Chapter 27 Gravimetric and Combustion Analysis Dr S

Chapter 27 Gravimetric and Combustion Analysis Dr. S. M. Condren

Representative Gravimetric Analyses Dr. S. M. Condren

Dr. S. M. Condren

Precipitation Ag. NO 3 + Na. Cl -----> Ag. Cl + Na. NO 3 Dr. S. M. Condren

Precipitation Ag. NO 3 + Na. Cl -----> ? Ag. NO 3 + Na. Cl -----> Ag. Cl + Na. NO 3 total ionic equation Ag+ + NO 3 - + Na+ + Cl-----> Ag. Cl + Na+ + NO 3 net ionic equation Ag+ + Cl- -----> Ag. Cl Dr. S. M. Condren

Precipitation. WHY? Solubility Rules 1. The common salts of the alkali metals (Group IA) and the ammonium ion (NH 4+) are soluble. 2. Salts of nitrate (NO 3 -), chlorate (Cl. O 3 -), perchlorate (Cl. O 4 -), and acetate (CH 3 COO-) anions are soluble. 3. All chlorides, bromides, and iodides are soluble, except those of Ag+, Pb+2, and Hg 2+2 (the form that Hg(I) exists in water). Dr. S. M. Condren

Precipitation. WHY? Solubility Rules 4. Sulfates (SO 4 -2) are soluble except those of Ba+2, Pb+2, Hg+2, and Hg 2+2. Dr. S. M. Condren

Precipitation. WHY? Solubility Rules 5. Most metal hydroxides are insoluble. The exceptions are the hydroxides of the alkali metals and the heavier alkaline earth metals (Ca, Sr, Ba). 6. All carbonates (CO 3 -2) and phosphates (PO 4 -3) are insoluble, except those of the Group IA metals and NH 4+ ions. Dr. S. M. Condren

Crystal Growth nucleation particle growth supersaturated Dr. S. M. Condren

Crystal Growth relative supersaturation = (Q - S)/S where Q => actual concentration S => equilibrium concentration Dr. S. M. Condren

Crystal Growth techniques to promote crystal growth 1. Raising the temperature to increase S and thereby decrease relative supersaturation. 2. Adding precipitant slowly with vigorous mixing, to avoid a local, highly supersaturated condition where the stream of precipitant first enters the analyte. 3. Keeping the volume of solution large so that the concentration of analyte and precipitant are low. Dr. S. M. Condren

Dr. S. M. Condren

Precipitation in the Presence of Electrolyte coagulate => “to change or be changed from a liquid into a thickened mass” Dr. S. M. Condren

Precipitation in the Presence of Electrolyte Dr. S. M. Condren

Precipitation in the Presence of Electrolyte adsorption => attached to surface absorption => penetration beyond surface Dr. S. M. Condren

Digestion process of keeping mixture warm while the size of the crystals increase Dr. S. M. Condren

Purity absorbed impurities inclusions - impurity ions which randomly occupy sites in the crystal lattice occulusions - pockets of impurity trapped inside growing crystal coprecipitaion Dr. S. M. Condren

Purity gathering agent - precipitating agent used to collect trace component (process gathering) masking agent post-precipitation peptization Dr. S. M. Condren

Product Composition • • product must be of known composition hygroscopic substance ignition thermogravimetric Dr. S. M. Condren

Thermogravimetric curve for calcium salicylate Dr. S. M. Condren

Example: What weight of Fe 2 O 3 can be obtained from 1. 63 g of Fe 3 O 4? 2 Fe 3 O 4 + [O] -----> 3 Fe 2 O 3 (1. 63 g Fe 2 O 3) #g Fe 2 O 3 = ---------- Dr. S. M. Condren

Example: What weight of Fe 2 O 3 can be obtained from 1. 63 g of Fe 3 O 4? 2 Fe 3 O 4 + [O] -----> 3 Fe 2 O 3 (1. 63 g Fe 2 O 3)(1 mole Fe 3 O 4) #g Fe 2 O 3 = -------------------(231. 54 g Fe 3 O 4) Dr. S. M. Condren

Example: What weight of Fe 2 O 3 can be obtained from 1. 63 g of Fe 3 O 4? 2 Fe 3 O 4 + [O] -----> 3 Fe 2 O 3 (1. 63)(1 mole Fe 3 O 4)(3 mole Fe 2 O 3) #g Fe 2 O 3 = ---------------------(231. 54)(2 mole Fe 3 O 4) Dr. S. M. Condren

Example: What weight of Fe 2 O 3 can be obtained from 1. 63 g of Fe 3 O 4? 2 Fe 3 O 4 + [O] -----> 3 Fe 2 O 3 (1. 63)(1)(3 mole. Fe 2 O 3)(159. 69 g. Fe 2 O 3) #g Fe 2 O 3 = ------------------------(231. 54)(2) (1 mol Fe 2 O 3) Dr. S. M. Condren

Example: What weight of Fe 2 O 3 can be obtained from 1. 63 g of Fe 3 O 4? 2 Fe 3 O 4 + [O] -----> 3 Fe 2 O 3 (1. 63)(1)(3)(159. 69 g. Fe 2 O 3) #g Fe 2 O 3 = ------------------ = 1. 69 g (231. 54)(2)(1) Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 if use all of the 2. 85 g Pb(NO 3)2 Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 if use all of the 2. 85 g Pb(NO 3)2 (2. 85 g Pb(NO 3)2 #g Pb. I 2 = ------------Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 if use all of the 2. 85 g Pb(NO 3)2 (2. 85 g Pb(NO 3)2)(1 mol Pb(NO 3)2 #g Pb. I 2 = ----------------------(331 g Pb(NO 3)2) Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 if use all of the 2. 85 g Pb(NO 3)2 (2. 85)(1 mol Pb(NO 3)2)(1 mol Pb. I 2) #g Pb. I 2 = ----------------------(331) (1 mol Pb(NO 3)2) Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 if use all of the 2. 85 g Pb(NO 3)2 (2. 85)(1)(1 mol Pb. I 2)(461 g Pb. I 2) #g Pb. I 2 = ----------------------(331) (1 mol Pb. I 2) Dr. S. M. Condren

EXAMPLE: What mass of Pb. I 2 will precipitate if 2. 85 g Pb(NO 3)2 is added to 225 m. L of 0. 0550 M KI(aq)? Pb(NO 3)2 + 2 KI ------> Pb. I 2 + 2 KNO 3 if use all of the 2. 85 g Pb(NO 3)2 (2. 85)(1)(1)(461 g Pb. I 2) #g Pb. I 2 = ---------------- = 3. 97 g (331)(1)(1) Dr. S. M. Condren

Dr. S. M. Condren