Adsorption titration

Il e degree of adsorption saturation and the area occupied by an emulsifier molecule in the satiuated adsorption layer on the surface of latex particles were determined by means of the adsorption titration method (16). Sizes of latex particles were determined... 

The synthesis of latexes for adsorption titration was conducted in a thermostated glass reactor provided with a stirrer and an attachment for feeding in inert gas. The polymerization tempei ature is 328 K in the case of UA, and increases to 333 K, when EA. BA and St are used. Phase ratio was 1 4 (by weight). Ccuaposltions used for latex synthesis and latex characteristics are listed in Table I. 

The results of adsorption titration of latexes, presented in Fig e 4, show that the equilibrium adsorption is significantly cmd regularly decreased with an increase of monomer solubility in water. The area occupied by cm emulsifier molecule in the saturated adsorption layer is correspondingly increased this is seen in Figure 5 showing this area to be the function of the surface tension o monomer-... 

The object of this study was to clarify some aspects of the mechanism of shear-induced flocculation in colloidal dispersions. Vinyl chloride homopolymer and copolymer latices were prepared by emulsion polymerization using sodium dodecyl sulphate as emulsifier. Agglomeration behavior in these latices was studied by measuring the mechanical stability using a high speed stirring test. The latex particle size was measured by an analytical centrifuge. Molecular areas of emulsifier in the saturated adsorption layer at the surface of homopolymer and copolymer latex particles were estimated from adsorption titration data. 

Surface tension measurement. Adsorption titration, also called soap titration, (2.3) was carried out by the drop volume method at different polymer concentrations. The equivalent concentration of salt was held constant. The amount of emulsifier necessary to reach the critical micelle concentration (CMC) in the latex was determined by each titration. The total weight of emulsifier present in the latex is the weight of emulsifier in the water plus the weight of emulsifier adsorbed. The linear plot of emulsifier concentration (total amount of emulsifier corresponding to the end-point of each titration) versus polymer concentration gives the CMC as the intercept and the slope determines the amount of emulsifier adsorbed on the polymer surface in equilibrium with emulsifier in solution at the CMC (E ). 

In Figure 3 the same stability data are plotted versus the surface concentration of emulsifier. The surface concentration is given as per cent of total coverage of the particle surface, as determined by adsorption titration. 

The area occupied by each adsorbed emulsifier molecule at the polymer-water interface (A ) was estimated from the mean particle size of the latices, determined by the analytical centrifuge, and the amount of emulsifier adsorbed at the interface corresponding to full coverage, determined by adsorption titration. 

Several investigations have determined tbe absorption behavior of surfactant adsorption on particles of aqueous polymer dispersions by adsorption titration. The results have been similar to those observed by Wolfram for adsorption on a planar polymer surface determined from the wettipg angle. Thus, Paxton (1969) established that the area occupied by a sodium dodecylbenzylsulfonate molecule in a saturated adsorption layer (ylsiim) the surface of PMMA latex particles is 1.31 nm, whereas on the surface of polystyrene latex particles it is only 0.53 nm. The author considers that previous studies of adsorption of this emulsifier, which gave adsorption area Msitm) were carried out on interfaces with similar adsotption... 

Adsorption calorimetry Enthalpy and entropy of adsorption (titration method) fm jacbe molecules and reactants Limited to simple systems (well-defined surface, one adsorptive) no... 

In this survey, it is not attempted to show how successful surface complexation models are. Rather, it is attempted to show what can be done with them, what will be done with them in the future and what should not be done with them. The experimental aspects (e.g., the input data to the models) are discussed whenever judged important (certainly without full coverage but rather with focus on those aspects, which have not yet been addressed or details which have been of interest to the present author). In particular, the importance of combining methods and data is stressed. In recent years, there has been an increased interest in linking surface complexation models, which are traditionally based on macroscopic (adsorption, titration, and other) data, with structural information obtained with modem spectroscopic methods such as x-ray absorption spectroscopy (XAS). It is expected that the closer the agreement of the thermodynamic formulation of a surface chemical reaction with the actual structure of a surface complex is, the more reliable a prediction of the system behavior under more or less strongly varied conditions will be. 

Rxp, Surface Chemistry Macroscopic data Adsorption, titration... 

Fajans method The titration of Cl" with Ag using fluorescein as an adsorption indicator. At the end point the precipitate becomes red. 

Figure V-8 illustrates that there can be a pH of zero potential interpreted as the point of zero charge at the shear plane this is called the isoelectric point (iep). Because of specific ion and Stem layer adsorption, the iep is not necessarily the point of zero surface charge (pzc) at the particle surface. An example of this occurs in a recent study of zircon (ZrSi04), where the pzc measured by titration of natural zircon is 5.9 0.1...

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

Table 11.30 lists standard solutions for precipitation titrations and Table 11.31 lists specific reagents as indicators, adsorption indicators, and protective colloids for precipitation titrations. 

Titration methods using adsorption indicators, based on the precipitation of insoluble iodides, have also been proposed (81—84). The sensitivity of these methods is less than that for the thiosulfate titration. Electrometric titration of the reaction between iodine and thiosulfate (85) was not found to be practicable for routine deterrninations of minute quantities of iodine. 

After the addition of silver nitrate, potassium nitrate is added as coagulant, the suspension is boiled for about 3 minutes, cooled and then titrated immediately. Desorption of silver ions occurs and, on cooling, re-adsorption is largely prevented by the presence of potassium nitrate. 

A disadvantage of adsorption indicators is that silver halides are sensitised to the action of light by a layer of adsorbed dyestuff. For this reason, titrations should be carried out with a minimum exposure to sunlight. When using adsorption indicators, only 2 x 10-4 to 3 x 10 3 mol of dye per mol of silver halide is added this small concentration is used so that an appreciable fraction of the added indicator is actually adsorbed on the precipitate. 

For the titration of chlorides, fluorescein may be used. This indicator is a very weak acid (Ka = ca lx 10-8) hence even a small amount of other acids reduces the already minute ionisation, thus rendering the detection of the end point (which depends essentially upon the adsorption of the free anion) either impossible or difficult to observe. The optimum pH range is between 7 and 10. Dichlorofluorescein is a stronger acid and may be utilised in slightly acid solutions of pH greater than 4.4 this indicator has the further advantage that it is applicable in more dilute solutions. 

Other dyestuffs have been recommended as adsorption indicators for the titration of halides and other ions. Thus cyanide ion may be titrated with standard silver nitrate solution using diphenylcarbazide as adsorption indicator (see Section 10.44) the precipitate is pale violet at the end point. A selection of adsorption indicators, their properties and uses, is given in Table 10.8. 

Either the Mohr titration or the adsorption indicator method may be used for the determination of chlorides in neutral solution by titration with standard 0.1M silver nitrate. If the solution is acid, neutralisation may be effected with chloride-free calcium carbonate, sodium tetraborate, or sodium hydrogencarbonate. Mineral acid may also be removed by neutralising most ofthe acid with ammonia solution and then adding an excess of ammonium acetate. Titration of the neutral solution, prepared with calcium carbonate, by the adsorption indicator method is rendered easier by the addition of 5 mL of 2 per cent dextrin solution this offsets the coagulating effect of the calcium ion. If the solution is basic, it may be neutralised with chloride-free nitric acid, using phenolphthalein as indicator. 

Similar remarks apply to the determination of bromides the Mohr titration can be used, and the most suitable adsorption indicator is eosin which can be used in dilute solutions and even in the presence of 0.1 M nitric acid, but in general, acetic (ethanoic) acid solutions are preferred. Fluorescein may be used but is subject to the same limitations as experienced with chlorides [Section 10.77(b)], With eosin indicator, the silver bromide flocculates approximately 1 per cent before the equivalence point and the local development of a red colour becomes more and more pronounced with the addition of silver nitrate solution at the end point the precipitate assumes a magenta colour. 

Discussion. The Mohr method cannot be applied to the titration of iodides (or of thiocyanates), because of adsorption phenomena and the difficulty of distinguishing the colour change of the potassium chromate. Eosin is a suitable... 

Iodides can also be determined by this method, and in this case too there is no need to filter off the silver halide, since silver iodide is very much less soluble than silver thiocyanate. In this determination the iodide solution must be very dilute in order to reduce adsorption effects. The dilute iodide solution (ca 300 mL), acidified with dilute nitric acid, is treated very slowly and with vigorous stirring or shaking with standard 0.1 M silver nitrate until the yellow precipitate coagulates and the supernatant liquid appears colourless. Silver nitrate is then present in excess. One millilitre of iron(III) indicator solution is added, and the residual silver nitrate is titrated with standard 0.1M ammonium or potassium thiocyanate. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

Baviere et al. [41] determined the adsorption of C18 AOS onto kaolinite by agitating tubes containing 2 g of kaolinite per 10 g of surfactant solution for 4 h in a thermostat. Solids were separated from the liquid phase by centrifugation and the supernatant liquid titrated for sulfonate. The amount of AOS adsorbed is the difference between initial solution concentration and supernatant solution concentration at equilibrium. 

Alternative variations of the isothermal titration technique utilize CO adsorption followed by reaction with gaseous 02. Experiment has shown that similar Nc values are obtained.1,19... 

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