiEDTA Solutions. All glassware was cleaned with hot dichromate cleaning solution, rinsed three times with 3 g/liter EDTA solution, and finally rinsed with distilled water. All solutions were prepared using reagent grade chemicals and distilled water which was passed through a mixed anion-cation resin (Amberlite MB-3). Four test solutions were prepared. Borosilicate-glassstoppered bottles, 125-ml capacity, were cleaned and dried thoroughly. Into each bottle was pipetted 100 ml of O.O20M, pH 7.0, EDTA solution and 0.0, 1.03, 5.15, or 10.0 ml of a solution 0.00919M in NiEDTA and approximately 0.011 M in free EDTA to give solutions O.OM, 0.937 X lO-’M, 4.50 X 10-4M, and 8.36 X lO-4M in NiEDTA. Each test solution contained at least a 20-fold excess of EDTA which served to keep Ni in the complexed form and to buffer the system at pH 7.0. Procedure. All absorbance readings were taken at 590 nm. Transfer of solutions was accomplished using a glass syringe with polyethylene “needle.” Before filling, cells were rinsed twice with the solution to be observed. Modified Spectrometer, Dark current was zeroed by placing opaque material in the cell and adjusting the balance control of the sample amplifier until its output was near zero. Distilled water was used for the reference blank as 0.020M EDTA had an insignificant absorbance at 590 nm (+O.OooOo7 f O.ooOo14 absorbance unit). Transmittance readings were taken from a Hewlett-Packard 3460-A digital voltmeter, corrected for dark current, divided by the interpolated 100% T readings (reference blank) to yield % T, and then converted to absorbance. Cary 14. All measurements were made against 0.020M EDTA in the reference cell. The absorbance of each solution was recorded for 2 minutes on 11-inch-widechart paper repVOL. 41, NO. 7, JUNE 1969
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rnobrity NiEDTA x lo4
molarity NIEDTA x 104
Figure 3. Absorbance VS. concentration of NiEDTA
Figure 4. Absorbance us. concentration of NiEDTA
Modified spectrometer Slope: +7.531 liter mole-’ cm-l Intercept: +0.000035 A.U. Standard error of estimate: O.oooO56 A.U. Dashed lines represent f one standard error of estimate
Cary 14 Slope: +7.711 liter mole-’ cm-l Intercept: +O.M”)77 A.U. Standard error of estimate: 0.000259 A.U. Dashed lines represent f one standard error of estimate
resenting 0.1 absorbance unit full scale. Noise was averaged visually to obtain an absorbance value. Interpolative corrections for changes in blank readings were made to obtain a final, corrected value of absorbance.
(recorded directly on tracing paper) of the spectrometer response at 522 nm, illustrating typical short term characteristics. The noise level is observed to be well within 0.0125% T. The sharp noise spike near the right hand side of the figure caused by an electric motor starting near the spectrometer is indicative of the response speed of the system. The spectrometer is observed to exhibit short term drift. If the drift shown in Figure 2 were to continue in the same direction, it would represent a drift of f0.06’% T per hour. In practice, the drift changes directions periodically and tends to oscillate, with a period of about 45 minutes, about a gradually drifting point. Long-term drift was determined at each of three wavelengths and is summarized in Table I. Drift is defined here as plus or minus one-half the width of the envelope enclosing the signal trace (noise included) on the Sargent Model SR recorder. Results were obtained over consecutive hourlong periods. Data representing the worst, best, and typical performance characteristics are included for the three wavelengths. The response characteristics are observed to be a function of wavelength with comparable performance obtained at 425 nm and 522 nm and slightly poorer performance at 600 nm. Also, the spectrometer has a temperature coefficient. No attempt was made to evaluate an exact temperature coefficient of the spectrometer or the source($ of the temperature sensitivity. Some improvement over the data in Table I can be achieved by controlling variations in room temperature to be below f1 OC per hour. Equilibrium Data. Plots of absorbance us. concentration of NiEDTA at 590 nm are shown in Figure 3 for the modified spectrometer and in Figure 4 for a Cary Model 14 spectrophotometer for comparison. Quoted specifications for the latter instrument (Bulletin 100, Cary Instruments, Monrovia, Calif.) are: photometric reproducibility, 0.0005 absorbance unit with 04.1 scale; photometric accuracy, 0.0005 absorbance unit with expanded scale. The dashed lines in Figures 3 and 4 represent plus and minus one standard error of estimate (6) based on the nine
RESULTS AND DISCUSSION Spectrometer Characteristics. The spectral response of the spectrometer described under Instrument Design is shown in Figure 1. For these measurements, the control amplifier (OAl in Ref. 5) was saturated causing the bulb voltage to be fixed at 6.2 V by the Zener diode. The feedback resistance of the sample beam amplifier was 20 M ohms. If the bulb is operated near its maximum intensity and the maximum feedback resistor in the sample amplifier is 55 M ohms, then an output signal of 1 V representing 100% T is obtainable over the wavelength range from 400 to 625 nm. Spectrometer performance data are reported for three wavelengths (425,522, and 600 nm), representing the optimum (522 nm) and two wavelengths on the rapidly changing portions of the spectral response curve. Figure 2 is a recording
Table I. Long Term Stability of Modifled Spectrometer
Number Drift T per hour)” of hours Wavelength observed Highest Lowest Average &0.008 f0.013 11 &0.018 425 nm f0.007 f0.013 14 f0.033 522 nm f0.036 10 f0.055 f0.025 600 nm Room temperature variations of f1 “C per hour.
(z
Table 11. Comparative Data on Two Spectrometers Standard error of Drift, estimate, Av std absorbance Instrument A.U. dev, A.U. units per hour O.ooO267 0.001“ Cary 14 0. ooO259 Modified 0.oooO38 f O .ooo1 spectrometer 0,000O56 a Bulletin 100,Cary Instruments, Monrovia, Calif.
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(6) W. E. Deming, “Statistical Adjustment of Data,” Wjley, New York, 1943,p 167.
points (triplicate samples at each of three concentrations) shown in each graph. Similar results are obtained by taking the average standard deviations of the three NiEDTA solutions. Comparative data for the two spectrometers are shown in Table 11. The standard error of estimate for the modified spectrometer (0-000056 A.U.) indicates that species with an absorbance of 0.0002 may be detected with 99% confidence. The modified spectrometer has beem shown to possess
ultra-high stability and high precision necessary for the kinetic measurements discussed at the beginning of this report. The instrument is currently being used to measure precisely the rates of enzyme catalyzed reactions. RECEIVED for review February 3, 1969. Accepted February 17, 1969. This work was supported in part by PHS Research Grant No. G.M. 13326-03 and in part by a grant from the Indiana Elks Association to F’urdue University.
Electrolytic Dissolution of Tantalum, Niobium, and Other Refractory Metals Abraham Aladjem Soreq Research Center, Yavne, Israel INthe analysis of tantalum, niobium, zirconium, tungsten, and other refractory metals, the specimens are usually prepared for analysis by dissolution in a mixture of nitric and hydrofluoric acids followed by several evaporation-dissolution stages. This procedure is cumbersome and laborious, and a new electrolytic dissolution technique developed by us offers greater simplicity and dispenses with the use of hazardous reagents; it would be especially valuable in cases in which the presence of the fluoride ion is objectional. Electrolytic dissolution techniques have found a limited use in analytical chemistry, notably in the case of uranium ( I ) . EXPERIMENTAL
Reagents. A 5 solution of NH4C1 in methanol is used as the electrolyte: analytical-grade reagents are used, without special precautions to ensure the absence of moisture, because the electrolyte tolerates a moisture content of up to 4 % without adverse effects. Procedure. The dissolution is carried out in borosilicate glass containers, without separation of anolyte from catholyte. Tantalum or platinum wire 0.4 mm in diameter is used as the cathode. The metal to be dissolved serves as the anode, either on a suitable graphite support, or directly connected to the leads. Dissolution occurs with a current efficiency of over 50% and is accompanied with an evolution of O2on the anode. N o metal deposition takes place on the cathode. The initial anode-to-cathode surface ratio should be about 1 :1 but naturally it decreases in the course of the electrolysis. The anode current density is not critical and may range from 3 to 150 mA/sq cm, but a density of 20-30 mA/sq cm is needed to provide reasonably short dissolution times, and densities beyond 40-50 mA/sq cm are to be avoided because of excessive heating. At current densities of the order of 40 mA/sq cm the time needed to dissolve a tantalum or niobium foil 0.1 mm thick is about 10 minutes; to avoid precipitation (probably of the oxychloride) at least 50 ml of the solution must be used for every gram of metal taken. It is preferable to use a power supply with controlled-current dc output instead of constant voltage because the conductivity of the solution increases with increasing temperature, and as some heat is evolved in the dissolution the temperature of the electrolyte may rise and the current density may rapidly increase beyond the optimum range, causing boiling of the electrolyte. (1) R.P.Larsen, ANAL.CHEM., 31, 545 (1959).
The electrolyte temperature should be kept below the boiling point but it is not critical except to avoid excessive losses of solvent by evaporation. A clear solution should be obtained at the end of the dissolution, the only possible residue being very thin (30-40 A) flakes of the natural oxide covering the metal; in most cases these flakes are also dissolved in the process. RESULTS AND DISCUSSION
The method has been used to dissolve tantalum, niobium, tungsten, zirconium, vanadium, hafnium, titanium, and their alloys-e.g., Ni-14% Ta, Ta-Nb-and no difficulty was experienced with materials from different sources (Fansteel, Heraeus, Metallwerke Plansee) and of varying degrees of purity. Although the method is especially suitable for the dissolution of massive samples or wires which may be connected directly to the leads, crushed samples or even finely divided powders may also be dissolved by supporting them on a graphite plate or preferably on a heavily-anodized (to an oxide thickness of 4000-5000 A) tantalum dish, which is not attacked at current densities below 50 mA/sq cm provided that the applied voltage does not exceed 10-12 volts. In some cases the analysis may be continued directly in the methanol solution (preliminary experiments show that colorimetric measurements are possible in such solutions), but in general the methanol is evaporated and the residue is dissolved in the aqueous medium needed for subsequent analytical steps-e.g., HC1 or H 2 S 0 4 solutions-and separation or determination of impurities is carried out by conventional methods. It should be recalled that some of the refractory metal chlorides are rather volatile; hence, care should be taken not to overheat the residue from the methanol evaporation stage -[.e., the evaporation should be carried out over a water bath. ACKNOWLEDGMENT
The author thanks Yitshak Marcus of the Hebrew University, Jerusalem, for his valuable comments. RECEIVED for review November 5, 1968. Accepted December 30, 1968. VOL. 41, NO. 7 , JUNE 1969
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