in this laboratory for the reduction of chromium(V1) in acid solution using platinum electrodes, regardless of the pretreatment used or the direction of voltage change. This is believed to be due to the oxidation of the electrode surface by the chromium(VI), and formation of an oxide layer. The existence of this film has been shown by Kolthoff and Tanaka (8) and Ross and Shain (11). The oxide film probably lowers the rate of the electrode reaction by acting as a barrier in the electron transfer process. Catalytic reactions, in which platinum is first oxidized by chromium(V1) and then reduced a t the electrode surface, probably do not occur to any great extent, because of the low rate of the platinum oxide reduction reaction. The behavior of gold electrodes is very similar to platinum electrodes. In the absence of complexing media, gold electrodes have greater stability toward oxidation.
I-
z
W
a LT
3 0
LITERATURE CITED
Armstrong, G., Himsworth, F. R., Butler, J. A. V., Proc. Roy. SOC. 143A, 89 (1933). Bockris, J. O W . , “Modern Aspects of Electrochemistry,’’ p. 260, Academic Press, New York, 1954. Hickling, A., Trans. Faraday SOC.42, 518 (1946).
I
I
06
04
I 02
VOLTS VS
Figure 5. duction o f acid
I 0
I -02
I -04
S C E
Effect of electrode pretreatment on re-
5 X 1 0-SM chromium(V1) in 1M perchloric
A. Preanodized a t 2.5 volts and shorted a t 0 volt for 5 seconds B,C. Re-use of gold electrode without further pretreatment
ACKNOWLEDGMENT
The authors %-ish to thank E. I. du Pont de Nemours & Co. and the Wisconsin Alumni Research Foundation financial assistance.
I 08
D. E.
After repeated trials without further pretreatment Residual current with same pretreatment as A
Jirsa, F., Buryanek, O., Chem. Listy 16, 189 (1922). Kolthoff, I. M., Jordan, J., AKAL. CHEM.24, 1071 (1952). Kolthoff, I. M., Jordan, J., J. Am. Chem. SOC.74, 4801 (1952). Kolthoff, I. )I., May, D. R., ANAL. CHEM.18,208 (1846). Kolthoff, I. bl., Tanaka, X., Ibid., 26, 632 (1954);( Latimer, W. M., Oxidation States of the Elements and Their Potentials in Aqueous Solutions,” 2nd ed., Prentice-Hall, Yew York, 1952.
Lord, S. S., Jr., Rogers, L. B., A N ~ L . CHEM.26, 284 (1954). Ross, J. W., Shain, I., Ibid., 28, 648 (1956). Silvestroni. P.. Ann. chim. (Rome) 44,464 (1954). Silvestroni. P.. Troili., hf.., Ricerca Sci. 24, 116 (1954). RECEIVED for review May 17, 1956. Accepted September 26, 1956. Based on the Ph.D thesis of Frederick Baumann, University of Wisconsin, 1956.
Differential Thermal Analysis and Thermogravimetry Applied to Potassium PerchIorate-AIuminumBarium Nitrate Mixtures VIRGINIA D. HOGAN, SAUL GORDON, and CLEMENT CAMPBELL Pyrofechnics Chemical Research laboratory, Picafinny Arsenal, Dover, N. J.
b A thermogravimetric study indicated that barium nitrate catalyzes the decomposition of potassium perchlorate to potassium chloride. Differential thermal analyses of the system yielded curves characteristic of the individual compounds and their relative quantities. These complementary techniques have been used to develop a simple 306
ANALYTICAL CHEMISTRY
but rapid method for analyzing the pyrotechnic composition potassium perchlorate-aluminum-barium nitrate. Thermogravimetric curves provided the data for determining the optimum temperature for perchlorate decomposition. A filtering crucible was used to permit the quantitative aqueous removal of potassium chloride and bar-
ium nitrate, leaving a residue of aluminum. Barium nitrate may be determined b y difference or b y titration for barium ion.
D
IFFERENTIAL thermal
analysis has been widely used in the study of minerals, clays, and soils (9, 16).
Recently this experiinental tool has been extended to the characterization of inorganic and organic systems and to investigation of reaction mechanisms (6, 8, 10, 14). The method involves heating the material under study and a thermally inert reference material to elevated temperatures a t a constant rate while continuously recording the temperature ,difference between them as a function of sample temperature or time. The curves obtained may be used to characterize the system under study in terms of its thermal reactions, both physical and chemical. These curves can also be used to determine qualitatively the composition of mixtures, since they exhibit many of the features characteristic of the individual components. Integration of the areas under endothermal bands has been used to obtain semiquantitative estimates of one or more of the substances present (8,17).
I
O
"
"
r
"
-
I
'
7
100
gravimetric curves are quantitative representations of weight changes, they can be related to the chemical and physical changes taking place in the sample as it is heated, and can often be used to determine the nature of the products. Differential thermal analysis and thermogravimetric studies of the binary oxidant system potassium perchloratebarium nitrate indicated that barium nitrate catalyzes the decomposition of potassium perchlorate to potassium chloride. This phenomenon, which has been reported for several other cntalytically active additives (1, 5$ 1 1 ) , suggested a means for developing a simpler, rapid method for the analysis of the standard military pyrotechnic photoflash material of the following composition: 30% potassium perchlorate, 40% aluminum, and 30% barium nitrate. This paper presents differential thermal analysis and thermogravimetric curves for the above composition and its binary mixtures, resulting thermogravimetric analyses for the mixture, and semiquantitative estimates of per cent potassium perchlorate based upon an integration of the crystalline transition peak areas found on the differentid thermal analysis curves.
EXPERIMENTAL
Reagents. Potassium perchlorate, analytical reagent grade (J. T. Baker Chemical Co.) ; barium nitrate, analytical reagent grade (Mallinckrodt Chemical Works) ; atomized aluminum (18); and 98.0% free metallic aluminum (Metals Disintegrating CO.).
loo~
t , , , , , , , , 0
600
300 TEMPERATURE,
OC.
Figure 1. Thermogravimetric analyses a t heating rate of 15' C. per minute A.
B.
KClOr, 0.2959 gram 50% KClO4 50% Ba(NO&, 0.3990
C.
gram Ba(NOJ2, 0.3069 gram
+
Thermogravimetry involves continuously weighing a material as it is heated either a t a constant temperature or t o elevated temperatures a t a constant rate. Curves are obtained as a function of time or temperature. The applications of this technique have been largely confined to studies of dry corrosion of metals and thermal behavior of precipitates recommended for gravimetric analysis (4). Because the thermo-
The reagents for the volumetric titration of barium ion with standard potassium sulfate, using tetrahydroxyquinone indicator and sodium chloride-silver nitrate end point sharpener, were prepared as specified by Deal (3) from analytical reagent grade materials, Preparation of Mixtures. A series of binary and ternary mixtures of the above reagents was prepared by weighing sufficient quantities of the components to make 50 grams of each mixture, and blending them together in a ball mill using rubber stoppers. The samples for each individual run were taken from these previously prepared mixtures. Instrumentation. The differential thermal analysis apparatus employed was a modification of t h a t described by Gordon and Campbell ( 8 ) . The resistance furnace used was a Hevi-Duty Electric Co., Type 84, with special heating coils rated for 710 watts a t 89 volts. The differential temperature was recorded as a function of the sample temperature on a Moseley Autograf X-Y recorder. To monitor the linearity of the nominal 15" C. per minute heat-
ing rate, the sample temperature was simultaneously plotted as a function of time on a Brown Electronik potentiometric strip chart recorder (6). The apparatus used for the thermogravimetric analyses was a photographic Chevenard thermobalance converted to electronic recording ' 7 ) . The heating rate of 15" C. per min. was controlled by a Gardsman stepless programming and controlling pyrometer (West Instrument Co.) and monitored by recording the temperature in the furnace above the sample as a function of time on a Brown Electronik potentiometric strip chart recorder (6). The 4 X 4 x 4-inch muffle furnace used in the analytical procedure was maintained a t a constant temperature by means of a Gardsman proportional indicating and controlling pyrometer, using a thermocouple inserted through a hole in the door and in close proximity to the crucibles. The thermogravimetric curves were obtained with a Moseley Autograf X-Y recorder, which plots the change in weight as a function of furnace temperature. These furnace temperatures were about 5" to 30" higher than the sample temperatures. Procedures. Two portions of the sample of ternary composition are n-eighed into Selas No. 3001 porcelain filtering crucibles. One portion is ignited in a muffle furnace a t 515' C. for 20 minutes t o decompose the potassium perchlorate, and allowed t o cool in a desiccator prior to \\-eighing. The other portion is filtered and washed with hot water to remove the potassium perchlorate and barium nitrate quantitatively, leaving a residue of aluminum, which is dried and iveighed. The barium nitrate is determined by difference. This analysis for potassium perchlorate by a catalyzed thermal decomposition is similar to a Joint Army-Navy specification analysis (IS),which requires the addition of ammonium chloride (15) to accelerate the thermal decomposition of perchlorate. An alternative procedure involves performing the entire analysis on a single sample. The sample is ignited and weighed to determine the perchlorate content, then filtered and washed. The aluminum residue is dried and weighed. Barium nitrate may be determined by difference, or by titration for barium ion in the filtrate. I n the latter case, the filtrate is quantitatively transferred to a 250-ml. volumetric flask through a Whatman No. 42 ashless paper to remove the negligible quantity of ultrafine aluminum which interferes with the titration, and then diluted to volume with water. Twenty-five-milliliter aliquots of this solution are titrated with a standard 0.0249N potassium sulfate solution using tetrahydroxyquinone indicator with a sodium chloride-silver nitrate end point sharpener, according to the procedure described by Deal ( 3 ) . Four-gram samples are used for the differential thermal analysis. An equal volume of alumina is used as reference material. For thermogravimetric analyses a 400-mg. sample is used, in order VOL. 29, NO. 2 , FEBRUARY 1957
307
to obtain the maximum deflection over a 200-mg. range for the compositions richest in potassium perchlorate. DISCUSSION
Thermogravimetric curves for the individual oxidants and a representative binary mixture, 50% potassium perchlorate and 50% barium nitrate, are shown in Figure 1. The curves for potassium perchlorate and barium nitrate show weight losses indicating their stoichiometric decomposition to potassium chloride a t a furnace temperature of about 600' C., and to barium oxide a t about 650" C., respectively. The curve for the mixture shows weight loss equivalent to the quantitatire loss of oxygen from potassium perchlorate a t 520' C., a temperature significantly lower than that observed for the pure potassium perchlorate and well below the decomposition temperature of barium nitrate. The subsequent loss in weight is due to the decomposition of nitrate ion. The thermogravimetric results for a series of binary oxidant mixtures over a wide composition range are summarized in Table I. I n all cases the absolute weight losses observed experimentally agree to n-ithin 0.5y0 with those calculated for the quantitative decomposition of potassium perchlorate to potassium chloride. This is observed at a heating rate of 5" C. per minute as well as a t 15" C. per minute. An examination of the crucibles used to contain these samples of binary oxidant mistures indicated that the oxygen loss from potassium perchlorate proceeds smoothly in the presence of barium nitrate. Differential thermal analysis curves 1%-ereobtained for all of these mixtures. The curves for the individual oxidants and a representative mixture consisting of 50y0 of each compound are shown in Figure 2. They exhibit endothermal and exothermal bands and peaks characteristic of the individual compounds and of their relative quantities. The curve for potassium perchlorate has an endothermal peak a t 300" C. due to the crystalline transition from the rhombic to the cubic labtice. I n the range of 600' to 700" C., two sharp endothermal peaks are followed immediately by a large exothermal peak. These are due to successive stages of fusion and decomposition, culminating in complete decomposition to potassium chloride. Barium nitrate exhibits two endothermal peaks, a t 600' to 700" C., corresponding to fusion and subsequent decomposition to barium oxide. The curve of the mixture displays a summation of several features of the curves of the individual compoundsLe., the crystalline transition of potassium perchlorate a t 300" C., the
308
ANALYTICAL CHEMISTRY
mal effect may be the result of a vigorous, somewhat decrepitative decomposition. The additional endothermal peak a t 465" C. is a t a temperature below the melting and decomposition points of both potassium perchlorate and barium nitrate, and appears on the
fusion and decomposition of potassium perchlorate a t 550" C., and the decomposition of nitrate ion above 600' C. The loop in the endotherm a t 550' C. for this 50:50 composition is unique, in that none of the other binary oxidant mixtures exhibit it. This strange ther-
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20
1
lot
!-
To
" 0
-
Figure 2. Differential thermal analyses at heating rate of 15" C. per minute, using &gram samples
Table I.
KC104, %
AI, %
Thermogravimetric Analyses
Ba(SOa)r, %
Calcd. Loss"
Exptl. Loss
37.0 32.3 27.7 23.1 18.5 13.9 9.2
37.4 32.4 28.1 23.3, 22.6c 19.0 13.8 9.7
Oxidant Binaries 20 30 40 50 60
80 TO 60 50 40 30 20
TO
80
Aluminum, Oxidant Binaries 10 20 40 80
30 90 80 60 20
TO
28.96 4.6 9.2 18.5 37.0
22.6 3.7 7.9 16.9 41.3
Aluminum, Oxidant Ternaries 40 20 30 0
b c
40 40 40
-
18.5 9.2 13.9
20 40 30
-4ssuming KC104 + KC1 f 202 f . Assuming 2 Ba(NO3)g 2 BaO 4- 2 r\Tz t f 502 t Heating rate 5'/min.
.
18.2 9.0 13.4, 1 3 . 8
The mixture of 30% potassium perchlorate, 40% aluminum, and 30% barium nitrate is a standard military protechnic composition. These thermoanalytical data for the binary oxidant systems have been used to develop a simple, rapid method for the analysis of this composition. Therniogravimetric curves for the binary mixtures of aluminum with potassium perchlorate and with barium nitrate, summarized in Table I, indicate weight losses a t furnace temperatures of 600" and 630" C.; which are in poor agreement with those calculated for the stoichiometric de-
differential thermal analysis curves for all the binary oxidant mixtures. It is also lower than the thermogravimetrically determined temperature a t which the binary mixtures lose tveight owing to the stoichiometric loss of oxygen from potassium perchlorate. Since partial melting of the sample is noticeable nhile this endotherm is occurring, it is assumed to be the fusion of a eutectic mixture of potassium perchlorate and barium nitrate. The thermal instability of the eutectic mixture precludes a further study of this system by conventional cooling curve techniques.
I
10
4
100
200
300
500
400
TEMPERATURE
600
700
rC1
800
903
'
Figure 3. Differential thermal analyses at heating rate of 15" C. per minute, using 4-gram samples
Table 11.
Detn. 1 2 3
Analytical Results
[Mixtures contain: 30Yc KClOa, 40% AI, 30% Ba(S08)*] Ba(K03)~,% KC104," Al, G y By difference By titration Using Procedure 1 30.5 39.7 29.7 ..
4 5
30.5 30.7 30.7 30.7
6 7 8 9 10 11 12
30.7 30.2 29.9 30.7 31.1 29.5 30.8
39.6 39.6 39.5 39.5
29.9 29.7 29.8 29.8
, . t
.
.. ..
Using Procedure 2
0
Assuming KC104
-
40.1 41.7 40.9 41.4 40.1 40.2 41.7
KC1 -t 202. t
29.2 28.1 29.2 27.9 28.8 30.3 27.5
..
.. ..
29:5 29.4
composition of the oxidants. This may be due to reaction of the salts with aluminum. However, thermogravimetric curves of the ternary mixtures exhibit weight losses that are equivalent to the quantitative decomposition of potassium perchlorate to potassium chloride at about 540" C. and a further loss due to the decomposition of nitrate ion, as in the case of the binary oxidant mixtures. The agreement between the calculated and experimental weight losses, shown in Table I, is within a few tenths of 1%. This is true for a heating rate of 5" C. per minute as well as for the usual 15' C. per minute. The differential thermal analysis curves of all the binary and ternary mixtures, three of which are shown in Figure 3, exhibit the expected bands and peaks. The curves for the binary aluminum-potassium perchlorate mixtures display the potassium perchlorate transition, its decomposition pattern around 600' C., and the fusion of aluminum at 650' C. The aluminumbarium nitrate curve consists of the barium nitrate fusion peak, the fusion of aluminum a t 650" C. immediately followed by the overlapping endothermal decomposition of nitrate ion and an exothermal reaction. The curve for the ternary mixture exhibits the potassium perchlorate transition, the fusion of the eutectic mixture a t 465' C., the fusion of aluminum, and the decomposition of nitrate ion. I t was found that for a constant v,-eight of sample, the area under the potassium perchlorate transition peak a t 300" C. on the differential thermograms is linearly proportional to the amount of potassium perchlorate present, The areas obtained by planimetering this endotherm on the curves of all the mixtures containing potassium perchlorate were plotted as a linear function of the per cent potassium perchlorate. The composition of the 4-gram samples ranged from 10 to 100% potassium perchlorate. This technique may be used to obtain semiquantitative estimates of the potassium perchlorate content of a sample, since the per cent potassium perchlorate read from Figure 4 is accurate to 1 5 % . The thermogravimetric curves provide data for determining the temperature a t rhich to ignite the ternary mixtures. This temperature, because of the barium nitrate present, is well below the temperature a t which aluminum will air oxidize or react with the oxidants. The loss on ignition is due to the quantitative decomposition of potassium perchlorate to potassium chloride and thus is equivalent to the per cent potassium perchlorate present in the sample, assuming that the sample contains no volatile impurities. Since the residue after filtration consists only VOL. 29, NO. 2, FEBRUARY 1957
309
of aluminum, the per cent aluminum is obtained directly. The per cent barium nitrate can be determined either by difference or by titration. Analytical results are listed in Table 11. The procedure which uses two samples is both faster and more accurate than the procedure using a single sample. The results for barium nitrate determined by difference, using single samples are poor because of the cumulative errors inherent in the successive procedures involved. However, by direct titration these results are shoivn to be comparable in accuracy to those obtained for aluminum and potassium perchlorate using this procedure. Analyses of this type yielding accuracies are often sufficient to within =tO.5yG for analytical specifications where speed and simplicity are of primary importance.
I
I
I
KC104 Transition 300’C
I
I
I
I
I
A r e a of Endotherm
I
d
% KC104
Figure 4. Semiquantitative analysis for potassium perchlorate from areas on differential thermograms. Endothermal transition a l 300’ C.
ACKNO WLEDGMENl
The authors are indebted to E. D. Crane, who prepared all the mixtures used in this work. LITERATURE CITED
Asociacion de productores de Yodo de Chile, French Patent 680,116 (Aug. 9, 1929). Carthew, A. R., Am. Mineralogist 40, 107-17 (1955).
Deal, S. B., ANAL. CHEM.27, 109 (1955).
Duval, C., “Inorganic Thermogravimetric Analysis,” Elsevier Publishing Co., Amsterdam, 1953. Glasner, A., Weidenfeld, L., J. Am. Chem. SOC.74, 2467 (1952). Gordon, S., Campbell, C., ANAL. CHEM.27, 1102 (1955).
( 7 ) Zbid., 28, 124 (1956). (8) Gordon, S., Cam bell, C., Fifth
Symposium on d’ombustion (Proceedings), pp. 277-84, Reinhold, New York, 1955. (9) Grim, R. E., Ann. N . Y . Acad. Sci. 53, 1031-3 (1951). (10) Grim, R. E., Machin, J. S., Bradley, W. F., “Amenability of Various
Types of Clay Minerals to Alumina Extraction by the Lime Sinter and Lime Soda Sinter Processes,” Illinois State Geol. Survey, Bull. 69, 41-4, 71-3 (1945).
(11) Ishikawa, F., Murooka, T., Hagisawa. H.. Sci. Reds. TGhoku Imv. Univ: First Se;. 22, 1207-28 (1933). (12) Joint A4rmy-?javy Specification, JanA-289 (Jan. 30, 1946), aluminum
powder, flaked, grained, and atomized.
Joint Army-Navy Specification, JanP-217 (May 29, 1945), potassium perchlorate. Morita, Hirokazu, ANAL.CHEM.28, 64 (1956).
Moser, L., Marion, S., Ber. 59B, 1335-44 (1926).
Smoth?;s, W. J., Chiang, Y., Wilson, A,, Bibliography of Differential Thermal Analysis,” Univ. Arkansas Inst. Sci. and Technol. Research Ser. KO. 21 (November 1951).
Speil, S., Berkelhamer, L. H., Pask, J., Davies, B., “Differential Thermal Analyses,” U. S. Bur. Mines, Tech. Paper 664, 5-8 (1945). RECEIVED for review June 15, 1956. Accepted Xovember 2, 1956. First Delaware Valley Regional Meeting, ACS, Philadelphia, Pa., February 16, 1956.
Chromatographic Separation of Fluoride and Phosphate SIR: The determination of fluoride in biological materials requires its separation from interfering ions, chief among which is phosphate. This may be accomplished by the Willard and Winter distillation (9) or by the diffusion technique of Singer and Armstrong (8).
Recent advances in ion exchange and partition chromatography suggest that these procedures may also be feasible for the separation of fluoride and phos310
ANALYTICAL CHEMISTRY
phate, as the relative affinities of these ions for a n anion exchange resin differ greatly. Funasaka, Kawane, and Kojima (5)have reported the separation of fluoride from phosphate (1 to 60), using Amberlite IRA 410 in the hydroxyl form. I n the present studies, as little as 25 y of fluoride have been separated quantitatively from 500 times as much phosphorus on Dowex-1 (OH-), using phosphorus-32 as tracer indicator. I n
addition, fluoride has been separated‘ semiquantitatively from phosphate by a simple paper partition chromatographic technique.
EXPERIMENTAL A N D RESULTS
Ion Exchange Chromatography. Dowex 1-X10, 100-200 mesh, chloride form was cycled three times, using 100 ml. of 3 N sodium hydroxide and
