Hygroscopicity

The calorimetric RH perfusion cell can be used to characterise the hygroscopic nature of a compound. A useful complementary technique for studying hygroscopicity is DVS [35]. The prineiple of operation is similar to the calorimetric RH perfusion technique except that it measures mass change as a function of RH whereas the calorimetric RH perfusion device measures enthalpy change as a function of RH. The application of both techniques to the study of hygroscopicity is sufficiently similar so that the methods of operation are described together. 

Using these techniques hygroscopic materials can be characterised by increased water sorption at high RH values. The experimental method can be extended to examine the mechanism of the water sorption process. At 90% RH, the target RH is then taken down to 0% RH so that each equilibrium desorption step mirrors the adsorption steps. The mass change associated with water sorption onto hygroscopic materials should be equal for the adsorption and desorption at any given RH. The quantity of water taken up by each step is 

Salt Relative humidity in the enclosed space above the solutions, % 

The adsorption and desorption curves for povidone powders at room temperature are not the same. The two curves are shown for comparison in Fig. 16 [140]. 

It has been reported by Will that for nitrocelluloses with nitrogen content from 9.0 to 13.3%, dried to a constant weight at the temperature of 40°C, the percentage sum of the nitrogen content and of the absorbed mixture was constant equal, on average, to 14.6  

In spite of the variability of the sum calculated from the nitrogen content and hygroscopicity the effect of the nitrogen content on the hygroscopicity can be expressed by a straight line. In Fig. 109 (after de Pauw) straight-lines corresponding with different relative moistness of atmosphere (f/F) are shown. 

Water absorption from a 40% calcium nitrate solution was studied by Gukhman, Petrov and Yakovlev [120], who found that a nitrocellulose film immersed in such a solution was capable of absorbing up to 50% of water. After 5 min the water content reached 15-20%, and after 15 min it amounted to 25-30%. 

From the practical point of view the absorption of adds by nitrocellulose is of major importance for the question of stabilizing nitrocellulose. It has been established (Wiggam [121]) that nitrocellulose absorbs dilute nitric acid and dilute sulphuric acid to the same extent, the absorption of hydrochloric acid being somewhat less. 

The capacity of nitrocellulose to absorb vapours of organic solvents from the 

Although, all these problems can be resolved, generally, the amorphous forms are to be avoided unless the differences in solubility make a significant impact on the bioavailabihty. 

Water molecules have polar ends, and readily form hydrogen bonding. As a result, several compounds interact with water molecules by surface adsorption, condensation in capillaries, bulk retention, and chemical interaction, and are called hygroscopic. At times, the interaction between the compounds and water is so strong that the interacting water vapors result in dissolving the compound. This process is called deliquescence, wherein a saturated layer of solution is formed around the 

Generally, a compound that is very hygroscopic would be less desirable, but if studies show that despite moisture uptake the compound stays stable and workable in the formulation studies this is an important consideration. 

Solubility is a function of hygroscopicity, polymorphism, and chemical nature or pKa of the salt. If the pfCa is at least two units lower than the pH of the medium. 

The dissolution of solid particles of salts can be inhibited if the parent acid or base precipitates at the surface of the particles undergoing dissolution. For example, stearate salts show reduced dissolution if stearic acid layer precipitates on the surface in an acidic pH environment. 

Some solvents, particularly those containing hydroxyl groups (e.g., alcohols and glycol ethers) are hygroscopic they absorb moisture from the atmosphere until an 

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CgHijClNjOj. Colourless, hygroscopic, m.p. 2I0-212 C (decomp.). Prepared from fi-chloroethyl carbamate and trimethylamine. It has a physiological action similar to that of acetylcholine, but more prolonged, as it is less readily hydrolysed. It is used for intestinal atony following operations, and can be given orally. 

Used industrially to cross-link hydroxylic polymers, polyethyleneimine. Possesses some carcinogenic properties. Polyethyleneimine is a hygroscopic liquid used in paper manufacture to confer wet strength and in textiles, alkylated derivatives also form useful polymers. 

HOCHj CHjOH. Colourless, odourless, rather viscous hygroscopic liquid having a sweet taste, b.p. 197 C. Manufactured from ethylene chlorohydrin and NaHC03 solution, or by the hydration of ethylene oxide with dilute sulphuric acid or water under pressure at 195°C. Used in anti-freezes and coolants for engines (50 %) and in manufacture of polyester fibres (e.g. Terylene) and in the manufacture of various esters used as plasticizers. U.S. production 1979 1 900 000 tonnes. 

Cj jH,2N202. Used as its sodium salt, which is a white hygroscopic powder. Unstable, readily absorbing carbon dioxide and liberating phenytoin. Made by treating a-bromodi-phenylacetylurea with alcoholic ammonia. It has a mild hypnotic and strong anticonvulsant action, and is used in the treatment of grand-mal and focal epilepsy. 

TMED, (CH3)2NCH2CH2N(CH3)2. B.p. 122 C a hygroscopic base which forms a hydrocarbon-soluble stable chelate with lithium ions and promotes enhanced reactivity of compounds of lithium, e.g. LiAlH4, UC4H9, due to enhanced kinetic basicity of the chelate. Used in polymerization catalysts, tetramethyl lead, TML 5 lead tetramethyl. 

Thallium lII) chloride, TlCl3,4H20. Formed by passing CI2 through a suspension of TlCl in water. Hygroscopic, loses CI2 at lOO C. The [TlCl2l ion is stable chloro complexes up to [TlClfi] are formed. 

Tin iy) fluoride, Snp4, stannic fluoride. Polymeric solid formed Sn plus F2 or SnCU plus HF. Very hygroscopic, forms fluoro-stannales(IV) containing [SnFgp ions. 

The ammonia gas is used again and the only by-product, calcium chloride, is used to melt snow, prevent freezing of coal in transit and as an antidust treatment since it is hygroscopic and forms a solution of low freezing point. 

If an excess of magnesium is used, magnesium silicide, Mg2Si, is also produced.) The silicon obtained is a light brown hygroscopic powder. Crystalline or metallic silicon is obtained industrially by the reduction of silica with carbon in an electric arc furnace ... 

A) Rectified ethanol, contains 95-6% ethanol. 4-4 ., water. Care in using it with hygroscopic substances. 

Hygroscopic. Hot liquid gives pungent fumes. Frequently used to dissolve strong oxidising agents (p. 259). 

The absolute ethanol obtained in this way should have d., o 794, It is very hygroscopic, and the bottle in which it is stored should preferably have a well fitting ground-glass stopper alternatively, the bottle can be closed with a tightly fitting rubber stopper but not with a cork, as corks contain appreciable quantities of water. 

A mist of condensed water on the upper portion of the tube A indicates the presence of hydrogen. To detect the presence of hydrogen in this way, however, the copper oxide must first be strongly heated in a crucible and then allowed to cool in a good desiccator otherwise the water normally absorbed by the very hygroscopic copper oxide will always give a mist on the tube A. 

Physical Properties. All colourless liquids, completely miscible with water, except benzyl alcohol and cyclohexanol, which are slightly soluble. Pure glycol and glycerol have high viscosity, which falls as the hygroscopic liquids absorb water from the air. 

Benzene. Pure benzene (free in particular from toluene) must be used, otherwise the freezing-point is too low, and crystallisation may not occur with ice-water cooling alone. On the other hand, this benzene should not be specially dried immediately before use, as it then becomes slightly hygroscopic and does not give a steady freezing-point until it has been exposed to the air for 2-3 hours. Many compounds (particularly the carboxylic acids) associate in benzene, and molecular weights determined in this solvent should therefore be otherwise confirmed. 

Nitrobenzene. Usually rather a troublesome solvent, as it is markedly hygroscopic and the freezing-point of the solvent itself tends to fall steadily moreover it is, like benzene, an associating solvent. For a modification of the above freezing-point apparatus, by which the determination may be made in the absence of water-vapour, the student should consult an advanced textbook of practical physical chemistry. 

Glacial Acetic Acid. Acetic acid is very hygroscopic, and its freezing-point is considerably affected by the water absorbed. Satisfactory results cannot be obtained with the apparatus shown in Fig. 79, p. 433, and a specially modified apparatus must be employed (see Nitrobenzene). 

Rectified epirit should always be used in preference to absolute ethanol wdiidt is strongly hygroscopic. 

Use approximately 0 5 ml. of glycerol, and cork the flask A securely when weighing the glycerol by difference, owing to the very hygroscopic nature of the latter. Heat on the water-bath for 60 minutes instead of 30 minutes. Excellent results are obtained by this method if a freshly opened sample of anhydrous glycerol is available a sample which has been exposed to the air for even a short period will absorb sufficient water to give inaccurate results. 

Pyridine. The analytical reagent grade pyridine will satisfy most requirements. If required perfectly dry, it should be refluxed over potassium or sodium hydroxide pellets or over barium monoxide, and then distilled with careful exclusion of moisture (compare Fig. 77, 47, 2). It is hygroscopic, and forms a hydrate of b.p. 94-5°. Pure pyridine has b.p. 115-5°/760 mm. 

The anhydrous compound is not appreciably hygroscopic, is readily soluble in acetone and amyl alcohol, and insoluble in benzene, toluene, xylene and chloroform it is also readily soluble in absolute methyl or ethyl alcohol, but a trace of water causes immediate hydrolysis with the formation of an opalescent precipitate. 

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