Spectrophotometric Determination of Cesium Using 12-MoIybdophosphoric Ac id Florence Huey and L. G. Hargis Department of Chemistry, Louisiana State Unicersity, New Orleans, La. 70122 VERYFEW spectrophotometric methods for the determination of cesium have been published. Since cesium is commonly encountered only in small amounts, a spectrophotometric approach is especially :appropriate as a means of determination. Because of the close similarity in the chemical behavior of potassium, rubidium, and cesium, nearly all of the proposed spectrophotometric methods for cesium are somewhat applicable to potassium arid rubidium (1); consequently, serious interferences are encountered when these ions are present. Burkser and Feldman (2) used 12-molybdosilicic acid to determine cesium. The cesium precipitate was separated, dissolved, and reduced to a heteropoly blue for colorimetric measurement. The quantitative effects of many solution variables were not given. Krochta and Mellon (3) described an indirect method for cesium by precipitating the cesium with excess 12-tungstosilicic acid and subsequent development of a heteropoly blue from the excess precipitant. Although Krochta and Mellon made a thorough study and evaluation of the solution variables, their method suffers several disadvantages. Their indirect procedure is of relatively low sensitivity. In addition, 12-tungstosilicic acid solutions are not stable for long periods of time and their exact concentration cannot easily be determilied. Since it is this excess precipitant that is actually measured, it becomes necessary to prepare fresh solutions and a new calibration curve at frequent intervals. Furthermore, the: titanium trichloride reductant used, due to its instability, was not easy to work with and had to be prepared fresh every fl3w days. We have developed a rapid spectrophotometric method for the determination of small amounts of cesium which can tolerate the presence of at least small amounts of potassium and rubidium. The method is based on the precipitation of cesium 12-molybdophosphate which was separated by centrifuging and simultaneously dissolved and reduced to a heteropoly blue or simply dissolved in a basic buffer to liberate molybdate ions, which absorb in the ultraviolet. EXPERIMENTAL
Apparatus. All spectrophotometric measurements were made in matched 1.000-cm silica cells with a Beckman DU-2 spectrophotometer. Reagents. All solutions were prepared from reagent grade chemicals and distilled water. The 12-molybdophosphoric acid, H3P (M03C110)4.xHzO, (Mallinckrodt, analytical reagent) was filtered after dissolution to remove small amounts of insoluble molybdenum trioxide. This solution was prepared fresh each week. Procedure. PRECIPITATION : By means appropriate to the cesium sample prepare a solution containing 0.05 to 1.20 mg
(1) L. G. Bassett and W. Iiyerly, “Sodium, Potassium, Rubidium, and Cesium,” in “Analytical Chemistry of the Manhattan Project,” C. J. Rodden, ed., McGraw-Hill, New York, 1950, pp. 345-8. (2) E. S. Burkser and R. V. Feldman, Zacodskaya Lab., 7, 166 ( 1938). (3) W. G. Krochta and Rd. G. Mellon, ANAL. CHEM., 29, 1181 (1957).
of cesium in 5-10 ml of solution. Transfer the sample to a 50-1111 centrifuge tube, add 7 ml of 6N perchloric acid solution and dilute to 16 ml with distilled water. Add 5 ml of the 12-molybdophosphoric acid solution ( 1 4 z w/v), mix, and let the solution stand for 15 minutes. HETEROPOLY BLUE METHOD. Centrifuge the precipitate and wash once with about 25 ml of a 1.2N perchloric acid solution containing 2.5 (w/v) sodium molybdate. Mix the precipitate with 10 ml of 1.2N perchloric acid, add 5 ml of 1.2N perchloric acid containing 2 . 5 z molybdate, and finally 10 ml of hydrazine sulfate solution (0.6 mgiml). Mix well and heat in a water bath at 100” C for 15 minutes. Cool to room temperature, transfer the solution to a 50-ml volumetric flask (rinsing with 1.2N HClOa solution), and dilute to the mark with 1.2N perchloric acid. Measure the absorbance in 1.000-cm cells a t 805 mp against a reagent blank solution. The amount of cesium can be determined by reference to a calibration plot of absorbance us. concentration cesium, prepared by following this procedure using aliquots of a standard cesium solution. MOLYBDATEMETHOD. Centrifuge the precipitate and wash once with about 25 ml of a 1.2N perchloric acid solution (containing no sodium molybdate reagent). Mix the precipitate with about 30 ml of a borate buffer (pH 9) and allow 5 minutes for complete dissolution. Transfer to a 100-ml volumetric flask and dilute to the mark with the buffer solution. Measure the absorbance in 1.000-cm silica cells a t 226 or 208 mp against the buffer solution as a reference. The concentration of cesium can be determined by reference to a calibration graph of absorbance us. concentration cesium, prepared by following this procedure using aliquots of a standard Cesium solution.
z
RESULTS
As the acidity of the solution increases, the rate of precipitation of cesium 12-molybdophosphate increases and its solubility decreases. The effect of solution acidity on the precipitation, based on the absorbance of the heteropoly blueobtained after separation and reduction of the precipitate, was evaluated. Solutions with final acidities which varied over the range of 0.14 to 2.86N HC104 were studied. Solutions with acidities of 1.4 t o 2.9N HC104 gave virtually identical results and 2.ON was considered satisfactory for further work. A substantial amount of molybdophosphoric acid was required to effect a quantitative precipitation. No less than the equivalent of 5 ml of a 10 solution should be used, At this concentration of precipitant, a digestion time of 15 minutes is sufficient to ensure complete precipitation. It was found best to wash the precipitate with an acidic solution (preferably 2.0N) to eliminate loss of precipitate due to peptization and increased solubility. Two successive washings with 25-ml solutions of 2.ON perchloric acid gave identical results as obtained with one 25-ml washing, At this point in the procedure, two different methods were developed. The first involves a reduction (and accompanying dissolution) of the precipitate to a heteropoly blue and subsequent measurement in the NIR at the 805-mp absorbance maximum. The absorption spectrum obtained for a typical VOL. 39, NO. 1 , JANUARY 1967
125
Ion
Added as
Ag+
AgNOa CaClz CdCls NaCl CUClZ NaF FeCh
Ca2+ Cd2+ c1-
cu2+ FFe 3+ K+
KC1
Nos-
LiCl MgClz NaCl NHiCl NaN03
Rb+
RbCl
Li+ Mg2’
Na+ NH4+
so42-
Zn2+
NasSOa ZnCL
Table 1. Effect of Diverse Ions Heteropoly blue method Amount % R.E. Tolerance Amount added, mg 805 mp mg added, mg 25 3.8 20 25 2.6 125 >125 125 125 -4.2 90 125 125 >125 0.8 125 -4.2 125 90 125 0 125 >125 125 0 125 >125 125 25 3.8 20 25 1.1 >125 125 125 125 4.4 85 125 125 0.8 >125 125 2.5 25 30 25 -2.5 125 >125 125 3.8 0.5 0.4 0.5 -2.8 125 >125 125 -1.9 125 >125 125
determination is that of a standard heteropoly blue ( 4 ) . Although there are numerous reductants that can be used to produce heteropoly blues, hydrazine is reportedly among the best few (5, 6) and was found entirely suitable for this study. It is generally known that in order to obtain maximum color formation for a heteropoly blue, often some excess molybdate must be present (7). The reason for this is not fully understood, Reduction was accomplished by mixing the precipitate with about 10-15 ml of 1.2N perchloric acid solution and 10 ml of reducing agent (0.6 mg hydrazine sulfate, N2H4‘ H2S04,/ml prepared in 1.2N HC10,). The color development is slow at room temperature and should be carried out in a water bath at 100 C. The presence of molybdate during reduction is necessary for the production of a satisfactory heteropoly blue but too much molybdate may result in formation of some “molybdenum blue” (a reduced molybdic acid, blue in color) which will cause serious interferences. The amount of molybdate that can be tolerated depends somewhat upon the acidity: the more acidic the solution, the less tendency to form molybdenum blue. Ideally a 3-dimensional diagram of molybdate concentration, acidity, and absorbance should be prepared in order to find the optimum conditions. Experience has shown that the best acidity for reduction of 12-molybdophosphoric acid is around 1.ON(4,8). The amount of molybdate necessary to give maximum absorbance was from 4.0 to 8.0 ml of a 2.5 sodium molybdate solution added to the precipitate prior to reduction. Lesser amounts of molybdate resulted in decreased absorbances while larger amounts resulted in the formation of “molybdenum blue” in the blanks. To determine the most desirable acidity for reduction, the acidity range from 0.7N to 3.ON was studied. Below 0.7N some molybdenum blue was formed in the blanks. The absorbance was virtually constant in the range of 0.9 to 1.5N in perchloric acid, decreasing somewhat above 1.5N in HC104. On the basis of this data an acidity of 1.2N was chosen as the optimum acidity. The amount of hydrazine sulfate reducing agent had no effect O
(4) L. G. Hargis and D. F. Boltz, ANAL.CHEM., 37,240 (1965). (5) L. Duval, Chirn. Anal., 45, 237 (1963). (6) R. P. A. Sims, Analyst, 86, 584 (1961). (7) I. Berenblum and E. Chain, Biochem. J., 32,286 (1938). (8) F. D. Snell and C. T. Snell, “Colorimetric Methods of Analysis,” Vol. IV, Van Nostrand, New York, 1954. p. 482. 126 *
ANALYTICAL CHEMISTRY
Ultraviolet molybdate method % R.E. % R.E. Tolerance 226 mp 208 mp mg 5.4 6.9 15 2.8 3.1 >125 -1.3 -1.5 >125 -0.1 0 >125 5.8 6.4 65 -8.0 -7.4 45 4.8
5.1
3.6
80
3.9 -0.6 4.4
20 >125 95 >125 25 >125
-0.8
4.0 -0.1
0 3.0 - 1.o 4.1
3.0
-1.5 3.4 -0.8
-0.6 1.6
1.1
0.5
>125 >125
on the final absorbance so long as at least a 12-fold excess of reductant was used. As stated previously, the formation of the heteropoly blue was slow at room temperature and therefore was carried out in a water bath at 100” C. Maximum color is produced within 10 to 15 minutes. Prolonged heating should be avoided as this resulted in decreased absorbances. The final colored solutions were stable for a t least 12 hours when the recommended procedure was used. A study was made to determine the amounts of various ions which could be present without interfering with the determination of 0.25 mg of cesium. Errors less than + 3 were considered negligible. Table I summarizes the results of this study. Conformity to Beer’s Law for the overall process was observed for solutions corresponding to 0.02 to 1.20 mg cesium (based on final volume of 50.00 ml). The optimum concentration range, based on a Ringbom plot was 0.06 to 2.00 mg of cesium at 805 mp. This second method utilizes the same precipitation procedure developed for the heteropoly blue method. It differs in that the precipitate is washed once with 25 ml of a 1.2N perchloric acid solution containing no molybdate reagent, Then the precipitate is dissolved and decomposed in a basic buffer solution to yield the simple ions-ie., molybdate, phosphate, cesium. The amount of cesium can be determined indirectly by measuring the ultraviolet absorbance of the molybdate ions ( 4 ) . Measurements can be made at either a 226-mp shoulder or a 208-mp peak. Although the absorbance at 208 mp is larger (almost twice as large), absorption in the blank and decreased photometric accuracy at this shorter wavelength make it a less sensitive wavelength for measurement. It has been shown that a borate buffer is the most suitable for dissolution and decomposition of the precipitate because of its low wavelength cut-off ( 4 ) . Borate buffers of pH 8, 9, and 10 gave identical results and the pH 9 buffer was chosen for use. The precipitate dissolves completely in this buffer in less than 5 minutes. The final solutions were stable for at least 24 hours when the recommended procedure was used. A study was made to determine the amounts of various ions which can be present without interfering with the determination of 0.25 mg of cesium. Errors less than + 3 z were considered negligible. Table I summarizes the results of this study. A careful study was made of the interference effects of the alkali metal and ammonium ions, as these constituents are commonly found in the presence of cesium and usually inter-
z
fere with its determination. Lithium and sodium d o not interfere; however, ammonium, potassium, and rubidium must not be present in excess of the amounts specified, as they will coprecipitate with the cesium. Reducing agents will turn the 12-molybdophosphoric acid blue prior t o precipitation but if not present in large amounts they generally d o not interfere in the precipitation since a large excess of precipitant is used. Interference of some of the transition metals in the ultraviolet method was probably the result of coprecipitation of the mctal resulting in its contribution t o the ultraviolet absorption. Conformity t o Beer’s law for the overall ultraviolet procms was observed for solutions corresponding to 0.05 to 1.413 mg of cesium (based on final volume of 100.0 ml). The optiinum concentration ranges, based on Ringbom plots were 0.1 ‘io0.7 mg of cesium at 226 mp and 0.1 to 0.5 mg cesium at 208 nip. 1)ISCUSSION
An indication of the precision of thismethod wasascertained from the results of 12 samples, run according to the recommended procedure, each containing 0.25 mg of cesium. For the heteropoly blue procedure the mean absorbance value was 0.522 at 805 mp; i.he range was 0.514 t o 0.534. The standard deviation was 0.0082 giving a relative standard deviation of 1.6%. About 50 minutes were required for a series of four determinations. For the molybdate procedure the mean absorbance values
were 0.584 and 1.127 a t 226 and 208 mp, respectively. The range was 0.573 to 0.596 a t 226mp and 1.075 to 1.166 at 208 mp. The standard deviations were 0.0081 and 0.0330 at 226 and 208 mp, respectively, giving corresponding relative standard deviations of 1.4 and 2.9 %. About 30 minutes were required for a series of four determinations. The composition of the precipitate was determined by finding the amount of molybdate resulting from the decomposition of a precipitate and comparing it with the amount of cesium taken initially. A standard calibration curve was prepared using the absorbances from known amounts of molybdate dissolved in the borate buffer. The amount of molybdate in a precipitate was determined by comparing its resulting ultraviolet absorbance with the standard calibration curve. Molar ratios of molybdate t o cesium of 5.8 to 1 and 5.9 to 1 were obtained when the absorbances were measured at 226 mp. These values correspond closely to a forfor~ ~the ) ~ precipitate (theoretical mula of C S ~ H P ( M O ~ O Mo/Cs = 6.0/1). This formula corresponds to that obtained for the thallium (4) and ammonium ( 9 ) salts of 12-molybdophosphoric acid prepared under similar conditions. RECEIVED for review August 18, 1966. Accepted November 7, 1966. Work supported by a grant from the National Science Foundation for Undergraduate research participation. (9) W. W. Wendlandt, Anal. Chim. Acta, 20, 267 (1959).
Determination of Lanthanide Distribution in Rocks by Neutron Activation and Direct Gamma Counting James C. Cobb Department of’ Chernistrj, Brookhacen National Laboratory, Upton, N . Y .
THELANTHANIDES are a group of elements which are of great geochemical interest because chemically they are so similar, that any relative variations among them, which are thought to be due principally to differences in the trivalent ionic radius, should be indicative of the evolution of various rock types. The separation of the lanthanides is a formidable analytical problem, and until recently, little work has been done. In recent years, the technic ue of neutron activation analysis, coupled with separations by ion exchange chromatography, has been applied to varims rocks by several groups (1-3). Russian geochemists have also been active in this field. Their analytical technique involves a group separation of the rare earths and analysis hy x-ray spectrography (4, 5 ) . It is apparent from the published work that relative variations among the lanthanides are of much more interest than absolute abundances. When plotted against the trivalent ion radius, the relative abundances invariably follow a smooth trend. Therefore, it is not necessary to perform analyses on (1) L. Haskin and M. A. Grhl, J . Geopliys. Res., 67, 2537 (1962). (2) R. A. Schmitt, R. H. Smith, J. E. Lasch, A. W. Mosen, D. A. Olehy, and J. Vasilevskis, Geocliim. Cosmocliim. Acta, 27, 577 (1963). (3) D. G. Towell, J. W. Winchester, and R. V. Spirn, J . Geophys. Res., 70, 3485 (1965). (4) Y . A. Balashov, A. B. Ronov, A. A. Migdisov, and N. V. Turanskaya, Geochemistry (English trans.), 1964, p. 995. (5) L. K. Gavrilova and N. \’. Turanskaya, Zbid.,1958,p. 163.
11973
all fourteen lanthanides in order to establish the variations in any individual rock. It is the purpose of this paper to present a method of analysis of six or seven lanthanide elements in various rock types by neutron activation analysis, followed by direct gamma counting of the sample, without any chemical separations, on high resolution lithium-drifted germanium semiconductor detectors. Some of the properties and uses of Ge(Li), detectors with particular reference to nuclear spectroscopy, are described by Ewan and Tavendale (6). Additional descriptions and applications t o neutron activation analysis of impurities in aluminum foil are given by Prussin, Harris, and Hollander (7). The resolution achieved by these detectors is more than an order of magnitude better than the conventionally used Na(T1) scintillation detector. The increased resolution makes it possible to identify gamma photopeaks even though they are close together in energy. The number of elements which can be measured by a combination of neutron activation analysis and direct gamma counting vary with rock type, but gamma photopeaks due to the lanthanide elements are invariably present. The lanthanides which can be measured represent a (6) G. T. Ewan and A. J. Tavendale, Can. J . Pllys., 42, 2286 (1964). (7j S . G. Prussin, J. A. Harris, and J . M. Hollander, ANAL.CHEM., 37, 1127 (1965). VOL. 39, NO. 1 , JANUARY 1967
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