Anal. Chem. 1986, 58, 1152-1156
1152
the unsophisticated system, while the threshold limit value for silane is 5 ppmv. So, although a study of interferences from minor or trace atmospheric components was not carried out, the proposed method has the possibility to detect silane concentrations below the threshold limit value with only 1mL of sample, without any preconcentration. It should, however, be noted here that since the present method is not specific to silane, the combination with a separation technique, like gas chromatography, is strongly recommended for the selective determination. Registry No. SiH4,7803-62-5; SiOz, 7631-86-9.
LITERATURE CITED 1
L
5
0
10
15
SiH4 Concentration
20
(x4.5ppmv)
Figure 9. Analytical calibration curve for mode 111: symbols (A-D) indicate the same meaning as those of Figure 8.
sphere, the standard silane gas was diluted with air from a cylinder. I t should be noted that the values on the abscissa in Figure 9 indicate the concentrations of silane before being mixed with ozone. Silane at the concentration of 3 ppmv gave a signal intensity about 2 times the blank signal even with
(1) (2) (3) (4)
Nonaka, I.; Kato, Y. Amen Kogaku 1983, 22, 163. Tao, H.; Mlyazaki, A,; Bansho, K. Anal. Chem. 1988, 58, 202. Emeleus, H. J.; Stewart, K. J . Chem. SOC.1935, 1182. Okabe, H. “Photochemistry of Small Molecules”; Wiley: New York, 1978; p 241. (5) Harada, Y.; Murrell, J. N.;Sheena, H. H. Chem. Phys. Lett. 1988, 1 , 595. (6) Vasilyeva, L. L.; Drozdov, V. N.; Repinsky, S.M.; Svitashev, K. K. Thin Solid Films 1978, 55, 221. (7) Thoien, A. R. Acta Metall. 1979, 2 7 , 1765.
RECEIVED for review September 30,1985.
Accepted December
10, 1985.
Ultraviolet Spectrophotometric Determination of Gadolinium in Concentrated Solutions of Nitrate Salts Louis LBpine, Roland Gilbert,* and Guy BBlanger
IREQ (Institut de recherche d’Hydro-Qudbec), 1800 montBe Sainte- Julie, Varennes, QuBbec, Canada JOL 2PO
Ultravlolet spectroscopy was successfully applled for determining the gadollnlum concentration In Gd( NO,), solutlons. The 8S,/2 transltlons of the Gd3+ Ion, whlch are responslble for the flne structure at 272.8 nm, were selected as a basis for the lnvestlgatlon of three analytlcal modes, one based on three wavelength measurements (mode ABS) and the others on first- and second-derivative measurements (modes a A / a X and d * ~ / X2). d For the particular problem concerned here, namely, to deal wlth nltrate interference In the spectral reglon of Interest (270-280 nm), the best performance was shown by the flrst-order derlvatlve mode, whlch had a precision of f0.035 g L-’, a dlscrlmlnatlon limit of 0.071 g L-’, and an accuracy of 0.60% for an 8 g L-’ Gd solution. This study shows that it Is posslble to adopt this princlple for quantltative analysis of the Gd( NO,), solutions (0.0509 M or 8 g L-‘ Gd) used In the emergency shutdown systems of CANDU-PHW (Canada deuterium uranium-pressurlzed heavy water) reactors. +
Nuclear reactors are usually equipped with two completely independent emergency shutdown systems, involving either the introduction of liquid shut-off rods or the injection of a neutron absorber in solution (1). Traditionally, boron solutions have been used for the latter purpose, although the use of gadolinium dates back over 20 years to 1965 when Heinrich (2) reported its use for the reactors at Savannah River Plant in the U.S. More recently, it has been employed for Canadian reactors, including that a t Gentilly 2 (600-MW CANDUPHW), which Hydro-QuBbec commissioned in 1983. In this
particular case, gadolinium nitrate was chosen for ita solubility and ion-exchange properties, both of which are far superior to those of boric acid. The reason for choosing a nitrate, rather than other available salts, was to avoid the presence of certain anions such as chloride in the stainless steel reservoirs. The gadolinium nitrate solution in the Gentilly 2 emergency shutdown system is stored in reservoirs directly connected to the moderator, with no valve between, which means that there is always a possibility of the gadolinium drifting toward the moderator in the presence of pressure transients. According to Canadian licensing practice, control of the gadolinium concentration of the reservoir solutions is mandatory and grab samples are therefore collected periodically for laboratory analysis by flame emission spectroscopy. Continuous on-line monitoring without the need for manual sample preparation would obviously relieve laboratory personnel of this time-consuming procedure and avoid exposure to radiation. This would involve the use of a technique capable of detecting a 10% reduction in the concentration of a Gd(N03)3solution in heavy water containing 8 g L-’ Gd without affecting the physicochemical conditions of the samples; such requirements severely limit the choice. Most of the Gd analyses reported in the literature involve either sample destruction ( 3 , 4 )or the addition of complexing reagents (5,6). None of the electrochemical techniques based on the only known reaction of the trivalent Gd3+ion (Gd3++ 3e- F? Gd’) can be employed because this reduction takes place at a more negative potential than that of the reduction of water (7). The approach based on measuring the ion conductivity of solutions cannot be used because it is not specific for gadolinium. The
0 1986 American Chemical Society 0003-2700/86/0358-1152$01.50/0
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
10
08
Scan speed 0 3 n m t m i n Spectrol bandwidlh 008 nm Response 5 s
Q
Gd Cl3
Scan speed i5nmImin Spectrol bondwidth 025 nm
Response i s
08
-
300
325
350
375
400
Wavelength Inmi
Figure 1. Fine structure of 61 transitions of the Gd+3ion measured with solutions having equivalent concentrations of Gd (0.0509 M or 8 g L-’ Gd).
1153
data corresponding to the different excited states predicted by Russell-Saunders coupling for the 4f7 configuration of the Gd3+ion ( I I , 1 2 ) ,and the energy levels of the fine structure measured by Schwiesow and Crosswhite (13),all the bands observed can be attributed to transitions toward the 61excited states of the atom (Figure la). In the case of perturbed levels of the 611,jzand 611312 states, which are both involved in the main band a t 272.8 nm, these are partially overlapped and cannot be clearly distinguished with a 0.1-nm spectral resolution. The band resulting from the overlap of the latter transitions is the most attractive for quantitative analysis of gadolinium. The absorbance a t the maximum is of the order of 0.92 and the bandwidth a t half-peak height of the component is 0.76 nm for an 8 g L-’ Gd solution (measured under the conditions of Figure la). However, the spectrum shown in Figure l a was recorded for a chloride salt (GdC13)and the anion here does not absorb in the characteristic cation region. This is not the case with the nitrate salt, since the anion has a very wide absorption band ranging from 260 to 340 nm, with a maximum located a t about 300 nm. The 6115/2, 6113j2 8S7/, transitions of gadolinium are superimposed on the intense nitrate band, as may be seen in Figure lb, implying that the measurement technique must be able to eliminate the nitrate interference. Analytical Techniques. The simplest approach for subtracting the nitrate contribution to the absorption at 272.8 nm would be to measure nitrate absorption at other wavelengths (for example, a t 300 nm, the apex of its absorption band) and to apply a multiplication factor. This technique would have to assume that the ratio of the extinction coefficients for nitrate a t these two wavelengths will remain constant, but unfortunately, because of the equilibria characterizing the ionic associations of the different species present, such is not the case. These equilibria not only vary as a function of the salt concentration of the solutions but also can be influenced by the addition of other ions such as nitrates of nitric acid used to adjust the pH of the reservoirs. For example, the proportion of nitrates associated in the form of the GdN03’+ complex in an 8 g L-l Gd solution of Gd(N03)3 is of the order of 20% (14). Although these associations do not affect the 6116/2, 61,3j2 sS712transitions of the gadolinium because of the screening effect of its 5s and 5p electrons over the 4f electrons, they influence the nitrate absorption band. The bandwidth a t half-peak height of this structure shows an increase of 529 cm-l in passing from the NaN03 solution, where there is only free nitrate, to the Gd(NO3I3solution of equivalent nitrate concentrations (9.462 g L-’ NO3). However, a good approximation of the nitrate contribution can be obtained if the absorption is measpred on both sides of the gadolinium band. The net absorption due to gadolinium would then be calculated by +-
ideal analytical technique would therefore be neutron capture, which supplies the neutron-absorption capacity of a solution directly, although it does involve the development of instrumentation which, according to specialists in the field, can cost up to several tens of thousands of dollars. In view of the rather limited commercial potential of this so-called “ideal” technique, there would be more justification, economically at least, for using UV spectroscopy, a technique for which existing instruments could easily be adapted to meet specific needs. In the following sections, therefore, it will be shown how advantage can be taken of the spectral properties of gadolinium in the UV region for quantitative analysis of this element in concentrated Gd(N03)3solutions. The analytical difficulties encountered with the use of a nitrate salt will be presented, together with the means envisaged for overcoming them. The results yielded by three analytical modes, all based on the principle of UV spectroscopy, will be compared from the point of view of their sensitivity, precision, discrimination limit, and accuracy.
EXPERIMENTAL SECTION Spectroscopic Measurements. A Cary 17H spectrophotometer supplied by Varian was employed to investigate the electronic transitions of the Gd3+ion. Spectra with a 0.1-nm resolution were then used to select analytical modes that could be used to eliminate the unwanted nitrate background absorption. A Perkin-Elmer Lambda 5 spectrophotometer equipped with a photomultiplier and microprocessor-based electronics was used for the experimental validation of the analytical modes. This instrument was chosen because of its wavelength-programmerand spectral-derivative options. Chemicals. All the chemicals used for this study were commercial-grade products having the following purity: 99.9% for the Gd(N03)3.6H20(Research Chemicals) and Sm(N03)3.5H20 and GdCl3-xH2O(Alfa Products) and ACS certified for NaN03 (Fisher). The solutions were prepared in triply distilled demineralized water. RESULTS AND DISCUSSION Spectral Assignment of the Gd3+Ion. The electronic spectrum of gadolinium in aqueous solution was reported half a century ago (8). The transitions from the ground state 8S7/2 of the Gd3+ion to the 61excited states give rise to very narrow, relatively intense bands between 270 and 280 nm, whose spectrum is shown in Figure la. Moeller and Moss (9) have revealed the complexity of this spectrum in this particular region, which is seen to contain more bands than expected from the nonperturbed 61states of the Gd3+ion. Yost et al. (IO)attribute this phenomenon, typical of rare-earth salts, to the Stark effect. Thus, on the basis of the calculated energy
-
A272.8,Gd3f
=
A272.8
-
A272.8+Ah
+ A272.8-AX
The results obtained for the right-hand side of this equation by varying AA from 0.8 nm to 10 nm are shown in Figure 2. It can be seen from this figure that the maximum response is obtained from a AA of the order of 1.8 nm. At this interval, the signal is constant over a range of 0.5 nm, a value that is important because it must be higher than the wavelength accuracy and repeatability of the measuring instrument. Furthermore, as shown by the curve for Sm(N03)3in the same figure, it appears that the nitrate interference has been totally eliminated. The value of 1.8 for A x was therefore selected for the experimental validation of this analytical mode (mode ABS). Another way of dealing with nitrate interference is to use the derivative of absorbance with respect to wavelength. This
1154
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Cn-7
0.15
0.4
1
0.4
NoN03
03
40
20 0
2 s
011-
4
-
I
I
l
I
I
1
I
I
I
I
0.02
l
,
l
,
l
l
l
i
i
,
O'O20
I
2
3
4
8
9 IO
l
5 6 7 A x (nm)
-
40
g -20
11
Figure 2. Variation in the absorption signal of the Gd3+ion (611512, 611312 after subtraction of nitrate interference by the ABS mode as a function of the value of AX; @(NO,), and Srn(NO,), solutions have equivalent nitrate concentrations (0.0509 M or 9.462 g L-I NO,).
Comparison of Analytical Modes ABS, dA /ax, and
d2A/ah2. The calibration curves in Figure 4 were plotted from the application of modes ABS, dA/dX, and d2A/dX2 to analyze different Gd concentrations in Gd(NO& solutions. The interval of each point is based on the calculated average of 15 successive readings at 99% confidence. All the tests were performed under the following identical experimental conditions: band-pass, 0.25 nm; scan speed, 15 nm/min; response time, 1 s. The position of the monochromator for the ABS mode was programmed for measurements at 272.8 nm (A272.8), 271.0 nm (A272.&AX), and 274.6 nm (A272,8+AX), and the signals recorded at these wavelengths were used to calculate the absorbance according to eq 1. The derivative spectra were recorded by scanning the wavelengths in the range from 268 to 282 nm. The signals recorded at 272.2 nm and 272.8 nm
-20
- -40 uu 1'401 \I 1 268
+
technique is becoming increasingly popular as a background correction technique, to reduce the effect of spectral background interferences in quantitative analytical spectrophotometry (15). On a theoretical basis, a full mathematical description of the conditions in which derivatives of absorbance are linearly proportional to the concentration has been given in the literature (16-19). This is certainly an attractive method for freeing the peak of interest from the broad curve resulting from the presence of the nitrate. The first- and second-order spectral derivatives for NaN03 and Gd(N0J3 solutions of equivalent nitrate concentrations are given in Figure 3 (9.462 g L-l NO3). The curves for NaN03 illustrate the progressive elimination of nitrate interference, from the absorption signal (Figure 3a) to the first derivative (Figure 3b, curve with a slightly positive slope) and the second derivative (Figure 3c, zero slope). In Figure 3e, the maximum value of the dA/dX signal, around 272.2 nm, corresponds to the point of inflection on the high-frequency side of the absorption band at 272.8 nm of the Gd3+spectrum. The maximum value of the d2A/dh2signal in Figure 3f appears as a negative value at g72.8 nm and results quite simply from the inversion of the gadolinium absorption spectrum. It can be seen from the last two figures (Figure 3, parts e and f) that the derivative operation enhances the fine structure, which should lead to an improvement in the resolution or the discrimination limit.
1
Quartz cell : 4.Ocm Scan speed : 7.5 nmlmin Spectral bandwidth: 0.25 nm Response 0.2 s
272
276
280 268 272 Wavelength ( n m l
276
280
Figure 3. Subtraction of nitrate interferekein the region between 268 and 282 nm by the first- and second-order spectral derivative modes; NaNO, and Gd(N03)3soiutlons have equivalent nitrate concentrations (0.0509 M or 9.462 g L-' NO,). d2AldX2, ABS
0201
,
I
I
4
j48 j54
-46
-
-
-44 7 5 0
-42
46
-42 -I38 -
-40
-36 -38
-
134J , -32
-
I30 -I3'
Flgure 4. Calibration curves for gadolinium according to the ABS, dA 1,and d ' A A', modes as a function of the Gd concentration in Gd(N03)3solutions (the d ' A A' data are negative values).
/a
/a
/a
corresponding to the maximum and minimum values of dA/dX and d2A/dX2, respectively, were tabulated for Figure 4. The most striking feature of the curves in Figure 4 is their linearity in the range of Gd concentrations of interest (around 8 g L-l f 10%);a slight deviation is detected only at concentrations over 9 g L-' Gd. Thus the derivative spectra, like the absorption measurements, obey Beer-Lambert's law despite the relatively elevated concentrations involved. Table I presents the values obtained for the precision using the results of Figure 4. The sensitivity, defined as the 6 g L-I/ 10 g L-1 signal ratio, is constant from one mode to another, as expected from applying these different techniques to treat the same set of data. Maximum precision is obtained with dA/dh mode, although it is quite acceptable for the other two modes (Table I). The detection limit for each mode was calculated
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1980
1155
Table I. Measuring Sensitivity and Precision of the Analytical Modes Based on the Average of 15 Readings at 99% Confidence Gd concn, g L-'
5.999 7.002 7.999 8.997 9.993
ABS mode signal, AU" precision g L-'
aA/aX mode
f0.058 f0.036 f0.049 f0.129 f0.066
0.112 0.130 0.149 0.167 0.181
a2A/ah2mode
signalb
precision, g L-'
signalb
precision, g L-'
25.394 29.718 34.026 38.287 42.240
f0.016 f0.024 f0.035 f0.051 f0.035
36.705 42.920 48.919 54.942 60.630
f0.045 f0.057 f0.046 f0.064 f0.046
Absorbance units. *Arbitraryunits. Table 11. Short-Term Reproducibility for the Different Analytical Modes Based on Data from Three Series of Measurements ABS mode no.
solution
1
Gd(N03)3*6H20
2
Gd(N03)3*6HzO Sm(N03)3.5Hz0 NaN02 Gd(NO&6HzO Sm(N03)3.5Hz0 NaNO, Gd(NOJ3.6HZO Sm(N0J3.5HZO NaN03 Gd(N03)3*6HzO Sm(N03)3.5Hz0 NaN03
3 4 5
amt of Gd, g L-' 7.5 7.7 7.8 8.2 7.0
true value of Gd, g L-'
am' of Gdr g L-' 1 2 3
aA/ah mode
S
amt of Gd, g L-I 1 2 3
a2A/aX2mode
S
amt of Gd, g L-' 1 2 3
S
7.44 7.70 7.79 8.21
7.25 7.67 7.70 8.14
7.39 7.62 7.66 8.16
7.42 7.70 7.75 8.08
0,090 0.040 0.045 0.042
7.47 7.78 7.92 8.17
7.29 7.59 7.71 8.16
7.31 7.66 7.71 8.25
0,098 7.56 0.096 7.77 0.121 7.82 0.049 8.16
7.32 7.60 7.68 8.14
7.42 7.58 7.77 8.27
0.120 0.104 0.071 0.070
7.00 6.99
6.91 6.91
6.86 6.83
6.98 6.95
0.060 0.061
7.30 7.04
7.32 7.08
7.29 7.02
0.015 0.031
6.92 6.98
6.85 6.87
6.93 6.93
0.043 0.055
7.40 7.40
7.30 7.35
7.18 7.34
7.22 7.34
0.061 0.007
7.55 7.47
7.50 7.31
7.45 7.37
0.050 0.080
7.41 7.39
7.20 7.43
7.38 7.60
0.113 0.111
7.60 7.60
7.46 7.52
7.52 7.53
7.50 7.48
0.030 0.026
7.74 7.56
7.65 7.44
7.60 7.64
0.071 0.100
7.49 7.68
7.58 7.44
7.44 7.56
0.071 0.120
7.79 7.79
7.75 7.64
7.42 7.61
7.82 7.75
0.213 0.104
7.90 7.83
7.56 7.65
7.77 7.70
0.171 0.093
7.68 7.81
7.47 7.63
7.81 7.73
0.176 0.090
7.4 7.6 7.8
using the equation DL = 3/4(Nb/s), where N b and s correspond, respectively, to the background noise and sensitivity of an 8 g L-l Gd solution. We observe that the dA/dX mode also offers the best resolution; the smallest discernible quantity in concentration is 0.071 g L-l corresponding to a variation of 0.89% , whereas the values for the ABS and d2A/aX2modes are 0.153 g L-l (or 1.92%) and 0.103 g L-' (or 1.28%))respectively. This improved resolution compared with the ABS mode is due to the enhancement of the fine structure at the cost of loss of the background signal as noted in Figure 3. The slight loss in discrimination between the first and second derivatives can be attributed to the increase in noise, which is normally observed when going progressively to higher derivative orders (15). Furthermore in this work the response time was intentionally kept constant from one mode to another for comparison purposes. Table I1 presents typical results obtained from measurements made over a period of 3 days to test the accuracy and short-term reproducibility; the instrument was reset to zero at the beginning of each test series. The solutions measured contained either gadolinium nitrate at concentrations of interest (7.5,7.7,7.8, and 8.2 g L-l Gd) or mixtures of this salt with sodium or samarium nitrate. These two salts were chosen to simulate the two extreme cases: one where the complexing reactions of Gd are unchanged (Sm(N03)3),the other where the equilibria are displaced through complexation (NaN03). The mixtures were formulated such that the total content of NOS remained equivalent to that of the target solution, i.e., 9.465 g L-' Nos. The data presented show that the standard deviation varies between 0.09 and 2.73%) 0.21 and 2.19%, and 0.61 and 2.25%, respectively for the ABS,dA/dX, and d2A/dX2 modes. The reproducibility can therefore be considered to be adequate for our purpose from one mode to another; furthermore, it is barely affected by the nitrate sources. For the three tests reported in Table 11, the largest calculated difference with respect to the true value was 1.58%) 0.60%)
and 0.85% for the ABS, dA/dX, and d2A/dX2 modes, respectively, for a solution of 8.2 g L-l Gd. The presence of nitrate salt mixtures in the solution causes only a slight reduction in this accuracy. In the particular application of this technique to on-line monitoring of Gd concentrations in the solutions used for the emergency shutdown system of CANDU reactors, all three modes studied could be used to meet the analytical requirements. However, the first-order derivative (dA/dX) approach gave the best results as far as the precision, accuracy, and discrimination limit are concerned. It should be borne in mind that these parameters were obtained from well-controlled laboratory samples and that the three modes should therefore be compared in a real system in order to assess the applicability of the final choice. Furthermore, the results suggest the possibility of using this technique to trigger alarms in the control room or to activate a completely automatic system that would readjust the gadolinium content by adding a concentrated stock solution. The required optical performance could be achieved by using one of the relatively low-priced instruments on the market, providing that it has a band-pass of C0.76 nm, a wavelength accuracy of
