Automatic Spectrophotometric Titrations Determination of Milligram Quantities of Thorium H. V. MALMSTADT and E. C. GOHRBANDT Chemistry Department, University o f Illinois, Urbana,
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This investigation was undertaken to develop a spectrophotometric titration procedure in which a plot of absorbance z's. milliliters of titrant is automatically recorded and to apply the automatic spectrophotometric titration technique for the precise, accurate, and rapid determination of very small quantities of thorium. The Cary spectrophotometer was used to oLtain the automatically recorded titration curves, and a new type of titration cell with quartz windows was designed and constructed; 1 to 50 mg. of thorium in 100 ml. of solution were determined with an accuracy within 1 part per thousand. The strong ultraviolet absorption band of copper-Versenate was used to determine the end point.
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XCELLENT spectrophotometric titration procedures have recently been reported by Sweetser and Bricker (11, 12), who used the Beckman Models DU and B spectrophotometers to perform manual spectrophotometric titrations in both the visible and ultraviolet spectral regions. Their procedure was to add titrant to a specially constructed titration vessel containing the reactant, mix the solution, and then read absorbance values after each increment of titrant. A plot of absorbance us. milliliters of titrant was made; straight lines were drawn through the resulting points, and the lines were extrapolated to the end point. Several other tvorkers have employed photometric titration procedures to increase the precision of end-point determination.. 4 review of this work is given by Osburn, Elliot, and Martin ( 8 ) , and references to subsequent works may be found in the bibliography of a paper by Goddu and Hume ( 3 ) . I n spectrophotometric titrations greater precision and accuracy are the main advantages, compared to the direct measurement of absorbance. Precision is not determined by the measurement of a single absorbance value but rather by the measurement of volume of titrant which is determined by the intersection of two lines, each of which is the average of several points. If a titrant is available which will react rapidly and stoichiometrically with the reactant and if a good buret is employed, accuracies of the order of 0.1% are obtainable. Small amounts of impurities in the sample may cause large errors in the ordinary colorimetric or spectrophotometric procedures, if these impurities have absorption bands coincident with those of the substance being analyzed. If this type of impurity did not react with the titrant, it would only cause a shift of the spectrophotometric titration curve along the absorbance axis and would have no effect on the precision of end point determination. Muller and Partridge (7) have described an apparatus suitable for automatic photometric titrations. This automatic system, however, requires an indicator which has an appreciable color change a t the equivalence point. Spectrophotometric titrations, in which absorbance is plotted against milliliters of titrant, do not require large absorbance changes a t the end point. Extremely dilute solutions of reactant and titrant usually exhibit only slight changes of absorbance a t the end point. A plot of abssrbance against milliliters of titrant, therefore, should be especially applicable to micro and semimicrotitrations. It is possible to use the change in absorbance of the titrant, the reactant, the reaction product, or an added indicator substance for the spectrophotometric end-point determinations.
The ude of the ultraviolet spectral region also makes it possible to obtain high sensitivity and hence accurate determinations of very small quantities of substances. Evidence of the high sensitivity of spectroph3tonetric titrations in the ultraviolet ia given by the work of Sweetser and Bricker ( 1 2 ) and by the thorium procedure described herein. The manual spectrophotometric titrations performed by Sweetser and Bricker (12) are somewhat time-consuming. It was considered desirable to perform this same type of titration automatically. Since a Cary recording spectrophotometer can plot absorbance against time automatically a t a constant wave length, it i4 only necessary to have a suitable titration cell, a continuous stirrer, and a constant-flow buret to add the titrant. Thus it is possible to perform automatic titrations giving a recorded curve of absorbance against milliliters of titrant. RIost previous volumetric methods for thorium have been either inaccurate or rather involved. Recently, Blaedel and Malmstadt (1 ) described an accurate indirect volumetric method using high frequency titrimetry, but this method requires an apparatus which is not generally available. Wore recently Fritz and Ford ( 2 )have accurately titrated small quantities of thorium ( 6 to 50 mg. in 15 to 20 ml. of solution) with disodium dihydrogen Versenate (ethylenediamine tetraacetate) using alizarin Red S as indicator. Neither of these methods gives as precise and accurate results for extremely small quantities of thorium (1 to 10 mg. in 100 to 150 ml. of solution) as are possible by the automatic spectrophotometric titration procedure. APP 4R 4TUS
A Cary recording spectrophotometer, capable of plotting absorbance against wave length or absorbance against time a t a constant wave length, was used for all titrations. The only modifications on the spectrophotometer were a new cover for the titration cell compartment and a V-shaped notch on the cell
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Figure 1. Cross-Sectional Views through Light Path of Titration Cell
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V O L U M E 2 6 , N O . 3, M A R C H 1 9 5 4 position shaft. This notch was cut exactly between the normal two notches on the shaft. A new type of titration cell suitable for automatic titrations in the ultraviolet and visible spectral regions was constructed and a constant flow buret was used for the addition of the titrant. Titration Vessel. Figure 1 s h o w a top and side cross-sectional view through the light path of the titration cell. The cell war made from a polystyrene rod 8 cm. in diameter and 10 cm. in length. .k hole 4.80 cm. in diameter was bored vertically into the center of the rod to within 1.00 em. from the bottom. A hole 1.90 cm. in diameter was cut horizontally to the bottom through both sides of the resulting vessel; the center of this hole ]vas 2.95 em. from the bottom of the vessel. Two holes of larger diameter (3.0 cni.) were cut into the vessel walls, A , concentric to the 1.90-em. hole; these holes aere not cut completely through the walls, but just far enough to leave a 0.1-em. shoulder, D,as shown in the side cross-sectional viem-. Cylindrical metal ring inserts, B , were screwed into the vessel walls, A . The metal inserts, B , were threaded on the inside wall and had an inqide diameter of 2.38 em.
443 with a small paddle on one end fitted into the end of the sleeve extending below the cover and was held in position by a setscrew. A metal rod \vas attached to the other sleeve and connected to the shaft of a stirring motor.
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Figure 3.
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40 35 ML OF 001233 M
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Automatic Spectrophotometric Titration of Versenate with Standard Copper
This arrangement made it unnecessary to adjust the stirrer height before each titration. The stirrer always extended into the solution to within about 1 em. of the top of the quartz windows. The metal sleeves served to keep the stirrer in nearly a constant position. This arrangement also prevented any light from entering the sample compartment. Constant Flow Buret. .in automatic syringe microburet, basically the same as that described by Lingane (6), was used for all titrations. Modifications in the buret drive assembly have been made by Ziegler ( 4 ) . The tip of the buret was positioned as closely to the stirrer as possible to ensure rapid mixing of the titrant a i t h the solution in the titration vessel. The buret delivered titrant a t approuimately 0.6 ml. per minute. SOLUTIONS
Figure 2. Ultraviolet Absorption Spectra 4. B. C.
5 X 10-4M solution of copper-Versenate complex 0.01M solution of disodium dihydrogen Versenate 0.01M solution of copper nitrate
-1paper-thin Teflon gasket ring, 2.3 cm. in outer diameter and 1.9 cm. in inner diameter, and a standard quartz spectrophotometric cylindrical window, 2.30 em. in diameter and 0.3 em. thick, were seated in each side of the two wells against the shoulder, D. A rubber gasket ring, inside diameter 1.9 cni., was next seated in each well and the combination of Teflon gasket, quartz window, and rubber gasket was held firmly by screwing brass inserts C into brass inserts B . A rectangular base platform was made from polystyrene and was screwed onto the bottom of the cell. The bottom of the base, E , was notched so that when the Cary cell holder was removed the base of the titration vessel ivould fit snugly into position in the platform pegs of the Cary sample cell compartment. The light path through the solution of the titration vessel was 5.00 cm. A 5.00-em. Bausch & Lomb absorption cell was used as the solvent cell in the standard cell compartment. The outside of the polystyrene vessel was painted flat black to prevent reflection of stray light into the photocell compartment. Cover for Sample Cell Compartment. The cover n-as identical with the standard cover on the Cary spectrophotometer except for two holes drilled near the center. These two holes \!-ere fitted tightly Tith rubber stoppers, one bored to permit passage of a stirrer and the other bored to permit the passage of the buret capillary into the titration vessel. Stirrer. A brass rod, 3 inches long and 1/4 inch in diameter, lubricated with a light coating of oil, mas attached to one rubber stopper in the cover of the sample cell compartment. .I brass sleeve was fitted snugly onto each end of the brass rod. Each sleeve was held firmly to the rod with a setscrew. ,4 glass rod
Standard Copper Solution (0.01233-TI). An accurately weighed sample of electrolytic copper (0.7836 gram) was dissolved in approximately 6 ml. of concentrated nitric acid and diluted to esactly 1 liter in a volumetric flask. Standard Thorium Solution (0.00967.1I). A thorium nitrate solution (about 0.01M) was prepared by dissolving Baker’s reagent grade thorium nitrate in 1 liter of distilled water containing 2 to 3 ml. of concentrated nitric acid to prevent hydrolysis. The thorium was standardized gravimetrically by precipitating it as the oualate and igniting to the oxide for weighing (10). Sodium Acetate Buffer. .I solution was prepared which was about 0.2M in sodium acetate and 0.2M in acetic acid. Standard Disodium Dihydrogen Ethylenediaminetetraacetate (0.00962M). .ibout 3.7 grams of reagent grade disodium dihydrogen Versenate dihydrate (Hach) was dissolved in 1 liter of distilled water. The solution was standardized against standard copper solution using the spectrophotometric end-point procedure. Saeetser and Bricker (fd) have shown that precise and accurate results can be obtained in the titration of Versenate with copper by using this method. These authors used the visible absorption band a t 745 mp. For the standardization of the dilute Versenate solutions i t was necessary to use the strong ultraviolet absorption band of the copper-Versenate complex (Figure 2). SPECTROPHOTOMETRIC ESD-POINT DETER\IINATIONS
The spectrophotometric titration of dilute Versenate qolution n ith standard copper solution, or the reverse reaction, depends
on the formation of the stable copper-I‘ersenate complex and the *trong ultraviolet absorption of this complex Plumb et al. (9) have reported the visible absorption rpectra of the copperVersenate complex. Figure 2, curve A , shoiis the ultraviolet absorption band of a dilute (approuimately 5 X 10-4M) solution of the copper-Versenate complex obtained on the recording spectrophotometer with 1-em. quartz cells. Curves B and C show the abrorption bands of 0.01M Versenate and copper nitrate solutions, respectively. The absorption of light between about 260 and 350 m,u is small
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ANALYTICAL CHEMISTRY
when only copper Versenate is present, but the absorption increases greatly for small concentrations of copper-Versenate complex. Figure 3 is a trace of an automatically recorded spectrophotometric titration curve of an aliquot of Versenate solution with standard copper solution a t a wave length of 320 mp. In the titration, 5.00 ml. of 0.00962M Versenate were diluted with about 100 ml. of distilled water, buffered with 1 ml. of sodium acetateacetic acid buffer ( p H 4.2), and titrated with 0.01233M copper nitrate solution. This curve shows that the absorbance of the copper-Versenate complex increases linearly with concentration. As soon as all of the copper is complexed, the absorbance of the solution remains essentially constant. At this point a sharp break appears in the curve of absorbance vs. titrant. The small amount of “noise” on the curve indicates that mixing is almost instantaneous and also indicates the feasibility of the automatic procedure. The spectrophotometric titration of small quantities of thorium with Versenate solution depends on the formation of the stable thorium-Versenate complex. The thorium-Versenate complex has a negligible absorbance in the ultraviolet down to about 250 mp. This is shown in Figure 4, curve A , for a 5 X 10-4Jf solution of the complex in a 1-rm. cell as recorded on the qpertiophotometers.
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Figure 4. Ultraviolet Absorption Spectra
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Figure 5.
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Direct Titration of Thorium w-ith Standard Versenate
nate is added to the thorium Polution and titrated with standard copper solution. Direct Titration of Thorium with Versenate. Figure 5 shoas an automatic spectrophotometric direct titration curve of thorium with standard Versenate in the presence of copper as indicator: 5 00 ml. of 0.00967M thorium nitrate plus 1.00 ml. of 0.01233M vopper nitrate were diluted to 100 ml. and titrated with 0.0096231 versenate solution a t p H 3.1 and 290 mp wave length. As the Versenate is added, the absorbance remains small until nearly all of the thorium is complexed. Then the titrant combines with copper to form the copper-Versenate complex, and the absorbance increases linearly with concentration of the complex. The results obtained by the direct procedure are fairly good (within 5 parts per thousand) for between 20 to 70 mg. of thorium in 100 ml of solution. However, for smaller amounts of thorium the errors are as high as 1 or 2%. This error is partly due to the low pH a t which the titrations are performed. It is necessary to maintain the p H belolv 3.5, so that thorium oxide does not precipitate. The errors are not apparently the result of slow equilibrium in formation of the complex because the same results are obtained when long periods (30 to 40 minutes) are allowed for equilibrium to be reached. The direct procedure fakes longer to perform than the indirect procedure-when using the constantflow svringe microburet, the direct titration requires about 20 minutes for a 10-ml. delivery. However, the greatest advantage of thr indirect method is the high precision and accuracy, even for 1 to 10 mg. in 100 ml. of solution. Indirect Titration of Thorium. In the indirect method a known excess of Versenate solution is added to the thorium solution, the solution is diluted to about 100 to 150 ml. and buffered, a suitable wave length is selected (usually 290 or 320 mp), and the excess Versenate is titrated with standard copper solution. A simple method of determining when a small excess of Versenate is added is described.
A . 0.01M thorium nitrate solution B . 5 X 10-4M thorium-Versenate complex
Curve 4,B shows the absorption curve for a 0.01X thorium nitrate solution. The thorium-Versenate complex is more stable than the copper-Versenate complex. This is readily shown by mixing equal concentrations of copper nitrate, Versenate, and thorium nitrate solution and measuring the absorbance of the solution a t approximately 290 mp. The absorbance is nearly zero, indicating that the thorium-Versenate complex is formed in preference to the copper-Versenate complex. These facts make i t possible to determine thorium either by direct or indirect titration procedures, using Versenate to complex the thorium and the absorption of the copper-Versenate complex as end-point indicator. In the direct titration procedure the thorium is titrated with standard Versenate solution in the presence of a small amount of copper. In the indirect procedure an excess of standard Verse-
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Figure 6. Indirect Titration of Excess Versenate with Standard Copper
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V O L U M E 26, N O . 3, M A R C H 1 9 5 4 Table I.
Indirect Thorium Titrationsa
Thorium, Mg. Taken Found 44.89 22.44 22.44 11.22 11.22 11.22 2.244 1,122
44.95 22.43 22.43 11.23 11.22 11.23 2.242 1.122
Error
hIg. 0.06 -0.01 -0.01
0.01 0.00 0.01 -0.002 0.000
% 0.15 -0.05 -0.05 0.10 0.00
0.10
-0.10 0.00
a A measured excess of 0.009962.11 or 0.00096?M Versenate was added to aliquot8 of 0.00967M or 0.000967.M thorium solutions. The excess Versenate was automatically titrated with 0 01233.V copper solution. The wave length was 290 or 320 mp depending o n whether a relatively small or large excess of Versenate was added.
Figure 6 shows the automatic spectrophotometric titration curve of a small excess of Versenate with standard copper solution a t a wave length of 290 mp. I n the titration, 11.00 ml. of 0 . 0 0 9 6 2 i ~Versenate were added to 10.00 ml. of 0.00967M thorium nitrate. The solution was diluted to about 100 to 150 ml. in the titration cell buffered with 1 ml. of sodium acetateacetic acid buffer, and the excess Versenate automatically titrated with 0.01233M copper solution. This curve shows a continual rise in absorbance after the end point, caused by the absorbance of copper acetate a t this wave length. This rise is not found a t somewhat higher wave lengths (Figure 3, wave length 320 mp). Experimental data for indirect thorium titrations are given in Table I. The results are precise and accurate to within 1 part per thousand PROCEDURE FOR INDIRECT TITRATION
An aliquot of 0.00967M thorium is pipetted into the titration cell, and a known excess of standard disodium dihydrogen Versenate is added. One milliliter of sodium acetate-acetic acid buffer is added and the solution diluted to about 100 to 150 ml. in the titration cell. The titration cell is set in the sample compartment of the spectrophotometer, and the cover, stirrer, and buret are moved into position. The spectrophotometer is balanced to zero absorbance against distilled water in a 5.00-em. cell. The buret motor and chart are turned on simultaneously. The time required to reach the end point is determined from the recorded titration curve. The time necessary to reach the end point is easily converted to milliliters, because the syringe buret is calibrated in milliliters per second. T o determine when a small excess of reagent has been added, a small known amount (about 1.00 ml.) of O.OlI1.f copper solution is added to the unknown thorium solution. The solution is diluted to about 100 ml., the titration cell is set in the sample cell compartment of the spectrophotometer, and the cover and stirrer are moved into position. The 11-ave length is set a t 320 mp. The tip of a suitable stopcock buret is inserted in the hole in place of the tip from the syringe microburet. The standard Versenate solution is added from this buret, while stirring, to the unknown thorium solution containing the known amount of copper. The addition of Versenate is stopped
when the absorbance of the copper-Versenate complex is indicated. Approximately 0.5-ml. additions of Versenate are then continued until the absorbance does not increase after a n addition, indicating that all the copper is the form of copper-Versenate complex and all the thorium is in the form of thoriumVersenate complex. The quantity of Versenate is determined from the milliliters added from the stopcock buret. The buret is removed, 1 ml. of sodium acetate-acetic acid buffer is added, and the excess Versenate is accurately determined by automatic spectrophotometric titration with standard copper solution delivered from the syringe microburet. It is, of course, necessary to add the millimoles of copper originally added to the thorium solution to the millimoles of copper required in the back titration before subtracting from the millimoles of Versenate added to the solution: Total ml. of standard Cu = ml. of Cu originally added of Cu used in titration
+ ml.
Mg. of T h = 232.1 (ml. of Versenate X molarity of Versenate total ml. of Cu X molarity of Cu)
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INTERFERENCES
According to Fritz and Ford ( 2 ) no detectable interferences were caused by the following ions: K f , Na+, Li+, B a + +, Mg + +, Sr+-, Mn++, Co++, Cd++, Zn++, AI++1 Ag+9 Cr+++, La+++, and TrOZ++. S o work by the authors has been done in this direction. As the conditions used in this procedure were similar to those reported by Fritz and Ford, it is believed that practically no interference will be caused by these ions. Unknown concentrations of the following ions interfere: Pb++, Cu++, Xi++, B i t + + , Fe+++, ZrO++, Sn++, SII+~,and Ce+++. Levine and Grimaldi ( 6 ) have shown that a single extraction with mesityl oxide separates thorium from all metals except zirconium, uranium, and vanadium. A Versene titration, therefore, results in an almost specific analytical method for thorium. LITERATURE CITED
Blaedel, W. J., and M a l m s t a d t , H. V., ANAL. CHEM.,2 3 , 471 (1951).
Fritz, J. S., a n d Ford, J. J., Ibid., 2 5 , 1640 (1953). Goddu, R. F., a n d Hume, D. N., Ibid., 22, 1314 (1950). Laitinen, H. A., a n d Ziegler, W., University of Illinois, Ph.D. thesis, 1952. Levine, H., a n d Grimaldi, F. S., U. S. Atomic E n e r g y Commission, AECD-3186 (1950). Lingane, J . J., AKAL.CHEM.,20, 285 (1948). RIuller, R. H., a n d Partridge, H. RI., I n d . Eng. Chem., 2 0 , 423 (1928).
IND. ENQ. CHEY.,ANAL.ED.,1 5 , 6 4 2 (1943). P l u m b , R. C., Hartell, d. E., a n d Bersworth, F. C., J . Phys. Colloid Chern., 5 4 , 1 2 0 8 (1950). Scott, W. W,, “Standard l l e t h o d s of Chemical .%nalysis,” pp. 94G-53, New I’ork. D. Van Nostrand Co., 1939. Sweetser, P. B., a n d Bricker, C. E., Aix.4~.CHEM., 2 4 , 409 Osburn, R. H., Elliot, J. H., a n d M a r t i n , A. F.,
(1952).
I b i d . , 2 5 , 253 (1953). RECEIVED for review June 13, 1953. Accepted Sovember 24, 1953.
Spectrophotometric Determination of Telluric Acid LAWRENCE W. SCOTT’ and GUY WILLIAM LEONARD, JR. Department of Chemistry, Kansas State College, Manhattan, Kan.
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ELLURIC acid has been determined by various methods including refractometric analysis, acid-base titration, and oxidation-reduction methods (a, 4, 6, 7 ) . These methods are involved and time-consuming, and require careful control of the experimental conditions. Since telluric acid solutions absorb in the ultraviolet, the possibility of a spectrophotometric determination was investigated. 1 Present address, Department of Chemistry, Oregon State College, Corvallis, Ore.
REAGENTS AND EQUIPMENT
The telluric acid for this investigation was prepared by the method of Horner and Leonard ( 3 ) and purified by repeated recrystallization from water. The other reagents were reagent grade. The Beckman Model DU spectrophotometer, equipped with a set of thermospacers, vias used with 1-em. silica absorption cells for measuring the absorbancy of the samples. The Beckman thermospacers and lamp housing cooling coils were connected
