Humidity equilibration

Humidity Equilibration. The clays were humidified at 25°C in desiccators using a series of eutectic salts. Each dessicator was controlled at a different relative humidity using one of the saturated salt solutions listed in Table II. The samples were weighed repeatedly until equilibrium (constant weight) was reached. After the spectra were obtained, the samples were broken into three aliquots, reweighed and dried for -24 hours at 105°C to obtain the dry weights of the clay, from which the water contents were calculated. The water contents, as percent dry weight of the clay, are also shown in Table II. 

Figure 7.12. IMEP-20. Participants results for Hg according to sample humidity equilibration.

Mass concentration units for ambient measurements are mass (/xg) per unit volume (m ). Size classification involves the use of specially designed inlet configurations, e.g., PMjq sampling. To determine mass concentration, all the particles are removed from a known volume of air and their total mass is measured. This removal is accomplished by two techniques, filtration and impaction, described in Chapter 13. Mass measurements are made by pre-and postweighing of filters or impaction surfaces. To account for the absorption of water vapor, the filters are generally equilibrated at standard conditions T = 20°C and 50% relative humidity). 

Ascending, one-dimensional development in a trough chamber. After sample application the HPTLC plates were equilibrated in a conditioning chamber at 42% relative humidity for 30 min and then developed immediately. 

Ascending, one-dimensional multiple development method (stepwise technique, drying between each run) in two mobile phase systems in a twin-trough chamber without chamber saturation (equilibration 30 min at 20-22°C) at a relative humidity of 60 — 70%. 

When pure P-endosulfan was allowed to equilibrate in the apparatus, the ratio of the P-isomer to the a-isomer in the gas phase became 8 92 at 20 , suggesting that the P-isomer converts to the a-isomer (Rice et al. 1997). Several investigators have reported rapid initial losses of endosulfan residues from treated plant surfaces due to volatilization (Archer 1973 Terranova and Ware 1963 Ware 1967). One research group (Willis et al. 1987) attributed the limited runoff losses found in soybean fields treated with endosulfan to early losses of the compound during application and to volatilization/degradation of the compound from plant surfaces. Air sampling performed in a wind tunnel under defined conditions (20 air velocity 1 m/sec relative humidity 40-60%) showed that 60% of the initial dose of endosulfan is volatilized from Trench bean surfaces after 24 hours (Rudel 1997). Influences of various pesticide application formulations were not tested. 

SPFM experiments were performed on sulfuric acid deposited on the surface of aluminum films on silicon. A macroscopic droplet was first deposited and then rapidly dispersed using a jet of gas. This produced submicrometer-sized droplets. The initial concentration of the sulfuric acid ranged from 20 to 98 wt.%. However, the acid droplets equilibrate rapidly with the ambient water vapor. For example, at room temperature and RH = 30%, the concentration of sulfuric acid is 55 wt% at 90% RH, it is 20 wt%. The increase in droplet volume as they equilibrate with the ambient humidity is shown in Figure 35. 

Ultraviolet-visible (UV-vis) diffuse reflectance spectra of supported WOx samples and standard W compounds were obtained with a Varian (Cary 5E) spectrophotometer using polytetrafluoroethylene as a reference. The Kubelka-Munk function was used to convert reflectance measurements into equivalent absorption spectra [12]. Spectral features of surface WOx species were isolated by subtracting from the W0x-Zr02 spectra that of pure Z1O2 with equivalent tetragonal content. All samples were equilibrated with atmospheric humidity before UV-vis measurements. 

As another criterion of purity, the amino acid content of heparins should be determined. This is usually performed by ion-exchange88 or liquid89 chromatographic analysis of hydrolyzates. Reasonably pure heparin preparations contain < 1% of total amino acids, mostly L-serine and glycine. Heparin preparations should also be analyzed for residual solvents, and analytical (as well as biological) data be expressed on a dry basis. (Heparins equilibrated with atmospheric humidity contain up to 15%, or even more, of water.) Unless volatile materials are completely removed or accounted for, elemental analyses of heparin are meaningless. 

Water is introduced into closed pharmaceutical systems either accompanying the input materials or in the headspace as relative humidity [79]. Whatever water is contained within the dosage form and its container will ultimately equilibrate among the components according to its affinity for the solid ingredients and the number of association sites. The Sorption-Desorption Moisture Transfer model has been used to evaluate the thermodynamically favored state that will result after the equilibration process is complete [79]. 

Lot to lot variations of several drugs and excipients are shown in Table 9. Specially crystallized lots of ibuprofen, for example, show substantial changes in Brittle Fracture and Bonding Indices. Phenacetin shows a significant increase in brittleness at higher relative humidity. It was observed by Hiestand and Smith [31] that compacts of dried phenacetin did not fracture, while the lot equilibrated at 40% relative humidity did, consistent with the Brittle Fracture Index change. 

Since water molecules occupy regular positions within the lattice of a hydrate with a specific stoichiometry (e.g., 1 1 monohydrate, 2 1 dihydrate, 5 1 pentahydrate) to the solid, relatively large quantities of water are sorbed. Figure 3 shows a moisture uptake isotherm for ipratropium bromide [39]. This substance undergoes an apparent hydration of the crystal between 63% and 75% relative humidity. Above 75% relative humidity, approximately 4.6% water is sorbed (theoretical monohydrate is 4.4 g/g). Interestingly, as anhydrous ipratropium bromide is equilibrated for extended time periods (e.g., 2 months and 5 months respectively, as shown in Fig. (3), hydration of the crystal appears to occur at... 

Combining solids that have previously been equilibrated at different relative humidities results in a system that is thermodynamically unstable, since there will be a tendency for moisture to distribute in the system so that a single relative humidity is attained in the headspace. As shown in Fig. 7, moisture will desorb into the headspace from the component initially equilibrated at a higher relative humidity and sorb to the component initially equilibrated at a lower relative humidity. This process will continue until both solids have equilibrated at the final relative humidity. The final relative humidity can be predicted a priori by the sorption-desorption moisture transfer (SDMT) model [95] if one has moisture uptake isotherms for each of the solid components, their initial moisture contents and dry weights, headspace volume, and temperature. Final moisture contents for each solid can then easily be estimated from the isotherms for the respective solids. 

Fig. 7 Schematic representation of moisture transfer between solid components A and B with (a) headspaces isolated from one another and (b) headspaces allowed to equilibrate. Ra and Rb = initial relative humidities above A and B VA and VB = headspace volumes above A and B Rf and VT = final relative humidity and headspace volume above A and B. (From Ref. 95.)...

The second timescale involves equilibration of a food sample with the air or known relative humidity environment (e.g., saturated salt solutions, in the case of obtaining an isotherm). In this case, equilibration depends on the size of a sample, the measurement method used, and the nature of the sample being measured (this factor is related to the first timescale discussed previously, as illustrated with butter). For example, measuring the of 2 g of corn... 

In the patterning experiments, chlorinated poly(styrene) films were baked at 120 C for 15 mins, in a forced air oven, equilibrated for -12 hrs. inside the humidity-controlled glove box, exposed to deep-UV radiation and treated with TiCU under usual conditions. No significant variation in the lithographic parameters was observed by varying the relative humidity in the 30-60% range in the glove box. 

Figure 6. MaxEnt reconstruction of SAXS data from as-received Nafion equilibrated under (a) ambient humidity and (b) 100% RH. (Reprinted with permission from ref 63. Copyright 2000 American Chemical Society.)...

---