1. Chapter 12: Gravimetric Methods of Analysis

2. 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.

3. 12A Precipitation gravimetry
The 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:
After cooling, the precipitate is weighed and the mass is determined.

4. 12A-1 Properties of Precipitates and Precipitating Reagents
A gravimetric precipitating agent should react specifically or at least selectively with the analyte and give precipitates that is:
Enough particle size for retaining on filter paper
High purity (free of contaminants)
Low solubility that no significant loss of the analyte occurs during filtration and washing
Unreactive with air (stable)
Known stoichimetric composition
* Specific reagents: react only with a single chemical species.
Selective reagents: react with a limited number of species.

5. 12A-2 Particle Size and Filterability of Precipitates
Characteristics of Ion, colloid and particle
Name
Diameter
Characteristics
Ion
~10-8 cm
Dissolved
Colloid
10-7~10-4 cm
Suspended
Crystalline
>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.
Nucleation: 成核

6. Relative supersaturation (RSS) (von Weimann's theory):
Q: the concentration of the solute at any instant
S: the expected concentration solute at equilibrium
RSS
Process
Result
Large
Nucleation dominate
Smaller particles
Small
Particle growth dominate
Larger Particle

7. 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

8. 12A-2 Colloidal Precipitates
Colloidal 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.
Amorphous: 非晶形的
Coagulation: 凝聚

9. 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)
Neutral aqueous solution
Counter-ion layer
(- charged)
Adsorption: 吸附
Absorption: 吸收
* 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.

10. 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.

11. Coagulation of Colloids
Coagulation 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.
A coagulated colloid
* Digestion (standard for some time) result in a denser mass precipitate.

12. Peptization of Colloids
Peptization: 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.
Peptization: 膠溶

13. 12-4 Crystalline Precipitates
Crystalline 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.

14. 12-5 Coprecipitation
Coprecipitation: 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.

15. 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

16. 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

17. Problem solving: Digestion
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.)
Occlusion: 包藏

18. Problem solving: Digestion
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
mechanical entrapment: 機制捕捉

19. 12A-6 Precipitation from Homogeneous Solution
Homogeneous 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 precipitant
CO(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) + SO42–(aq)
* Generation rate can be controlled by pH

21. 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.

22. 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.

23. 12B Calculation of results from gravimetric data

26. 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:

29. 12C Applications of gravimetric methods
12C -1 Inorganic Precipitating Agents

30. 12C-2 Reducing Agents
(convert an analyte to its element form for weighing)

31. 12C-3 Organic Precipitating Agents
Some 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.

33. 12C-4 Organic Functional Group Analysis

34. 12C-5 Volatilization Gravimetry
Figure 12-8 Apparatus for determining the sodium hydrogen carbonate content of antacid tablets by a gravimetric volatilization procedure.

35. Homework (Due 2013/11/7) End of Chapter 12
Skoog 9th edition, Chapter 12, Questions and Problems
12-1
12-2
12-4
12-5
12-7
End of Chapter 12