Cahn microbalance

The TG thermobalance structure is illustrated in Figure 10.20 it includes a microbalance, furnace, temperature programmer and computer. The key component is the microbalance, which measures the mass change. A typical microbalance is able to measure mass change of 1 ptg with maximum mass of 100 mg. A commonly used microbalance is the null-point type. The null-point microbalance can maintain the sample in a vertical position when its mass changes. Figure 10.21 shows the structure of the commonly used null-point type microbalance, the Cahn microbalance. 

Figure 10.21 Structure of a Cahn microbalance with capability to measure the mass loss of a sample without the sample moving downward. (Reproduced with permission from R.F. Speyer, Thermal Analysis of Materials, Marcel Dekker, New York. 1993 Taylor Francis Group Ltd.)...

Does the Cahn microbalance balance mass change of a sample using a counter weight Why ... 

After carbonate removal using 6N HCl the samples were extracted with chloroform and methanol (1 1) (Fig. 1 Simoneit et al., 1981). Extract concentrates were separated by TLC on silica gel after esterification of the carboxylic acids (Fig. 1). Fractions corresponding to hydrocarbons, carboxylic acid esters, ketones and polar compounds were isolated for instrumental analysis (Simoneit et al., 1981). The amounts of material in the hydrocarbon, ester, and ketone fractions from the TLC separations were determined by weighing replicate aliquots of a few microliters each on a Cahn microbalance and, after averaging, calculating the yields. 

The following list of equipment and the specifications were obtained from the manufacturers. The author was able to identify 19 equipment manufacturers to measure the physical adsorption isotherm. Prices are not included since they can vary considerably. The author does not have personal experience with any of the equipment since he has constructed all of his own equipment in the past with the exception of a large number of Cahn microbalances (some models are still available) used for long-term studies. 

In order to measure the solubility and diffiisivity of low molecular weight vapours in PTMSP, sequences of dififerential sorption experiments were performed. The mass uptake as a nction of time for each sorption step was measured and recorded using a Cahn microbalance. Details of the experimental equipment are discussed in a previous publication (15). Prof G.Costa of the CNR-IMAG kindly provided the polymer, which was synthesised using TaCU as catalyst, and had an intrinsic viscosity of 4.5 dl/g in toluene solution at 30 C. 

In the DVS the sample is placed on a microbalance which is exposed to a continuous flow of air of known humidity. An ultrasensitive Cahn microbalance allows vapor soiption measurements on sample sizes as small as 1 mg to be an yzed with a resolution of 0.1 pg or as large as lOOg with a resolution of 10 fig. 

Magnetic Susceptibility. Magnetization as a function of applied field was determined at various temperatures according to the Faraday method. A Cahn model RG microbalance and an Alpha model 4800 magnet were used In this respect. A detailed description of the technique used can be found in reference 4. 

X-ray diffraction powder patterns were recorded on a CGR Theta 60 instrument, using monochromated CuKa radiation. The adsorption capacities for several adsorbates were measured at room temperature by gravimetry, using a Cahn RH microbalance as proposed by Vaughan and Lussier (3 ). The samples were first treated in air for 5 hours at 480°C. The experiment was performed by passing, over the sample, a stream of nitrogen saturated by the vapor pressure of the sorbate at room temperature, the relative pressure P/Po was then equal to 1. 

Adsorption equilibria and uptakes were gravimetrically measured with two balances, i.e., Cahn 1100 microbalance and Magnetic Suspension Balance (Rubotherm Co.). Adsorbents were loaded and regenerated in the balance under the flow of ultra high purity Helium gas at ISO 3S0 C, then adjusted to the measuring temperature and evacuated to 10 mmHg by using a turbo molecular pump. Adsorbates used were as follows propane (min.99.5), isobutane (min.99.5%), n-butane (min.99.S%), 1-butene (min.99.5%), isobutene (min.99.5%) and t-2-butene (min.99.8%). 

Details can be found in Bdlare (ref. 3). The experimental equipment consists of a cumene reservoir, a thermogravimetric analyzer (TGA) and a gas chromatograph (GC). The hdium-cumene mixture enters the TGA, a Cahn System 113DC with a Cahn 2000 Recording Electrobalance, a quartz tubular reactor, and an external split-shell furnace. The catalyst is placed in the sample pan of the microbalance inside the quartz reactor, kept at a controlled temperature in the center of the split-shell furnace. The incremental weight due to coke deposition on the catalyst is monitored by an IBM PC. The reactor exit stream is injected into a Varian 3700 GC using FID. 

Fig. 10. Schematic diagram of a recording microbalance. From L. Cahn and H. R. Schultz, in Vacuum Microbalance Techniques, Ed. K. H. Behmdt, 3 (1963) 30.

Supplemental gravimetric sorption/desorption data for CX>2 pressures bebw atmospheric were detained by conventional procedures using a Cahn recording vacuum microbalance, for greater precision in the low solubility range. 

The thermogravimetric measurements were performed in microbalance systems (CAHN TG-121 and Sartorius 4433). The experimental conditions were the following sample mass 100-150 mg, sample temperatures 300°C to 700°C, temperature ramps 2 and 5 K/min. The relative error of the thermogravimetric results was <10%. 

H2O, n-hexane and cyclohexane sorption capacities of SAPO-31 were determined gravimetrically using a vacuum microbalance (Cahn Instruments, USA). The size and the morphology of the crystals of SAPO-31 were examined using a JEOL (JSM-840 A) scanning electron microscope. The BET surface area was determined using a volumetric adsorption apparatus ( Model Omnisorb lOOCX, Coulter, USA). 

The latest versions of the Cahn recording microbalance are the Models 1000 and 2000. The former has a mass capacity of 100 g with a sensitivity of 0.1 jug and is capable of operation at pressures of 10 7 Torr to 50 atm. The Model 2000 is similar, except that the mass capacity is 1.0 or 2.5 g with a mass sensitivity of 0.L or 1.0 xg for the t.O or 2.5 g load, respectively. This microbalance is used in the Cahn 113 TG system. The furnace on ihis system has a maximum temperature of 1100°C with heating rates of 1 -25 C min. 

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