Evaporation, measurement

While the volume of concentrate can be determined based on plant capacity and recovery, the evaporation rate at any given site varies with climate. To determine the evaporation rate of fresh water at certain locations, a standard pan evaporation measurement is taken. Evaporation pans are small, open air pans filled with water from which losses in water due to evaporation are measured. Standard size Class A evaporation pans are most commonly used, which are 1.207 m in diameter and 0.25 m in depth. The daily change in depth, minus any rainfall, is used to determine the evaporation rate in mm/day. This rate takes into account the effects of climate on evaporation rate, but corrections for pond area and salinity must be made when determining the evaporation rate of a specific evaporation pond. 

For a pure component droplet of unknown vapor pressure, Eq. (104) together with Eqs. (101), (106), and (108) constitute a set of relationships involving three unknown parameters, pf, C , and three parameters. Data obtained by Davis and... 

SnOj-reactive evaporated, measured at 450°C 0.92 Poor selectivity and stability... 

Monitoring performance of steam consuming units such as process reactors, multiple effect evaporators (measuring their steam consumption and condensate recovery)-,... 

It has been demonstrated by various authors [18] that the surface available for evaporation per volume of solvent applied in the test and the type of substrate can have a great effect on the evaporation rate even when other conditions are identical. Over the years there has therefore been some discussion on the question in how far evaporation measurement from filter paper relates to practical conditions e.g. from smooth paint films. 

Pu5/92138 issued by AERE, Harwell (Hamilton et al. 1989). The isotope fractionation at each step is corrected by reference to the certified isotope ratios. The idea was further developed by Dubois et al. into a dynamic multidetection measurement mode (Dubois et al. 1989), which practically eliminates mass fractionation effects and possible mismatches of cup efficiencies with a 2-isotope internal standard and MIC/TIMS. According to the results presented, precisions and accuracies of 0.01 % are achievable with this procedure. An ultimate refinement has been introduced by performing total evaporation measurements with peak tailing correction in dynamic multicollection mode, using a MIC/TIMS with magnetic sector equipped with a dispersion quadrupole (Goldberg et al. 2002 Richter and Goldberg 2003). 

Fiedler R (1995) Total evaporation measurements experience with multi-collector instruments and a thermal ionization quadrupole mass spectrometer. Int J Mass Spectrom Ion Processes 146-147 91-97 Fiedler R, Donohue D (1988) Pocket sensitivity calibration of multicollector mass spectrometers. In Symposium on trace dement analysis by mass spectrometry, Regensburg, 30 Sep 1987—2 Oct 1987 Fresenius Z Anal Chem 331(2) 209—213 Filippov VI, Moritz W, Terentjev AA, Vasiliev AA, Yakimov SS (2007) IEEE Sens J 7(2) 192-196 Fisher D (1997) History of the International Atomic Energy Agency, the first forty years, STI/PUB/1032, IAEA, Vienna... 

Acid evaporation measurements Acid losses [pg/(m s)] References... 

This rule is justified by the need to limit the benzene emissions from evaporation (Tims, 1983) Figure 5.25 shows that emissions increase linearly with the benzene content of the fuel. It is noteworthy that current legislation limits the measured evaporation to 2 g per test conducted in accordance with a standard procedure (Sealed Housing for Evaporative Determination, or SHED). Yet for a fuel containing 5% benzene, an evaporation of 0.7 g benzene /test is observed. 

Volatility of LPG NFM 41-012 ISO/DIS 6620 ASTM D 1837 Measurement of residue temperature after 95% evaporation... 

The material of interest is dissolved in a volatile solvent, spread on the surface and allowed to evaporate. As the sweep moves across, compressing the surface, the pressure is measured providing t versus the area per molecule, a. Care must be taken to ensure complete evaporation [1] and the film structure may depend on the nature of the spreading solvent [78]. When the trough area is used to calculate a, one must account for the area due to the meniscus [79]. Barnes and Sharp [80] have introduced a remotely operated barrier drive mechanism for cleaning the water surface while maintaining a closed environment. 

Barnes and Hunter [290] have measured the evaporation resistance across octadecanol monolayers as a function of temperature to test the appropriateness of several models. The experimental results agreed with three theories the energy barrier theory, the density fluctuation theory, and the accessible area theory. A plot of the resistance times the square root of the temperature against the area per molecule should collapse the data for all temperatures and pressures as shown in Fig. IV-25. A similar temperature study on octadecylurea monolayers showed agreement with only the accessible area model [291]. 

Not all molecules striking a surface necessarily condense, and Z in Eq. VII-2 gives an upper limit to the rate of condensation and hence to the rate of evaporation. Alternatively, actual measurement of the evaporation rate gives, through Eq. VII-2, an effective vapor pressure Pe that may be less than the actual vapor pressure P. The ratio Pe/P is called the vaporization coefficient a. As a perhaps extreme example, a is only 8.3 X 10" for (111) surfaces of arsenic [11]. 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

Delchar T. Eberhagen A and Tompkins F C 1963 A static capacitor method for the measurement of the surface potential of gases on evaporated metal films J. Sci. Instrum. 40 105-7... 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

Extraction Eiltering limits particulate gravimetry to solid particulate analytes that are easily separated from their matrix. Particulate gravimetry can be extended to the analysis of gas-phase analytes, solutes, and poorly filterable solids if the analyte can be extracted from its matrix with a suitable solvent. After extraction, the solvent can be evaporated and the mass of the extracted analyte determined. Alternatively, the analyte can be determined indirectly by measuring the change in a sample s mass after extracting the analyte. Solid-phase extractions, such as those described in Ghapter 7, also may be used. 

Several standard methods for the quantitative analysis of food samples are based on measuring the sample s mass following a selective solvent extraction. For example, the crude fat content in chocolate can be determined by extracting with ether for 16 h in a Soxhlet extractor. After the extraction is complete, the ether is allowed to evaporate, and the residue is weighed after drying at 100 °C. This analysis has also been accomplished indirectly by weighing a sample before and after extracting with supercritical GO2. 

For any one ion type (e.g., Cs ), measurement of its abundance in a sample requires the sample to be evaporated over a period of time. The total yield of ions is obtained by integrating the area under the ion-yield curve (Figure 7.8c). 

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

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