NMR and titration (correlation coefficient = 0.998) with most of the comparative analyses differing by 2%or less. As revealed in Figure 4, when deviations occurred, they were predominantly below the unit slope line indicating a lower acid content by titration-as would be the case if moisture, solvent, or comonomers were retained. Indeed, in the majority of cases where deviation did occur, IH and I3C NMR showed the presence of some residual solvent or comonomer. Thus the use of the carbon spectra to determine the composition of these copolymers is well-founded. In addition to the advantage that the results are not influenced by the usual sources of contamination for these copolymers, another advantage in using 13C NMR is that the solubilities of the copolymers do not necessarily affect the results. A homogeneously swollen sample can yield quantitative data even though individual peaks are poorly resolved. This is illustrated for a series of spectra of a copolymer, Figure 5, which contained inorganic contaminants. The copolymer insolubility resulted from network formation via cationic cross-linking. The first spectrum shows the carbonyl region of a copolymer which contained 4000 ppm Ca, 400 ppm Mg, > 8000 ppm Na, and small amounts of other metal ions. The second spectrum was obtained after heating to make the sample more homogeneous and the third spectrum was taken after the sample had been treated with dilute HC1 and reprecipitated. The acid contents were found to be 25%, 26%, and 25%, respectively, by I3C NMR, and 25.5% by titration. Compositional analysis via I3C NMR has the further advantage over wet methods that microstructural information can be ascertained from the same spectrum (8-13). For example, the a-CH3, region divides into resonances arising from
isotactic, heterotactic, and syndiotactic triads (see Figures 1 and 2) and thus can be used as a quantitative monitor of the copolymer tactic distribution. I t is also possible to obtain sequence information on these copolymers from the same spectrum; however, the analysis is not as straightforward. The importance of the above observation is that often the physical properties of copolymers are affected by this “microstructural composition” as well as by the overall comonomer composition. Thus, the utility of obtaining information on both facets of the copolymers more than compensates for the additional time (relative to wet methods) required for the 13C analysis. LITERATURE CITED (1) (2) (3) (4)
(5)
(6) (7) (8) (9) (10) (1 1) (12) (13)
J. Schaefer, Macromolecules, 2, 210 (1969). J. Schaefer, Macromolecules,4, 107 (1971). E. B. Whipple and P.J. Green, Macromolecules,6, 38 (1973). R . L. Vold, J. S. Waugh, M. P. Klein, and D. E. Phelps, J. Chem. fhys., 48, 3831 (1968). J. R. Lyerla, Jr., and G. C. Le@, “Topics in Carbon-13-NMR Spectroscopy”, John Wiley, New York, 1974, p 81. S. J. Opella, D. J. Nelson, and 0. Jardetsky, J. Chem. fhys., 64, 2533 (1976). G. A. Gray, Anal. Chem., 47, 546A (1975) and references therein. L. F. Johnson, F. Heatly, and F. A. Bovey, Macromolecules, 3, 175 (1970). Y. Inoue, A. Nishioka, and R. Chujo, folym. J., 4, 535 (1971). A. Zambelli, G. Gatti, C. Sacchi, W. 0. Crain, Jr., and J. D. Roberts, Macromolecules, 4, 475 (1971). C. J. Carman, Macromolecules,6, 725 (1973). I. R . Peat and W. F. Reynolds, TetrahedronLett., 14, 1359 (1972). E. Klesper, A. Johnsen, W. Gronski, and F.Wehrli, Makromol. Chem., 176, 1071 (1975).
RECEIVEDfor review June 10,1976. Accepted September 20, 1976.
Colorimetric Determination of Niobium in Sodium Metal with Thiocyanate P. F. Sattler’ and 1. E. Schreinlechner Osterreichische Studiengesellschaftfur A tomenergie, lnstitut fur Metallurgie, Lenaugasse 10, A- 1082 Vienna, Austria
The niobium determination using the spectrophotometric measurement of the thiocyanate complex is applled for the determination of trace amounts of nloblum In sodlum. The effects of the concentration of the reagents on the absorbance were studled resulting in the optlmizatlon of the condltlons for the color development of the complex. The procedure is recommended in the range of up to I O pg niobium, whereby a detection limit of 0.040 pg Nb ml-l cm-l is obtained.
The application of sodium metal as a heat transfer medium a t elevated temperatures in nuclear fast breeder reactors causes a number of corrosion problems. Since it is a well known fact that the impurities in liquid sodium play an important role in the corrosion process, the niobium content of liquid sodium is of special interest when niobium-containing alloys are used as construction materials. For quantitative determination of small amounts of niobium, atomic absorption spectrometry, x-ray fluorescence, and spectrophotometric methods are discussed. Since the detection limit of atomic absorption spectrometry with the flame is not favorable and flameless excitation with the graphite furnace is not possible because of niobium carbide 80
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1 9 7 7
formation, x-ray fluorescence ( I ) and spectrophotometric methods (2) are employed as alternative methods. For our purpose the method of Lauw-Zecha et al. ( 3 ) and Freund et al. (4)-the spectrophotometric determination of the niobium thiocyanate complex-seemed to be the best one. In the present work, this method was adapted for the determination of niobium in sodium metal. There exists a considerable dependence of the concentration of the reagents on the sensitivity of the spectrophotometric measurement. This influence on the thiocyanate complex was studied. Results indicate that using other concentrations of reagents than described by the literature yields an increase in sensitivity and an improvement of the detection limit. The optimum conditions for the formation of the colored niobium thiocyanate complex are also studied. EXPERIMENTAL Apparatus. A Zeiss PMQ I1 Spectrophotometer was used for the spectrophotometric measurements. Reagents. All reagents were of analytical grade. Reagents for the preparation of the sodium sample were potassium pyrosulfate; tartaric acid solution, 1 M. Niobium Stock Solution. A melt is prepared by fusing pure niobium
- 0
10
20
30
LO
,
Q:50
STANNOUS CHLORIDE
SOL. ['/.I
Figure 1. Effect of stannous chloride concentration
0"'
0,5
TARTARIC
1,O ACID SOL.
375
40
425
w
175
FINAL NORMALITY OF KL
Figure 4. Effect of hydrochloric acid concentration
2,O
1.5
IMI
Figure 2. Effect of tartaric acid concentration MICROGRAM NIOBIUM
r
Flgure 5. (A) Calibration curve according to recommendedprocedure. (B) Calibration curve according to literature (3)
plex is extracted twice with 7 ml of diethyl ether. The extracts are collected in a 25-ml volumetric flask and filled up with diethyl ether. After 1 h the absorbance is measured in a 5-cm cell a t a wavelength of 385 nm.
RESULTS AND DISCUSSION POTASSIUM THIOCYANATE
SOL
1%)
Flgure 3. Effect of potassium thiocyanate concentration pentoxide with potassium pyrosulfate. It is dissolved in 1M tartaric acid solution. One ml of this solution contains 200 pg of niobium. Niobium Standard Solution. The stock solution is diluted with 1 M tartaric acid solution to a standard containing 20 pg niobium per ml. Stannous Chloride Solution. A 25% stannous chloride solution is prepared by dissolving the reagent in 4.5 N hydrochloric acid. Hydrochloric-Tartaric Acid Solution. The solution must be 9.9 N on hydrochloric acid and 1 M on tartaric acid. Potassium Thiocyanate Solution. A 30% potassium thiocyanate solution is prepared by dissolving the reagent in water. The solution must be prepared fresh every day. Diethyl Ether. The diethyl ether (peroxide content less than 3.10-5%) is saturated with 4.5 N hydrochloric acid. Recommended Procedure-Sample Preparation. Sodium samples are taken in alumina crucibles. The excess sodium, about 6 g, is distilled off in high vacuum. The residue is fused with potassium pyrosulfate. A large excess of fusion material must be avoided because of a disturbance of the color development for the final measurement. The quantity of the pyrosulfate taken for fusion must be calculated in such a way that the final sulfate concentration in the aqueous sample should not exceed 6 mg (pg of Nb)-l m1-I. If a larger amount of material is necessary, the calibration curve should be prepared under the same conditions. The melt is dissolved and filled up to 10 ml with tartaric acid. Photometric Determination. One ml of the sample solution is transferred into a separatory funnel. It is mixed with 3 ml of stannous chloride solution, 5 ml of hydrochloric-tartaric acid, and 5 ml of potassium thiocyanate solution. The final solution, 14 ml, has to be 4.5 N in hydrochloric acid. Within 5 min, the niobium thiocyanate com-
In order to achieve the best conditions for the development of the colored complex, the influence on the sensitivity by varying the concentration of the reagents was studied. To 1 ml of niobium standard solution, containing 10 pg of Nb, three reagents were added with constant concentration whereas the fourth was varied in its concentration. The final volume of 14 ml of aqueous solution before extraction was always kept constant. The results are summarized as follows. Effect of the Concentration of Stannous Chloride. In Figure 1 the variation of the concentration of stannous chloride vs. the absorbance is shown. There is a maximum of sensitivity a t a concentration of 25% which was chosen for the determination. Effect of the Concentrationof Tartaric Acid. As shown in Figure 2, the intensity of the color decreases with increasing concentration. If the concentration of the tartaric acid is too low, hydrolysis of the niobium is possible. A 1 M solution is suitable for final use because the possibility of hydrolysis of niobium has been decreased adequately while the loss in sensitivity is still acceptable. Effect of the Concentration of Potassium Thiocyanate. An increase of this reagent also increases the sensitivity of the determination, as shown in Figure 3. Yet there is a limitation, because a higher concentration causes a higher possibility of hydrolysis. With a solution of 30%of potassium thiocyanate, the results are satisfactory. Effect of the Concentration of Hydrochloric Acid. Hydrochloric acid is added not only with the tartaric acid solution but also as an ingredient of the stannous chloride solution. In Figure 4 the effect of the final normality of the ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
81
Table I. Mean Values and Standard Deviations for Calibration Curve A Std No. of Nb, Absorbance dev, % samples 0.5 0.045 f 0.003 6.7 12 1 0.084 f 0.004 4.8 18 2 0.166 f 0.007 4.2 18 4 0.327 f 0.015 4.6 12 6 0.487 f 0.018 3.7 10 8 0.648 f 0.019 2.9 10 10 0.818 f 0.023 2.8 12
Table 11. Mean Values and Standard Deviations for Calibration Curve B Std No. of Nb, vg Absorbance dev, % samples 1 0.050 f 0.006 12 8 5 0.225 f 0.007 3.1 6 10 0.440 f 0.021 4.8 6
solution with respect to hydrochloric acid is demonstrated.
A sharp increase in sensitivity is observed up to a normality of 4.3 N. In the recommended procedure the final normality of the aqueous solution was adjusted with 4.5 N hydrochloric acid. Calibration Curves. Calibration curves are prepared for the range of 0.5 to 10 pg of niobium with the aid of the niobium standard solution. The improvement of the accuracy of the method is demonstrated in Figure 5, where A denotes the calibration curve achieved by the recommended procedure, whereas B results when following the procedure according to literature ( 3 ) .Calibration curves A and B are based on the values given in Tables I and I1 together with their reproducibility. Interferences. The determination is interfered by many metallic elements and some anions ( 2 ) .Iron, chromium, and nickel are the main metallic impurities in our sodium samples. These elements are present in a concentration range up to 10 ppm. We added iron, chromium, and nickel stock solutions corresponding to a concentration range five times higher than the upper limit of the impurity level. At this concentration range, no disturbance was detected. The results are shown in
82
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
Table 111. Effect of Certain Ions on the Niobium Determination Nb, m No. of Nb, wg Added mg of recovery, % samples 1 0.03 Fe3+ 102 f 3 5 1 0.03 Cr3+ 100 f 2 5 1 0.03 Ni2+ 101 f 3 5 1 12 ~ 1 3 + 100 f 3 5 1 84 Sod299 f 2 5
Table 111. Aluminum as well as sulfate ions are introduced during the sample preparation by fusing the residue with potassium pyrosulfate in an alumina crucible. Several samples with known addition of aluminum and sulfate stock solutions were analyzed. The added amounts correspond to the maximum concentration which results from the fusion. For neither substance a disturbance a t these concentration levels was detected, as shown in Table 111. Analysis of Test Loop Sodium. Several sodium samples from a high temperature loop at Seibersdorf were analyzed for niobium. Over a period of several months, samples yielded values between 0.40 and 0.87 ppm Nb in sodium. During a following corrosion run with niobium test rings, the concentration of the niobium impurity in sodium increased to values up to 100 ppm Nb. Precision and Detection Limit. The precision of the analysis and the detection limit were determined by the method suggested for spectrophotometric measurements ( 2 ) yielding values of 0.0024 pg N b ml-l cm-l for the sensitivity and 0.040 pg N b ml-1 cm-' for the limit of detection. According to the conditions in our work (6 g of sodium, 25 ml of measuring solution, 5-cm cells) the detection limit amounts to 0.3 ppm niobium in sodium. LITERATURE C I T E D (1) H. Schneider and H. U. Borgstedt, Proceedings of BNES Conference "Liquid Alkali Metals", Nottingham, April 1973, p 77. (2) 0. G. Koch and G. A. Koch-Dedic, "Handbuch der Spurenanalyse", Springer Verlag, Berlin, 1974, p 849. (3) A. B. Lauw-Zecha, S. S. Lord, and D. N. Hume, Anal. Chem., 24, 1169 (1952). (4) H. Freund and A. E. Levitt, Anal. Chem., 23, 1813 (1951).
RECEIVEDfor review June 25, 1976. Accepted September 7, 1976.
