-
Quantitative Determination of Ferrous and Ferric Ion Using Mossbauer Effect Tohru Yoshioka, Yohichi Gohshi, and Hiroko Kohno
Central Research Laboratory, Tokyo Shibaura Electric Co., Ltd., Kawasaki, Japan
The Mossbauer effect was applied quantitatively to a nondestructive oxidation state analysis. With careful background correction, the absorption area was able to be used to obtain the concentration of the resonant atoms. It was assumed that the f values of Fez+and Fe3+were identical in the same absorber. I n the case of thin absorber, the concentration ratio of Fe3+to Fez+ is equal to the ratio of their absorption areas. The iron phosphate glass was used as a sample, and the result by this method was compared with the result of the chemical analysis. Accuracy of this method was f10%.
ALTHOUGH A KNOWLEDGE of the valence state of the various atoms in solid materials is very important in materials science, the determination of a multivalent element in its various formal oxidation states is very difficult. Recently, several chemical methods have been reported by Nadalin (1) Sachse (2, 3), and Cheng (4). As these are essentially destructive methods, there is always the possibility of change in the formal oxidation state during the decomposition process. These methods are not applicable for this determination except when a sample contains only one multivalent element, which occurs in two formal oxidation states. On the other hand, most physical methods are nondestructive. Especially, ESR and NMR spectra can reflect chemical states, but these spectra are not directly related to the formal oxidation state, and the intensity of the resonance absorption is influenced by various conditions (5) besides the amount of the element. Consequently, these methods are not very suitable for a quantitative formal oxidation state analysis. The Mossbauer effect has a possibility for application to the nondestructive formal oxidation state analysis, because isomer shift depends on the formal oxidation state of the resonance atoms and quadrupole splitting can frequently offer important information about it (6). A quantitative analysis has not yet been successful (3,because the apparent magnitude of the Mossbauer resonance depends not only on the thickness of the absorber but also on the recoil free fraction of the absorber. By establishing the background correction, we applied the Mossbauer effect to the nondestructive formal oxidation state analysis. Iron phosphate glass was used as a sample. Concentration of the ferrous and ferric ions in the sample was determined quantitatively by Mossbauer effect, and was compared with the results of the chemical analysis. It was concluded from this investigation that the concentration of the ferrous and
(1) R. J. Nadlin and R.I. B. Brozda, Anal. Chim. Acta, 28,282 (1963). (2) H. B. Sachse, ANAL.CHEM., 32, 529 (1960). (3) H. B. Sachse and G. L. Nichols, Ibid.,33, 1349 (1961). (4) K. L. Cheng, Ibid.,36,1666 (1964). (5) G . E. Pake, “Paramagnetic Resonance,” W. A. Benjamin, New York, 1962, Chaps. 2 and 6. (6) V. I. Goldanskii, “The Mossbauer Effect and Its Application In Chemistry,” Consultants Bureau, New York, 1964, Chap. V. (7) J. R. DeVoe and J. 3. Spijkerman, ANAL.CHEM., 38, 382 R ( 1966).
n , -
Vibrator
Source
Detector
Function Generator
I
l l
Modulator
1
1
100 Channel Pulse Height, Analyzer (
Figure 1. A block diagram of Mossbauer spectrometer
ferric ions can be determined to an accuracy of f10% with an appropriate background correction. EXPERIMENTAL
Apparatus. Details of the spectrometer used here are shown in Figure 1. Random pulses from the detection system were modulated by a triangular wave form which was generated simultaneously with the sinusoidal wave. The original sinusoidal wave and the wave from the pick-up coil were adjusted in phase. The transmitted gamma rays were detected by a scintillation counter which consisted of very thin NaI crystal (30 mm X 0.25 mm) and the Toshiba 7696 photomultiplier. 67C0 diffused in a stainless steel foil was used as a source. The velocity of the source was calibrated by observing the spectra obtained with ferrous sulfate and sodium nitroprusside absorber. Sample Preparation. ‘The iron phosphate glass was prepared according to the direction of Munekata (8). The concentration of ferrous and ferric ions was varied with the change of the melting time of the glass. The composition of glass samples is shown in Table I. Fez-cwas determined by titration with potassium permanganate after decomposition in inert atmosphere. Total iron was determined by the com-
(8) M. Munekata, “Research Report No. 638” (Japan Electrotechnical Lab.). Table I. Composition of Glass
Sample a b C
d
e
Composition of glass 8 BaO 62P205 30F&Oa 8 BaO 72PZOs 20FezOa 15F&03 10BaO 75 PzOs 10 Fez03 15 BaO 75 Pz05 5Fez03 20BaO 75PZO6
Preparation conditions Temperature, Time, “C min 1350 50 1350 30 1350 30 1350 30 1200 1300 15
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VOL. 40, NO. 3, MARCH 1968
603
velocity
:b)- METHOD(2)
(01- METHOD ( I
-I
I
0
2
3
mmhc
.. .. i..‘
a
Figure 3. A typical MBssbauer spectrum of iron phosphate glass
Figure 2. A method of background estimation plexometric titration. The amount of Fez03was calculated as a difference between total iron and FeO. The glass was pulverized and dispersed in polymethylmethacrylate, which was made into a thin disk. The surface density of the iron in the sample was varied between 3 mg/cm2 and 12 mg/cm2. A METHOD OF Ih’TENSITY ANALYSIS OF MOSSBAUER SPECTRUM
Area Method (9, 10). It is better to use the area method for the quantitative analysis of the Mossbauer spectrum, because it is independent of source line shape and instrumental velocity resolution and saturates less rapidly with increasing absorber thickness than does the associated depth. Although the analytical treatment of the area method was dealt with by Lang (IO), he did not show how to correct background counting rates. Considering background counting rates, an experimental shape of Mossbauer spectrum E(u) can be given as follows, Z(m)
=
Eex*(o)
- Z(V) - - Z(m)
- Z(u) . -1 p
I(m)
P=
Z(m)
(1)
-B
I(=J)
where Z( a ) and Z(u) refer to the transmitted intensity for infinite (off resonant absorption) and for resonant velocity, respectively. B is the background in the 14.4-keV window. P may be called a purification fraction. An analytical expression for Equation 1 is shown ~ ~ ~= ( 0 )
(E
+ v)[i
- e - n o ( ~ ) ld~
=
E,,,
(O)/P
(2)
so that the absorption area of the experimental spectrum is given by (10)
-
A
=
(-1)E+’ L(t)=C 2-1 1! -a-
1
-
P
?r
‘ f .- - L ( t ) 2
(21 - 3)!!
(21 - 2)!!
(3) t
t =nuof’
where some parameters in Equation 3 are the same as in Lang’s paper. In the above treatment, it is assumed the resonance line shape is Lorentzian. Consequently, the magnitude of the resonance area can be related to the con(9) R. M.Housley, Nucl. Instr. Methods, 27, 29 (1964). (IO) G.Lang, Zbid.,24,425 (1963).
604
0
ANALYTICAL CHEMISTRY
centration of resonant atoms through Equation 3. Experimental measurement of A and P, and the knowledge of ro, f,andf’ allow the determination of n. Becausef’, in general, is unknown, n cannot be determined. But the concentration ratio of the ferrous and ferric ions can be determined without the knowledge of f’ in the same samples. The desired quantity n(Fe3+)/n(Fe*+) is derived from A(Fe”)/A(Fe*) by the following procedure, A(Fe3+) L(Fe3+) t(Fe3+) n(Fe3+) -=-=-=__ (4) A(Fe2+) L(FeZ+) t(Fe2+) n(Fe2+) Here we assumed that thef’ (Fe3+) andf’(Fe2+) are equal in the same sample. This assumption will be proved to be right later. The magnitude of the obtained spectrum area was measured by numerical integration within f5 %. Determination of the Purification Fraction. Several techniques for background correction of Mossbauer spectroscopy were introduced by Housley (9). His method, however, is usable only when the composition of the absorber is known. Unless the composition of the absorber is known, it would be impossible to use. A conventional technique for background correction is an extrapolation of a smooth curve under a gamma ray peak, shown in Figure 2a. This method gives a fair approximation only when the absorber is sufficiently thin. Here we used another method, familiar in nuclear physics, to avoid this difficulty. This method is based on the fact that the photo peak of detected gamma rays shows the Gaussian profile and the energy distribution of Compton scattered gamma rays in the 14.4-keV window is constant (Figure 2b). The following experiments were carried out in order to find out which method is better to determine the purification fraction. A stainless steel foil (AIM1 304) of 15-11 thickness was used as an absorber. Several sheets of acryl resin of 0.6-mm thickness were put on the stainless absorber to change the purification fraction. The obtained spectrum area was corrected by these two methods, and the results are shown in Table 11. It may be concluded that the present method is superior to the conventional one. An error of the estimation of the purification fraction was within = t 5 % . RESULTS AND DISCUSSION Figure 3 shows averaged typical spectrum of iron phosphate glass for several experiments. The statistical error of this measurement is about 0.3%:. The absorption areas corresponding to ferrous and ferric ion were obtained easily, because the shallow absorption of the right-hand side is due to the ferrous ion, and the asymmetric doublet absorption is due to
1’
: z 3c
:
I
Io
5
15
25
20
30
(mol%)
Concentration of samples
/
Figure 5. Effect of concentration on the recoilless fraction of absorber 0 Fea+
0.5
effective 1.0 thickness
Mbssbauer analysis
Figure 4. Relation between the concentration and effective thickness in iron phosphate glass 0 30mol 0 15 mol
A
1Omol 5mol
the sum of the ferrous and ferric ion. After the background correction, the effective thickness of ferrous and ferric ion was obtained directly from the measured area using Equation 3. In that procedure, it was assumed that the recoil free fraction of 57c0(stainless) source was 0.69 (11). The obtained concentration from Mossbauer spectrum was compared to the value from chemical analysis, that is shown in Figure 4. Iff’ is constant, a linear relation will exist between the value of Mossbauer analysis and that of the chemical analysis. The reason of this nonlinear relation between the two is explained by assuming the modification of the recoil free fraction of the absorber. This saturation effect was not found for the samples of the same composition even if the thickness of the absorber was increased. Then it is expected that the value off’ will be changed when the composition is varied. In Figure 5 , the value off’ in the absorber is not explained sufficiently at the present time. It may be noticed from Figure 5 that the behavior of thef’(Fe3+) and f’(Fe2+) is similar; therefore the assumption which is used to derive Equation 4 was proved to be right. An equality off’(Fe3+) andf’(Fe2+) would be ascertained also by investigating the temperature dependence of the resonance area. The value off’, however, is too small compared to the expected value (12). From the results it is considered that the density distribution of the iron phosphate glass in the absorber disk is not so homogeneous that the 14.4-keV gamma rays from the source cannot interact with iron atoms completely. Therefore value off‘ looks smaller than the expected values. In Table 111, the concentration ratio of the ferrous and ferric ions from Mossbauer analysis and chemical analysis is shown for various samples, The error of this analysis is a standard deviation of the single determination. In the above discussion, the Goldanskii effect was neglected. It was also assumed that the line shape of the source and the absorber is Lorentzian. If the line shape approaches a (11) P. Debrunner and R. J. Morrison, Rev. Mod. Phys., 36, 463 ( 1964). (12) A. A. Belyustein, Soviet Physics-Solid State, 7 , 1163 (1965).
0
Fez+
Table 11. Comparison between Method 1 and Method 2 Absorption Method 1 Method 2 Sample area Alp Alp Stainless steel foil 7.86 18.50 14.02 Foil 0.6 mm resin 6.47 21.05 15.36 Foil 1.2mm resin 4.35 25.4 14.85
+ +
Table 111. Concentration Ratios n(Fe3+)!n(Fe2+) from Mossbauer Analysis (M) and Chemical Analysis (C) Sample M Std dev C Std dev a 1.6 0.2 1.78 0.02 b 2.1 0.2 2.06 0.02 C
d
e
2.0 2.3 4.5
0.2 0.2 0.4
1.97 2.66 5.00
0.02 0.04 0.10
Gaussian profile because of perturbations in the absorber, a quantitative treatment will be impossible without some modifications, except for a thin absorber ( t < 1). CONCLUSION
A quantitative determination of ferrous and ferric ion in the iron-phosphate glass was carried out nondestructively to an accuracy of 10% using the Mossbauer effect. A concentration ratio of ferrous and ferric ion in an absorber is obtained easily, because it does not need to consider the value off’. If the magnitude of the resonance area and the purification fraction are determined more precisely, the concentration of the resonant absorber will be obtained easily. As Equation 3 is applicable for all Mossbauer atoms, the same treatment is allowed for other Mossbauer elements, with different assignment for the constants. ACKNOWLEDGMENT
The authors thank T. Kishii and H. Endo for preparation of iron phosphate glass and chemical determination of ferrous and ferric ions.
RECEIVED for review June 12, 1967. Accepted September 29, 1967. VOL. 40, NO. 3, MARCH I968
605
