Determination of Fixed Water in Rocks by Infrared Absorption Irving A. Breger and John C. Chandler US.Geological Suruey, Washington, D.C. 20242 An infrared absorption technique has been developed for the quantitative determination of “fixed water” (H20+) in rocks. Potassium bromide disks containing 2-mg samples are scanned in the 3-pm spectral region and absorption at 2.96 pm is determined. Although the exact nature of this peak is not known, other than that it is caused by an interaction between the potassium bromide and hydroxyl groups and water, it can be used for quantitative analysis. Rock samples, other than those containing significant percentages of clay minerals, can be analyzed with a standard deviation of 0.26%.
WATERCONTENT of rocks and minerals is customarily reported in two forms: HzO- (“water minus”), and H2O’(‘‘water plus”). The first, “water minus,” refers t o loosely held water normally expelled from the sample when it is heated to 110 “C for a specific time, usually 30 minutes to several hours. Part or all of the water of minerals such as gypsum (CaS04.2Hz0) may be lost, as well as part or all of the interlayer water of clays. The degree of dehydration is frequently dependent upon the temperature t o which a sample is heated as well as on the duration of the heating period. “Water plus” ( H 2 0 +or fixed water) is that which is recovered when a rock sample is heated above 110 “C to a high temperature, usually about 1000 “C, with or without a flux. Such water includes that tightly held in the crystal structure as well as that formed by degradative elimination of hydroxyl (-OH) groups. Removal of such structural entities destroys the crystal lattices. Most analyses for the water content of rocks are based upon variations of the Penfield fusion technique ( 1 ) . It is standard practice t o heat a rock specimen to about 1000 “C to determine total moisture, and then to determine HzO- on a separate 1-gram sample by heating it for a predetermined time at 110 “C. “Water plus” (HzO+) is calculated as the difference between the two values. The Penfield method requires fairly large samples for accurate determinations. Peck ( I ) specifies the use of 1-gram samples, as d o Shapiro and Brannock in their “rapid” modification of the Penfield technique (2). As water values diminish, the error becomes greater thereby introducing a n element of doubt in the validity of low results obtained in routine techniques. Moreover, on heating to high temperatures, sulfur may be expelled from a sample as oxides of sulfur and weighed as water. Other possible errors are inherent to the technique. Another less frequently used method for the determination of water is one in which the rock is heated to 1450 “C by means of a n induction coil, the water released is converted to hydrogen gas by passing it over uranium turnings at 600 “C, and the hydrogen is determined manometrically. Success of the method depends, among other things, upon the availability of sufficient sample to yield 3-10 mg of water (3). Although the Penfield technique and variations based on it are normally used, large samples are required and it is difficult to assure the total elimination of water and hydroxyl groups (1) L. C.Peck, U.S. Geol. Survey Bull., 1170, 61 (1964). (2)L. Shapiro and W. W. Brannock, ANAL.CHEM., 27, 560 (1955). (3) I. Friedman and R. L. Smith, Geochim. Cosmochim. .4cta, 15, 218 (1958). 506
ANALYTICAL CHEMISTRY
from certain samples in which these components are particularly firmly bound. Development of a n alternate .method for the determination of water in small samples has long been desirable. Small amounts of water and hydroxyl groups can readily be detected in both organic and inorganic specimens by infrared absorption analysis. Study of rocks and minerals has been conducted by grinding a sample in mineral oil or a fluorinated oil that serves as a dispersing agent. This method, however, is only qualitative in nature. The familiar potassium bromide disk technique has several advantages over the mineral oil-suspension procedure. First, considerable scatter of incident light is eliminated by embedding the sample in a matrix (KBr) of similar index of refraction; second, the method can be adapted to quantitative analysis; and third, there is no interference from the potassium bromide, which is transparent to infrared radiation between wavelengths of from 2 t o 25 pm. Hydroxyl contents of organic substances have been determined quantitatively by this technique ( 4 , 5), but until now a number of difficult problems have precluded the development of a satisfactory quantitative method for water in either organic or inorganic substances (6-9). A major factor in the failure to develop a satisfactory quantitative procedure for water is likely related to the presumed hygroscopic character of potassium bromide and the appearance of a rather broad absorption at 2.96 pm (3380 cm-1) that has been attributed t o water. This absorption overlaps that for the symmetrical vibration (vl) for water at 3400 to 3450 cm-1 (2.90 to 2.94 pm) (9, IO), and normally interferes in the quantitative determination of water in a sample distributed in a potassium bromide disk. Numerous studies have been conducted of the absorption by potassium bromide at about 3400 cm-1 (2.94 pm), and numerous explanations for the phenomenon have been suggested. Friedel and Retcofsky (11) attributed the peak at 2.96 pm to an interaction between potassium bromide and water. Durie and Szewczyk, however, investigating the same observation with potassium chloride, concluded that the absorption is not related to water but may be caused by the substitution of hydroxyl ions in the potassium chloride lattice (12). We, as others before us, have also failed to eliminate the absorption of potassium bromide at 2.96 pm, even after many attempts at careful dehydration of the salt. As a result of our work, we have concluded that this persistent absorption most likely results from the interaction of potassium bromide with both water and hydroxyl groups. Although this problem has been extremely troublesome, we have devised appropriate (4)R. A . Durie and S. Sternhell, .4ustra/iarz J . Chem., 12,-205 (1959). (5) S. Friedman, M. L. Kaufman, W. A. Steiner, and I. Wender, Fuel, 40, 33 (1961). (6)K. Nakamoto, M. Margoshes, and R. E. Rundle, J . Amer. Chem. Soc., 77, 6480 (1955). ( 7 ) P. J. Lucchesi and W. A.Glasson, ibid, 78, 1347 (1956). (8) G.V.Yukhnevich, Russ. Chem. Rev. (in English), 32,619 (1963). (9)C.Rocchiccioli, Chim. Analytique, 46,452 (1964). (10)D.F. Hornig, H. F. White, and F. P. Reding, Spectrochim. Acta, 12, 338 (1958). (11) R. A. Friedel and H. Retcofsky, Proc. Conf. Carbon, 5th, Univ. Park, Penna., 2, 149 (1963),Pergamon Press, Oxford. (12) R. A. Durie and J. Szewczyk, Spectrochim. Acta, 13,593 (1959).
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Figure 1. Four types of absorption peaks in 3-pm region encountered in scanning potassium bromide disks containing rock samples
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methods to eliminate interference from this absorption by the potassium bromide used for analysis. Any absorption at 2.96 pm that is reported in this paper, therefore, reflects both the water and hydroxyl content only of the sample dispersed in the potassium bromide. This work was conducted to develop a procedure for the quantitative determination of HzO+in 2-mg samples of rocks. An analytical technique was sought which would (a) be rapid, (b) require very small samples, (c) give results comparable to those obtained by conventional techniques, and (d) be useful for the widest possible variety of rock samples.
the die. The disk is set on a small Teflon supporting ring and placed in a drying oven at room temperature. The oven is turned on, permitted to reach 110 "C over a period of several hours, and then maintained at this temperature overnight or for about 16 hours. The dehydrated disk and ring are then fitted into the holder that has been preheated to 110 "C, and the disk is scanned between 2 and 4 pm while hot and with a similarly heated 300-mg potassium bromide disk in the reference beam. Base-line absorbance is read at 2.96 pm, and the average of duplicate determinations is used to obtain per cent H20+from the calibration curve of Figure 3.
EXPERIMENTAL
Infrared Methods, An undesirable feature of the pressed disk method is the occasional interaction between a sample and the potassium bromide that leads t o absorption peaks that are difficult to interpret. Similarly, some peaks, such as those for hydroxyl groups or water in the 3-pm region, may shift or disappear entirely, perhaps to reappear at 2.96 pm. The work described in this paper has taken advantage of such an otherwise undesirable feature, the interaction of potassium bromide with water and hydroxyl groups, in the development of an analytical technique. The usual absorption by water at 2.96 pm is shown in Figure l A , where the technique of base-line measurement is also shown. Figures 1B and 2E, F,G, and H show examples of multiple absorptions that have been observed in the 3-pm region. In each instance, the potassium bromide-interaction peak at 2.96 pm or a change in slope related to it, is evident. Multiple absorptions are, however, rare for rock samples, and a discussion of their significance will be reserved for a future paper. Fortunately, nearly all rock samples containing up to 10% H20+ have absorptions similar to that of Figure l A , and the method described in this paper is restricted to analysis of only this type of absorption, a single peak at 2.96 pm. Few rocks contain more than 10% HzO+,so this limitation presents no particular problem. Figures 1 C and D illustrate the effects of heating a sample with high water and hydroxyl content a t 110 "C for several hours. Here the interaction peak at 2.96 pm nearly disappears (Figure 1 0 ) in accord with the removal of loosely held water. The unchanged absorption at 3.15 pm, however, illustrates other features of the water-hydroxyl system that are not thermally sensitive at 110 "C. The absorption peak at 6.15 pm,
Equipment. A Perkin-Elmer Model 21 double beam infrared spectrophotometer with sodium chloride optics was used in this study. To minimize interference from the absorption that normally accompanies potassium bromide at 2.96 pm, both the sample and the reference disks (300 mg) were maintained at 110 "C during analysis by use of heating units similar to those described by Longworth and Morawetz (13). When this technique was used, a differential scan ofheated pure potassium bromide disks in both the sample and reference beams led t o no appreciable absorption at 2.96 pm. Thus, analysis of a disk containing the sample, with a pure potassium bromide disk as a reference and maintaining both disks at 110 "C, doubly ensures the spectral reading at 2.96 pm against any significant interference. Standard Procedure. Approximately 50 mg of 80-mesh rock sample is pulverized in a Wig-L-Bug vibratory grinder for 2 minutes. Stainless steel sample holder and hammer are used, and grinding is conducted in four 30-second increments with 10-second interim pauses to prevent possible heating of the sample. The ground sample (2 0.01 mg) is carefully weighed and mixed with 298 mg of potassium bromide in a similar stainless steel container with hammer. Again with the Wig-L-Bug, the mixture is ground for two 30-second intervals with an interim 10-second pause. The hammer is then removed, all adhering mix is returned to the container, and the container and contents, without the hammer, are shaken in the Wig-L-Bug for 30 seconds to assure homogeneity. The sample is then removed quantitatively and pressed under vacuum at 15 tons/square inch for 8 minutes. Pressure is then relieved and reapplied for 4 minutes, after which the 13-mm disk is removed from (13) R. Longworth and H. Morawetz, Chem. Znd. (London), 1955,
1470.
RESULTS AND DISCUSSION
VOL. 41,NO. 3, MARCH 1969
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shown in Figures 1 and 2 , is related to the water content of the sample. Preparation of Disk. Microscopic analysis has demonstrated that in every instance approximately 80% of the sample ground for analysis had a particle size of 2 pm or less. The standard procedure led to translucent, homogeneous disks and guaranteed the preparation of disks having the same thickness. The disks were heated at 110 "C overnight (16 hours) in order to dehydrate the samples (remove HzO-) in accord with the chemical procedure (2) to which the infrared results were to be compared. Duplicate disks were prepared and analyzed for each sample. Grinding Time. Earlier work in our laboratory had demonstrated that lengthy grinding of certain samples could lead to the sorption of water which was then difficult to remove, even at 110 OC. Failure to grind samples fine enough, on the other hand, might lead to reflection rather than transmission of incident infrared radiation from the sample particles, especially in the 2-pm wavelength range. It was necessary, therefore, to determine an optimum particle size that would satisfy analytical requirements for most types of samples. In this respect, it ~
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must also be noted that some rocks are tough and difficult t o grind, whereas others are soft and can be readily pulverized. Samples 2, 3, 5, 6, 10, 11, 13, and 17 of Table I were chosen for the study of grinding times inasmuch as these samples both covered a wide range of H 2 0 +values (0.17 to 9.6%) and also represented a broad spectrum of rock types. A program of pauses between periods of grinding (Table 11) was set up to make certain that the samples did not inadvertently undergo heating. A fresh, 80-mesh, 50-mg quantity of each sample was ground for each period of time to be tested-Le., 1, 2, 3, 4, 5 , 6, or 7 minutes. Duplicate disks were made from each powdered sample and scanned at wavelengths between 2 and 4 pm as in the standard procedure. Results were evaluated by plotting absorbances against fixed water contents, determined chemically, for the eight samples pulverized for the same length of time. A least-squares line was drawn, and a standard deviation was calculated. A standard deviation was then determined for each grinding time, and as shown in Table 111,the optimum time of grinding of a sample was found to be 4 minutes. The data of Table 111 also show, however, that grinding a sample for 2 minutes
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Table I. Chemical Analyses for Fixed Water and Corresponding Base-Line Infrared Absorbances a t 2.96 pm for Rock Samples Sample Rock Fixed water,a Disks scanned, No. type % No Absorbance 0.00 2 0.039 k O.OOlb 1 Dolomite 0.17 2 0.046 -i- 0.005 2 Vitrophyre 0.46 2 0.054 f 0.OOO 3 Diabasec 0.54 2 0.033 -i- 0.008 4 Tuff 0.80 2 0.075 f 0.005 5 Andesi ted 1.5 2 0.063 & 0.003 6 Basalt, columnar 1.5 4 0.079 k 0.011 7 Basalt, columnar 1.9 4 0.064 F 0.009 8 Ironstone, hematitic 9 Ironstone, manganiferous 2.2 4 0.123 k 0.007 10 Basalt, pillow 2.9 2 0.149 f 0.005 11 Tuff 3.2 2 0.142 k 0.004 12 Porcelanite 3.8 3 0.158 2 0.016 4.7 3 0.205 k 0.004 13 Soapstone 14 Basalt, pillow 5.3 3 0.223 0.012 15 Saprolite from gneiss 6.3 3 0.222 0.007 16 Phosphate porcelanite 6.8 3 0.293 0.016 17 Saprolite from amphibolite 9.6 2 0.371 f 0.009 a H20+, analyses by L. Shapiro and staff of U.S. Geological Survey using "rapid methods" (2). b Average deviation from mean. c Ref. H. W. Fairbairn et al., US.Geol. Survey Bull., 980 (1951). d Ref. F. J. Flanagan, Geochim. Cosmochim. Acta, 31, 289 (1967). -~
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Table 11. Grinding Programs for Rock Samples Program Overall grinding SchedSchedSchedSchedtime,min. ule Pause ule Pause ule Pause ule 1 Aa 2 Bb 3 B 1 rnin A 4 B 1 min B 5 B 1 min B 1 min A 6 B 1 rnin B 1 rnin B 7 B 1 rnin B 1 rnin B 1 rnin A a Schedule A: Grind, 30 seconds; pause, 10 seconds; grind, 30 seconds. b Schedule B: Grind, 30 seconds; pause, 10 seconds; grind, 30 seconds; pause, 10 seconds; grind, 30 seconds; pause, 10 seconds; grind, 30 seconds.
Figure 3. Correlation of absorbances a t 2.96 pm to values for H 2 0 - obtained by rapid rock analysis methods (2) Data for samples listed in Table I shown as open circles; data for pseudo-tachylites (Table IV) shown as solid circles leads t o results nearly as good as those obtained on 4-minute grinding; longer periods of grinding lead to more scatter (standard deviation 0.51 at 7 minutes), confirming previous observations that some samples may irreversibly sorb water on prolonged pulverization. To confirm these results, each of the 17 samples shown in Table I was ground for 2 minutes and disks were prepared for analysis according t o the usual procedure; the same was carried out with each of the 17 samples ground for 4 minutes. Now, on the basis of 17 samples, the standard deviation for 2-minute grinding was 0.26, whereas that for 4-minute grinding was 0.70. The values based on 17 samples represent a reversal of the data of Table I11 and demonstrate, where more samples are used, that 2-minute rather than 4-minute grinding leads t o more consistent results. In actuality, there are n o significant differences at the 9 5 z level between the variances for the 1-, 2-, 4-, and 5-minute runs based on the 8 samples of Table 111. The 2- and 4-minute runs, however, with the minimum standard deviations, warranted further analysis and, as was demonstrated, the 2-minute grinding period leads t o more significant data. Heating Time. As infrared absorbances were to be compared to H?O+ values determined on the samples by the Shapiro-Brannock technique (2), it was desirable, for consistency, t o preheat the rock samples for 16 hours at 110 "C prior to making infrared measurements. This procedure is followed in the chemical analysis to remove loosely held water (H20-), and in the procedure (2) the sample is merely heated overnight in a n open container. Where the sample is used for infrared absorption measurements, however, handling following drying may result in re-hydration. For this reason, it was best to prepare the sample disk and then place it in a drying oven at room temperature, raising the temperature t o 110 "C over a period of up to 2 hours. The oven was then maintained at 110 "C for 16 hours, and the disk was transferred directly to a heated holder and scanned while hot. Inasmuch as dehydration of a sample is dependent upon the rate of diffusion of water through pressed potassium bromide, it was necessary t o establish the time required t o assure completion of the process. To d o this, three samples were chosen (Table I, Nos. 2, 4, and 6), disks were prepared, and absorbances at 2.96 pm were measured after heating the disks
Table 111. Evaluation of Optimum Time of Grinding Based on Eight Samples Standard deviation, % Time of grinding, min. 1 0.33 2 0.24 4 0.19 5 0.26 7 0.51 Table IV. Comparison of Chemical and Infrared Analyses for Pseudo-tachylites Chemical Infrared Sample Disks scanned. H,O+,a H,O+. o/ No. No. Absorbance % / O 2 18 0.25 0.042 k 0.000t 0.28 19 0.33 0.041 i: 0.003 0.28 2 20 0.043 k 0.001 0.28 2 0.34 2 21 0.36 0.038 -C 0.001 0.28 22 2 0.40 0.040 zk 0.002 0.25 23 2 0.045 k 0.001 0.29 0.49 24 2 0.54 0.043 k 0.003 0.28 25 0.54 0.053 zk 0.005 0.61 2 26 2 0.55 0.048 k 0.002 0.45 27 2 0.043 k 0.001 0.28 0.56 28 2 0.60 0.046 i: 0.000 0.27 29 2 0.090 zk 0.009 1.62 1.25 30 2 1.31 0.044 & 0.009 0.29 4 31 1.47 0.071 i: 0.018 1.09 32 2 0.153 i: 0.001 3.41 2.74 2 0.171 i: 0.004 3.94 33 3.49 a Values obtained from calibration curve of Figure 4. b Average deviation from mean. for periods ranging from 1 to 38 hours. Dehydration of all three samples was virtually complete at the end of 10 hours. The rapidity with which dehydration of these samples occurs, in contrast t o the comparatively slow loss of water from C a S 0 4 . i H 2 0 in potassium bromide disks at 150 "C as reported by Farmer (14), may merely reflect the relative degrees of retention of water by the CaS04 and the rock samples. Evaluation of Data. Corresponding chemical analyses and infrared absorbances for each of the 17 samples of Table I are plotted as open circles in Figure 3. The calibration curve of Figure 3 is based on a least-squares calculation; standard deviation is 0 . 2 6 z . This value includes errors involved in the rock analyses conducted chemically for which the average deviation from the mean is a b o u t + O . l z , as well as those errors inherent in the infrared technique. This relatively low standard deviation is particularly significant because it includes data for many kinds of samples. The intercept for the least(14) V. C. Farmer, Spectrochim.Acta, 22, 1053 (1966). VOL. 41. NO. 3, MARCH 1969
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squares line is 0.033 absorbance unit, and the slope of the line is 0.034 absorbance unit/per cent H20+. To evaluate the infrared technique further, a suite of 16 pseudo-tachylites was analyzed. These samples are of Precambrian age from South Africa and are part vitreous-part crystalline products derived from altered gneiss. Penfield analyses for total water were conducted by standard semimicro techniques, and H20- was calculated on the basis of loss of weight on heating each sample for 16 hours at 110 “C. Fixed water (H,O+) was recorded as the difference between the two values. Chemical values for fixed water are plotted against the corresponding absorbances as the solid circles of Figure 3; comparison of chemical values with the values for fixed water determined from the absorbances and the calibration curve of Figure 3 are shown in Table IV. X-ray analysis has demonstrated clay to be present in some of the samples of Table IV and, as already noted, the presence of clays presents an analytical problem. There is evidence for small percentages of kaolinite in samples 29 and 32, and it has also been noted that duplication of H20- values for some of the other samples is difficult, indicating the presence of constituents that dehydrate inconsistently. In view of the possible presence of small amounts of clay in these samples, and in spite of the fact that an alternate method was used for the chemical determination of H 2 0 + values, it is particularly significant that the standard deviation for the data of Table IV, when comparison is made of results obtained chemically and by the infrared technique, is 0.26%; this value is identical to that for the much more broadly based samples of Table I from which the calibration curve of Figure 3 was originally calculated. Advantages of Infrared Technique. The relative simplicity of the infrared technique, in which it is not necessary to drive water out of the sample, is self-evident. Moreover, using a
2-mg sample containing 0.2% H20+, the infrared method is able to detect as little as 4 pg of water quantitatively. Where samples are rare, they can also be recovered after analysis by dissolving the potassium bromide in water. The method requires no equipment other than that normally available in any infrared installation and follows a simple procedure. The method also lends itself to mass production operations where such may be necessary. Only small amounts of sample are necessary, and nearly all rock samples, except those containing over about 10% H20+or having appreciable amounts of clay minerals can be analyzed. With minor additional work, it should be possible to adapt the technique to the determination of total water (H20+plus HzO-) in rock samples. To the best of our knowledge, water in any form has not previously been determined quantitatively using the infrared pressed-disk techniques except in one instance where an attempt was made to determine the water content of a coal sample by measurement of absorbance at 2.95 pm (15). ACKNOWLEDGMENT
Frank Cuttitta, M. K. Carron, and Dennis Ligon kindly supplied the pseudo-tachylites and the corresponding analyses. Leonard Shapiro made all the other samples available along with the “rapid rock” analysis for each, and F. J. Flanagan was our consultant in statistical problems. All are from the U.S. Geological Survey and are our friends. We thank them.
RECEIVED for review May 22, 1968. Accepted December 16, 1968. Publication authorized by the Director, US.Geological Survey. (15) R. A. Friedel in “Applied Infrared Spectroscopy,” D. N. Kendell, Ed., Wiley and Co., New York, N.Y., 1966, p 317.
Determination of Nickel( I I) by Quenching of the FIuoresc ence of AI uminum-1 - (2- Py ridy Iazo)-2- NaphthoI and Direct Fluorometric Determination of Cobalt George H. Schenk, Kenneth P. Dilloway, and John S. Coulter Department of Chemistry, Wayne State Unioersity, Detroit, M i d . 48202 After a general investigation of the reported fluorescence colors of various metal-PAN [1-(2-pyridylazo)-2naphthol] complexes, it was found that the nickel(l1)PAN did not fluoresce, but that nickel(l1) could be determined in the 10-9 to 10-7M range by the fluorescence quenching of the aluminum(lll)-PAN complex in absolute ethanol. Measurements can be made after a 40-minute heating period or after four hours at room temperature. This method is far more sensitive for traces of nickel(l1) than atomic absorption spectrometry or the present colorimetric methods, and has few serious interferences. The same general investigation led to the discovery that a fluorescent species develops when cobalt(l1) air oxidizes in the presence of PAN dissolved in 95% ethanol. This oxidation occurs more rapidly in absolute ethanol, and a direct fluorescence method of determination was developed in both solvents.
CHELATES of lighter metals, such as aluminum(III), and organic ligands, such as PAN [1-(2-pyridylaz0)-2-naphthol],are known to fluoresce efficiently as symbolized below : I(Al-PAN)+ (AI-PAN)o 510
ANALYTICAL C H E M I S T R Y
+ hy
(1)
where I(A1-PAN) is the first excited singlet and (Al-PAN)o is the ground state chelate. In contrast, very few complexes of transition metal ions with partly filled d sublevels are fluorescent in fluid solutions. Since most coordinated transition metal ions are paramagnetic, the usual rationalization ( I ) given is that the rate of intersystem crossing (IC) in the chelated excited ligand is enhanced by the influence of the unpaired electrons of the metal ion. This may be represented for Ni(I1)-PAN as: l(Ni-PAN)- (IC)-.
YNi-PAN) - (Q)-
(Ni-PAN)o
(2)
where the first excited singlet crosses over to the triplet state, YNi-PAN), which may then undergo some efficient type of quenching(Q) in fluid solutionto return to the ground state ( I ) . Also, in other systems, heavy atoms such as diamagnetic transition metal ions with partly filled 4 or 5 d sublevels, are known to increase spin-orbit coupling which also increases the rate of intersystem crossing ( I ) . This again leads to quenching (1) D. M. Hercules, “Fluorescence and Phosphorescence Analysis,” Interscience-Wiley, New York, N.Y., 1966, pp 151-65.
