Table I. Results of Mixture Analyses by the Linear Plotting Method
l o S_ c l ( ~_ w)
xa A
CN
C1 C1
a
_ B
C1 H
F
kA/kB Taken
8.5 2.7
2.0
3.18 2.12 1.06 3.41 2.28 1.13 2.53 3.32 1.10
1o5c;(~w)
~
Found
Taken
Found
2.89 2.04 0.96 3.31 2.26 1.35 2.23 3.34 1.28
0.99 1.98 2.98 1.07 2.13 3.18 1.46 0.73 2.18
0.98 2.05 3.06 1.27 2.03 2.76 1.31 0.59 1.86
Fraction B Taken Found
0.24 0.48 0.74 0.24 0.48 0.74 0.37 0.18 0.67
0.25
0.50 0.76 0.28 0.47 0.67 0.37 0.15 0.59
X in X-C,H,COOC,H,-NO,.
contribution from volumetric factors or concentration conversions. The principal source of error in this graphical method is in the k A and k B values required to make the plot. These must be determined in separate experiments on the individual components, and the assumption is made that the same values apply in the reaction of the mixture. Aside from normal experimental uncertainty, changing reaction conditions may weaken this assumption. It is also possible that the two compounds interact in their mixture, with mutual kinetic effects. Some complication of this kind apparently occurred in these systems, degrading the analytical accuracy. For example, the semilogarithmic extrapolation method applied to a mixture of the cyanobenzoate and the chlorobenzoate yielded k A = 0.0335 sec-I and k B = 0.00324 sec-I, whereas the independently measured values (which were used to make the linear plot) were k A = 0.0310 sec-l and k B = 0.00364 sec-l. Figure 3 shows the analysis of a mixture of the cyanobenzoate ( A ) , chlorobenzoate ( B ) , and benzoate (C) esters. These results are found from the plot: lo5 C i , taken 1.41, found 1.45; lo5 COB, taken 1.51, found 1.31; lo5 C& taken 1.41, found 1.60. These experiments reveal that this new graphical method is capable of application to some mixtures that cannot be treated by the semilogarithmic extrapolation method. When k A / k B is small or when ci/cg is large, the new meth-
O5I
Figure 3. Analysis of a three-component mixture; details as described in the text
od is superior. When k A l k B is large, the semilogarithmic plot will usually be the preferred method. Thus, of the mixtures reported in Table I, the cyanobenzoate-chlorobenzoate system can be handled with semilogarithmic plotting, whereas the other two systems require the new method. Although this graphical technique requires that the rate constants be independently measured prior to its application and that Cg be known, its notable advantage is that it can make use of all of the kinetic data, and it nicely complements the semilogarithmic graphical method.
ACKNOWLEDGMENT The ester samples were kindly furnished by Joseph R. Robinson.
LITERATURE CITED (1) H. B. Mark, Jr., and G. A. Rechnitz, "Kinetics in Analytical Chemistry", Wiley-lnterscience, New York, N.Y., 1968. (2) H. C. Brown and R . S. Fletcher, J. Am. Chem. SOC.,71, 1845 (1949). (3) J. B. Worthington and H. L. Pardue. Anal. Chem., 44, 767 (1972). (4) B. G. Willis, W. H. Woodruff, J. R. Frysinger, D. W. Margerum, and H. L. Pardue, Anal. Chem., 42, 1350 (1970). (5) R. J. Washkuhn, V. K . Patel, and J. R. Robinson, J. Pharm. Sci., 60, 736 (1971).
RECEIVEDfor review July 7, 1975. Accepted October 1, 1975. This work was supported in part by National Science Foundation Grant GP-36567.
Thermoparticulation Analyses of Malonic Acid Compounds J. D. B. Smith, D. C. Phillips,' and T. D. Kaczmarek Westinghouse Research Laboratories, Pittsburgh, Pa. 15235
Decomposition studies of malonic and related carboxylic acid compounds were carried out using the new technique of organopartlculation analyses (OPA). When an ion chamber detector technique Is used, malonic acid and seven of its alkyl and aryl carbon-substituted analogues are strong emitters of particulates at temperatures below 200 'C. Chemically similar carboxylic acids, such as oxalic and maleic, do not show this behavior. This particulatlon was found, in several instances, to be in close proximity to the llterature decomposition temperatures for these compounds. However, in other cases, particulation Is observed well beneath and sometimes well above the literature decomposi-
tion values. Mass spectral studles on the effluent arlslng from the decomposition of these compounds were carried out in an attempt to characterize the exact nature of the particulates. Indications are the particulates may consist of aerosol size particles of volatile carboxylic acid "ollgomers" suspended In an atmosphere of carbon dioxide.
Thermoparticulate analysis, which was used by Doyle ( 1 ) for polymer degradation studies, is a technique based upon the detection of condensation nuclei evolved from polymeric materials undergoing decomposition when s-ubjected to ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
89
ThermPcoupleSf
,-Insulating
Stand-off
I
Linear Temperature Programmer
Detector
A
I
Recorder
=
1
p J k Flgure 1. System utilized for generating particulates
programmed heating. Recently, we have been successful in adapting this technique to follow the thermal decomposition mechanisms of well-known organic compounds such as diazoniums (2). The new technique is termed Organic Particulate Analysis (OPA) and enables a new physical property, namely, the temperature at which particulates are emitted from organic substances, to be measured. This analytical procedure makes it possible to detect the initiation of thermal decomposition at significantly lower temperatures than were previously assigned to such organic compounds. In addition, the extreme sensitivity of the instrumentation used in this new technique has revealed that some of the organic compounds investigated are very strong sources of particulate emission, sometimes at temperatures well beneath their literature decomposition temperatures. This paper describes the studies which have been carried out on malonic acids and related compounds using OPA. Malonic acid and its alkyl- and aryl-substituted derivates are known to be temperature sensitive, exhibiting decomposition temperatures in the range 100-200 O C ( 3 ) .This instability suggested that this particular group of organic compounds could be likely sources of particulates close to their decomposition temperatures and therefore be amenable to investigation using the OPA technique.
EXPERIMENTAL Two very sensitive analytical instruments are commercially available for the detection of vapor-borne particulate matter. (Both instruments are marketed by Environment One Corporation, 2773 Balltown Road, Schenectady, N.Y. 12309.) One instrument, an ion chamber detector ( 4 ) , utilizes the change in current output of an ion chamber as a function of particulate concentration, whereas the other instrument, a condensation nuclei detector (5, 6), utilizes the principle of a cloud chamber in which water is condensed upon submicroscopic particles to produce visible, micron sized droplets. Although both techniques have been found suitable to investigate the thermal decompositions of malonic acids, most of the work reported here was carried out with the ion chamber detector. Mode of Operation of Ion Chamber Detector (4).Submicron particles can be detected by their influence on the output current of an ion chamber arranged to collect the small ions produced by a low level radiation source in the gas stream containing the particles. In the absence of particles, almost all the ions are collected, and this results in a maximum output current of a magnitude determined by the strength of the radiation source and the ionization properties of the gas stream. When particles are present in the ionized gas stream, some ion-particle combinations take place. Because the particles are much larger than the ions, the mobility of the resultant charged particle is less, and only a few of the species are collected in the ion chamber. The result is a decrease in the output current of the ion chamber; this decrease being a function of the particle concentration and particle size. Concentrations as low as 2 X g/l. can be detected. Mode of Operation of Condensation Nuclei Monitor. In a Condensation Nuclei Monitor, water vapor is caused to condense 90
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
on particulate matter and the opacity of the vapor is then measured and related by the electrical signal to particle concentration. A constant-flow gas sample is periodically diverted into a humidifier, where its relative humidity is raised to 100% with water. The sample then passes through a rotating valve into a cloud chamber, where it is expanded adiabatically, causing the sample to cool and the relative humidity to rise to a supersaturation of 400%. The cloud attenuates a light beam that is focused on a solid-state light-sensitive element. As the light value is decreased, an electrical pulse is created which is amplified and rectified into a dc signal proportional to the condensation nuclei concentration in the sample. After expansion, the cloud chamber is pressurized to the original atmospheric condition and then flushed with a new supply of gas. The instrument provides a continuous measurement of gasborne particle concentrations in a size range down to 0.001 p. The concentration range of the instrument is from 10 to 10’ particles per cm3 with a response time in the order of 1 to 2 sec. Sample Preparation. The organic compounds under investigation were incorporated into a special air-drying epoxy polymer matrix which serves to prevent the compound from producing “dusting” during testing in the gas stream; this dusting effect could produce false particulate signals on the instrument (Le., the particulates are not derived from decomposition of the organic compounds). The special polymeric epoxy material was shown to particulate a t temperatures well above that shown by the malonic acid compounds under investigation. This ensured the minimum of interference from particulates derived from the epoxy matrix with those originating from decomposition of the organic compounds. The most convenient method of sample preparation was to incorporate the malonic acid compound into an air-drying styrenated, alkyd-modified epoxy varnish using the following composition: Malonic acid compound, 100 parts; epoxy, 100 parts; cobalt naphthenate solution, 1.0 parts; and lead naphthenate solution, 0.25 parts. All of the alkyl C-substituted malonic acids were obtained from Pfaltz and Bauer. Malonic acid was from Eastman-Kodak, and the other carboxylic acids were from Fisher Scientific and Aldrich Chemicals. The epoxy (B-276 resin) was obtained from Westinghouse and contained 50% by weight of toluene solvent. The cobalt naphthenate solution (6% in “Nuodex”) and lead naphthenate (24% lead w /w) were added as “driers” for the epoxy varnish. It was found convenient to add these driers prior to the addition of the malonic acid compound to ensure uniform mixing of the naphthenate solutions with the epoxy varnish. The malonic acid-epoxy mixtures were brushed onto thin section aluminum strips (1 in. X 3 in.) and allowed to air-dry before placing in an oven a t 60 “C for a 16-hr period to remove the last traces of solvent. The carboxylic acid-epoxy matrix, prepared in this fashion, was well-bonded to the aluminum metal surface with no indication of dusting or loosening of the material within the matrix. Small portions were then cut off from these samples (usually measuring 1 in. X in. and weighing -0.5 g), and the thermoparticulation analyses were run as described in the next section. Measurement of “Organoparticulation Temperature”. Figure 1 shows typical apparatus used to study the thermal decomposition of the organic compounds. Accurate temperature measurements were made through a Chromel-Alumel thermocouple attached to a stainless steel boat which rested directly on a strip heater. The entire assembly was mounted on insulating stand-off pedestals within a stainless steel tube (1-inch 0.d.). A phase controlled temperature regulator and programmer, connected through a sealed end-plate to the boat, acted as a temperature control on the heater. The output of the thermocouple and an ion chamber detector was monitored on a two-pen potentiostatic recorder. Hydrogen, a t a constant flow rate of 6 l./min, was passed over the samples contained in the boat. A 6 OC/min heating rate was maintained in each experiment. Two temperatures were read from the charts; the threshold temperature which corresponded to the onset of organoparticulation (as shown by an initial fall-off in amplified ion current) and the temperature which signified a 50% decrease in the ion current (usually 0.8-0.4 mA). These values enabled an organoparticulation “temperature range” to be determined for each sample. Characterization of Effluent by Mass Spectrometry. The total vapor pyrolyzate was collected on Porapak R (a modified divinyl-benzene-type adsorbent obtainable from Waters Associates, Inc.). The analytical instrumentation consisted of the PerkinElmer Model 270 gas chromatograph/mass spectrometer. In this analysis, the gas chromatographic mode was not used. The trapped
~~~
Table I. Organoparticulation Data for Malonic Acid/Epoxy Resin Samples
Sample No.
Sample composition
0.8
m
Organop articulation temperature range, "C (OPTR)
Malonic acid/epoxy resina 126-132 SC-5/21A Malonic acid/epoxy resina 126-136 SC-5/21B Malonic acid/epoxy resina 125-1 30 sc-5/21c 127-133 SC-5122A Malonic acid/epoxy resina Malonic acid/epoxy resina 125-1 32 SC-5/23A 200+ SC-261/1 EpoxyrEesinQ SC-26112 Epoxy resin + 20% silicab 200+ Alkyd-modified styrenated epoxy varnish from Westinghouse Electric Corporation [ B-2761. b Using Cab-0-Sil. Q
pyrolyzate was flushed off the adsorbent a t 180 O C into the mass spectrometer. Mass spectra were taken when the total ion monitor of the Model 270 indicated that sample was reaching the ion source. In some instances, particulates were also collected on glass fiber filter discs and introduced into the mass spectrometer in similar fashion.
Timer, minutes Monitor I o n Chamber Detector H f l t q e n Flow Rate 6JJmln Heating Rate 6 min
Figure 2.
Organoparticulation pattern for malonic acid
RESULTS Organoparticulation from the Malonic Acids. Figures 2-4 are typical organoparticulation responses as shown by the ion chamber detector for malonic, ethylmalonic, and dimethylmalonic acid. As shown in these diagrams, all these materials gave very strong particulation emission; in every instance, the amplified ion current fell well beneath the 50% ion current level on the chart. The data in Table I confirm the reproducibility of the measurements for malonic acidlepoxy samples. Five different samples, three from the same batch (Le., SC-5/21A, SC-5/21B, and SC-5/21C) and two from other batches (i.e., SC-5/22A and SC-5/23A), were evaluated with the ion chamber detector. As the data in Table I show, the organoparticulation temperature ranges agreed to within a few degrees of each other for the five test samples. The data also show that the epoxy resin alone (sample Sc-261/1) did not exhibit organoparticulation beneath 200 "C. Addition of 20% of silica filler to the epoxy resin did not have any noticeable effect on the organoparticulation properties of the epoxy resin. This latter sample demonstrates that altering the mechanical properties of the epoxy resin by addition of an inert filler material is not sufficient to produce particulate matter, i.e., physical rupture of the epoxy resin matrix under thermal stress would appear not to be a viable mechanism producing organoparticulation. Table I1 gives a comparison of the organoparticulation behavior of a series of aliphatic carboxylic acid compounds, some with structures very similar to that of malonic acid, e.g., maleic, fumaric, oxalic, ketomalonic acid (dihydroxy malonic acid), and cyanoacetic acid. Included in Table I1 are the literature melting points (or decomposition temperatures) for each of these compounds. It is noted that none of the compounds, with the exception of malonic acid, exhibit thermoparticulation activity below 200 "C (as detected by the ion chamber technique). I t would be logical to try to reconcile this organoparticulation characteristic of malonic acid with the fact that this acid decomposes about the same temperature as the particulates are detected. However, dihydroxymalonic acid also undergoes low temperature decomposition and no particulates were detected from this particular compound. Table 111 shows the organoparticulation temperature data for seven aryl- and alkylsubstituted malonic acid compounds investigated in this work compared with that from malonic
m
0. 8
7 -1
I15
1%
0.6 d
u
121 0
$ f
9 20.4
im
2
-x E.
15
m
0. 2
20 15 IO 5 Timer minuter Monitor Ion Chamber Detector 6Jlmin Hfldnqen Flw Rate Healing Rate 6'Cmin
25
Figure 3.
Organoparticulation pattern for dimethylmalonic acid
0. 8
2 0.6
-
-I
u
-
E-
go.4
4
0.;
20
I5
IO
5
Timer mtnu(er Monitor I o n Chamber Detector H @ r q e n Flow Rate 6 limin Heating Rate 6'C min
Figure 4.
Organoparticulation pattern for ethylmalonic acid
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
91
Table 11. Comparison of the Organoparticulation Temperature of Malonic Acid with That of Related Carboxylic Acids Compound
Malonic acid
Chemical structure0
FOOH CH*
Organoparticulation temperature range, 250
1 .2H20 COOH HOOCCH
None
101-102
II
None
299-300 (sublimes)
COOH Oxalic acid dihydrate Fumaric acid
CHCOOH CHCOOH None 134-136 I1 CHCOOH HO, ,COOH C Ketomalonic acid monohydrate None 118-120 (dec) ‘COOH HO Cyanoacetic acid None 65-67 KECCH~COOH a Samples prepared as 100 parts/100 I: rts air-drying epoxy resin. Ag d overnight (- 16 hr) at 60 C. b Using an ion ch mber detector, Maleic acid
’
Table 111. Correlation between Organoparticulation Temperature Ranges and Decomposition Temperatures for the Malonic Acid Group of Compounds Compound0
Chemical formula
Organoparticulating temperature range, ‘Cb
Literaturec decomposition temperature, Oc
Malonic acid CH,(COOH), 126-132 132 Methylmalonic acid 127-131 135 CH,CH(COOH), Dimethylmalonic acid 147-151 190 (CH, ),C(COOH), Ethylmalonic acid C,H,CH(COOH), 118-125 112 Diethylmalonic acid 168-180 125 (C,H,),C(COOH), 134-1 37 163 Di-n-propylmalonic acid (C,H,),C(COOH), Benzylmalonic acid C,H, CH,CH( COOH), 143-151 120d Phenylmalonic acid 150-157 153 C,H,CH(COOH), a Samples prepared as 100 partsilo0 parts air-dry epoxy resin. Aged overnight (16 hr) at 60 ’ C. b Using an ion chamber detector. c E. H. Rodd, “Chemistry of Carbon Compounds,” Vol. l B , Elsevier Publishing Company, New York, N.Y., 1952, p 962. d Melting point. acid. The literature melting point or decomposition temperatures are also shown for each of the compounds investigated. I t is noticed that with some of these materials, particulate emission is detected by the ion chamber technique a t temperatures very close to their literature decomposition temperatures (e.g., with malonic, methylmalonic, ethylmalonic, and phenylmalonic acids). However, with other compounds, such as dimethylmalonic and di-n-propylmalonic acids, strong organoparticulation is detectable a t temperatures well below the literature decomposition values; in some cases by as much as 40 OC. On the other hand, diethylmalonic and benzylmalonic acids give thermoparticulation well abooe their literature decomposition temperatures. Thus, these data indicate that there is no clear-cut correlation between the organoparticulation temperatures and decomposition temperatures, and it would seem that we are measuring an entirely new physical property of the organic compound. I t would be logical to assume that the particulates are derived from the molecular decomposition of these compounds. However, it would be difficult to recon92
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
cile this interpretation with the compounds that exhibit particulation well beneath or well above the literature decomposition values. Mass Spectral Data. In an effort to characterize the nature of the particulates emanating from malonic acid, samples which were trapped on glass fiber filter discs were analyzed by mass spectrometry; the spectra are in Figure 5 . The peaks a t mle 43 (CH3--CO-)+, 45 (-COOH)+, and 60 (CH3-COOH, acetic acid) are very distinctive in this particulate pattern and represent the expected decomposition products from a compound such as malonic acid ( 3 ) .However, another malonic acid sample, which contained an equimolar amount of sodium bicarbonate, did not show any particulation below 200 OC with the ion chamber detector, whereas the mass spectrum (shown in Figure 6) still exhibited strong peaks a t mle 43, 45, and 60. This then suggests that the particulates detected by the ion chamber technique may be species other than the fragments shown by these peaks. Mass spectral analyses were also carried out on the vapor effluent obtained from methylmalonic, dimethylmalonic, ethylmalonic, diethylmalonic, and di-n-propylmalonic
I
4
yl
M 70
m m im
110 120
ia
ido 1% 160 ITO
Ion MISto Charge Ratio Idel
I.,,
40
50
60
I,
,
,
im
190
Mass spectra of particulates evolved by malonic acid plus sodium bicarbonate in an epoxy resin heated to 150 O C Flgure 6.
80 W 1W 110 120 130 140 150 160 Ion hbrr to Charge Ratio l m i e i
70
Flgure 5. Mass spectra of particulates evolved by malonic acid in an epoxy resin heated to 150 O C
acids; these are shown in Figure 7 . An attempt was made to identify the major components in each of these samples; this is shown in Table IV. The degree of agreement expected between experimental and literature data in this table can be illustrated by the differences among three reference spectra for n-butyric acid as reconstructed for Table V (7). It is possible that the spectra of effluent from diethylmalonic acid exceeds a permissible degree of agreement suggesting there may be another acid present along with the 2-ethylbutanoic acid. In compiling the data for Table IV, the mass spectral peak a t mle 44 in the experimental spectra was ignored as presumably being due to the carbon dioxide (mass 44) formed in the decomposition of malonic acids. Assuming loss of carbon dioxide from these compounds on heating (as is the case with malonic acid), the following thermal effluent products could be expected: 1.
cH3\c/c00H H/
40
60 70 80 90 100 110 M a s s to Charge Ratio lm/el
50
40
Fig. 7a-Methylmalonic acid
60 70 Bo 90 la, 110 Mass t o Charge Ratio ( d e l
50
Fig. 7bDimethyimaionic acid
.-”Ex
LA
40
50
*
Mass t o Charge Ratio ( d e )
60 70 80 90 im 110 Mass to Charge Ratio ( d e )
Fig. 7c-EthylmaIonic acid
Fig. 7d -Diethylmaionic acid
60
70
80
100 110
90
40
50
I
L O O H
Methylmalonic acid
Propionic acid
2.
Isobutyric acid
40
3.
50
60
70 80 90 1W 110 120 130 Mass to Charge Ratio h / e I Fig. 7e-Di-n-propylmalonic acid
Mass spectra of vapor from homologues of malonic acid in epoxy resin Figure 7.
Ethylmalonic acid
n-Butyric acid
presence of a peak at m/e 60 (without a peak of comparable height at mle 45) suggests the presence of n-butyric acid. This may suggest a homologous impurity in the as-received dimethylmalonic acid.
4.
4 Diethylmalonic acid
DISCUSSION
5.
+ co* Di-n-propylmalonic acid
2-Propylpentanoic acid
It will be observed in Figure 7 that there are no peaks found a t the molecular weight of the expected acid product in the mass spectral patterns of the higher molecular weight alkyl substituted malonic acids. The mass spectra appear to be dominated by ions which are derived by cleavage of the molecule a t a carbon atom which is p to the carbonyl carbon. This is particularly the case with the mass spectral patterns observed with dimethyl-, ethyl-, diethyl-, and di-n-propylmalonic acids. In Figure 7 b , the mass spectral pattern for dimethylmalonic acid shows the presence of a peak at mle 88 which is probably due to the formation of isobutyric acid. The
The exact nature of the particulates derived from these organic compounds is still under investigation. The fact that the addition of sodium bicarbonate to malonic acid suppressed particulation from this compound indicates that the acetic acid formed during decomposition is somehow associated with particulate formation. Presumably the basic sodium bicarbonate is able to react with the “nascent” acetic acid, either by a chemical or physical interaction, thereby reducing particulate formation. However, as shown by the mass spectral data in Figure 6, acetic acid, albeit a t a reduced concentration, is still detectable in the particulates collected from this particular sample. Indications are that the sodium bicarbonate, although not preventing the formation of acetic acid, might be inhibiting the types of particulates, which are detectable on the ion chamber detector, from being formed. Such particulates are probably of the dimensions of an aerosol type species. ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
93
Table IV. Composition of Vapor Effluent from the Alkyl Substituted Malonic Acid Group of Compounds from Mass Spectral Data Dominant Peaks in Mass Spectral Data Acid used in e p o x y resin matrix
Methylmalonic acid
Propionic acid
Dimethylmalonic acid
Isobutyric acid
Ethylmalonic acid
a
Major component expected in vapor effluent
74 45 73 57 55 56 43 41 73 45 88 42
n-Butyric acid
Relative peak height Experimental
Literature (7)
100 69 63 35 19 27
100 71 61 38 22 20 100 46 32 14 13 11 100 31 23 22 20 17 100 67 56 36 25 24 Not available
1000
lOOb
50 53 16 14
45 37 18 16
17
7
60
100
73
29
41
22
42 16 43 16 45 12 Diethylmalonic acid 2-Ethylbutanoic acid 43 100 88 90 73 84 41 38 a7 33 55 34 Di-n-propylmalonic acid 2-Propylpentanoic acid 73 100 102 36 43 33 57 23 41 21 88 18 Experimental data uncorrected. Experimental data with a correction for n-butyric acid content.
Reference Figure
7a
7b
7c
7d
7e
*
Table V. Variations in Relative Intensities of Dominant Peaks in Several Literature Mass Spectra of n-Butyric Acid (7) Mass/charge ratio, m / e
60 73 41 42 43 45
Relative intensities of Peaks Spectra API 0649
API 0303
Dow 0724
100
100 31 23 22 19 17
100 30 17 16 15 13
27 24 25 22
19
Possibly, the particulates are in the form of a very fine spray or mist of acetic acid or other carboxylic acid species dispersed in a cloud of carbon dioxide; carbon dioxide gas is always liberated when compounds such as these decompose. Recent work in this laboratory has indicated that the particulates have to be above a certain critical size to be detected by the ion chamber method. This size was determined by passing a series of saturated fatty acids (CnH2,02, where n = 8 to 18) through the ion chamber instrument. The break in the detectability occurred between Clo and C11. Since the molecular dimensions of the fatty acids are well established ( 8 ) , an approximate value of 25-30 8, was established as the critical size of molecule which can be detected by the ion chamber instrument. This compares very favorably with a figure of 25 8, established recently for the detectability limit of a Condensation Nuclei Monitor (9). 94
Mass/charge ratio
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
Since 25-30 8, is well above the molecular size of acetic acid, it would seem that acetic acid aggregates, possibly hydrogen-bonded oligomers, would have to be formed in the carbon dioxide atmosphere to be detectable. In a very recent publication (10) on the thermodynamic properties of acetic acid in the vapor state, Choa and Zwolinski show that, because of the presence of hydrogen bonding between molecules, acetic acid vapor contains polymeric species, (CH3COOH),. The formation of these acetic acid oligomers might possibly explain why dihydroxymalonic acid (Table 11) did not form particulates that were detectable. Although this organic compound decomposes a t 118-120 "C, the products of decomposition are COn and H20 with no acetic acid formation. The particulates derived from the dimethylmalonic acid sample have been photographed using a scanning electron micrograph a t a magnification of X2500. The particulates in this photograph (Figure 8) appear to have an average size of 2 microns which is close to the dimensions of particulates found in typical tobacco smoke ( 2 1 ) . Using the onset of organoparticulation as a criterion for assessing the thermal stabilities of these compounds, the order of thermal stability would be as follows: diethyl > phenyl > dimethyl > benzyl > di-n-propyl > methyl = malonic > ethyl. This would appear to be an acceptable relative order of thermal stability since the more highly alkyl-substituted or aromatic-containing acids would be expected to be more thermally stable than the unsubstituted or mono-alkyl ones. I t is interesting to note that this order would not be
Conversely, organic compounds that undergo a partial decomposition beneath their actual melting point may not be easily detectable by visual observations. The ion chamber technique described here would appear to provide a sensitive means whereby organic compounds, which undergo partial decomposition before melting, can be monitored. It would appear that methyl-, dimethyl-, and di-n-propylmalonic acids would fall into this class of compound. Compounds which would appear to decompose ajter melting are exemplified hy diethyl- and benzylmalonic acids. Thus, organoparticulate analysis, using an ion chamber detector technique, would appear to be a very sensitive and useful technique to monitor the thermal stabilities of organic compounds. I t would seem to offer distinct advantages over conventional techniques, such as melting point determination, TGA, mass spectrometry, infrared and UV spectrometry, color and visual changes, etc., which have been used previously to study the thermal decompositions of organic compounds. Other types of organic compounds are now under investigation and will he the subjects of future publications in this area.
LITERATURE CITED (1)
Figure 8. Scanning electron microscopy photograph (2500 magnifi-
cation) of dimethylmalonic acid particulates collected on a glass fiber disk the same as that obtained using melting point or other visual change methods to evaluate thermal stability (refer to Table 111).The melting point procedure, however, does not take into account the possibility of these organic compounds thermally decomposing ajter they have melted.
C. 0. Doyle. "Evalmtion of Experimental Polymers", WADD Tech. Rep. 60-283, U.S.A.F.. Wright-Patterson AFB, Ohio, May 1960.
(2) J. 0. 8. Smith and 0.C. Phillips. Micrchem. J., in press. (3) E. H. Rodd, "Chemistry 01 Carbon Compounds", VoI. 18. Elsevier Publishing Co.. New York. N.Y.. 1952, p 962. (4) G. F. Skala. J. Rech. Atmos., 189 (1966). (5) C. E. Murphy and C. 0.Doyle. Appl. Poiym. Symp., 2, 77 (1966). (6) G. F. Skala, Ami. Chem.. 35,702(1963). (7) E. Stenhagen, S. Abrahamsson. and F. W. McLafferty, "Atlas 01 Mass Spectral Data", Val. 1, Interscience Publishers, New York, N.Y.. 1969. (8) A. I. Kitaigorodskii. "Organic Chemical Crystallography". Consultants Bureau Publishers. New Y a k . N.Y.. 1961.
(11) F. W. van Luik, Jr.. and R. E. Rippsre. And. Chem., 34, 1617(1962)
RECEIVEDfor review July 16, 1975. Accepted October 6, 1975.
Study of Lead(l1)-Manganese(l1) Energy Transfer in Sodium Chloride Pellets Richard 0. Delumyea' and George H. Schenk'
Department of Chemistry, Wayne State Univenky, Detroit, Mich. 48202
The solid state lead(I1)-manganese(l1) energy transfer system has been studied in sodium chloride pellets from a quantitative viewpoint. Lead(ll) and manganese(l1) are quickly coprecipitated from a saturated sodium chloride solution by adding ethanol. After filtration and drying, the sodlum chlorlde matrlx Is compressed into a crystalline pellet using standard pellet techniques. The excitation maxima at 275 and 303 nm are those of lead(ll) and the broad emlssion maxlma at 610 nm Is that of manganese(l1). The coprecipltation of lead(ll) In an excess of manganese(l1) is repro-
'
Present address, Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48104
ducible enough to yield a reproducible analytical lumlnescence curve. A speclal pellet holder was devised for the Turner filter fluorometer to permlt routine fluorometric measurement of traces of lead(l1).
Fluorescence and phosphorescence are used routinely in qualitative and quantitative determinations of organic compounds. Few purely inorganic systems, however, luminesce in solution and hence inorganic luminescence measurements are based largely on the use of fluorescent organic chelating complexing agents ( I ) . There are exceptions, of course; for example Bozhevlo'nov and Solov'ev (2) developed a luminescent method for determining lead in ANALYTICAL CHEMISTRY, VOL. 48. NO, 1, JANUARY
1976
95
