V O L U M E 28, NO. 7, J U L Y 1 9 5 6
lo73
PROCEDURE FOR ANALYSIS OF m K N O W N
Select a pair of well matched cells for absorbance measurements. Introduce the solution of unknown concentration into one of them and make an approximate determination of its concentration. To do this, adjust the width of the slit until the instrument is set a t zero. Then, by reference to a table similar to Table I but prepared for the particular spectrophotometer being used, find the approximate concentration that corresponds to the required slit width. Place a reference solution of somewhat lower concentration than that indicated for the unknown in the second cell, and, with the reference solution in the light path, set the spectrophotometer to zero. Then move the unknown solution into the light path and measure its relative absorbance, A r . While making the measurement of relative absorbance, control the temperature as described in the section on control of variables. Calculate the concentration of uranium in the uncl, as esplained known by means of the equation, c 2 = F X Ar previously.
concentrations of uranium. It is especially suitable for solutions which are free from compounds that have strong absorption banda in the wave length region near the 4 1 & m ~peak. Microgram quantities of impurities do not seriously interfere. The presence of sulfuric acid leads to results which are slightly low, unless compensated for. ACKNOWLEDGMENT
The authors are grateful to H. P. House for his assistance in the preparation of this manuscript. LITERATURE CITED
+
Bacon, A., Milner, G. W. C., Harwell, Berks, Atomic Energy Research Establishment, Rept. C/R 1637 (1955). Ibid., C / R 1749 (1955). Bastian, R.,ANAL.CHEM. 21, 972 (1949). Ibid., 25, 259 (1953). Bastian, R., Weberling, R., Palilla F., Ibid., 22, 160 (1950). Hiskey, C . F.,Ibid., 21, 1440 (1949). Hiskey, C . F.,Young, I. G., Ibid., 23, 1196 (1951). Moudy, L.,Silverman, L., Ibid., 28, 45 (1956). Rodden, C. J., “Analytical Chemistry of t h e M a n h a t t a n Project,” McGraw-Hill, New York, 1950. Rodden, C. J.,ANAL.CHEM.25, 1598 (1953). Scott, T.R.,Dixon, P., Analyet 70, 462 (1945). Young, I. G.,Hiskey, C. F., ANAL.CHEM.23, 506 (1951).
PRECISION O F METHOD
The test results for a series of samples are presented in Table 111, together with the uranium concentration as determined by a gravimetric method. For 11 samples, the average standard deviation is 0.37,. One of these samples contained microgram quantities of impurities, chromate, ferric, nickel, and permanganate ions, and one was 0.25N with respect to sulfuric acid. No significant error was caused by the impurities. The presence of acid in relatively high concentrations increased the error. CONCLUSION
The differential spectrophotometric method provides a rapid and accurate procedure for the determination of relatively high
RECEIVED for review November 29, 1955. Accepted April 9, 1956. Southeastern Regional Meeting, ACS, Columbia, S. C., November 5, 1955. Work carried out under Contract No. W-7405-eng-26 a t Oak Ridge National Laboratory, operated by Union Carbide Nuclear Co., a division of Union Carbide and Carbon Corp., for the Atomio Energy Commission.
Raman Spectrometer Assem bled from Available S. M. DAVIS, H. C. LAWRENCE, and G. L. ROYER American Cyanamid Co., Bound Brook, N. 1.
R. F. STAMM American Cyanamid Co., Stamford, Conn.
Raman spectroscopy offers advantages for certain applications, but its advance has been slow, partly because of complicated instrumentation. A Raman spectrophotometer assembled by modifying available components has resolution reproducibility and intensities that make the instrument of practical use.
R
AMAN spectroscopy some years ago was the leader in the
measurement of vibrational spectra, but it has fallen far behind the infrared in analytical applications. Part of the reason for this lies in problems such as color, fluorescence, solubility, size of sample, and the like, which are inherent in the Raman technique, but generally cause no difficulty in the infrared. Another major reason for the development of infrared rather than Raman spectroscopy has been instrumentation. Infrared has moved far ahead in having commercially available recording spectrometers and spectrophotometers with adequate resolution and precision for adaptation to analytical control as well as structural studies. These instruments have been made compact and rugged enough for satisfactory plant and laboratory operation. In this approach to the problem, it was decided that any instrument would have t o be ready to operate a t all times with a minimum of preparation, be rugged enough to be placed in a
plant labofatory area, and record fast enough to give a complete spectrum in a reasonable time. The present instrument, made from conlmercially available parts, is compact and rugged enough to run in a laboratory in the plant area for some time with practically no special precautions. It is ready for use a t any time and a spectrum can actually be running within 5 to 10 minutes from notice that it is desired. 9 complete spectrum can be recorded within a half hour. Its actual performance approaches the achievements of some of the better instruments now in use. DESCRIPTION OF INSTRUMENT
The basic design of the instrument is similar to that built earlier a t the Stamford Laboratories of the American Cyanamid Co. (4, 6),but numerous modifications and simplifications have been made. The light source is a Toronto-type arc (purchased from the Applied Research Laboratories). It is of borosilicate glass and is permanently evacuated. Figure 1 shows the arc mounted in position. The power for the arc is supplied from a direct current source, an available motor generator set of 15 kw., 250 volts, a t 60 amperes. This is far in excess of the requirements, as the arc itself has approsimately a 100-volt drop at 15 amperes a t its optimum operating efficiency ( 1 ) . Any stable direct current source can be used, if it can supply 230 volts a t 15 amperes. Immediately adjacent to the arc in Figure 1is a double-jacketed glass cylinder. This is a light filter. In the large outside com-
1076
ANALYTICAL CHEMISTRY can be seen in Figure 3 w m made in the shop and is painted
01,
the inside with a water-base paint of barium sulfate and carhorbaxyniethylcellulose, which givos a highly reflective matte finish. The monochromator is a modified I'erkin-Elmer Model 83 universal monochromator. The modifications can best be shown by a schematic drawing of the opticel system shown in Figure 1. The usual aluminized off-axis paraboloid is M 2 . The prism, ~8 in the standard Perkin-Elmer Model 83. The Littrow mirror has been replaced b an aluminized plane replica grating of 15,000 lines per inch. (8urrently a grating of 45 700 lines per inch is being used satisfactorily.) This plane graking is rotated by 1~ modification of the meohanism ordinarily employed for the standard Littrow mirror, hut it is oriented so that its dispersion augment,sthat of the prism, and rotation of the grating changes the wave length of the radiation going through the exit slit, S2. From the grating the light is further dispersed by another pass through the prism, and it impinges again upon the pnrabola. The light is then sent to plane mimor, M1, which reflects it through the exit slit. Before going through the exit slit, the light passes through a lbeyele chopper of special design, the flag being driven by the installation of a special cam upon the standard chopper drive. The position was chosen to permit chopping of the light a t its focal point. This resulted in the sharpest rive and decay time of the energy pulses a t the photomultiplier detector, The 13-cycle chopping was used because the standard equipment-monochromator and amplifier-was commercially available. The detector, an RCA 1P21 photomultiplier, was mounted behind the exit slit a t the focus of the entrance slit image. The photomultiplier voltage WILR obtained from a regulated power supply (made by Fnrst Electvonics).
PR, is of glass and is in the mme position
Figure 1. Are light source, glass double-jacketed light filter cylinder, and sample tuhe
Figure 2. Positioning of light source, light filter, and ~arnpletube with respect io optics of speetromekr
Other modifiestionp of the monochromator were: matching the curvatwe of the entrance slit s i t h the image curvature prodtreed by the prism and grating cornhination, and installing Imffles inside the monochromd.ottor, as it was found of primary importance to eliminate as much &ay light as possible. The stray light, being the major oause of noise, limits the useful sensitivity of the detector. The monochromator as it wa8 used is shown in Figure 5. Because the standard 13-cycle chopped light was used, it was feasible to use the standard Perkin-Elmer Model 81 amplifier with only a slight change in the response. In anticipation of high noise levels, 1-, 2-, 5-, and 12-second time constants were made available as compared to the 2-second and less usually found on the Model 81. However, this change was actually superfluous, inasmuch as it has not been necessary to use anything longer than 2 seconds, even though other time constants were tried, as can be seen in Figure 6. Full gain of the amplifier is ahoot 32 X.
partment saturated sodium uitrite is circulated. This m v e s a double purpose, inasmuch &s it acts as a heat exchanger, keeping as muoh heat as possible f w m the sample tube, and as a filter for ultraviolet and violet radiation. The inner and smaller campartment of the filter cylinder may be used fqr rhodamine, dinitrotoluene, e t a ; the choice of the filter solution depends upon the type of sample and the region of the spectrum being surveyed. The 4358 A. mercury line was nsed to excit.e the Raman spectra in this installation. The sample tuhe iu shown in Fieure 1 next to the filter jackz. This tube has a concave, hack-surface, spherical mirror on the bottom of the stopper which fits into the top uf the tube. The radius of curvature is equal to the length of sample tube. The light scattered in both the forward and backward direations is collected in the monoahromator; hence the signal is doubled. The sample tube is rigidly positioned so that it is vertical and in the middle of the light source (Figure2). The scsttered r a d i a t i o n passes through the window a t the bottom of the vertical sample tube, impinges on n right-angle prism with the hypotenuse itluminiaed, and is sent off a t right angles in t,he horizontal direction. A Ions then focuses an image of t,he r e a end of the sample tube upon the entranceslit of Figure 3. Over-all view of assembled Raman spectrophotometer with controls the monochromator (Figure and recorder 2). The light housing which
1077
V O L U M E 28, N O . 7, J U L Y 1 9 5 6 The amplified signal is recorded on a modified extended range Leeds & Northrup Speedomax. The modification consisted of an
signal frdm less than 1 ta 880 mv. The complete assemlrly shown to the left in Figure 3. OPERATIOX O F INSTRUlMENT
Stat-Up. The starting or me-striking process hnn generally lieen laborious and time-consuming. I n this instrumrnt it >\-as made an automatic time sequence operation, neecssitat.ing only the momentary depressing of a contact switch. Basically, tho mtomatic controls consist of a 3-minute interval timer and B direct current relay. The program sequences are started when the button on the main control panel is depressed. This starts B rhain of events conbrolled by interval timer and current relay.
.
r
ou8 cheeks of the reprodurihilit3- of this instrument have been made. The possible factors causing nonreproducibility have been classified into three main categories-rror in sampling, light source, and electronics. Giving colorless, nonturbid samples, the sampling procedure seems to be almost completely reproducible when all normal precautions are taken and thus should contribute no errors. The deviations accountable to the light murm would be of two types. The first would be a gradual error attributed to possible darkening of the tube. This is corrected by running standard cirves periodically and applying correction factors. The other possible error is due to variations in the arc current and has to be more or less accepted, unless iionio monitor system is used. The extent of this error can be estimated, as it has been found that the intensity of a Raman line varies approximately a8 the current through the light source. The arc system has a minimum amperage deviation of shout 0.S ampere, ahic.h means a calculated uncertainty of about ,tlq', on any line intensity. Actual experiment shows that the average error attributable to the light source i 8 about =t0.5%. The error doe to the elect.ronics can be identiiied as the noise due to the detector, amplifier, or recorder. Actually over 90% of the noise found may be identified a8 "light noise" in the detector, which is an iinrefrigeratcd 11'21 photomultiplier. The nar-
U
Figure 4. Schematic drawing of optical system of monochromator used in Raman instrument
1 . Interval timer starts. 2. Direct ourrent (250 volts) is applied throuah a suitable dropping resistance fop current ebntroiof arc lamp.'
4..
An electric heater of 4050-watt capacity, which is located
amdient to 100" C. in 10 to 15 secbnds and maintain this tempera-
rtial is maintained for 10 seconds or until the arc strikes (wliichever is shorter) and maintains an 3sc through the lamp. 7. The direct current flow through the arc actuates a current relay which turns off the air heater and high voltage source. It turns on cooling water t o fingers in mercury pools of the lamp. 8. After 3 minutes and 15 seoands, the interval timer shuts itself off and resets itself to the starting position. This program of sequences has unfailingly started the helical mercury arc lamp in this installation. It is possible to start running a curve within 1 or 2 minutes after the arc comes on, but the amperage drifts somewhat and has to be closely checked throughout the running of the initial spectrum. Within 30 to 60 minutes the current will stabilize snd require little or no further attention. Operating Variables. The most important feature of any :inslytical technique is its reproducibility. Accordingly, numer-
Y
Figure 6. Two Raman spectra of carbon tetrachloride Slit width 5 crn-1. Right. Slit width 20 0m-l
L&.
ANALYTICAL CHEMISTRY
1078 row band pass of the alternating current amplifier made it possible to use a photomultiplier of high quality a t room temperature. The alternating current component of the dark current is never observed a t the amplification levels employed. Some selection of the photomultiplier tube may be desirable.
BENZENE 1-SECOND TIME CONSTANT
SPEED 50CM:' PER MINUTE
992 CM:'
SLIT CA. 5CM.l
to within about 2 to 37, can be obtained. This is shown in Figure 8, where the 991-, 1004-, and 1 0 3 0 - ~ m . -lines ~ of toluene were run three times a t approximately 15-minute intervals. The maximum deviation is about 0.25 mv. with a signal of over 8 mv., making an average deviation of about 2%. Weaker lines, of course, have poorer reproducibility, while some stronger lines should reproduce to within *l.O%. Figure 9 shows the spectra for a 50% benzene-50% toluene mixture, where the 992-cm.-l line of benzene is easily resolved from the 1004-cm.-l line of toluene. This shows a possible use of Raman spectroscopy in the analysis of mixtures and the detection of small quantities of one in the presence of another. The grating ghost a t 991 em.-' interferes with a direct quantitative measurement. However, this is easily corrected for by knowing the ratio of the intensity of this line to that of the Rayleigh scattered 43,580 line. The actual correction is then obtained by measuring the intensity a t 43,580 (or O-cm.-l) and applying the aforementioned ratio. Other analytical applications and
TOLUENE CURVES RUN AT 15-MINUTE INTERVALS
FREOUENCV (CM:l)
Figure 7.
-
Two Raman spectra curves of benzene
Left. High amplifier gain Lower amplifier gain
Right.
Actual calculation of this error due to the noise level is rather difficult, inasmuch as the actual ratio of signal to noise is dependent upon the intensity of the Raman line to be studied, and this in turn is dependent upon the slit width used. These factors can best be demonstrated by actual Raman curves. Figure 6 shows the 459-em. -1 line of carbon tetrachloride with a wide (20 cm. -1) spectral slit, and the signal-to-noise ratio for this band is well over 500 to 1; on the other hand, next to it is a curve of the same sample of carbon tetrachloride run under identical conditions, except that a 5-cm.-I spectral slit was used. A much lower signal-to-noise ratio of 100 to 1 is obtained, but the band width is much narrower, thus making possible much greater resolution. In referring to noise for signal-to-noise ratios, the maximum noise or randomness found in the background near the peak is used. Operation of Range Changes. The stronger line of carbon tetrachloride shown in Figure 6 serves to demonstrate the manner in which the range changer works. The peak is on the third scale and has a signal of 45 mv. This is obtained from the range changer going continuously from 0 to 10 mv. on range 1, 10 to 30 mv. on range 2, and 30 to 80 on range 3. There are six ranges, which give a total of 880 mv. Thus it is possible to record signals having ratios of greater than 10,000 to 1 on the same paper a t a given amplification setting. Resolution. The resolution is best shown by a study of the 992-em. -l line of benzene. Using a wide slit of 20 em. -I, signalto-noise ratios can be obtained in the neighborhood of 1000 to 1, but no indication of the line due to the carbon-13 isotrope is noted, By going to very narrow slits of about 5 cm.-', the fine structure of the 992-em. line can be resolved partially (Figure 7 ) . The left-hand curve was made a t a higher amplifier gain, but in both plots the lines at 979 and 984 em.-' are resolved. Reproducibility. If it is recognized that the error in evaluating the true height of a peak is a direct function of signal-to-noise ratio, then the reproducibility has a maximum within about ~ & l . O (limited 7~ by the light source variations), and probably has no real or absolute minimum value. Taking an average line having a signal-to-noise ratio of about 300 to 1, reproducibility
Figure 8. Three Raman spectra curves of toluene run at &minute intervals to show reproducibility 991 om.-' G line i5 a ghost on this instrument and will be background on other spectra in this region shown in this paper
50% BENZENE 50% TOLUENE I-SECOND TIME CONSTANT SPEED 50CM:' PER MINUTE SLIT CA. 5CM.I
W I U E N C Y (CM:l)
-
Figure 9. Raman spectra of 50-50 mixture of benzene and toluene Showing two major lines from these compounds that could be uaed for analysis
V O L U M E 28, NO. 7, J U L Y 1 9 5 6
1079
limitations of the Raman technique have been discussed by Stamm (3). ACKNOWLEDGMENT
(2) Salzman, C. F., Jr., Rev. Sci. Instr. 22, 59 (1953);
U. S . Patent
2,661,260 (Dec. 1, 1953).
(3) Stamm, R. F., IND.ENG.CHEM.,ANAL.ED. 17, 318 (1945). (4) Stamm, R. F., Salsman, C. F., Jr., J . O p t . SOC.Amer. 43, 126
The authors wish t o acknowledge electronic help received from G. E. Gerhardt and C. W. Warren.
(1953). (5) Stamm, R. F., Salzman, C. F., Jr., Mariner, T., Ibid., 43, 119 (1953).
LITERATURE CITED
RECEIVED for review January 20, 1956. Accepted April 14, 1956. Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1954, and Instrument Society of America.
(1) Kemp, J. W., Jones, J. L., Durkee, R. W., J . Opt. SOC.Amer. 42, 111 (1952).
Spectrophotometric Titration of Telluric Acid GUY WILLIAM LEONARD, JR.,
and
ROBERT W. HENRY
Department o f Chemistry, Kansas State College, Manhattan, Kan.
A s no direct, simple, and precise volumetric methods are available for the determination of telluric acid alone or in the presence of other acids, the possibility of a spectrophotometric titration was investigated. The tellurate ion absorbs at a longer wave length in the ultraviolet than does telluric acid. Therefore, the production of the tellurate ion during an acid-base titration can be followed spectrophotometrically. A plot of absorbance vs. milliliters of titrant gives a smooth curve. The linear portions of the curve can be extrapolated to indicate the stoichiometric point of the titration. The spectrophotometric titration serves satisfactorily for the determination of telluric acid alone and in the presence of hydrochloric, sulfuric, nitric, selenic, and acetic acids.
B
ECAUSE telluric acid is a very weak polyprotic acid, the usual acid-base titration with an internal indicator is not feasible. Rosenheim and Weinheber ( 4 )suggested adding neutral glycerol to the telluric acid solution. The glycerol forms a complex with the telluric acid which behaves as a stronger monoprotic acid. This complex can then be titrated with a standard sodium hydroxide solution using phenolphthalein as the internal indicator. However this method gives results ranging from 96.87 to 1037, for a single sample. Another modification also suggested by Rosenheim and Weinheber is the addition of barium chloride to a solution containing the telluric acid sample plus an excess of standard sodium hydroxide. Barium tellurate precipitates and the excess sodium hydroxide is back-titrated with a standard oxalic acid solution. This modification gives results ranging from 98.97 to 100.270. Of the oxidation-reduction techniques, the most widely accepted method is the procedure proposed by Gooch and Howland ( 2 ) . Telluric acid is reduced t o tellurous acid by bromide ion in a strongly acid medium. The resulting bromine is distilled into a solution of potassium iodide. The iodine is then titrated with a standard sodium thiosulfate solution, using starch solution as the internal indicator. Although this method is complicated, good results can be obtained. Scott and Leonard (6) used the absorbance of the tellurate ion in the ultraviolet region for the spectrophotometric determination of telluric acid. Therefore the possibility of spectrophotometrically titrating telluric acid in the presence of other acids with a standard base was investigated. REAGENTS AND EQUIPMENT
Commercial grade tellurium dioxide was purified by dissolving it in a sodium hydroxide solution, filtering, and reprecipitating the tellurium dioxide by the addition of nitric acid.
The purified tellurium dioxide was oxidized with hydrogen peroxide in a dilute solution of sodium hydroxide. The telluric acid was precipitated by the addition of nitric acid, and purified by three recrystallizations from water solutions. Stock solutions were prepared from the purified telluric acid and standardized by the method of Gooch and Howland (2). Stock solutions of the other acids and bases were prepared and standardized. The normality of the ammonium hydroxide solution was checked frequently; no noticeable change was found in its concentration over a month's time. The Beckman Model D U spectrophotometer, equipped with a set of thermospacers, was used with 1-cm. quartz absorption cells for measuring the absorbance of the samples. RECOMMENDED PROCEDURE
Place the sample of the telluric acid solution in a 250-ml. Erlenmeyer flask and add sufficient water to bring the volume to 50 ml. With an eyedropper transfer a portion of this solution t o a quartz cell of 1-cm. light path. With water as a blank, determine the absorbance of the solution. Then return the sample of the acid t o the titrating flask by the same method of transfer. Add a small amount of standardized ammonium hydroxide to the flask, stir the contents thoroughly, and transfer a sample to the quartz cell. After each addition of base, use some of the solution t o rinse the cell, return it to the flask, again stir the contents, and then transfer a sample t o the cell and determine the absorbance. Continue the process for the entire titration. If the volume of titrant added, after the absorbance begins to increase, is greater than 1% of the total volume just before the absorbance increase begins, the absorbance must be multiplied by the dilution factor, ( V v)/V.
+
The corrected absorbances are plotted against milliliters of base added. This type of titration gives a very smooth curve which is easily extrapolated to give the stoichiometric point of the reaction. Klingman Hooker and Banks ( 3 ) have shown recently a modification of the cell compartment of a Beckman Model DU spectrophotometer which would speed up the titration as used in this investigation. The cell compartment should be equipped with a set of thermospacers and maintained a t the temperature of the solution being titrated. EXPERIMENTAL AND DISCUSSION
I n the ultraviolet region of the spectrum, sodium hydroxide and potassium hydroxide solutions, because of the presence of impurities such as carbonate, were found to have greater absorbances than corresponding solutions of ammonium hydroxide. Therefore ammonium hydroxide was selected as the standard solution in this investigation. The change in the absorption spectrum of telluric acid upon the addition of ammonium hydroxide is shown by curves I and I1 in Figure 1, while curves IT and I11 show the change with concentration of tellurate ions. Effect of Wave Length. The effect of wave length upon the ease of detecting the end point was studied. A series of samples, each consisting of 10 ml. of a 0.2M telluric acid solution, was
