Aerod yncr mic Forces in T hermogravi metry LEE CAHN and HAROLD SCHULTZ Cahn lnstrumenf Co., Paramount, Calif.
In thermogravimetry, small samples permit better accuracy and resolution for the same scanning rates. With balances of adequate sensitivity, aerodynamic “noise” is of interest. Aerodynamic noise is found to be strongly affected by hangdown tube diameter and pressure, proportional to the horizontal area of the sample pan, only slightly affected by changes in sample temperature, and independent of sample weight. Precision to the 1-pg. level can be obtained at atmospheiic pressure with veiy small diameter tubes, or with large tubes at pressures below 150 torr.
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small samples offer substantial advantages. Sample temperature is more homogeneous, and exposure to atmospheres present is more uniform. Transitions are sharper, and both transition temperature and mass are better defined. I n addition, the transition temperature value obtained will generally be closer to the true value for the molecule. An example is shown in Figure 1. T o measure thermogravimetric curves for samples of a few milligrams or tens N THERMOORAVIMETRY,
of milligrams with good accuracy, precision of the order of 1 pg. is desirable. The first requirement for such work is a controlled-atmosphere balance of adequate sensitivity. Several such instruments have been described by the authors (1, 2). With such an instrument, the question arises as to the level of precision to which the phenomena themselves permit measurement. Much important thermogravimetric work requires significant partial pressures about the sample. Heating in such an atmosphere will cause aerodynamic forces, which introduce uncertainty into the balance readings. The magnitude and nature of these forces, the effect of operating parameters upon them, and the conditions for optimum precision were investigated. The aerodynamic forces applied to the balance can be divided into two components, a constant offset and a varying component. The varying component always looks like the “noise” seen on spectrophotometer traces (Figure 2). Like that noise, i t can be characterized in magnitude by the peak-to-peak variation for the degree of precision signifi-
1 C u S O , . 5H,O
- 0 . 4 2 6 mg - - - - 18 00 mg
-.
cant here. The uncertainty in this measure of the uncertainty is about &20%. The rule of thumb that an average value can be defined to within one fifth of the peak-to-peak ( p p ) value by visual estimation from a noisy trace was applied. The constant offset was never larger than the p p noise in these experiments. It could be determined by a blank run and corrected for, if desired, so it is not considered further here. The resulting situation can be compared to that in infrared spectroscopy, where resolution, signal to noise ratio, and scanning time are all important and interrelated. Both resolution and scanning time can be improved by using smaller samples, but a t the possible expense of signal to noise ratio. TO achieve both high resolution and high signal to noise ratio may require longer scanning times. Two well known boundary conditions may be noted. The first is weighings in air at ambient temperature and pressure. These are the conditions under which balances are commonly rated, and much data is available on them. While
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g 80 3 sp
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Figure 1. Thermogravimetric curves for copper sulfate pentahydrate in air a t atmospheric pressure with samples of 0.426 mg. and 18-00 mg. under identical conditions Heating rate was 13’ C./minute.
Two moles
of water a r e lost a t about 75’ C. and two more a t about 100’ C., iri separate reactions. Resolution and reaction temperature values a r e much better with the smaller sample. Temperature error with the larger sample was mare than 40’ C.; mass error a t the break about 20% of the true change
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AREA Figure 2. Typical noise curves a t 100’ C. and atmospheric pressure in a 41 -mm. diameter tube; sensitivity 0.01 rng./div. (1) no hangdown wire (2) hangdown wire only (3) 100 mg. vertical foil a d d e d (4) hangdown wire, stirrup, and 12-mm. pan (5) 25-mm. p a n
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Figure 3. Blank run on platinum pan using RG Electrobalance and No. 2006 TGA kit Hongdown tube is 8 mm. i.d., rconning rote about 20' C./minute. correction must be mode for work finer than 1 2 pg.
air currents are believed to be a limiting factor, they do not prevent stahility at the 0.1-pg. level in a properly designed balance. The second is weighings in high vacuum, as is often done in surface chemistry. Values to 0.1 pg. have been reported for such weighmgs (6). In the pressure range from about 10-3 to 10 torr there is another phenomenon, thermomolecular flow, which also exerts real forces on heated samples and hangdown wires. It is discussed elsewhere (4,5). Measurements in this work were confined to the pressure ranges where thermomolecular flow is not a factor. EXPERIMENTAL
The curves in Figure d and Figure 3 were taken using an RG Electroba1auce"'and No. 2006 TGA accessory. All other measurements were made using an RG Electrobalance mounted in a glass vacuum bottle with hangdown
Figure 5. Enclosure for sample pan and stirrup is suspended from bottle IO hangdown tube can b e removed easily
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ANALYTICAL CHEMISTRY
Blank
tubes (Figure 4). This instrument is sensitive to 0.1 pg., and can be set to full scale recorder ranges from 0.02 mg. to 1 gram. The sample was suspended near the bottom of the hangdown tnhe, which is 500 mm. long and 41-mm. i.d., for all runs except one noted below. The suspension wire w&s 0.1-mm. diameter nichrome. The sample space was heated by a heating tape. Temperature values were measured outside, underneath the heating tape, and corrected hy the results of a run made with a thermocouple inside a t the sample position. The difference between inside and outside temperature values did not exceed 10' C. There was no thermal insulation between the glass hottle and the heating tape. Time constant w a s 0.5 second for all runs except Figure 3. More filtering would reduce noise further. The fmt rnn was made without anything suspended from the halance heam to evaluate the effects of air currents flowing up the tube and impinging directly on the heam, of heat conduction np the tube, and of radiation directly from the heating tape to the halance mechanism. Sample space temperature was 100' C., and pressure was atmospheric. Noise was less than 1 pg. pp, and thus negligible for this study. All subsequent forces measured are believed to he real forces acting on the sample and its suspension, and not balance errors. Area. All of the following runs were made a t 100" C. and atmospheric pressure. The second run was made with hangdown wire only. The noise increased to about 40 pg. p p . Then a vertical foil weighing 100 mg. and about 2 sq. em.
Figure 4. RG Electrobalance weighing mechanism in glass vacuum bottle as used in measurements
in area was added. There was no perceptible increase. With a small stirrup and 12-mm. diameter aluminum pan in a horizontal plane, the noise increased to about 100 pg. p p . With a large stirrup and 25-mm. pan, it increased to about 350 pg. p p . The conclusion drawn was that vertical area has no effect on the noise, hut horizontal area suspended from the balance increases it almost proportionally. Geometry. Two W e r e n t geometries were investigated in an attempt to reduce the noise a t atmospheric pressure. In the first approach, the stirrup with 12-mm. pan was put inside a metal enclosure suspended from the hottle (Figure 5 ) . It was completely enclosed except for a small hole about 1-mm. diameter in the top to pass the wire. At loo" C. with 100-mg. sample, the noise was reduced from 100 @g.p p to 40 pg. p p by the
PRESSURE, m a
Figure 6. Effect of air pressure on aerodynamic noise, pg. ppvs. absolute pressure in torr
enclosure. This is tht: amount observed on the hangdown wire alone. It appears that the endosure eliminated the forces on the pan, leaving only those on the hangdown wire, which set a limit to further improvement in this direction. In the second approach, a much smaller diameter hangdown tube was used (7-mm. i.d. and 150 mm. long) (9). A special quartz pan, 5-mm. i.d. :tnd weighing about 50 mg., was suspended by the same wire, with no additional load. Temperature was 100’ C., pre3wre was atmospheric. Xoise was only 1.2 pg. p p , pl?rmitting measurements to about 0.24 pg. This geometry appears ideal for samples below about 20 mg., eliminating all concern with aerodynamic noise. Temperature. T’ie effect of temperature was determined with the 12-mm. pan, 100-mg. load, a t atmospheric pressure in the enclosure (Figure 5). At amtlient temperature, 25’ C., the noise was only 0.4 pg. p p , which appears consistent with a large body of performancc: data considering the effect of the uniisually long hangdown wire. -4t 100‘ C., the noise had increased to about 40 pg. p p . In other work the authors have often observed major increasos in noise immediately after beginning to heat a t sample s C. At 300’ temperatures as low : ~40” C., the noise had on11 increased to 50-60 pg. p p . Similar results were obtained without the enclosure and in other hangdown geometries a t Iemperatures up to 1250’ C. It was ccncluded that rela-
tively slight heating causes major amounts of noise, and that further increases in temperature cause only minor increases in noise. This appears to make other measurements a t 100’ C. applicable to much higher temperatures. Mass. The effect of mass in the pan was evaluated with the 12-mm. pan a t 100” C. and atmospheric pressure, in the enclosure, with samples of 10, 100, and 1000 mg. An effect was expected since the wire and pan had moved in other experiments. However, no difference was observed between the three runs. The noise remained about 40 pg. p p for all loads. For maximum precision as a fraction of total load, the sample should then be as heavy as possible. If to one fifth of the p p noise can be read, a precision of 10-5 of total load can be obtained in the large hangdown tube at atmospheric pressure, using an enclosure. Pressure. The effect of pressure n-as determined a t 100” C. in air with a 100-mg. sample, in the enclosure (Figure 6). C p to about 150 torr, the noise was not more than 1 pg. p p . There seems to be a slight minimum just above 100 torr. Above 200 torr, the noise increases rapidly, so that 500 torr is just as bad as 760. Identical results were obtained without the enclosure up to 200 torr; beyond that, the noise increased in a similar way to 100 pg. a t 760, as expected. A pressure of 150 torr is enough atmosphere for many experiments. Where it is, aerodynamic noise can be eliminated easily, even in large tubes.
Blank. A blank run on a 40-mg., empty platinum pan with 8-mm. i.d. hangdown tube, in air a t atmospheric pressure is shown in Figure 3. The indicated weight varies 1 2 pug. between 50’ and 650’ C. Such blank curves are reproducible, and blank corrections should be made for the finest work. An electrical filter period of 2.5 seconds in this run eliminated all noise. At this sensitivity, the recorder chart can be read to about 0.4 pg. With quartz pans, the effect of reduction in air buoyancy produces a blank curve ranging from 0 to 40 pg. which agrees with theoretical values within *2 pg. LITERATURE CITED
(1) Cahn, Lee, Schultz, Harold, “Vacuum Microbalance Techniques,” Vol. 2, p. 7,Plenum Press, New York, 1962. (2) Cahn, Lee, Schultz, Harold, “Vacuum Microbalance Techniques,” Vol. 3, p. 29, Plenum Press, New York, 1963. (3) Cahn Instrument Go., Paramount, Calif., Catalog Nos. 1392 and 1393.
Original design by Mr. E. Dosch, Technical Equipment Corp., Denver, Colo. (4)Poulis, J. A., “Vacuum Microbalance Techniques,” Vol. 3,p. 1,Plenum Press, New York, 1963. (5) Wolsky, S. P., Zdanuk, E. T.,; “Vacuum Microbalance Techniques, Vol. 1, pp.111, 129, 143, 145, Plenum Press, New York, 1961. (6) Ibid., Vol. 2, p. 35, 1962. RECEIVED for review April 22, 1963. Accepted July 25, 1963. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 44,1963.
Spectrophotometric Determination of Bromide Ion in Water FRED ZITOMER’ and JACK L. LAMBERT Department o f Chemistry, Kansas State University, Manhattan, Kan.
b Bromide ion is determined spectrophotometrically in water as a function of its inhibition in the chlorination of ammonia to form irichloramine. The determination is rapid and applicable to concentrations of bromide ion in the range of 0.02 to 1.20 p.p.m. Reactions proceed in solutions buffered to pH 5.6 between 23’ and 26” C., and the concentration of bromide ion is measured as a function of the ammonia lost because of oxidation b y hypobromite to nitrogen. A simplified preparation for a modified cadmium iodide-linear starcli reagent, which produces the blue starch-triiodide complex used for spectrophotometric determination, is described.
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N A RECENT PAPER (11) the
authors established the feasibility of determining ammonia spectrophotometrically in the parts per billion range as tri-
chloramine, through the starch-iodide reaction. As part of that study, it was found that bromide ion interfered because of its oxidation by hypochlorite to hypobromite which then oxidizes ammonia to nitrogen; thus the yield of trichloramine is decreased leading to low values for ammonia. Interference due to bromide ion was eliminated by the use of salicyclic acid, but the fact that its interference was significant and specific at concentrations of less than 1 p.p.m. suggested the applicability of this reaction to its determination in water. With some slight modification in the original procedure for ammonia, the method has been adapted to the determination of the bromide ion in the range of 0.02 to 1.20 p.p.m. Under these circumstances, ammonia becomes the standard reagent, and bromide ion is estimated as a function of the amount of ammonia not oxidized to nitrogen.
A determination is made by adding an accurately measured amount of ammonia to the bromide ion sample buffered to pH 5.6 between 23’ and 26’ C. Upon addition of hypochlorite, competitive reactions between ammonia and hypobromite, which is formed almost immediately, result in the simultaneous formation of trichloramine and nitrogen. The amount of each product formed is dependent upon the quantity of bromide ion present initially if the ammonia and hypochlorite concentrations remain constant fora series of determinations. Unreacted hypochlorite and hypobromite are then selectively destroyed with nitrite ion and the trichloramine-nitrite oxidation product is measured spectrophotometrically as 1 Present address, Celanese Research Laboratories, Box 1O00, Summit, N. J.
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