Chapter 12: Gravimetric Methods of Analysis 1/35.

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Chapter 12: Gravimetric Methods of Analysis 1/35

Transcript of Chapter 12: Gravimetric Methods of Analysis 1/35.

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Chapter 12: Gravimetric Methods of Analysis

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Gravimetric methods: The quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.• Precipitation gravimetry: The analyte is separated from a

solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed.

• Volatilization gravimetry: The analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition that can be weighed.

• Electrogravimetry: The analyte is separated by deposition on an electrode by an electrical current.

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12A Precipitation gravimetryThe analyte is converted to a sparingly soluble precipitate that is then filtered, washed free of impurities and converted to a product of known composition by suitable heat treatment and weighed.

Ex., for determining the [Ca2+] in water:+ 2-

3(aq) 2 2 4(aq) 4(aq) 2 4(aq)

2 2-2 4(aq) 2 4(s)

2 4(s) (s) (g) 2(g)

2NH + H C O 2NH + C O

Ca + C O CaC O

filtered, dried, ignited

CaC O CaO + CO + CO

After cooling, the precipitate is weighed and the mass is determined.

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12A-1 Properties of Precipitates and Precipitating ReagentsA gravimetric precipitating agent should react specifically or at least selectively with the analyte and give precipitates that is:1.Enough particle size for retaining on filter paper2.High purity (free of contaminants)3.Low solubility that no significant loss of the analyte occurs during filtration and washing4.Unreactive with air (stable)5.Known stoichimetric composition

* Specific reagents: react only with a single chemical species.

Selective reagents: react with a limited number of species.

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12A-2 Particle Size and Filterability of PrecipitatesCharacteristics of Ion, colloid and particle

Name Diameter CharacteristicsIon ~10-8 cm DissolvedColloid 10-7~10-4 cm SuspendedCrystalline >10-4cm Settled from solution (filterable)

Nucleation and Particle growth• Nucleation: The initial formation process in which a

minimum number of atoms, ions, or molecules join together to give a stable solid.

• Particle growth: The subsequent growth after nucleation.

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Relative supersaturation (RSS) (von Weimann's theory):

RSS Process ResultLarge Nucleation dominate Smaller particlesSmall Particle growth

dominateLarger Particle

SS-Q

RSS

Q: the concentration of the solute at any instantS: the expected concentration solute at equilibrium

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Experimental Control of Particle Size(the lower RSS value the larger particles) Increase S:

Heating pH adjustment (If the solubility of the precipitate

depends on pH) Adding complexing agent

Decrease Q: Using dilute precipitating solution Adding precipitating agent slowly and stirring Using homogeneous precipitation technique

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12A-2 Colloidal PrecipitatesColloidal Precipitates: The amorphous precipitates which is formed by decreasing the repulsion force between alike charged colloids and leading those colloids to be coagulated to give a filterable particles.

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A colloidal silver chloride (AgCl) suspended in a solution of excess AgNO3 for example – a electric double layer:

Primary adsorption layer on colloidal particle(+ charged)

Counter-ion layer(- charged)

* Adsorption: A process in which a substance (gas, liquid, or solid) is held on the surface of a solid.Absorption: A process in which a substance within the pores of a solid.

Neutral aqueous solution

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Effect of AgNO3 and electrolyte concentration on the thickness of double layer surrounding a colloidal AgCl particle in a solution containing excess AgNO3.

* The effective charge can be thought as a measure of the repulsive force that the particle exerts on like particles in the solution.

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Coagulation of ColloidsCoagulation can be hastened by heating, by stirring, and by adding an electrolyte to the medium because of decreasing the volume of the counter-ion layer.

* Digestion (standard for some time) result in a denser mass precipitate.

A coagulated colloid

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Peptization of ColloidsPeptization: The process by which a coagulated colloid

reverts to its original dispersed colloidal suspensions. (When a coagulated colloid is washed, some of the electrolyte responsible for its coagulation is removed, therefore, the repulsive forces as well as the original colloidal state are reestablished.)

Volatile electrolyte is used as washing solution, e.g., HNO3, NH4NO3, to avoid peptization.• Electrolyte prevent the losses of precipitate from

peptization.• Volatile electrolyte out to atomosphere once heating.

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12-4 Crystalline PrecipitatesCrystalline precipitates are generally more easily filtered and purified than are coagulated colloids.Particle size of crystalline solids can often be improved significantly by minimizing Q (by using dilute solutions) or maximizing S (precipitating from hot solution or by adjusting the pH), or both.Digestion improves the purity and filterability of both colloidal and crystalline precipitates.

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12-5 CoprecipitationCoprecipitation: A process in which normally soluble compounds are carried out of solution by a precipitate. (It may resulted in impurities within the desired precipitates.)

There are four types of coprecipitation: surface adsorption, mixed-crystal formation, occlusion, and mechanical entrapment.

Coprecipitated impurities may cause either negative or positive errors.

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Surface Adsorption: The impurity is chemically or physically adsorbed onto the surface of precipitates.Cause significant contamination in coagulated colloids but of no significance in crystalline precipitates.

Problem solving:• Washing a coagulated colloid with a solution containing a

volatile electrolyte• Reprecipitation

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Mixed-Crystal Formation: A type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal.EX. SrSO4 in BaSO4

Problem solving:• Change to another more selective precipitating agent• Reprecipitation

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Occlusion: A type of coprecipitation in which coprecipitated physically trap contaminated counter ions within a precipitate during rapid precipitate formation.

Problem solving: Digestion* Digestion: The process which has a waiting time to achieve

desired outcome. (The solubility–precipitation is in a dynamic equilibrium, digestion ensures the occluded material is eventually exposed to the supernatant solution.) 17/35

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Mechanical Entrapment: A type of coprecipitation in which coprecipitated physically trap a pocket of solution within a precipitate during rapid precipitate formation.

Problem solving: Digestion

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12A-6 Precipitation from Homogeneous SolutionHomogeneous precipitation: A process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution.

• Solids formed by homogeneous precipitation are generally purer and larger size particles.

Example 1: For generating OH– as precipitantCO(NH2)2(aq) + H2O(l) CO2(g) + 2NH3(aq)

NH3(aq) + H2O(l) NH4+

(aq) + OH–(aq)

* Generation rate can be controlled by temperature

Example 2: For generating SO42– as precipitant

NH2SO3H(aq) + 2H2O(l) NH4+

(aq) + H3O+(aq) + SO4

2–(aq)

* Generation rate can be controlled by pH

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12A-7 Drying and Ignition of Precipitates

A gravimetric precipitatate is heated until its mass becomes constant. This new compound is called the weighing form.

Thermogram: A graph show mass loss as a function of temperature.

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Figure 12-7 Schematic of a thermobalance: A: beam; B: sample cup and holder; C: counterweight; D: lamp and photodiodes; E: coil; F: magnet; G: control amplifier; H: tare calculator; I: amplifier; and J: recorder.

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12B Calculation of results from gravimetric data

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Gravimetric factor: A gravimetric factor is an algebraic expression that converts grams of a compound into grams of a single element. It is the ratio of the formula weight (FW) of the substance being sought to that of the substance weighed.

Example 1. Fe is sought, Fe2O3 is weighed:

Example 2. Fe3O4 is sought, Fe2O3 is weighed:

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12C Applications of gravimetric methods12C -1 Inorganic Precipitating Agents

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12C-2 Reducing Agents(convert an analyte to its element form for weighing)

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12C-3 Organic Precipitating AgentsSome organic reagents react with metal ions then produce

insoluble coordination compounds.Reagents that form coordination compounds of this type

are called chelating agents, and their products are called chelates.

Those metal chelates are relatively nonpolar and have low solubilities in water.

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12C-4 Organic Functional Group Analysis

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12C-5 Volatilization Gravimetry

Figure 12-8 Apparatus for determining the sodium hydrogen carbonate content of antacid tablets by a gravimetric volatilization procedure.

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Homework (Due 2013/11/7)

Skoog 9th edition, Chapter 12, Questions and Problems12-112-212-412-512-7

End of Chapter 12

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