Hygroscopic, definition

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

Discussion. The hydroxides of sodium, potassium, and barium are generally employed for the preparation of solutions of standard alkalis they are water-soluble strong bases. Solutions made from aqueous ammonia are undesirable, because they tend to lose ammonia, especially if the concentration exceeds 0.5M moreover, it is a weak base, and difficulties arise in titrations with weak acids (compare Section 10.15). Sodium hydroxide is most commonly used because of its cheapness. None of these solid hydroxides can be obtained pure, so that a standard solution cannot be prepared by dissolving a known weight in a definite volume of water. Both sodium hydroxide and potassium hydroxide are extremely hygroscopic a certain amount of alkali carbonate and water are always present. Exact results cannot be obtained in the presence of carbonate with some indicators, and it is therefore necessary to discuss methods for the preparation of carbonate-free alkali solutions. For many purposes sodium hydroxide (which contains 1-2 per cent of sodium carbonate) is sufficiently pure. 

Pure alkyl sulfuric acids are white crystalline solids with definite melting points but they are hygroscopic. Hygroscopicity increases as the chain length increases. Dodecyl, tetradecyl, hexadecyl, and octadecyl sulfuric acids were obtained by Maurer et al. [57] by chlorosulfation of the corresponding alcohols and the melting points obtained are shown in Table 2. 

The presence of water does not only create conditions for the existence of an electrolyte, but it acts as a solvent for the dissolution of contaminants [10], Oxygen plays an important role as oxidant element in the atmospheric corrosion process. The thickness of the water layer determines the oxygen diffusion toward the metallic surface and also the diffusion of the reaction products to the outside interface limited by the atmosphere. Another aspect of ISO definition is that a metallic surface is covered by adsorptive and/or liquid films of electrolyte . According to new results, the presence of adsorptive or liquid films of electrolyte perhaps could be not in the entire metallic surface, but in places where there is formed a central anodic drop due to the existence of hygroscopic particles or substances surrounded by microdrops where the cathodic process takes place. This phenomenon is particularly possible in indoor conditions [15-18],... 

The consistency of Napalm gel is variable and depends on many factors. However, 2-4% of Napalm gives a definitely stringy gel whereas 10-12% gives a very elastic gel which is almost solid. Napalm, while satisfactory as a thickener, is hygroscopic and also susceptible to oxidation by atmospheric oxygen resulting in a thickened gasoline with poor characteristics. In the presence of moisture, the soap may hydrolyze as shown in Equation 5.24 ... 

Dry boxes also offer significant advantages over vacuum line techniques in the study of mixed-solvent systems. It is difficult to mix two different solvents accurately on a vacuum line. In a dry box, the procedure is rendered almost trivial. Definite advantages of a dry box can also be envisioned for electrochemistry in low-temperature molten salts, which are often very hygroscopic. 

A hydrate Ca3P208.H20 has been described as a hygroscopic powder, but this also is not in equilibrium with a definite solution but gives solutions containing greater proportions of phosphoric acid and dissolves with hydrolysis, depositing lime.4... 

The hygroscopicity of a drug and pharmaceutical substances is a potential parameter to be considered in tablet formulation. The moisture uptake rate is quite variable depending on the type of drug and excipients as well as the environmental conditions. So, a concise definition of hygroscopicity is not possible. Powders can absorb moisture by both capillary imbibition and swelling. The instantaneous water absorption prosperties of pharmaceutical excipients correlate with total surface area while the total absorption capacity correlates with powder porosity [22],... 

Ferric hydroxide, dried at 100° C., gradually becomes rehydrated upon prolonged exposure to a saturated atmosphere.2 Although the above substances are usually described as hydrates of ferric oxide, it is by no means certain that all of them are to be regarded as definite chemical entities.3 They are mostly hygroscopic substances, the amount of water they contain at any moment fluctuating with the temperature and humidity of the atmosphere. The task of determining precisely how much of the contained water is merely physically attached to the oxide, and how much is chemically combined with it, is not easy. 

Three rules apply to all analyses (1) pay most attention to the strongest absorptions, (2) pay more attention to peaks to the left (shorter wavelength) of 1250 cm , and (3) pay as much attention to the absence of certain peaks as to the presence of others. The absence of characteristic peaks will definitely exclude certain functional groups. Be wary of weak O—H peaks because water is a common contaminant of many samples. Because KBr is hygroscopic it is often found in the spectra of KBr pellets. 

The three natural CDs form crystalline structures in the solid state that decompose above 200°C with no definite melting points. They are not considered hygroscopic, but they do form various stable hydrates. The water vapor sorption isotherms (Fig. 9) show two phases for the p- and y-CDs, and one phase for a-CD. At 11% RH, ot-CD absorbs 4 water molecules and upon long-term storage, forms a stable hydrate with 6 water molecules. The water content gradually increases with increasing humidity to a constant value of 6.6 water molecules per CD molecrJe at and above 79 /o Four different... 

The definition of M as given above is equivalent to the term regain as used for certain other hygroscopic materials such as textiles (J). The term moisture content is defined on a wet rather than dry weight basis. The wet basis moisture content is then related to M by... 

These highly purified chemicals may be weighed out directly for the preparation of solutions of selected concentration or for the calibration of solutions of unknown strength. They are supplied with a certificate of analysis for each lot. These chemicals must be stable substances of definite composition that can be dried, preferably at 104 to 110 C, without a change in composition. They must not be hygroscopic, so that water is not absorbed during weighing. 

Information about physicochemical and complexation properties of polyvinylsulfonic acid (PVSH) or, more correctly, polyethylenesulfonic acid (PESH) [11] is much less available than that of the polyamines. PVSH (or PESH) is a simple, strong polyacid (see Fig. 6.6), so that partial ionization and possible hydrolysis effects, encountered with weak polyacids and their salts (e.g., polyacryfic acid and its salts), do not complicate the picture [76]. Commercially available sodium salt, PVSNa, is a brittle hygroscopic solid, soluble in water, but insoluble in organic solvents (methanol, dioxane, etc.) [77]. Viscosity behavior of the alkali metal salts was studied by several groups [28, 52, 75-77], but data are too scattered to be trustworthy. The PVS polyanion shows a definite selectivity in ion binding with alkali Li, Cs metals in the presence of Na, K hah des [78]. 

White amorphous hygroscopic solid. No definite mp chars when heated to 300. +26 (water). Insol in... 

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