Gravimetric Methods of Analysis Chapter 8 Gravimetric Analysis Skoog Book Page 179-198 Do Problems: 1,2,4,9,10,11,14,16,21,27,30,33 Chapter 9 Electrolyte Effects Activities effective concentration and equilibrium Chapter 10 Complex Equilibria similar but a few steps we have not looked at
Gravimetric Analysis Gravimetric Analysis--quantitative technique based on the determination of the mass of a precipitated or volatized compound which the analyte is stoichiometrically related Analyte: the substance determined in the procedure. converted to an insoluble form, collected and massed on an analytical balance. Reaction: aa + rr ===> AaRr (s) Analyte Precipitating Agent Precipitate that we will dry, mass and relate to analyte Gravimetry
Vacuum Filteration Set Up Aspirator Buchner Funnel Filter Adaptor Filter Paper Rubber tubing Filter Flask Mother Liquor
Mechanicism of Precipitation Two Competing Processes Nucleation vs Particle Growth
Representative Gravimetry
Representative Gravimetry
Mechanicism of Precipitation Two Competing Processes Nucleation vs Particle Growth Desirable!
Creating Ideal Precipitates Now let s learn more about precipitate formation in particular colloids and crystalline precipitates precipitating agent AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq)
Colloidal Double Layer Model If we add Cl - we run out of Cl and NO3 - fills in
Precipitates Vary And Complicate! Particle Size of the precipitate is determined by a number of factors, such as! Relative supersaturation (RSS)! Mechanism of formation: nucleation vs particle growth! Experimental control: ph, temperature, mixing,! Supersaturation occurs when precip. agent is added to analyte. System reacts to reach equilibrium (Q > Ksp). It does this by nucleation. Nucleation is not desirable as the particles are small. More nuclei bring about entrapment or unwanted ions. Study shown that particle size is inversely proportional.
Nucleation Individual ions/atoms/molecules coalesce to form nuclei Particle Growth Condensation of ions/atoms/molecules with existing nuclei forming larger particles which settle out Colloidal Suspension Colloidal particles remain suspended due to adsorbed ions around a small particle yielding a net + or - charge Coagulation, agglomeration Suspended particles coalesce (lump together) to form larger filterable particles Peptization Terms Used In Gravimetry Re-dissolution and dispersion of coagulated colloids by washing and removing inert electrolyte (not good)
How Can We Create Ideal Precipitates Studies show that Particle Size is inversely proportional to Relative Supersaturation Relative Supersaturation = Q S S where: S = solubilty of precipitate at equilibrium, and Q = concentration of mixed reagents before precipitation. Large Q gives undesirable colloidal precipitates. Small Q gives large crystalline filterable particles
How To Keep RSS Low Best Practice Precipitate from dilute solution with good stirring, this keeps Q low Add dilute precipitating reagents slowly with good stirring (prevents local excess or saturation) Precipitate from hot solutions. This normally increases S (solubility)...followed by cooling for complete quantitative precipitation. Precipitate at lower ph as generally precipitates are more soluble in acid medium--this slows precipitation.
Somtimes Important We Factors Get for Colloids Gravimetric (not Analysis fun) When colloidal particles form we need to aggregate or coagulate them to form easily filterable particles. Two Common Approaches To Accomplish This Digest the colloidal sample by heating (1-hr) Let the heated precipitate age overnight Add an electrolyte (a non-interfering salt) Both reduce the number of stabilizing counterions and allow for particles to approach one another
Cl - adsorbs on the particles when in excess (primary layer). A counter layer of cations forms. The neutral double layer causes the colloidal particles to coagulate. Washing with water will dilute the counter layer and the primary layer charge causes the particles to revert to the colloidal state (peptization). So we wash with an electrolyte that can be volatilized on heating (HNO 3 ). Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) Fig. 10.2. Representation of silver chloride colloidal particle and adsorptive layers when Cl - is in excess.
Important Addressing Factors for Peptization Gravimetric Analysis Peptization occurs when one washes the precipitate to remove the counter-ions and as a result some of the colloid redissolves. Two Common Approaches To Accomplish This Digest the colloidal sample by heating (1-hr) Let the heated precipitate age overnight Add an electrolyte (a non-interfering salt) Both reduce the number of stabilizing counterions and allow for particles to approach one another
Important Crystalline Factors for Precipitate Gravimetric Analysis Much larger particle size Optimize by keeping RSS << 1 Easy to filter Ideal to work with Best done with dilute precipitation then digest
Other Important Problems: Factors for Co-precipitation Gravimetric Analysis Co-precipitation occurs when another soluble compound in the mix precipitates with your analyte unwantingly. 4-Types of Co-precipitation 1. Surface Absorption 2. Occlusion 3. Mechanical entrapment 4.Mixed crystal formation
Coprecipitation: Important Factors for Surface Gravimetric Adsorption Analysis Surface Adsorption: surface of the precipitate will contain some absorbed ions. Example: AgCl(s) + NO3 - adsorbed (mass will be high) Occurs mostly with colloidal precipitates as the surface area is larger alters the mass and alters the accuracy of the analysis How To Deal With It Digest for longer periods Washing--usually doesn t work as counter ion is strong Washing With Volatile Electrolyte (HCl followed by drying)
Important Occlusion Factors Traps for Gravimetric Counter-Ions Analysis
Important Mechanical Factors for Entrapment Gravimetric Analysis
Important Occlusion Factors and for Entrapment Gravimetric Analysis
Important Mixed Crystalline Factors for Gravimetric Precipitate Analysis Difficult to deal with. Alternate analysis?
Worked Example A 0.4500 g sample of impure potassium chloride was dissolved in water and treated with an excess of silver nitrate. A 0.8402 g of silver chloride was massed after digesting, collecting, washing and drying the precipitate. Calculate the percentage KCl in the original sample. (9712)