Aqueous solutions additives

The thermal gelation temperature for methylcellulose decreases as a function of concentration. The presence of additives can increase or decrease the thermal gelation temperature. The presence of drugs can influence the properties of methylcellulose gels. In addition, the viscosity of methylcellulose solutions can be modified by the presence of drugs or other additives. Aqueous solutions of methylcellulose can be frozen and do not undergo phase separation upon freezing. 

For surface tension, contact angle and water uptake measurements thin-film samples were prepared from coating polymers with surfactant addition. Aqueous solutions of different surfactants were also studied. Coated particle samples were prepared for drug release studies. The surface characteristics of some active ingredients in compressed form were monitored. 

For constraction products, cellulose ethers act as thickeners, binders, film formers, and water-retention agents. They also function as suspension aids, surfactants, lubricants, protective colloids, and emulsifiers. In addition, aqueous solutions of certain cellulose ethers thermally gel, a unique property that plays a key role in a variety of applications. 

We have considered the surface tension behavior of several types of systems, and now it is desirable to discuss in slightly more detail the very important case of aqueous mixtures. If the surface tensions of the separate pure liquids differ appreciably, as in the case of alcohol-water mixtures, then the addition of small amounts of the second component generally results in a marked decrease in surface tension from that of the pure water. The case of ethanol and water is shown in Fig. III-9c. As seen in Section III-5, this effect may be accounted for in terms of selective adsorption of the alcohol at the interface. Dilute aqueous solutions of organic substances can be treated with a semiempirical equation attributed to von Szyszkowski [89,90]... 

The larger cations of Group 1 (K, Rb, Cs) can be precipitated from aqueous solution as white solids by addition of the reagent sodium tetraphenylborate, NaB(C( H5)4. Sodium can be precipitated as the yellow sodium zinc uranium oxide ethanoate (sodium zinc uranyl acetate). NaZn(U02)3(CH3C00)y. 9H2O. by adding a clear solution of zinc uranyl acetate in dilute ethanoic acid to a solution of a sodium salt. 

A white gelatinous precipitate of aluminium hydroxide is obtained when an alkali is added to an aqueous solution of an aluminium salt. Addition of an excess of caustic alkali causes the precipitate to redissolve, the whole process being reversed by the addition of a strong acid the actual substance present at any time depending on... 

This reaction proceeds slowly in aqueous solution, so that the basic salt. Sn(OH)Cl, is slowly precipitated. Addition of excess hydrochloric acid gives the acids of formulae HSnCl3 and H2SnCl4. Salts of these acids containing the ions SnCl J and SnCl (chloro-stannates(II)) are known. 

Lead(II) in aqueous solution gives on addition of the appropriate anion (1) a white precipitate of lead(II) chloride, (2) a yellow precipitate of lead(II) chromate, and (3) a yellow precipitate of lead(II) iodide which dissolves on heating and reappears on cooling in the form of glistening spangles . 

Many of the reactions of halogens can be considered as either oxidation or displacement reactions the redox potentials (Table 11.2) give a clear indication of their relative oxidising power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself. Thus fluorine can displace all the other halogens from both ionic and covalent compounds, for example... 

Aqueous solutions containing titanium(IV) give an orange-yellow colour on addition of hydrogen peroxide the colour is due to the formation of peroxo-titanium complexes, but the exact nature of these is not known. 

How ever, the Mn(II) ion forms a variety of complexes in solution, some of which may be more easily oxidised these complexes can be either tetrahedral, for example [MnClJ , or octahedral, for example [Mn(CN)f,] Addition of ammonia to an aqueous solution of a manganese(II) salt precipitates Mn(OH)2 reaction of ammonia with anhydrous manganese(II) salts can yield the ion [MnfNH y T... 

Addition of an alkali metal hydroxide solution to an aqueous solution of a nickel(II) salt precipitates a finely-divided green powder. nickel(II) hydroxide NilOHfj on heating this gives the black oxide. NiO. which is also obtained by heating nickel(II) carbonate or the hydrated nitrate. Black nickel(II) sulphide, NiS, is obtained by passing hydrogen sulphide into a solution of a nickel(II) salt. 

From the standpoint of thermodynamics, the dissolving process is the estabHsh-ment of an equilibrium between the phase of the solute and its saturated aqueous solution. Aqueous solubility is almost exclusively dependent on the intermolecular forces that exist between the solute molecules and the water molecules. The solute-solute, solute-water, and water-water adhesive interactions determine the amount of compound dissolving in water. Additional solute-solute interactions are associated with the lattice energy in the crystalline state. 

Sodium dichromate is used instead of the potassium salt because it is far more soluble in water, and is not precipitated from its aqueous solution by addition of the ethanol. It is also cheaper than the potassium salt, but has the disadvantage of being deliquescent. 

Since hydroxylamine is usually available only in the form of its salts, e.g., the hydrochloride or sulphate, the aqueous solution of these salts is treated with sodium acetate or hydroxide to liberate the base before treatment with the aldehyde or ketone. Most oximes are weakly amphoteric in character, and may dissolve in aqueous sodium hydroxide as the sodium salt, from which they can be liberated by the addition of a weak acid, e.g., acetic acid. 

Reactions of Aspirin, (i) Distinction from Salicylic acid. Shake up with water in two clean test-tubes a few crystals of a) salicylic acid, (0) aspirin, a very dilute aqueous solution of each substance being thus obtained. Note that the addition of i drop of ferric chloride solution to (a) gives an immediate purple coloration, due to the free —OH group, whereas (b) gives no coloration if the aspirin is pure. 

The crystalline sodium sulphide (NajS,9H20) used to prepare the disulphide is very deliquescent, and only a sample which has been kept in a well-stoppered bottle and therefore reasonably dry should be used. A sample from a badly-stoppered bottle may contain, in addition to the crystals, a certain amount of aqueous solution, in which hydrolysis and partial decomposition will have occurred such a sample should therefore be rejected. Add 4 2 g. of finely powdered sulphur to a solution of 16 g. of the crystalline sodium sulphide in 60 ml. of water, and boil the mixture gently for a few minutes until a clear solution of the disulphide is obtained. 

When diazoaminobenzene is added to a warm aqueous solution of hydrochloric acid, it tends to break up into its original components, i.e., to benzene-diazonium chloride and aniline, and an equilibrium is thus established. The diazonium chloride and the aniline, however, in addition to recombining to form diazoaminobenzene. also undergo direct condensation at the p-hydro-... 

Arylarsonic acids are most readily prepared by the Bart Reaction, in which a diazonium salt in aqueous solution is run into a solution of sodium arsenite in an excess of sodium carbonate. The addition of copper sulphate to the +. ... 

Note, (a) Aqueous solutions of formaldehyde and acetaldehyde give these addition products, which are so soluble that they rarely separate this reaction is therefore an unsatisfactory test for these aldehydes. (6) These addition products are also formed by ketones (p. 345). 

Physical properties. Majority are liquids except p toluidine and 1- and 2-naphthylamine. All are colourless when pure, but rapidly darken on exposure to air and light. All are very sparingly soluble in water, but dissolve readily in dilute mineral acids (except the naphthyl-amines, which are only moderately soluble in adds). They form colourless crystalline salts e.g., CjHjNH2,HCl) which are soluble in water these aqueous solutions usually have an add reaction owing to hydrolysis, and give the reactions of both the amine and the acid from which they are derived. Addition of alkali to the acid solution liberates the amine. 

The most satisfactory reagent is a saturated solution of sodium bisulphite containing some alcohol it must be prepared aa required since it oxidises and decomposes on keeping. Frequently, a saturated aqueous solution is used without the addition of alcohol. 

Dissolve 0 01 g. equivalent of the amino acid in 0 03 g. equivalent of N sodium hydroxide solution and cool to 5° in a bath of ice. Add, with rapid stirring, 0 -01 g. equivalent of 2 4-dichlorophenoxyacetyl chloride dissolved in 5 ml. of dry benzene at such a rate (5-10 minutes) that the temperature of the mixture does not rise above 15° if the reaction mixture gels after the addition of the acid chloride, add water to thin it. Remove the ice bath and stir for 2-3 hours. Extract the resulting mixture with ether, and acidify the aqueous solution to Congo red with dilute hydrochloric acid. Collect the precipitate by filtration and recrystallise it from dilute alcohol. 

In aqueous solution at 100° the change is reversible and equilibrium is reached when 95 per cent, of the ammonium cyanate has changed into urea. Urea is less soluble in water than is ammonium sulphate, hence if the solution is evaporated, urea commences to separate, the equilibrium is disturbed, more ammonium cyanate is converted into urea to maintain the equilibrium and evfflitually the change into urea becomes almost complete. The urea is isolated from the residue by extraction with boiling methyl or ethyl alcohol. The mechanism of the reaction which is generally accepted involves the dissociation of the ammonium cyanate into ammonia and cyanic acid, and the addition of ammonia to the latter ... 

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