Silver chloride fractional precipitation

The theory of the process is as follows. This is a case of fractional precipitation (Section 2.8), the two sparingly soluble salts being silver chloride (Xsol 1.2 x 10 10) and silver chromate (Kso] 1.7 x 10 12). It is best studied by considering an actual example encountered in practice, viz. the titration of, say, 0.1M sodium chloride with 0.1M silver nitrate in the presence of a few millilitres of dilute potassium chromate solution. Silver chloride is the less soluble salt and the initial chloride concentration is high hence silver chloride will be precipitated. At the first point where red silver chromate is just precipitated both salts will be in equilibrium with the solution. Hence ... 

Fractional precipitation The calculation as to which of two sparingly soluble salts will be precipitated under given experimental conditions may be also made with the aid of the solubility product principle. An example of great practical importance is the Mohr method for the estimation of halides. In this process a solution of chloride ions is titrated with a standard solution of silver nitrate, a small quantity of potassium chromate being added to serve as an indicator. Here two sparingly soluble salts may be formed, viz. silver chloride (a white precipitate) and silver chromate (which is red) ... 

From this relationship, we see that the iodide concentration decreases to a tiny fraction of the chloride ion concentration before silver chloride begins to precipitate. So for all practical purposes, silver chloride forms only after 25.00 mL of titrant have been added in this titration. At this point, the chloride ion concentration is approximately... 

Lead chloride precipitation Is hot quantitative for lead under normal conditions but can be useful for radiochemical purification of a lead fraction. It can also be used to remove the bulk of a large quantity of lead prior to more complete separations. Although sparingly soluble In cold water, lead, chloride (PbClg) Is readily soluble In hot water or In dilute, hot nitric add. This has been used as a separation from silver by precipitation of silver chloride from hot nitric add solutions (H3). Tlra (T1) reports that the condition for maximum yield of PbClg with hydrochloric add at 20 C occurs at pH 0.5... 

Exercise 6 In a gravimetric determination, 0.451 g of silver chloride is washed before ignition and weighed. Calculate the precipitate fraction lost during the washing ... 

The process of isolation finally adopted by the former authors consists in precipitating as reineckates the water-soluble bases contained in a methyl alcoholic extract of the curare. The mixed reineckates are further purified, by solution in acetone and precipitation with water as often as may be necessary. The product so cleaned represents the bulk of the biological activity of the crude drug the mother liquors may contain curine (p. 374), which indicates a menisperm as one of the components of such curares. The mixed reineckates are then fractionated chromato-graphically over alumina and the components isolated as chlorides by the use of silver sulphate and barium chloride in succession. This process has been modified in detail by Schmid and Karrer, who have also found that with their curare, the more soluble reineckate fraction includes less potent quaternary alkaloids. 

For the resolution work, meso-3. and the racemate of the tetraphenylborate were converted to chlorides via chromatography over Dowex IX-8(C1 ). The racemate of the chloride (hygroscopic) in methanol (anh) was treated with silver L(+)-hydrogendibenzoyltartrate (HDBT). A mixture of diastereomers precipitated, and fractional crystallization (HCClj-ether) gave pure 2 L(+)-HDBT isomer ([alp = + 60.5°... 

The first chemical work on calabash curare was carried out in 1897 by Boehm (8), who isolated a highly active amorphous material which was named curarine. This was soluble in water and insoluble in ether, so it is probable that Boehm was handling a mixture of crude quaternary alkaloids. Much later (1935), King described (9) the preparation of an equally active amorphous quaternary iodide from the bark of S. toxifera. However, the first isolation of well-characterized crystalline alkaloids was achieved by H. Wieland and his school (10-13). Calabash curares were extracted with methanol, and the water-soluble quaternary alkaloids in the extract were precipitated as the reineckate salts this mixture was then fractionated by adsorption chromatography on alumina. The various reineckate fractions so obtained were converted into the corresponding chlorides by successive treatment with equivalent quantities of silver sulfate and barium chloride some of the quaternary alkaloids then crystallized as the chlorides or as the picrates. C-Curarine1... 

A mixture of 250 g. of dry, pure acetone and 180 g. (1.20 moles) of dry sodium iodide is heated under reflux while 100 g. (1.24 moles) of ethylene chlorohydrin is added over a 4-hour period. The mixture is refluxed for an additional 12 hours, and the precipitated sodium chloride is filtered off. Ten grams (0.067 mole) of sodium iodide is added, and the mixture is heated under reflux for another 4 hours. If necessary the mixture is again filtered. The acetone is distilled off, and the residue is fractionally distilled to give 204 g. (96%) of 2-iodoethanol boiling at 85-88°/25 mm. The product has a brown color due to free iodine, which may be removed by agitation with mercury or finely divided silver. 

At the beginning of the addition of silver nitrate, a colloidal precipitate of halide chloride forms. This first fraction of precipitate tends to preferentially adsorb bromide ions (for example) and not the anionic anions A of the indicator. This phenomenon occurs until the silver ions are completely exhausted. 

Assemble the apparatus, introduce into the flask a number of fragments of Pyrex glass, 0-2 g of silver sulphate or sufficient to precipitate all the chloride in the portion of the sample being treated, 7 ml of water and 15 ml of 60 per cent w/w perchloric acid. Heat the flask until the temperature reaches 120 to 125 , connect the steam supply, regulate the gas and steam supplies so that the temperature of the distillation is maintained at 137 to 140 and distil 150 ml in twenty-five to thirty-five minutes, steaming out the condenser towards the end of the distillation discard the distillate. Distil a further 150 ml and titrate an aliquot part by the method given below. Calculate the amount of fluorine in the whole fraction. (This figure, which should not exceed 1-5 fig, may be termed the apparatus blank and should be approximately constant for any further 150-ml fractions.)... 

The substance present in the crude extract of leaves, is purified by repeated adsorption on norit, followed by elutions with ammonia or aniline. Folic acid is then precipitated from the eluates as the lead salt. It is redissolved in an ammonium sulfate solution, precipitated as a silver salt, and dissolved once again in an ammonium chloride solution. Folio acid is adsorbed at pH 1.0 on fullers earth, and eluted by ammonia water. It is again adsorbed on alumina, treated by a fractional elution first with aqueous methanol, then with aqueous methanol containing 2 % ammonia. The acidified solution is concentrated in vacuo and a precipitate forms on cooling to low tern-... 

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