for the determination of nitrites is satisfactory. Ten determinations using 9.80 p.p.m. of nitrite gave a mean absorbance value of 0.737 with a standard deviation of 0.004 or 0.53yG. A slight modification in procedurenamely, the use of excess nitrite instead of excess thiourea-enables small amounts of thiourea to be deterniined colorimetrically.
LITERATURE CITED
(1) .+-res, GI. H., .IY.~L. CHEM.21, 652 (1949). 1 2 ) Bent, H. E., French, C. L., J . A m . Cheni Soc. 63,568 (1941). ( 3 ) Coade. 11.E.. Weiner. E. I.. J . C'heiii. Soc.'103, 1221 (191i). ' (4) Kuemmel, D. F., Mellon. 11. G.. .\NAL. CHEY.28, 1674 (1956). (5) Ovenst,on, T. C. P., Parker, C. -I,, Anal. Chirn. Acta 3, 277 (1949).
( 6 ) Pappenhagen, J., liellon, 11. G., . i S A L . CHEM. 25, 341 (1953). (;j Rider, B. F., IIellon, 11. G., IND. E S G . CHEM., . ~ K A L . ED. 18, 96
(1946).
-\.,2 . anal. Chern. 115, 332 (1939). RECEIVEDfor revie\\- .April 1, 1957. -4ccepted August. 2, 1957. Pittsburgh Conference on Analytical Chemistry and .Ipplied Spectroscopy, Pittsburgh, Pa., l l a r c h 1957. (8) Ringbom,
Inexpensive Auto matic Recording The rmo baIa nce WESLEY W. WENDLANDT Department o f Chemistry and Chemical Engineering, Texas Technological College, lubbock, lex.
b The construction and operation of an inexpensive, automatic recording thermobalance are described. The instrument was built from a torsion balance, 0- to 102-mg. capacity. The null position of the balance was maintained b y a beam of light falling between two cadmium sulfide photocells. The apparent accuracy for a 1 00-mg. sample was approximately o.SQ/o. The reproducibility was 0.20y0. The advantage of this instrument over commercially available models was the low cost of construction. Excluding labor, the complete thermobalance cost about $400. The accuracy and reproducibility of the instrument agreed favorably with the more expensive models.
I
ated instrument (21). The null point of the balance is maintained by a beam of light falling between two sensitive photocells. The weight-temperature curve is pen recorded on a cylindrical drum. DESCRIPTION OF APPARATUS
Balance. A schematic diagram of t h e thermobalance is shoxw in Figure 1. The balance was a torsion-wire type instrument, 0- to 102-mg. capacity, made by T'ereenigde Draadfabrieken, Nijmegen, Holland. The smallest scale dirision was 0.2 mg.; thus weighings could be read t o 0.1 mg. Recorder. The recording drum consisted of a n aluminum cylinder, 3
. -
x'rmm'r in the thermogravimetric
of analytical precipitates has increased rapidly since the first tliermobalance was built by Hoiida ( 1 1 ) in 1915. The impetus given to this field by Duval and his coworkers ( 5 ) has resulted in a greater knowledge of the thermal stability of some 1000 analytical precipitates. The development of the therniobalance has been reviewed by Duval ( 6 ) . By far the most popular instrunient has been the Chevenard thermobalance (3, 5 ) . Gordon and Campbell (8) have described the conversion of the photographically recording model into a chart recording instrument. As interest in this field has increased, a iiumber of new thermobalances have appeared. 1IanunlIy operated ( 2 . 4, 9, 15, 17': 20, 21), as well a? recording instruments (1, 7 , 10, 12-14, 16, IS,19, 22), have been described. This paper describes an inerpeiisive, automatic recording therinohalance which has proved successful in studying the thermal decomposition of a number of analytical precipitat,es. The main components of the t'herniobalance are essentially those of the manually oper-
55
ANALYTICAL CHEMISTRY
inches in diameter and 10.5 inches in length, connected to t h e torsion wire shaft of the balance. The rotation of the drum was controlled by a 1 r.p.in reversible synchronous motor. How ever, as this speed caused too much overshoot, the shaft speed was reduced with a 4 to 1 reduction gear. The recording pen consisted of a size 000 Leroy lettering pen suitably mounted on a sliding carriage. This carriage was d r a n n across the slide bar with a 1 revolution-per-liour synchronous motor. The motor contained a friction clutch so that the pen carriage could be manually reset to the starting position. Furnace. The furnace was t h e sanie as previously described ( 2 2 ) . It n as constructed by first winding 15 feet of S o . 22 gage, Xichrome alloy V,
4
0
I---.. c.
Figure 1.
Schematic diagram of the thermobalance
Torsion balance, 0- to 102-mg. capacity light source 6. D. C. Adjustable slit and focusing lens Beom mirror Recording drum E. Reflecting mirror F. G. Photocells H. Drum motor
A.
Combustion tube joint Pen carriage Furnace M . Pen drive motor N. Thermocouple 0. Plotinum sample p o n P. Exhaust gos connection
1. K. 1.
resistance wire (1.01 ohms per foot), into a coil inch in diameter. This was then 11ound onto a n asbestos-covered T'j-cor glass tube.2.5 cni. in dianieter and about 25 em. in length. a t about 1/4-inch spacings. The completed windings n ere adequately covered with asbestos insulation. The sample was contained in a platinum pan, 1 cm. in diameter and 0.5 cni. in height, suspended in the furnace by a platinum wire connected t o the balance beam. The temperature rise of the furnace
was controlled by gradually increasing the input voltage by means of a 6revolutions-per-day synchronous motor connected to the shaft of a Powerstat. The motor-driven Powerstat-input voltage was controlled by means of another Powerstat. The heating rate of the furnace could be varied by altering the input voltage to the motor-driven Powerstat. K i t h an input voltage of 60 volts and the motor-driven Powerstat started a t 20 volts, the temperature rise of the furnace was approximately linear a t 5.4' per minute.
J Figure 2.
Electrical circuit of the thermobalance
1 00-kilo-ohm, ' / 2 watt 500-kilo-ohm potentiometer 1 megohm, '/? watt 750 ohms, 10 watt Furnace 8-mfd. condenser, 400 volt 500-mmfd. condenser 1 -mfd. condenser, 400 volt Miniature photocell (cadmium sulfide) 2D21 tube Filament transformer, 6.3-volt a t 3 amperes Powerstat, 0-1 3 5 volts Powerstat, 0-1 35 volts
Ri.
Rz. R3.
R4.
Rs. c1.
cz. Cb. Cll.
vi.
Ti.
Tz. T3.
Mi. M?. Ma.
s w1. S W2.
swp.
sw,.
sw5.
s w6. w1.
W?.
w3. 11.
1 r.p.m. reversible synchronous motor 1 r.p.h. synchronous motor 6 RPD synchronous motor SPST switch SPST switch SPST switch SPST switch SPDT switch SPST switch Lamp (pilot) Lamp (pilot) Lamp (photocell light source 5 0 W ) 1 0-kilo-ohm relay
Figure 3. Heating rate curve for t hermobalance furnace
0
100 PEN TRAVEL
200 MM
Optical System. 1 plane mirror, 0.25 inch in diameter, was attached t o t h e weight hook of t h e balance beam. A beam of light from a 50watt lamp was focused on this mirror by means of a lens and adjustable slit. The reflected light from the mirror was focused by a lens mounted on the balance case and then reflected d o l m m r d onto the two photocells by means of another plane mirror. The photocells m-ere enclosed in a light-tight cabinet t o prevent interference from stray light. Electrical System. A schematic diagram of the electrical circuit is given in Figure 2. Several types of photocells were t'ried; best results n-ere obtained with the cadmium sulfide crystal type (Claires Type CL-2, 100 pa. a t 100 volts at 2-footcandle sensitivity). The relay circuits werp described by Groot and Troutner ( I O ) . Calibration. The temperature rise of the furnace is proportional t o the horizontal pen-travel distance on t h e recording drum. Hence, i t was necessary t o calibrate the pen-trawl distance with the furnace temperature. This was done by measuring the temperature of the furnace with a calibrated iron-constantan thermocouple, using an ice bat'h as the reference junction, at certain pen-travel interrals. The heating rate curve so obtained is given in Figure 3. The curve w:is linrw :at 5.3" per minute up to 265" C., while beyond this, the heating rate was still linear but a t 5.5' per minute. An average heating rate of 5.4' per minute was thus adopted. Two calibration runs are presented iTith agreement' t o iyithin of each other. The weight cliaiige is recordrd as tlie drum rotates. It was found that the pen traveled 2.40 mm. per mg. of weight change. During a typical decomposition run, a s l o stream ~ of air is passed t,hrough the furnace by means of a water aspirator connected to tlie loiver part of blie furnace. A blank run showed that there was no apparent weight change (less t,haii 0.1 mg.) for a temperature rise from ambient t o 850" C. Operation of Thermobalance. The platinum pan is first' cleaned and then ignited for 5 minutes with a Meker burner. After cooling, t h e pan is suspended on t h e plwtinuin Ivire, a 5O-nig. n-eight added. and the furnace raised into position. After checking to see that the pan sivings freely, the halance beam is released and the photocell circuits switched on. The aspirator is then adjusted to allon- a slow stream of air t o pass over the sample. The recording drum motor should then bring the balance pointer to exactly 50.0 nig. on the balance scale. If not,, this is brought about by adjustment of the balance zero-set screw. The furnace is then lowered, the 5O-nig. weight is removed. and about 90 to 100 mg. of the desired sample are i n h d u c e d into the pan: The furnace is then raised into position and the phobocella are a l l o w d t o bring the beam to the null position. The thermal decomposition curve is then begun by switching on the furnace VOL. 30, NO. 1 , JANUARY 1 9 5 8
0
57
gestions by R. C. Wilhoit and G. K. Estok are also gratefully acknowledged.
Table 1. Thermogravimetric Analysis of Disodium Hydrogen Phosphate 12-Hydrate
LITERATURE CITED
Composition, Based on Assumed NazHPOc. 12Hz0,% Product Theoret. Obsd. Diff. NazHP04 39.64 39.5 -0.1 39.5 -0.1 NarPzOl 37.13 37.3 $0.2 37.2 +0.1
1L ' 20 MG.
circuits. If the sample changes weight, the appropriate photocell will be activated by the beam of light and thus rotate the recording drum back to the null position. With proper adjustment of the photocell sensitivity potentiometers, a smooth curve is recorded.
375
DISCUSSION
The thermal decomposition curve of disodium hydrogen phosphate 12-hydrate (Na2HPO4.12HzO) is given in Figure 4. Water of hydration began to come off a t a little above room temperature with a break in the curve corresponding t o the 7-hydrate a t 105" C. The anhydrous salt, disodium hydrogen phosphate (NazHPOd), was obtained from 185" to 325' C., which then decomposed to give the sodium pyrophosphate (NaaPzO,) level, beginning a t 375" C. The decomposition temperatures agreed qualitatively with those reported previously (6). However, as no weight data have previously been given and no heating rate specified, it is difficult to make a rigorous comparison. The reproducibility of the thermobalance is given by the data in Table I. The sensitivity (the minimum weight change that can be detected) is about 0.05 mg. with a response time of 2.4 seconds per mg. The maximum weight change that can be recorded is 102 mg. The apparent accuracy for a 100-mg.
TEMPERATURE 'C.
Figure 4. Thermal decomposition curve of sodium dibasic phosphate dodecahydrate
sample is approximately 0.50'%. The reproducibility over the same weight range is approximately 0.2 mg. or 0.20%. The reproducibility of the decomposition temperatures, for duplicate sample weights, is about 1%. The advantage of this instrument over several of the commercially available models is the low cost of construction. The complete thermobalance, excluding labor, cost about $400. The accuracy and reproducibility of the instrument agreed favorably with the more expensive models.
Japan, Pure ( ( 1951). Zbid., 74, 642 (1953). Murthy, A. R. V., Bharadwaj, D. S., Mollya, R. M., Chem. & Znd. (London) 1956, 300. Orosco, E., hfznzsterzo Trabalho ind. e com., Znst. nacl. tech. (Rio de Janeiro) 1940. Spinedi, P., Riccrca sci. 23, 2009 (1953). Wahl, K., Klemm, W., Wehrmeyer, G. W., 2. anorg. allgem. Chem. 285, 333 (1956). Wendlandt. 'cv..-4NAL. CHEM.27, 1277 (1955). ' Zagorski, Z., Przemysl Chem. 31, 326 (1952).
w.
ACKNOWLEDGMENT
The author would like to acknowledge the assistance of Warner Kendall and David Hayes in construction of the thermobalance. The many helpful sug-
Brefort, J., Bull. SOC. chim. France 1949, 524. Buriel Marti, F., Barcia Goyanes, C., Anales real SOC. espaii. fh.y quim. 47B, 73 (1951). Chevenard, P., Wach6, X., de la Tullage, R., Bull. SOC. chim. France 11, 41 (1944). Clark, G. L., Sprague, R. S., ANAL. CHEM.24, 688 (1952). Duval, C., "Inorganic Thermogravimetric Analysis,'' Elsevier, Houaton. Tex.. 1953.' Zbid., p. 6. Eyraud, C., Eyraud, I., Laboratories ( P a r i s ) 12, 13 (1955). Gordon, S., Campbell, C., A NAL. t3\ CHEM.28, 124 (195L,. Gregg, S.J., Winsor, G. W., Analyst 70. 336 (1945). Groot, C.,' Troutner, V. H., U1. s. Atomic Energy Commission, HW41007 (Jan. 20, 1956). Honda, K., Sci. Repts. Tohoku Imp. liniv. 4 , 9 7 (IC)15). Hyatt, E. P., Cutter, I. B., Wadsworth. M. E.. Am. Ceram. SOC. Bull. 35, 180 (1956). Izvekov, I. V., 9
RECEIVED for review February 23, 1957. -4ccepted June 26, 1957.
Thermal Decomposition of Scandium, Yttrium, and Rare Earth Metal Oxalates WESLEY W. WENDLANDT Department of Chemistry and Chemical Engineering, Texas Technological College, lubbock, Tex. The thermal decomposition of the oxalates of scandium, yttrium, lanthanum, cerium(lll), praseodymium, neodymium, samarium, europium, gadolinium, holmium, and erbium was studied on the thermobalance. The hydrate water began to come off in the temperature range of 40" to 60" C. In certain cases, intermediate weight levels were found for the 6-
58
ANALYTICAL CHEMISTRY
and 2-hydrates. The anhydrous oxalates appear to b e very unstable; no weight levels were found with this composition. The oxide levels were obtained in the temperature range of 360" to 800" C.
T
rare earth metal ions readily form slightly soluble oxalates in acid solution. Because of the selecHE
tivity and insoluble nature of the metal oxalates formed, this procedure has proved useful for separation as well as for gravimetric estimation of these elements. The oxalates, however, cannot be weighed directly as precipitated because of the varying amount of hydrate water present (6). This hydrate vater was found t o be dependent on the temperature of precipitation. For dys-
