D. Harvey - Modern Analytical Chemistry (794078), страница 73
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Early in the precipitation, when NaCl is the limitingreagent, excess Ag+ ions chemically adsorb to the AgCl particles, forming a positively charged primary adsorption layer (Figure 8.5). Anions in solution, in this caseNO3– and OH–, are attracted toward the surface, forming a negatively charged secondary adsorption layer that balances the surface’s positive charge. The solutionoutside the secondary adsorption layer remains electrically neutral. Coagulationcannot occur if the secondary adsorption layer is too thick because the individualparticles of AgCl are unable to approach one another closely enough.Coagulation can be induced in two ways: by increasing the concentration of theions responsible for the secondary adsorption layer or by heating the solution.
Oneway to induce coagulation is to add an inert electrolyte, which increases the concentration of ions in the secondary adsorption layer. With more ions available, thethickness of the secondary absorption layer decreases. Particles of precipitate maynow approach one another more closely, allowing the precipitate to coagulate. Theamount of electrolyte needed to cause spontaneous coagulation is called the criticalcoagulation concentration.1400-CH08 9/9/99 2:17 PM Page 243Chapter 8 Gravimetric Methods of Analysis(b)243(c)(a)Figure 8.6(e)Tear(f)(d)Heating the solution and precipitate provides a second way to induce coagulation.
As the temperature increases, the number of ions in the primary adsorptionlayer decreases, lowering the precipitate’s surface charge. In addition, increasing theparticle’s kinetic energy may be sufficient to overcome the electrostatic repulsionpreventing coagulation at lower temperatures.Filtering the Precipitate After precipitation and digestion are complete, the precipitate is separated from solution by filtration using either filter paper or a filtering crucible.
The most common filtering medium is cellulose-based filter paper, which isclassified according to its filtering speed, its size, and its ash content on ignition. Filtering speed is a function of the paper’s pore size, which determines the particle sizesretained by the filter. Filter paper is rated as fast (retains particles > 20–25 µm),medium fast (retains particles > 16 µm), medium (retains particles > 8 µm), andslow (retains particles > 2–3 µm). The proper choice of filtering speed is important.If the filtering speed is too fast, the precipitate may pass through the filter paper resulting in a loss of precipitate.
On the other hand, the filter paper can becomeclogged when using a filter paper that is too slow.Filter paper is hygroscopic and is not easily dried to a constant weight. As a result, in a quantitative procedure the filter paper must be removed before weighingthe precipitate. This is accomplished by carefully igniting the filter paper. Followingignition, a residue of noncombustible inorganic ash remains that contributes a positive determinate error to the precipitate’s final mass.
For quantitative analytical procedures a low-ash filter paper must be used. This grade of filter paper is pretreatedby washing with a mixture of HCl and HF to remove inorganic materials. Filterpaper classed as quantitative has an ash content of less than 0.010% w/w. Qualitative filter paper typically has a maximum ash content of 0.06% w/w.Filtering is accomplished by folding the filter paper into a cone, which is thenplaced in a long-stem funnel (Figure 8.6). A seal between the filter cone and theProper procedure for filtering solids usingfilter paper. The filter paper circle in (a) isfolded in half (b), and folded in half again(c).
The filter paper is parted (d), and a smallcorner is torn off (e). The filter paper isopened up into a cone and placed in thefunnel (f). Note that the torn corner is placedto the outside.1400-CH08 9/9/99 2:17 PM Page 244244Modern Analytical ChemistrysupernatantThe solution that remains after aprecipitate forms.Figure 8.7Proper procedure for transferring thesupernatant to the filter paper cone.funnel is formed by dampening the paper with water and pressing the paper to thewall of the funnel.
When properly prepared, the stem of the funnel will fill with thesolution being filtered, increasing the rate of filtration. Filtration is accomplished bythe force of gravity.The precipitate is transferred to the filter in several steps (Figure 8.7).
The firststep is to decant the majority of the supernatant through the filter paper withouttransferring the precipitate. This is done to prevent the filter paper from becomingclogged at the beginning of the filtration process. Initial rinsing of the precipitate isdone in the beaker in which the precipitation was performed. These rinsings arealso decanted through the filter paper. Finally, the precipitate is transferred onto thefilter paper using a stream of rinse solution. Any precipitate clinging to the walls ofthe beaker is transferred using a rubber policeman (which is simply a flexible rubberspatula attached to the end of a glass stirring rod).An alternative method for filtering the precipitate is a filtering crucible (Figure 8.8).
The most common is a fritted glass crucible containing a porous glassdisk filter. Fritted glass crucibles are classified by their porosity: coarse (retainingparticles > 40–60 µm), medium (retaining particles > 10–15 µm), and fine (retaining particles > 4–5.5 µm). Another type of filtering crucible is the Gooch crucible, a porcelain crucible with a perforated bottom. A glass fiber mat is placed inthe crucible to retain the precipitate, which is transferred to the crucible in thesame manner described for filter paper.
The supernatant is drawn through thecrucible with the assistance of suction from a vacuum aspirator or pump.Rinsing the Precipitate Filtering removes most of the supernatant solution. Residual traces of the supernatant, however, must be removed to avoid a source of determinate error. Rinsing the precipitate to remove this residual material must be donecarefully to avoid significant losses of the precipitate.
Of greatest concern is the potential for solubility losses. Usually the rinsing medium is selected to ensure thatsolubility losses are negligible. In many cases this simply involves the use of coldsolvents or rinse solutions containing organic solvents such as ethanol. Precipitatescontaining acidic or basic ions may experience solubility losses if the rinse solution’spH is not appropriately adjusted.
When coagulation plays an important role in deVentCrucibleto VacuumRubberadapterRubber hoseTrapSuctionflaskFigure 8.8Procedure for filtering through a filteringcrucible. The trap is used to prevent waterfrom a water aspirator from backwashinginto the suction flask.1400-CH08 9/9/99 2:17 PM Page 245Chapter 8 Gravimetric Methods of Analysistermining particle size, a volatile inert electrolyte is often added to the rinse water toprevent the precipitate from reverting into smaller particles that may not be retained by the filtering device.
This process of reverting to smaller particles is calledpeptization. The volatile electrolyte is removed when drying the precipitate.When rinsing a precipitate there is a trade-off between introducing positive determinate errors due to ionic impurities from the precipitating solution and introducing negative determinate errors from solubility losses. In general, solubilitylosses are minimized by using several small portions of the rinse solution instead ofa single large volume.
Testing the used rinse solution for the presence of impuritiesis another way to ensure that the precipitate is not overrinsed. This can be done bytesting for the presence of a targeted solution ion and rinsing until the ion is nolonger detected in a freshly collected sample of the rinse solution. For example,when Cl– is known to be a residual impurity, its presence can be tested for byadding a small amount of AgNO3 to the collected rinse solution. A white precipitateof AgCl indicates that Cl– is present and additional rinsing is necessary. Additionalrinsing is not needed, however, if adding AgNO3 does not produce a precipitate.Drying the Precipitate Finally, after separating the precipitate from its supernatant solution the precipitate is dried to remove any residual traces of rinse solution and any volatile impurities. The temperature and method of drying depend onthe method of filtration, and the precipitate’s desired chemical form.
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