Use of the Thermobalance in the Analysis of Soils and Clays MORRIS SCHNITZER, J. R. WRIGHT, and ISRAEL HOFFMAN Chemisfry Division, Canada Department o f Agriculture, Ottawa, Canada )The thermobalance has been used to study a number of soils differing widely in origin and composition. The behavior of inorganic carbonates, relatively pure clay minerals, and soils to which inorganic carbonates and clays were added is reported. The chief applications of thermogravimetry to soil analyses are the determination of hygroscopic moisture, organic matter, and inorganic carbonates in soils, and lattice water in clay minerals. Some possible sources of error in these determinations are discussed.
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years many compounds ranging from simple inorganic salts to organometallic complexes have been examined on the thermobalance. This paper describes the thermogravimetric behavior of numerous surface and subsurface soil samples differing nidely in origin and composition. Duval's (6) findings that moiature in wheat flour could be accurately determined on the thermobalance suggested the feasibility of similar measurements in soils. The work of Gibaud and Geloso ( 6 ) supported the possibility of a qualitative detection and quantitative determination of carbonates in soils. Of even greater interest was the possibility of the approximate estimation of the organic matter content of soils. Loss-on-ignition is still a idely used for this purpose for reasons of simplicity and convenience, but the temperatures and duration of ignition are not standard. I n this laboratory it has been customary to heat mineral soils for 3 hours and organic soils for 4 to 5 hours a t 450" C. According to the Association of Official Agricultural Chemists ( I ) , the oven-dry sample is ignited to full redness in a platinum dish, with occasional stirring until organic matter is destroyed. However, this method approximates the organic matter contents of sandy soils only. K h e n soils contain appreciable quantities of clay and/or carbonates, values for organic matter as estimated by loss-on-ignition may differ considerably from those obtained by multiplying the percentage of organic carbon found by dry combustion by the factor 1.724. The origin and accuracy of this factor is discussed by n'aksman ( I d ) , u-ho concludes that N RECENT
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ANALYTICAL CHEMISTRY
it comes close to representing the actual relation of humus to carbon in soil. The thermogravimetry of soils, relatively pure clays, crystalline carbonates, and soils to which knomn amounts of clays and carbonates were added, was studied. During this investigation, sharp breaks were observed in the pyrolysis curves of relatively pure clay minerals a t elevated temperatures, which suggested the use of the thermobalance for the determination of lattice water in such systems, and the possibility of the identification and even estimation of pure clays and simple clay mixtures. Research in clay mineralogy is a t present hampered because no reliable method exists for the determination of lattice water. The soil samples examined ranged in clay content u p to i5%, and in carbonate content up to 15%. Some preliminary findings have been reported ( 7 ) . EXPERIMENTAL
A Stanton recording thermobalance of 0.1Apparatus and Methods.
mg. sensitivity was used and t h e samples were held in platinum crucibles. T o eliminate random thermal currents and avoid migration of t h e hot zone, the top of t h e furnace was baffled with a silica cup containing silica 11001 and a silica lid 1%-asplaced on top. T h e weights ranged from 50 to 300 mg. of air-dry material ground in an agate mortar to pass a 0.5-mm. sieve. The rate of heating was 300' C. per hour. The clay contents were estimated with the hydrometer according to Bouyoucos ( 3 ) . The percentages of clay shown are average values of a t least four determinations carried out in four different laboratories ( I O ) . p H measurements were made with the calomel-glass electrode svstem using a soil-water ratio of 1 to 2.5 (4). Titrations with Karl Fischer reagent were done electronietrically using the deadstop end point. Sodium tartrate dihydrate mas used as primary standard for the Karl Fischer reagent (11). One gram of soil was suspended in 50 ml. of absolute methanol and titrated as quickly as possible, to avoid prolonged contact between soil and reagents. Organic carbon was determined by one or more of the following methods: dry-combustion (8); wet oxidation after Walkley as modified by Peech
et al. ( 1 2 ) ; and manometrically, according to T a n Slyke and Folch (13). Inorganic carbon was measured by treating the soil with 1 to 3 hydrochloric acid and absorbing the carbon dioxide evolved in barium hydroxide (9). Calcium and magnesium were determined volumetrically and gravimetrically, respectively, after fusion with sodium carbonate (4). All results are expressed on an air-dry weight basis. Materials. Table I shows the horizon, soil series, great soil group, geographical origin, pH, and clay content for each sample. I n addition t o dolomite (National Bureau of Standards KO. 88), crystalline calcite and magnesite rvere used. The Wyoming bentonite was purchased from Baroid Division, Sational Lead Co., Houston, Tex., and converted t o the H-clay b y bringing it into contact with IR-120 exchange resin in the H-form. The Fithian illite was obtained from Ward's Natural Science Establishment, Inc., Rochester, N. Y. RESULTS AND DISCUSSION
Determination of Hygroscopic Moisture in Soils. T h e pyrolysis
cuives in Figures 1 t o 3 show distinct breaks, often plateaus, starting from 150' t o 180' C. and extending t o 210' t o 240' C., depending upon the organic matter and clay contents. These straight portions might be interpreted as indicative either of the elimination of hygroscopic moisture, or of hygroscopic moisture plus the easily volatile loiv molecular weight organic compounds which are encountered in soils. The low-temperature losses were evaluated quantitatively and compared n ith moisture determinations by the Karl Fischer reagent and with drying overnight a t 105" C., the method most commonly used in soil laboratories. Table I1 s h o w that values obtained with the Karl Fischer reagent were the highest in all cases except one. This \vas most marked for soils 7 and 13, both containing approximately io% clay. I n addition to hygroscopic moisture, the Karl Fischer method appeared to include a portion of the lattice water of the clays. It has been stated ( 2 ) that the Fischer method gives higher results than do oven methods for dehydrated vegetables and for hydrated c r y s t a l h e materials such as the monohydrate of mon-
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Figure 2. Thermogravimetry of mineral soils containing less than 15% clay 1. 2.
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Figure 1. ganic soils 1. 2.
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Thermogravimetry of orSample weight 9 0 mg. Soil 18 3. Soil 20 Soil 1 9 4. Soil 21
usodiuni glutamate. I n general, the thermogravimetric values in Table I1 fell in between those obtained by the Karl Fischer titration and by oven-drying. Lacking a n absolute method for the determination of hygroscopic moisture in soils, it is not possible a t present to decide which of the three methods is the most reliable. Determination of Organic M a t t e r in Soils by Thermogravimetry. After t h e d a t a had been examined both qualitatively and quantitatively, t h e soils n ere grouped a s f o l l o w : organic soils-those containing more t h a n 60% organic matters; mineral soils containing less than 15% clay; mineral soils containing between 15 and 40% clay; mineral soils containing more than 40% clay. ORGANICSOILS. Figure 1 shows pyrolysis curves for samples of two A,, horizons, a peat and a muck soil. According t o curve 1, all hygroscopic moisture mas eliminated by 150" C. Organic matter started to burn off at 210" and the pyrolysis was completed by 520" C. Curve 2 s h o w that a temperature of 180' C. was required to drive off hygroscopic moisture. The decomposition of organic matter started a t 210" and was completed a t 550' C. Curve 3 is very similar to curve 1. Curve 4 does not show the usual plateau indicative of the elimination of hygroscopic moisture. Instead there is a continuous loss of weight u p to about 600" C . However, there is a distinct
Sample weight 9 0 mg. Soil 1 3. Soil 9 Soil 2 4. Soil 16
change of slope a t about 220' C., indicating the beginning of the pyrolysis of the organic matter. As Table I11 shon s, the agreement between the thermogravinietric and chemical methods was satisfactory, especially when the proximate nature of the factor 1.724 is taken into account. MINERALSOILS COKTBINISG LESS THAN15% CLAY. Curve 1 in Figure 2 shows that all hygroscopic moisture was eliminated at 150" C. Organic matter started t o burn off a t 230" C. and its ignition appears to have been completed at 520' C. Curve 2 s h o m that heating to 190" C. was necessary to remove hygroscopic moisture. Ignition of organic matter started a t 225' and was complete by 500" C. Curves 3 and 4 are similar to curve 1, except that the soils contained considerably less organic matter. Table I.
Sample Designation 1
2 3
4 5 6
7 9
10 11 12
13 14
15 16
17 18
19 20 21
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Figure 3. Thermogravimetry of mineral soils containing more than 1570 clay 1. 2.
3. 4.
Sample weight 9 0 5. Soil 11 Soil 4 6. Soil 1 2 7. Soil 1 4
mg. Soil 15 Soil 7 Soil 13
The agreement between the thermogravimetric and chemical methods as indicated by the data in Table I11 was satisfactory. RTINERAL SOILS TWEEP; 15 AND 40%
CONTAINING BECLAY. Curve 1 in Figure 3 shows elimination of hygro-
Key to Samples Examined, pH and Clay Contents of Mineral Soils
Geographical Origin Prince Edward Island Gray brown Ont. AI Podzolic C Carrol Chernozem Man. A, Darlingford Chernozem Man. Brown wooded B. C. C Ma>ooli AI Saline Sask. C Regina Dark brown Sask. 242 Breton Gray wooded Alta. C Penhold Chernozem Alta. A41 Bedford Dark gray Que. Gleizolic B Bedford Dark gray Que Gleizolic B Ste. Rosalie Dark gray Que. GleGolic A Interval Alluvial N. B. B Caribou Thin Podzol 3. B. Truro Podzol x. s. Queens Podzol N. s. Armadale Podzol P. E. I. 1,unenburg Brown Pods.s. zolic. Virgin peat Organic Xewfoundland Muck Ste. Clothilde Organic Que. Horizon A,
Great Soil Series Soil Group Charlottetown Podzol
Clay,
%
pH 5 8
11 87
6 5
985
8 6
17 26 20 76
7 6 8 9 8 3 5 4 8 6 5 9
17 10 19 85 71 06 11 62 21 20 18 40
7 1
27 SO
6 6
70 30
5.3 6.2 5.7 5.6
23.20 22.60 11.20 29.50
8 2
VOL. 31, NO. 3, MARCH 1959
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Figure 4. Thermogravimetry of mineral soils containing carbonates
Figure 6. minerals
Sample weight 90 mg.
Soil 3 Soil 5
1. 2.
3. 4.
1.
Soil 1 0 Soil 6
2. 3.
4.
Table 11.
Soil
No. 1 2 3 4 6 7 9 11 12 13
Hygroscopic Moisture in Mineral Soils
Hygrocopic Moisture, % ThermoKarl Drying graviFischer at metry titration loso C. 1.64 3.00 1.91 5.66 2.08 5.06 1.24 6.46 1,65 3.00
1.93 3.95 2.35 5.84 2.92 6.56 1.64 6.93 2.34 5.45
1,29 2 96 1.39 4.23 3.08 4.18 0.98 5.42 0.89 3.44
scopic moisture by 160" C. Organic matter started to burn off a t 220' C. with a continuous loss in weight until 650' C. Curve 2 s h o w similar behavior. Hygroscopic moisture was expelled by 180' C Organic matter started to decompose a t 220' and weight losses occurred up to 700" C. Because Figures 1 and 2 h a r e shown that, with these conditions of pyrolysis, organic matter is usually burned off by 500" to 550' C., the additional weight losses up to 700' C. were probably due to the expulsion of lattice n-ater from clav minerals. The validity of this argument was tested by adding Fithian illite to soil 2 to give final concentrations of 20 and 40% of the clay mineral. Thp beginning of the straight portion of the pyrolysis curve, not shown, was displaced from 500" to 680" C., and 720" C., respectively, thus indicating that weight losses beyond 500" C. Jyere due to lattice watm of the clay.
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ANALYTICAL CHEMISTRY
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Thermogravimetry of clay
Fithian illite (90.0 mg.) H-Wyoming bentonite (90.0 mg.) Mixture of Fithian illite (123.0 mg.) and HWyoming bentonite ( 1 25.4 mg.) Kaolinite (90.0 me.)
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tcr values in agreement with clicmical determinations. Determination of Inorganic CarFigure 5. Thermogravimetry of carbonates in M i n e r a l Soils. Figure 4 bonates a d d e d to soil and carbonates and Table I V provide information alone about t h e thermogravimetric determination of carbonates in soils. 1 . 90 mg. of soil 2 These contained approximately 20% 2. 85.2 mg. of sail 2 5.3 mg. of calcite 3. 80.1 mg. of soil 2 10.0 mg. of dolomite clay and represent typical moderat'ely 4. 82.0 mg. of soil 2 10.0 mg. of magnesite alkaline Canadian soils. Curres 1, 5. Calcite (90.0 mg.) 2: and 3 in Figure 4 show that these soils 6. Dolomite (90.1 mg.) began to lose carbon dioxide a t about' 7. Magnesite (87.2 mg.) 600" C. and the evolut'ionwas completed betneen 730" and 780" C. It was not possible to distinguish betnerln calcite Curves 3, 4, and 5 in Figure 3 show and dolomite on the basis of the pythat these soils also lost weight up to rolysis curves. Chemical analyses in650" to 700" C. Weight losses a t 500" C. dicated the presence of appreciable were read from their respective pyrolyamounts of dolomite in samples 3 and 5 sis curves for each of the five soils and (Table IT). Curve 4 was difficult' to incompared with carbon determinations terpret'. I n Figures 1 and 2 , platpaus by two different methods. Table I11 indicative of the elimination of organic shon s that for soils containing between matter usually started between 500" 15 and 40% clay, thermogravimetric and 550" C. Because chemical analysis values agreed more closely n ith dry comof soils containing up t'o 40% clay (Table bustion than n ith n e t oxidation data. ~IIXERA SOILS L COKTAINIKGNORE 111)substantiated that these losses were due to organic matter, it w s assumed THAS40% CLAY. Curves 6 and 7 in that, subsequent losses Kith increasing Figure 3 show the pyrolysis of t\To soils temperature could be attributed to careach containing approximately 7OYc bonates. The data in Table IV for soil clay. Curve 6 indicates the elimination S o . 6 shon-ed that this assumption was of hygroscopic moisture by 180" C. Orjustified because the agreement with the ganic matter started to burn a t 250" C. chemical value was satisfact'ory. Losses continued up to 700" C. Curve 7 Curve 1 in Figure 5 shows the pyrolys h o m expulsion of hygroscopic moisture sis of soil 2. Curves 2 and 3 show the by 160" C. Organic matter started to pyrolysis of soil 2 with small amounts of burn a t 220" C. Losses of weight conadded calcite and dolomite. Table T' tinued until 550" C. indicates that these carbonates were Table I11 compares organic matter dequantitatively recovered. Curve 4 demterminations by two chemical methods onstrates the qualitative detection in with weight losses a t 500" C., as read this soil of magnesite, contaniinated from the pyrolysis curve of each soil. with some dolomite. Because the beLosses in weight a t this temperature ginning of the deromposition of niagnccould not be used to obtain organic mat-
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Table Ill. Organic Matter Content of Soils by Thermogravimetry and Chemical Methods
Organic Matter, yc C Soil ThermoX Wet Xo. gravimetry 1 .724 oxidation 18 19 20 21
c l
Mineral Soils Containing < 157, Clay 1 4 50 4 48h 4.45 2 11.25 ll.OOh 11.67 9 0.95 1.16b 0.94 16 2.04 ... 2.05 Xineral Soils Containing between 15 and 40% Clay 11 27.4iC 26.17" 4 11.3lC 11.53h 10.88 6 2.76c 2.6sh 3.00 12 1 26c 1 0Ob 0 84 14 3 20c 3 45* 3 66 15 4 13c 3 83h 3 56
LL
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TEMPERATURE
800
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1.
-
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Goethite ( 9 2 mg.) Lepidocrocite ( 9 3 mg.) Haematite (91 mg.) Gibbsite ( 6 0 mg.) Diaspore (1 8 3 mg.) Bauxite ( 2 1 1 mg.) Boyerite ( 2 3 3 mg.)
site nrarly coincides wit'li the end of t'hat of the organic matter, quantitative niagriesite tlott.rminatjions in soils are more difficult. Curves 6, 6, and 7 show the pyrolysk of crystalline calcite, dolomite, and magnesite contaminaCed n-ith a small ainount of dolomit'e. Calcite and tloloniite gave very similar curves, in agreement with data published by Gibaud and Geloso (6) and it n a s not possible to dist'inguish between the two. Carbon dioxide from niagnesite s t a r t c ~ to l evolve a t 500" C.,in contrast to dolomite and calcite where decaomposition started a t 620" and 670" C., rwpectivc,ly. As shown in Table Y. the carhon dioxide yields for pure carImnatcs were quantitatiye. I n the case of the magnesite used, x-ray diffraction established the presence of some dolomite. C'hcmical analysis shoiT-ed 3.07, calcium. so that the amount added was Ixtsed on an e s t i m a t d 94% magnesite and 6% dolomite. The discrepancy betiwen the amount of carbon dioxide added and found for magnesite might be due to t h k estimate (Table V). P T R O L Y OF ~ I SCLAYAIIKERALS. The presencc of clay minerals is one of the main sourccs of error in the estimation of the organic mat'ter content of soils b y loss-on-ignition due to the elimination of lattice w t e r \vit,h increase in temperature. Consequently. a number of rela-
p
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400
p
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600
800
1000
T E M P E RATUR E "C.
OC.
Figure 7. Thermogravimetry of hydrous iron and aluminum oxides 2. 3. 4. 5. 6. 7.
0
Organic Soils 62 00 60.6%' 72.13 69 06O 86 74 82.42a 66 45 65 46a
Figure 8. Thermogravimetry of silicate minerals and gypsum 1. 2. 3. 4. 5.
Biotite ( 4 7 8 me.) Tremolite ( 4 6 8 mg.) Vermiculite (21 0 mg.) Allophane ( 9 4 mg.) Gypsum ( 1 3 4 mg.)
tively pure clay minerals n-ere exaniinecl on the thermobalance. Curve 1 in Figure 6 shows the pyrolysis of Fithian illite. Hygroscopic moisture was expelled by 160" C. Additional weight losses. apparently due to lattice water, began a t 460" and continued up to 750" C. Curve 2 describes the pyrolysis of a Kyoming bentonite (H-form). After the expulsion of hygroscopic moisture a t 180" C., losses apparently due to lattice water occurred from 640" to 710" C. Curve 3 shows the pyrolysis of a 1 to 1 mixture of Fithian illite and H-bentonite. It is possible to distinguish the t n o coinponents of the mixture. Curre 4 shows the pyrolysis of kaolinite. Hygroscopic moisture was eliminated a t 180" C. Keight losses apparently due to lattice n ater commenced a t 460" C., increased rapidly between 500" and 660" C., then continued a t a slow rate u p to 800" C. Because of controversy in the literature regarding the amounts of lattice water in clay mincrals and also because of the importance of finding a technique for its quantitative determination from the structural point of view. this aspect of the investigation is presently under further study. Sources of Possible Errors. When soils are known t o be high in hydrous oxides of iron and aluminum such as latosols, certain podzolic B horizons, etc., theinioprarimetric curves must
hlineral Soils Containing > 407, Claj 7 3 14 1 86h 1 58 13 2 35c 0 88b 0 76 a Alanometric T'an Slyke-Folch method. b Dry combustion c From pyrol>-sis curve at 500" C. Table IV. Inorganic Carbon Content of Mineral Soils Containing Carbonates
Inorganic Carbon, % TherSoil mograv- ChemNo. imetrj-a ically CaO AlgO 4.07 3 2.05 2.12 8.49 5 1.04 1.05 3.96 4.30 1.77 6 0.83h 0.88 5.04 10 2 08 1.98 9 84 195 Loss of weight X 12/44. b From curve a t 500" C. 5
Table V. Recoveries of Carbon Dioxide from Pure Carbonates Alone and Mixed with Soil
Material Calcite Dolomite
Air-Dry Weight, Mg. 190 2 90 0 50 0 90 1 87 2
Magnesite Soii+ Calcite 8 5 . 2 t5 . 3 Dolomite 80 1 10 0 Magnesite 82 0 10 0
+ +
a
coz, Mg. Added 83 2 39 4 23 6 42 8 45 0
Found 82 9 39 4
2.3 4 7 5 2
23 4.7 5 3a 4 6h
From curve at 450" C. From curve at 500" C.
23 3 42 6 43 8
be interpreted with special care because, as shown in Figure 7 , weight losses due to the decomposition of these components occur mainly in the region of organic matter losses. For example, if a podzolic B horizon contains 2% goethite or kpidocrocite, the absolute error in the value for organic matter VOL. 31, NO. 3, MARCH 1 9 5 9
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443
for a 90-mg. soil sample would be approximately 0.2%. The presence of 8% gibbsite in a soil would result in an absolute error of 2.8%. Allowances would have to be made in the case of tropical soils high in gibbsite, diaspore, bauxite, or bayerite. Figure 8 shows thermogravimetric curves for a number of silicate minerals and gypsum. Biotite, tremolite, and vermiculite (curves 1, 2, and 3) show high temperature losses. The presence of appreciable amounts of the first two minerals would result in inaccurate carbonate values. Large amounts of allophane would give erroneous results for organic matter. For subsoils developed on saline parent materials containing 1 to 2% gypsum, the absolute error in the hygroscopic moisture determination on a 90-mg. sample would range from 0.2 to 0.4%. Jt7here crystals of gypsum can be detected in the soil, (Bcshorizon), by the naked eye, corrections in the hygroscopic moisture values should be made. The presence of 20% illite would cause a positive absolute error in the organic matter determination of approximately 0.6%. While kaolinite and bentonite would not interfere with organic matter determinations, if present a t the 20% level, they could cause absolute positive errors in carbonate measurements of 1.5 and 0.770, respectively. However, kaolinite is seldom a significant constituent of calcareous soils. Hydrous silicates such as biotite, vermiculite, and allophane (Figure 8, curves 1, 3, and 4) when present in large amounts might cause high values for hygroscopic moisture. The data on a variety of soils, as taken directly from thermogravimetric curves, agreed satisfactorily with those obtained by established chemical methods. Where it is known that large amounts of interfering constituents such as those mentioned are present, caution should be exercised in interpreting the thermogravimetric curves.
SUMMARY
The pyrolysis curves of most soils examined showed plateaus starting a t 150’ to 180’ and extending to 210” to 240’ C., indicative of either hygroscopic moisture, or hygroscopic moisture plus easily volatile organic compounds. I n general, thermogravimetric values fell in between those obtained by the Karl Fischer titration and oven-drying a t 105’ C. This aspect needs further clarification. Organic matter started to burn off between 210’ and 240” C. I t s pyrolysis was usually complete a t 500” C. I n organic soils and those containing less than 15% clay, a relatively close estimate of the organic matter content rould be made from a pyrolysis curve. K h e n the clay content varied from 15 to 40%, the loss in weight a t 500’ C., read from a pyrolysis curve, usually gave an estimate of the organic matter content which was in satisfactory agreement R ith dry combustion and wet oxidation data. When soils contained more than 40% clay, it was not possible to distinguish between Lveight losses due to the pyrolysis of organic matter and those due to the elimination of the lattice water of clays. The thermobalance has proved useful for the detection and quantitative estimation of carbonates in pure carbonates, in soils to which carbonates were added, and in soils in which they occurred naturally. The data suggest the possibility that lattice water may be quantitatively determined at least in pure clays and in simple mixtures. Because lattice water came off at different temperatures with different clays, it may be possible to use this characteristic as an additional means of identification. -4 preliminary thermogravimetric study of a soil will give valuable information as to its constitution. I n addition, the thermobalance has proved a useful tool for the intelligent definition of
conditions of analysis by other methods. ACKNOWLEDGMENT
The authors are indebted to the National Soil Survey Committee (Canada) for supplying most of the soil samples and analytical data on clay contents, and to Ralph Grim of the University of Illinois for supplying the kaolinite. They also thank Richard Levick for technical assistance. LITERATURE CITED
(1) ,Assoc. of Offic. Agr. Chemists, “Offi-
cial .\lethods of .4nalysis,” 8th ed., p. 28, Washington 4, D. C., 1955. (2) Rarlow, J. W., Can. Food I n d . 28, 11 (1952). (3) Bouyoucos, G. J., Soil Sci. 42, 225 (1936). (4) ,Chemistry Division, Science Service, Canada Department of Agriculture, “Chemical Methods of Soil Analysis,!’ rev. ed., Otta,wa, 1949. ( 5 ) Dwal, C., “Inorganic Thermogravimetric Analysis,” p. 31, Elsevier, Xex York, 1953. (6) Gibaud, hfichelle, Geloso, 31. hf., Chinz. anal. 36, 153 (1954). (7) Hoffman, I., Schnitzer, M., Kright, J. R., Chem. & I n d . ( L o n d o n ) 261, 1958.
(8) Mahin, E. G., Carr, R. H., “Quantitative iigricultural Analysis,” 1st ed., pp. 249-51, McGraw-Hill, Xetv York, 1923. (9) Ibid., pp. 81-5, 249. (10) National Soil Survey Committee (Canada), unpublished report. (11) Neuss, J. D., O’Brien, AI. G., Frediani, H. A., A N ~ L .CHEU. 23, 1332 (1951). (12) Peech, Michael, Alexander, L. T., Dean, L. A., Reed, J. F., “AIet’hods of Soil Analvsis for Soil-Fertilitv Investigations,” U. S. Dept. Agr. ”Circ. 757, 6 (1947). (13) Van Slyke, D. D., Folch, Jordi, J . Biol. Chem. 136, 509 (19401. (14) Waksman, S. A., “Humus,” 1st ed., p. 243, Baillikre, Tindall and Cox, London W. C. 2, 1936. RECEIVEDfor review July 29, 1957. Accepted October.27, 1958. Contribution KO. 372, Chemistry Division, Science Service.
Transistorized Switch for Derivative Polarography JAN KRUGERS’ laborafory for Analytical Chemistry, University of Amsterdam, Amsterdam, Holland
b The mechanical switching mechanism suggested by Ishibashi and Fujinaga for use in derivative polarography may b e superseded to advantage by an electronic switching system. Several modifications are suggested which may be important in connection with theoretical considerations and trace analysis. A more general use of the circuit is for converting a small direct 444
ANALYTICAL CHEMISTRY
current into an alternating current of any desired frequency between 0.1 cycles per second and 10 kilocycles per second.
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method of derivative polarography, described by Ishibashi and Fujinaga (I), the voltage applied to the polarographic cell alternates between E and E - AE volts many times per secN THE
ond. 4 t the same time the connections of the galvanometer are reversed. Both current paths required to enable this are shown in Figure 1. Obvlously, when hE is zero, both currents have the same value. Their effects on the gahyanometer system cancel 1 Present address, T. N. 0. Studiecentrum Kernenergie (BRl), Mol, Belgium.
