Effect of Sodium Ion Impurity on Thermal Decomposition Reaction of Calcium Oxalate as Studied by Absorption Infrared Spectrometric and Thermoanalysis Techniques JOHN M. SCHEMPF, FRED E. FREEBERG,' and FRANCIS M. ANGELON12 Department of Chemistry, Whitmore Laboratory, The Pennsylvania State University, University Park, Pa.
This study demonstrated the usefulness of thermal methods in studying impurity effects on solid state decomposition reactions and shows the results of the presence of an impurity on the lowering of chosen decomposition temperatures and kinetic parameters of a selected compound as studied by TGA. It illustrates the use of independent techniques such as spectrometry to further describe the reaction phenomenon. Calcium oxalate monohydrate was selected for this study because of its extensive use as a calibration standard in both differential thermal analysis (DTA) and thermogravimetric analysis (TGA). This work shows calcium oxalate monohydrate is a poor standard unless the origin and history of the sample are known. DTA, infrared, and activation analysis were used to elucidate the nature and amount of the impurity and its effects on the decomposition reaction.
I
of thermal methods of analysis, TGA and DTA (4), has made evident that a calibration standard for use in comparison of both techniques and as an interlaboratory check would be of considerable value. Calcium oxalate monohydrate has found use as a so-called calibration standard ill thermoanalytical studies. This material has been shown to be sensitive to instrumental and environmental conditions by Simons and Newkirk (7). The authors have shown that lack of control of experimental variables such as atmosphere control and heating rate can greatly affect the experimental results. However, even in cases where extreme caution was observed, correlation is poor. The purpose of this 'research was t o investigate the effect of the method of preparation of calcium oxalate from various compounds on decomposition NCREASING USE
Present address, Procter & Gamble
Co., Ivorydale Technical Center, Cincinnati, Ohio 45217. 2 Present address, Koppers Co. Inc., Monroeville, Pa.
1704
ANALYTICAL CHEMISTRY
curves of a selected reaction and to attempt to determine what chemical impurities would most affect these results. These analyses were carried out under constant experimental conditions using the same equipment, thereby limiting the study to impurity effects. EXPERIMENTAL
A Chevenard automatic recording thermobalance No. 3 manufactured by SocietB A.D.M.E.L. Paris, France, was used throughout this study. Since atmosphere control is difficult with any thermobalance, particularly the Chevenard, the reaction studied was the thermal decomposition of calcium oxalate :
+ heat
CaC204
=
CaC03
+ Cot
The study was limited to consideration of this reaction because the reaction is irreversible; equilibrium considerations could therefore be neglected. I n order to study the effect of impurities introduced by sample preparation on this thermal decomposition, a series of calcium oxalate samples was prepared from various calcium salts and oxalic acid, maintaining standardized conditions of precipitation, temperature, and concentration throughout. These samples were decomposed on the balance a t a heating rate of 150' C. per hour in an ambient air atmosphere. The initial weight of all samples was 162 mg. The reaction temperatures chosen for comparison were the 10, 50, and 90% weight loss points. The reproducibility of the above weight loss temperatures was determined on 40- to 60-mesh, reagent grade calcium oxalate monohydrate obtained from Matheson Coleman & Bell Co. The temperature chosen was the 50% weight loss point for the oxalate to carbonate decomposition. The confidence limits for the mean temperature based on 6 measurements a t the 99% confidence limits was found to be 5" C. The precision in reading the millivolt scale from the recorder chart corresponded to a deviation of 2" C. This undoubtedly contributed to the precision value reported. Factors affecting the experimental results were studied in order to determine optimum operating conditions.
7 6802
These factors and their effects have been discussed in detail by Newkirk (5) and were held constant throughout this study. RESULTS A N D DISCUSSION
The results of the thermal decomposition of the samples of calcium oxalate in an ambient air atmosphere and 150' C. per hour, prepared from various starting materials, are listed in Table I. The differences observed in the decomposition temperatures show a considerable temperature difference caused by sample preparation for each of these selected points of the decomposition range amounting to as much as 4 3 O C. for the 90% point. A t this time no quantitative analysis of the samples for foreign ions was made because semiquantitative spectrometric analysis showed the presence of many metal ion impurities to a like degree. It was first necessary to isolate the impurity causing the temperature lowering and then investigate its effect under controlled conditions. With DTA runs a t a heating rate of 20' C. per minute, the carbon monoxide peak exhibited two reaction peaks, a small peak followed by the major decomposition peak. This behavior was found both in flowing air and inert atmospheres, Since the DTA system used enabled the flowing gas to pass through the sample and reference cells, the atmosphere could be easily con-
Table I.
Starting material (oxalic acid)
Effect of Starting Materials
Carbon monoxide loss decompositFn temperature, C. 10%
507, wt.
90%
wt. loss loss loss Ca(N08)2 445 473~ 485 CaF2 416a 436 446" CaC12 (31") (38") (43") 424" 443a 463" CaBre 447a 468 489= CaL 421 435" 462 Ca(OH)2 437 465 474 a Maximum temperature difference. wt.
FREQUENCY. (CM-1
L
I
I
I
200
430
490
TEMPERATURE, *C
Figure 1.
OL
DTA trace of reagent grade calcium oxalate
trolled. More extensive studies by Angeloni (1) showed that the behavior was due to sodium impurities in the sample and not experimental variables. Figure 1 shows a DTA trace of reagent calcium oxalate as obtained from Matheson Coleman & Bell. The small peak shown on this curve was also present in all samples prepared in our laboratory except that sample prepared from calcium nitrate and oxalic acid. It was noted that, under identical conditions, this small peak as well as the major decomposition peak occurre: at a reproducible temperature, regardless of the sample decomposed. This suggested that the same impurity was introduced in all the above preparations and the starting material, calcium nitrate, was sufficiently free of this impurity so that its effect could not be detected by DTA. In order to determine the impurity causing the anomalous behavior found in the DTA analysis, isothermal decomposition of various calcium oxalate preparations were studied using the I R pellet technique as reported by Hartman and Hisatsune (3). I n this method spectra are taken after heating in a constant temperature oven for varying time increments which enables one to follow the thermal decomposition of a compound dispersed in an inert pellet. Thus, the reaction is obviously carried out in an inert atmosphere. When samples showing the anomalous peak, as detected by DTA, were studied by this I R pellet technique, I R peaks a t 760 and 800 cm.-l were observed in addition to the normal 780 cm.-l band. Similar behavior was observed for other IR active oxalate bands. Figure 2 shows a spectrum from frequencies of 700 to 800 cm.-l for a sample partially decomposed to calcium carbonate. The first trace shows the results of the decomposition of a sample prepared in a KBr pellet, and the second a spectrum of a pellet
Figure 2.
Effect of N a + on IR spectra
IO
.VJ VJ
st r eY
-E
$ 5 0
90 450 TEMPERATURE. O C DECREASING Na' CONTENT
400
Figure 3. oxalate
500
---.
Sodium effect on decomposition of calcium
containing this sample to which NaBr had been added. Sodium was chosen because it is a very common impurity and initial neutron activation studies indicated the presence of detectable amounts of sodium in calcium oxalate prepared as shown in Table I. These extra I R bands appeared to be enhanced if sodium bromide was added during preparation of the pellet. A pellet containing a sample which did not exhibit the extra DTA peak also did not show the additional I R bands. The samples were decomposed over the same time increment. These runs suggest that sodium ion is the impurity causing the lowering of the decomposition temperature range and the appearance of the extraneous peaks observed in the DTA and I R . Additional information as to the nature of the small peak was obtained by
preparing a sample from calcium nitrate and oxalic acid. This product did not show the extraneous peaks found in the DTA and I R results of other samples. However, when sodium was added as an impurity to the solu-
Table 11.
Decomposition Results with Sodium Impurity
Mole 70 Na (in
sohNa tion) (p.p.t.) 0
5 10
20 30 50 60
>0.01 1.3
2.2 3.2 4.80 4.80 4.80
Decomposition temperatures in "C. for carbon monoxide weight loss 10% 445 435 425 413 415 412 412
50% 473 459 450 442 431 430 430
VOL. 37, NO. 13, DECEMBER 1965
90% 485 478 470 469 440 442 438
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Table 111.
Kinetic Results for CO Loss
Na ion, p.p.t.
Activation energy, Kcal./mole
>0.01 ~. -
74
1.3 2.2 3.2
70
Arrhenius factor, sec.-l 1 x ,. in18 -i.5 x 1018
58 38
5 1
x
x
1014 10'0
tion prior to precipitation, the small peak was observed in the material thus prepared. A series of samples were prepared from solutions of calcium and sodium nitrate of various mole per cent compositions to investigate t h e effect of sodium ion concentration on the decomposition rate. Oxalic acid was used t o precipitate the samples of calcium oxalate. Neutron activation analysis was used to determine the total sodium content of each sample and will be reported as parts per thousand (p.p.t.). The results obtained for the decomposition temperatures for these samples are given in Table 11. The sodium content of the sample prepared from solutions without the addition of sodium ion was found to be less than 10 p.p.m. ,411 samples were then decomposed on the thermobalance under identical conditions. Figure 3 shows the portion of the actual thermogram corresponding to the carbon monoxide loss for these samples. Not only do the decomposition temperatures decrease lvith increasing sodium ion content over the 10-90~o range,
but the curves also change shape indicating different decomposition rates. Kinetic analyses were made on these runs using a method based on that suggested by Newkirk ( 6 ) , and developed more fully by Freeberg ( 2 ) . The curves exhibiting precipitous rates of decomposition were not amenable to such studies. Kinetic results calculated from seven temperature programmed runs on reagent grade CaC204 gave a deviation from the mean activation energy of 1 6 Kcal./mole a t the 99% confidence limits. Kinetic activation energies calculated for runs made on samples containing added sodium ion are listed in Table 111. It should be noted that the activation energ) shows a decrease with increasing sodium content. Angeloni (1) has shown by the DTA method that the activation energy corresponding to the reaction responsible for the small peak was approximately 4 Kcal./mole. This supports the above observation that the lowering of the TGA activation energy is a result of sodium ion irnpurity. The results obtained from the samples that gave very rapid weight loss indicated that the sodium may catalyze the oxalate decomposition, thereby changing the kinetics of the reaction. CONCLUSIONS
This study has s h o m the usefulness of thermal methods to determine the effect of impurities on solid state decomposition reactions. Additional work would be required to determine whether the effect of the impurity results from the production of a crystal defect in the
calcium oxalate monohydrate or through an adsorption phenomenon. However, the results indicate that such impurities have a measurable effect on TGA curves. Although TGA constants and temperatures are not measured at equilibrium conditions, the trends observed appear to be real as verified by results obtained by independent techniques. The DTA results indicated that the effects produced can be isolated, thus giving insight into the cause of the observed behavior. Since sample preparation affects both the shape of the reaction curves and temperatures of reaction a t selected points for the decomposition of calcium oxalate, this strongly points up the necessity for ascertaining the purity of the material in addition to other experimental variables as shown by previous investigations (5, 6) before it can be used as a standard. LITERATURE CITED
(1) Angeloni,, F. M., Ph.D. thesis, The Pennsylvania State University (1965).
(2) Freeberg, F. E., Ph.D. thesis, The
Pennsylvania State University, University Park, Pa., 1965. (3) Hartman, K. 0 Hisatsune, I. C.,
J . Phys. Chem., 69,'583 (1965). (4) Murphy, C. B., AKAL. CHEM.36, 347 R (1964). (5) Newkirk, A. E., Ibid., 30, 162 (lY58). (6) Ibid., 32, 1658 (1960). (7) Simons, E. A,, Newkirk, A. E., Talanta 11, 549-71 (1964). RECEIVEDfor review June 29, 1965. Accepted September 22,1965. Abstracted
in part from Ph.D. thesis of Fred E. Freeberg and presented at 16th Annual Pittsburgh Conference on Analytical Chemistry and -4pplied Spectroscopy, hlarch 1965.
A Windowless Photoionization bource for High Resolution AnaIyticaI Mass Spectrometers C. E. BRION Department o f Chemistry, University o f British Columbia, Vancouver 8, B.C., Canada
b A windowless light source has been constructed for use with the Associated Electrical Industries high resolution mass spectrometer (Type M.S.9). Photons from the helium 584 A. (21.21 e.v.) emission are used for the photoionization of atoms and molecules. The photon flux is adequate for operation of the mass spectrometer at high resolving power to permit the resolution of mass multiplets and the accurate measurement of mass, The possible uses and applications of the device are discussed. Evidence is presented for the occurrence of thermal decomposition in the electron impact ion source and it is shown that these effects can b e minimized by the use of photoionization. 1706
ANALYTICAL CHEMISTRY
I
ANALYTICAL mass spectrometers ions are generally produced by electron impact. Electron impact sources are simple in operation and have the important property that a high ionizing flux is easily obtained. This results in large and easily measured ion currents. However despite its general usefulness, ease of operation, and appeal for a wide variety of problems there are some disadvantages of the electron impact source for certain applications. Due to the proximity of the hot tungsten filament (-2000°K.) the ion source usually operates a t 80-200' C. above room temperature depending on the construction of the particular source. I n addition to the general field of organic chemistry, mass spectrometry is
N
assuming an increasing importance in biochemistry. Here many delicate molecules may be subject to thermal decomposition in the mass spectrometer ion source and pyrolysis on the filament could be even more serious. I n addition, in physical chemistry and allied subjects, there are many problems where thermal decomposition is an operational hazard, frequently of an unknown magnitude since it is often difficult to ascertain whether a given fragment has been produced by thermal or electron impact induced dissociation. Numerous problems immediately suggest themselves where it would be of advantage to be able to operate a mass spectrometer ion source a t a series of closely controlled temperatures. All these applica-
