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The Mechanism of the Peptization and Precipitation of Negative Silver

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THB MECE&SISM OF THE PEPTIZATIOB AHB PRECIPITATION OF HEGATIVB SILVER IODIDE SOLS fcy William Joseph Coppoc % B. Sc* A Thesis Presented to the Faculty of the lice Institute of Houston, Texas in Partial Fulfillment of the Requirements for the Degree * of Master of Arts The Rico Institute IPS? CONTESTS ISTRGBUCTXGH 1 EXPERIMENTAL PROCEDURE (1) THE PEPTIZATION Of SILVER IODIDE 4 (B) THE ADSORPTION OP HIBRQXYL ION 7 (3) 1HI DETERMINATION OF PRECIPITATION VALDES 10 THEORETICAL DISCUSSION 17 S®IRX 24 BIBLIOGRAPHY 25 THE MECLIAHI&Vi OF THE PEPTIZATION AKB PRECIPITATION OF MBATIV& SlVm IODIDE SOLS Recent developments of theoretical colloid chemistry bring to the fore the question of the mechanism of peptization and precip¬ itation of colloidal particles; that is, the character of the double layer, the specific action of some ions in forming this layer, and tho change in the double layer accompanying peptization and precip¬ itation. This problem has been attacked from several theoretical and experimental viewpoints. Freundlioh (2), working with arsenious sulfide sols, proposed the adsorption theory for the precipitation of sols and also believed at that time that equivalent amounts of ions of different valence were necessary for the coagulation, leiser and co-workers {18, 21, 25} using arsenious sulfide, hydrous ferric oxide, and hydrous alumina sols, found that the adsorption of ions of varying valence need not be in equivalent amounts to effect precipi¬ tation and Freundlich {4} recognized in 1929 that the amounts of adsorption were not equivalent. Weiser {19) la 1951 contrasted the adsorption theory of the coagulation with the solubility theory of Duclaux (1) and Pauli ill, 18, 13) and proposed a quite complete mechanism for the coagulating action of electrolytes on sols of the hydrous oxides and, by inference, other colloidal solutions. Kruyt and van der Willigen (8) proposed their « isomorphism” theory in 1928 from the results of their experiments on silver iodide sols. According to this theory, a precipitate will be peptized by, and only by, an ion which will form a salt, with the oppositely 2 charged ion of the crystal lattice, which is isomorphous with the pre¬ cipitate. From their discussion, the term "isomorphous” is taken to mean "will form mixed crystals with”. Sound basis for this theory was found in the work of li. Mare (9) whose results showed that adsorp¬ tion is dependent on crystal structure in such a way that “adsorption takes place when adsorbent and adsorbed substance are isomorphous or of similar crystal habit** Thus, potassium chloride will peptize silver iodide forming a negative sol because silver chloride will form mixed crystals with silver iodide, inter work in this same laboratory (16, 17, 6, 7) has given results which, in the opinion of iCruyt and co-workers, support this theory, and the results of Versey and Iruyt (17) apparently contradict some parts of the adsorption theory. A true mechanism has, however, not bean proposed; that is, the state¬ ment is made that ieomorphous ions build the double layer but a picture of how this is done is not given. This theory then, as we shall see, reduces to a special case of the adsorption theory as Koltboff (5) has classified it in his comprehensive paper on "Adsorp¬ tion on Ionic lattices"• The apparent contradictions which Verwey and Kruyt observed were: (a) Some ions may be adsorbed on the particle but will not in themselves peptize the particle; (b) precipitation my occur before the maximum adsorption is attained; and (c) maximum adsorption is ^Sloat and lieazies (15) in a more recent study of the adsorp¬ tion from aqueous solution of six salts on PbS, found that the adsorp¬ tion was related much less closely, if at all, to the relative lattice dimensions of salt and adsorbent than to the aqueous solubility of the solute. 3 oometimea obtained long before the precipitation value of the sol is reached. As examples of these contradictions they cite: (a) hydroxyl iois% as sell as oxalate and phosphate ions, are adsorbed on silver iodide but are not in themselves "potential-deterraini ng " ions and ere therefore not able to peptize the silver iodide; (b) cerium ion, adsorbed on a silver iodide sol in the presence of a large amount of hydrogen ion, precipitates the sol long before sufficient cation has been added to effect the precipitation of the sol; and (e) barium ion on a negative silver iodide sol attained maximum adsorption at approx¬ imately. 0.2 to 0.4 lailliequivalents of barium per kilogram of sol and yet the sol was not precipitated until the concentration of barium was approximately 1.0 to 1.2 milliequivalents per kilogram of sol, feiser and Milligan (22) have explained case (b) from the fact that, to the undlalyzod sol with which Verwey and Kruyfc were work¬ ing, the large concentration of hydrogen ions would reduce the zeta potential to the coagulation value before the maximum adsorption of cerium ion had been attained. In this study, it is proposed to explain cases (a) and (e) to the light of the adsorption theory and to propose a mechanism, according to the adsorption theory, for the peptization and precipitation of negatively charged silver iodide sols. It Is important to observe that in all of these cases, the precipitation value plays quite an important role la determining the validity of any one theory. For example, Fennycuick and Best (14) and Weiner and Gray (20) have also used the precipitation value as a means of solving problems of theory. In the experimental portion of this paper there ia given: 4 (1.) The effect of various loss on the peptization of silver iodide. (2.) The adsorption of hydroxyl ion on silver iodide* (3) The determination of the precipitation value of several electrolytes for silver iodide sols. BgBBIHSna& (1) FEPTIZATIOH OF SILVER IODIDE. Since, as Kruyt and Oysouw 16} have pointed out, and as has been found experimentally daring this study, only the iodide ion will serve to peptize directly a well-washed silver iodide precipitate, it was decided to add the peptizing electrolyte to the potassium iodide solution before mixing the two solutions to form the sol* By this method, the final sol always contains a known concentration of potassium nitrate. The slight solubility of silver iodide is suffi¬ cient to allow quite rapid aging so that by the time the silver iodide, which has been precipitated from equivalent solutions of potassium iodide and silver nitrate, has been washed free from electrolyte, the aging of the particles has proceeded to a point where direct peptiz¬ ation is almost impossible* The solutions of silver nitrate and of potassium iodide were prepared from the "C. P.* salts and were found to contain no impurities which mi^ht interfere with the accuracy of our results. The solutions were made up approximately 0.05 Jf and standardized, then carefully diluted to 0,0223 H. Approximately 1.0 I? solutions of electrolyte were made up by 5 placing a known amount of the properly dried salt in water and dilut¬ ing to 100 cc. except in the case of potassium hydroxide which ©as made up approximately correct and then standardized against standard hydro¬ chloric acid. The proper amounts of these solutions to give the different concentrations of electrolyte desired ©ere added to enough water to make 10 cc. and this solution then added to the potassium iodide solutionj thus in each case, 48 cc. of silver nitrate solution, 45 cc. Of potassium Iodide solution plus 10 cc. of the electrolyte solution were mixed. Since the silver nitrate and the potassium iodide were both 0.0S22 H, 100 cc. of a sol containing 10 mMola of Agl per liter was obtained. The solutions were mixed In a "quick-mixing" device of 100 cc. capacity. This device consisted of a 50 cc. Pyrex beakex* placed inside a wide mouth glass chemical container with a bakelite, screw- top lid and held in place by a glass stirring rod which was of the proper length to reach exactly from the bottom of the beaker to the opposite side of the screw-top lid. The silver nitrate solution was placed in the beaker and the potassium iodide solution plus the peptizing electrolyte was placed in the container outside the beaker, the lid screwed on tightly and the whole then shaken vigorously for thirty seconds. This device assured probably the quickest and most complete possible mixing and was found to give easily reproducible results after a little practice. The electrolytes used were potassium bromide, potassium fluoride, potassium nitrate, potassium chloride, potassium sulfate, and potassium chlorate. According to the isomorphism theory of Kruyt, 6 oat *d *xi *0 «ti ts 0 0 0 Q m 0 4» 4* 4» 4> - P P 0 0 0 m 0 0 0 m m JH tJ H 4* 4* 49 49 49 49 SO %_ p 0 & 0 «D »H i*4 i*4 if4 *rl i4 ts3 m M s* $2 0 A P4 04 D< P* P* 4> 4» ** 4» *H *rl *r4 i4 *H *4 * •P m P 0 49 m 0 0 O 0 0 fj o a, P* 04 0 0 m 0 0 m 0 * 0 I 0 * A 5> A |ft 0* > & % a a aa *0 *0 *0 m *0 m o m m m 4» 4> 49 49 $$ 0 <8 0 0 *0 <& *0 4* 4* un 49 49 0 o 0 •*4 %4 n-4 «r4 n*$ «H m m N 0 P* 0* P* m o *r4 H *rl i~* ns *H *Hf *4 i4 * 49 £) 4> ,0 49 JP 0 O 0 o 0 0 o 04 fil PU 0 Oi 0 0 0 m 0 m o 0 49 © 4* 0 4* 04 m rk © A 0 a a aa *d *a *d *ts 0 0 0 0 0 0 4» 4» 42 42 42 42 0 0 0 0 0 0 *0 *d nd -nd *d *0 49 49 4» 42 4* 42 o 0 0 0 0 m m i4 **4 *r4 f4 •Ht «r4 S3 23 M fa t* 2a *a Pi a.
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  • Gravimetric Methods of Analysis

    Gravimetric Methods of Analysis

    GRAVIMETRIC METHODS OF ANALYSIS Gravimetric methods are quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related. Gravimetric methods of analysis are based on mass measurements with an analytical balance, an instrument that yields highly accurate and precise data. Precipitation Gravimetry In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed. Properties of Precipitates and Precipitating Reagents Ideally, a gravimetric precipitating agent should react specifically or at least selectively with the analyte. Specific reagents, which are rare, react only with a single chemical species. Selective reagents, which are more common, react with a limited number of species. In addition to specificity and selectivity, the ideal precipitating reagent would react with the analyte to give a product that is 1. easily filtered and washed free of contaminants 2. of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing 3. unreactive with constituents of the atmosphere 4. of known chemical composition after it is dried or, if necessary, ignited Particle Size and Filterability of Precipitates Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities. In addition, precipitates of this type are usually purer than are precipitates made up of fine particles. Factors That Determine the Particle Size of Precipitates The particle size of solids formed by precipitation varies enormously.