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Ch 7: Gravimetric and Combustion Analysis

Ch 7: Gravimetric and Combustion Analysis. General Gravimetric Procedure. aA (aq) + rR (aq) → A a R r (s). precipitating reagent (in excess). analyte. precipitate. weight of A determined using stoichiometric ratio between A and A a R r. moles A a R r. moles A. mass A.

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Ch 7: Gravimetric and Combustion Analysis

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  1. Ch 7: Gravimetric and Combustion Analysis

  2. General Gravimetric Procedure aA(aq) + rR(aq)→ AaRr(s) precipitating reagent(in excess) analyte precipitate weight of A determined using stoichiometric ratio between A and AaRr

  3. moles AaRr moles A mass A stoichiometric ratio FM FM Stoichiometry Calculations mass AaRr

  4. EXAMPLE: A Simple Gravimetric Calculation A 10.00-mL solution containing Cl- was treated with excess AgNO3 to precipitate 0.4368-g of AgCl (FM = 143.321). What was the molarity of Cl- in the unknown?

  5. Selected Analyses and "Masking"

  6. Impurities such as Ag+, Mn2+, Zn2+, Cd2+, Hg2+, Fe2+, and Ga3+ are masked using -

  7. EXAMPLE: When the Stoichiometry is Not 1:1 Solid residue weighing 8.4448-g from an aluminum refining process was dissolved in acid, treated with 8-hydroxyquinoline, and ignited to give Al2O3 weighing 0.8554-g. Find the weight precent of Al in the original mixture.

  8. Precipitates (Sec. 7-2) • large particle size ease of filtering • colloids 1-500 nm, charged (migrate in an electric field), don't settle out (suspended by Brownian motion), pass through filter paper • need to promote particle growth over nucleation • precipitation mechanisms are still poorly understood, but they do depend on - • solubility • temperature • reactant concentrations • rate of mixing

  9. Relative Supersaturation (RS) • Two competing mechanisms: nucleation vs. particle growth • supersaturated solutions result in nucleation (the formation of many small colloids) Q = supersaturated concentration S = equilibrium concentration

  10. Control of Particle Size = Minimize Supersaturation • elevate the temperature (increases S) • use dilute solutions (decreases Q) • slow mixing with lots of stirring (decreases Q)

  11. Precipitation in the Presence of Electrolyte (e.g. 0.10 M HNO3) • we want the particles to coagulate together to make bigger particles • since the colloids are charged however, they repel each other • the charge on the colloid depends on which lattice ions are in excess in solution, e.g. for AgCl the lattice ions are Ag+ and Cl- • if excess Ag+ in solution, then + colloids • if excess Cl- in solution, then - colloids • if unknown = Cl- and it's being precipitated out with Ag+, then initially the excess lattice ion is Cl- and the colloids are negatively charged. • after all of the Cl- is precipitated out, adding more Ag+ will change the colloid charge to +

  12. A AgCl colloid growing in a solution of excess Ag+and additional electrolyte HNO3 (H+ and NO3- ions) will end up being negatively charged - The + colloid will attract negatively charged counter ions (NO3-) into the "ionic atmosphere" surrounding the colloid, and the entire particle will be negatively charged.

  13. Stirring and heating adds kinetic energy to the colloids, while the added electrolyte shrinks the ionic atmosphere. Both effects reduce repulsion between particles and they can then coagulate together.

  14. Thermogravimetric Analysis The mass of the sample is measured as a function of temperature.

  15. For this gravimetric analysis, the precipitate must be heated to at least 800 oC in order to drive off all the water, volatilize the HNO3, and reduce the sample to CaO. = "heating to constant weight"

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