Effect of Potassium on Determination of Tin by Atomic Absorption Spectrophotometry J. R. Levine, S. G . Moore, and S. L. Levine International Business Machines Corporation, Systems Development Division, Poughkeepsie, N . I’.
ATOMICABSORPTION spectrophotometry was used to assay various lots of technical grade potassium stannate for tin content. Theoretically, 39.72% Sn is present in this compound, but all results were in the order of 10% (relative) high. The same effects were noted in the determination of Sn present in a n alkaline potassium stannate plating bath. The potassium stannate contains approximately 26 % of the alkali metal; this compound in addition to 37.5-60 g/l. of KOH was present in the plating bath. It was suspected that the potassium may have affected the tin absorbance values. Potassium has been shown to enhance the absorption of Ca and Ba in the ~ 3 0 0 0“C nitrous oxidc-acetylene flame, Little or no effect was noted in the cooler (=2300 “C) airacetylene flame ( I ) . The presence of ammonium iodide in tin solutions caused a slight enhancement of tin absorbance in the nitrous oxide flame (2). Sodium in high concentrations has been known to enhance the absorption of Sn; lower concentrations of sodium had no effect (3). Data are presented to show that potassium has effects similar to those of sodium and the ammonium ion, but to a slightly greater extent, on the absorbance values for tin. EXPERIMENTAL
‘Tin solutions were prepared by diluting a 1000-ppm stock solution prepared from high purity Sn metal (Mallinckrodt) in 10% HC1. The potassium was added as KBr with aqueous solutions prepared from recrystallized KBr (Mallinckrodt, AR) assayed to contain 67.1 1 % Br (theoretical value 67.15% Br). All analytical solutions contained equivalent amounts of acid, either 10% or 1% HCl, to minimize viscosity effects due to acid concentration during aspiration. The potassium stannate (M & T Chemicals and Vulcan, Tech. Grade) was dissolved, as is, in 10% HC1. After assaying was completed, alkaline stannate tin-plating baths were prepared from the potassium stannate to contain 37.5-45 g/l. of Sn and 37.5-60 g/l. of KOH. Samples were drawn from the -28-1. bath and diluted to get the tin into the working range of AAS standard plots. A fuel rich air-acetylene flame [the air-hydrogen flame, for better sensitivity for tin (4,was not available] w%sused to measure the absorbance of Sn at 2246 A and 2863 A using a Perkin-Elmer Model 303 atomic absorption spectrophotometer equipped with a Boling 3-slot burner, digital readout, and a Sn hollow cathode lamp. All other conditions were those specified by the instrument manufacturer (5). RESULTS AND DISCUSSION
Three series of 100 ppm of Sn solutions were prepared with varying amounts (0 to 205 pprn) of potassium added: two (1) D. C. Manning and L. Capacho-Delgado, Anal. Chim. Acta, 36, 312 (1966). (2) J. A. Bowman, ibid., 42,285 (1968). (3) E. J. Agazzi, ANAL.CHEM., 37, 364 (1965). (4) L. Capacho-Delgado and D. C. Manning, Spectrochim. Acfa, 22, 1505 (1966). ( 5 ) “Analytical Methods for Atomic Absorption Spectrophotometry,” Perkin-Elmer Corp., Norwalk, Conn., 1968. 412
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
contained 10% HCl and the third, 1 % HC1. Figure 1 is a plot of Sn absorbance us. K concentration at 2246 A. Low concentrations of the alkali metal have no effect, but a sharp increase in the absorbance of about 10 to 12 % (relative) for the outer two curves is noted. The increase in the middle curve, where the data were obtained in a slightly more fuel rich flame, is about 16% (relative). The rate of increase in Sn absorbance with >50 ppm of K is significantly lower. The good correlation among the curves indicates little or no effects due to acid concentration. The enhancement effect is also evident in Figure 2, plots of Sn absorbance at two wavelengths us. Sn concentration, with and without added potassium. In each case, the solutions prepared with sufficient potassium to simulate a solution of potassium stannate showed higher absorbance values than those containing no potassium. There are several possible explanations for this phenomenon. One of the species that can be formed in the flame during the excitation process is the metal oxide (see, for example, ref. 6). SnO is relatively stable having a dissociation of 5.7 eV (6). A very fuel rich flame is required for the atomic absorption analysis for tin, and any additive, such as KBr and/or KOH, inhibiting this oxide formation would significantly allow for more ground-state tin atoms in the flame. Although tin has a high ionization potential (7.33 eV) relative to potassium (4.38 eV) and tin ionization may be almost negligible in an air-acetylene flame, this may be another small factor in the enhancement. KBr and/or KOH are also more easily vaporized than tin. The presence of these materials in the tin solutions can increase the vaporization and/or dissociation of the tin specie in the flame portion in the optical path. More atomic tin is available more rapidly to interact with the resonant tin emission of the lamp causing enhanced absorption. The actual enhancement mechanism was not studied but may be any of the above or, more likely, a combination of those phenomena. In any event, the enhancement of tin absorption is significant and serious analytical errors can be encountered unless one compensates for the effect of the potassium. The extent of the analytical error that can be encountered in the AAS determination of tin in the presence of large concentrations of potassium is demonstrated by the data of Table I. The tin content of four lots of technical grade potassium stannate was determiaed using standard plots, as shown in Figure 2, for the 2246 A resonant line. The relative differences in the values obtained using standards with and without added potassium range from 8 to 12 %-quite significant percentages. The values determined with potassium present in the standards are quite good for a technical grade material. It should be noted that all values reported are the result of at least duplicate analysis to & 2 % precision. (6) J. E. Gibson, W. E. L. Grossman, and W. D. Cooke, ANAL.
CHEM., 33, 266 (1963).
Figurt 1. AS,, cs. ppm of K at 2246 A a . 100 ppm of Sn, 1% HCl b. 100 ppm of Sn, 10% HCl c. 100 ppm of Sn, 10% HCI (less fuel
IOOpprn Sn, 10% HCL IOOppm
D---o
IOOppm Sn, I% HCL
-
rich flame)
Sn, I O % H C L
bd
AT 2 2 4 6 %
NO K
o--O
Sn WITH
6---0
Sn WITH K
0--0
Sfl WITH
NO K
0-4
Sn WITH
K
Figure 2. Asn 6s. ppm of Sn; solutions for b and d contain sufficient K to simulate a KQSn(OH)6 solution a. at 2246 A with added K b. at 2246 A without added K c. at 2863 A with added K d. at 2863 A without added K
20
I
I
I
I
I
I
I
40
60
80
100
120
140
160
I
180
J
200
w m sn
One of the most common means of compensating for enhancement effects such as those noted here is the use of standard additions. This method was used on samples drawn from five alkaline stannate tin-plating baths; the results (duplicate analysis t o 3=3% precision) are shown in Table 11. It is immediately obvious that the cause of the enhancement is extensive enough that it was not compensated for by use of standard additions when one compares those results obtained using Sn-K standards and a n iodirnettic titration (7).
(7) “ASTM Methods for Chemical Analysis of Metals,” American Society for Testing and Materials, Philadelphia, Pa., 1956, p 308.
Table I. Effect of Potassium on Determination of Sn in Potassium Stannatea
% Sn Sample
No K added to stdsb K added to stdsc 42.3 38.8 43.4 38.6 43.5 38.1 39.1 43.4 39.72% Sn; 4 lots of technical grade material
K2Sn(OH)6 = used. * Standards contain 10 % HC1. 26% potassium added as KBr Standards contain 10% HCl (e.g., 100 pg/ml of Sn, 35 pg/ml of K used as a standard).
+
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
413
Table 11. Analysis of Alkaline Stannate Tin-plating Baths for Tina
Bath No.
Sn ( g / l J b Sn ( g / P Sn (g/lJd 112.0 44.3 42.4 ... 113.2 44.4 112.8 43.0 42.2 108.0 43.6 42.8 101.6 44.4 ... Nominally: 37.5-45 g/l. Sn as KiSn(0H)B and 37.5-60 g/l. 1 2 3 4 5
KOH. Used method of standard additions. Used calibration plot with K added to standards. Iodimetric titration,
The results in Table I1 also indicate that by adding potassium to the standard tin solutions in order to account for the enhanced absorption, the AAS analysis for tin compares
favorably with wet chemistry. The AAS results are within rt 5 of the titration results and were obtained in about one fourth the time. One can also use this enhanced absorption to advantage. When working with small quantities of tin by AAS, the addition of potassium to the standards and samples (provided no other problems are caused by the potassium) will enhance the normally weak signal obtained for tin in the air-acetylene flame. ACKNOWLEDGMENT The authors thank George E. Calley who performed the titrations and prepared the tin-plating baths.
RECEIVED for review August 22, 1969. Accepted December 22,1969.
Homogenized Fission Track Determination of Uranium in Whole Rock Geologic Samples David E. Fisher Rosenstiel School oj' Marine and Atmospheric Sciences, Unicersity of' Miami, 10 Rickenbacker Causeway, Miami, Fla. 33149
DATAON THE uranium contents of many different geologic materials show considerable discrepancies. Activation analysis of the same ultrabasic rocks have shown discrepancies of up t o a factor of three between different investigators ( I , 2). One lherzolite nodule was measured by both activation analysis and flame photometry and showed a variation in abundance of nearly a factor of two (3). Analyses by activation analysis and isotope dilution on three olivine nodules show discordancies of factors of two and three ( 4 ) . Activation analysis of different samples of the same achondritic meteorhes varies by a factor of two from one investigator to another (5-8). Determinations by both activation analysis and delayed neutron counting of chondritic meteorites generally show good agreement, although again variations of a factor of two have been noted (8,9). The U abundances in several standard rocks have been determined by many investigators using a variety of techniques. For some of these rocks the agreement is quite good but in others there are variations of up to an order of magnitude (IO).
(1) H. Wakita, H. Nagasawa, S . Uyeda, and H. Kuno, Earth Plunet. Sci. Lett., 2, 377--81 (1967). (2) V. Becker, J. H. Bennett, and 0. K. Manuel, ibid., 4, 357-62 ( 1968). ( 3 j D. H. Green, J. W. Morgan, and K. S . Keier, ibid., pp 155-66. (4) G. R. Tilton and G. W. Reed, Earth Sci. Meteorit., 31-42 (1963). ( 5 ) H. Von Konig and H. Wanke, Z . Naturforsch. 14 (1959). (6) J. F. Nix and P. K. Kuroda, Nature, 221, 726 (1969). (7) R. S . Clark, M. W. Rowe, R. Ganapathy, and P. K. Kuroda, Geochim. Cosmochim. Acta, 31, 1605-14 (1967). (8) J. W. Morgan and J. F. Lovering, Talanta, 15, 1079-95 (1968). (9) S. Amiel, J. Gilat, and D. Heymann, Geochim. Cosmochim. Acta, 31, 1499-1504 (1967). (10) F. J. Flanagan, ibid., 33, 81-120 (1969). 414
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
While a t least some of these differences are undoubtedly due to heterogeneous U distributions in these rocks, it is possible that there might be some deficiency in any of the several analytic techniques used. I have therefore thought it worthwhile to modify the technique of fission track analysis to make it suitable for whole-rock U determinations. The method as presented here is simple, direct, inexpensive, and rapid. Further, the results clearly indicate, though in a qualitative manner, the degree of heterogeneity of the U distributions within the total rock. In previous fission track studies (11, 12) a plastic detector was placed next to a polished section of rock and recoil fission fragments induced by a neutron irradiation were counted in the plastic. These studies showed that U is heterogeneously distributed within many geologic materials. The technique is useful for studying such distributions, but it is difficult t o analyze the data in terms of average U concentrations for the whole rock. Also, the necessary polishing may remove U from water-soluble U-rich minerals, or introduce contaminant U. The present technique uses a well-powdered homogenized rock surface to avoid this difficulty. The rocks are thoroughly crushed in an agate mortar and pestle and passed through a 100-mesh sieve. The gig used to mount the samples is shown in Figure 1. Milligram amounts of the powder are poured through B, leaving a -0.5-cm disk on A . B is removed and methyl cellulose powder is poured in through C. A pressure of -2000 1 b / h 2 produces a -0.1-cm thick disk with smooth, cohesive surfaces. Lexan, with a n inscribed circle of diameter 20.5 cm is then taped over the sample. A series of -50 disks, together with (11) R. L. Fleischer, Geochim. Cosmochlm. Acta, 32,989-98 (1968). (12) R. L. Fleischer, C . W. Naeser, P. B. Price, R. M. Walker, and U. B. Marvin, Science, 148,629-32 (1965).
