Gravimetry Analysis Part 2: Steps of Gravimetric Precipitation and Question and Answers

STEPS INVOLVED IN GRAVIMETRIC PRECIPITATION :-

1.  PREPARATION OF THE SAMPLE SOLUTION.
2. PRECIPITATION PROCESS
3. DIGESTION OR OSTWALD RIPENING
4. FILTRATION
5. WASHING
6. DRYING
7. IGNITING
8. WEIGHING
9. CALCULATION

• SAMPLE SOLUTION :- sample solution is prepared generally dilute solution is preferred. Adjustment of the volume, PH, and getting the desired property of the solution.

• PRECIPITATION FORMATION :-

Precipitation formation has three stages  1. Super saturation :-

It is defined as solution that contain more of the disolved material than could be dissolved by the solvent under normal circumstances.

It is first step of preparation process. After addition of first drop of precipitating agent supersaturation is occur.

Solute/ Analyte conc >> Equilibrium solubility.

2. Nucleation :-

Nucleation process is starting after supersaturation.

In this step aggregation of ions/ element to form sub – micro nuclei.

3. Precipitate particle growth :-

Sample amount of analyte will be added to nuclei that leading to particle growth.

FACTOR DETERMINING SUCCESSFUL PRECIPITATION :-

1.  Nature of precipitation :- physical nature of PPT will be determined by the relatively rate of Nucleation and particle growth.
2. Rate of nuclei formation :- more number of nuclei are formed that means small ppt are produced.
3. Von-weimarn :- Particle the of precipitate is inversely proportional to the relatively super saturation of the solution during precipitation process.
4. Rate of ppt formation :-  if rate of ppt formation is more than rate of nuclei formation so coarse ppt formed.

CONTAMINATION OF PRECIPITATES :-

1. Post Precipitation :- Deposition of impurity after PPT.

Ex. Ca- oxalate ppt in presence of Mg ion. After some time, Mg oxalate formed deposit on Ca- oxalate surface.

2. Co- Precipitation :- when two or more than two ion are simultaneously precipitated out in the sample solution, then the condition is known as the co Precipitation.

• Digestion :- 

1. The Preciptate is left hot ( below boiling point ) for 30 -60 minutes in order to digest the particle.
2. This digestion involves dissolution of small particle and re-precipitation on larger once when cooling slowly
3. This process is known as the OSTWALD RIPENING. Digestion is highly useful for colloidal precipitation.

MCQ

1. Which sentence is false about preparation of the solution ?

A. It has required PH.

B. It is diluted solution.

C. It is concentrated solution.

D. All of the above

2. What is a supersaturation ?

A. Solution containing low amount of solute

B. Solution containing equilibrium amount of solute

C. Solution containing more amount of solute than normal circumstances.

D. None of this.

3. In which step, ions or element are aggregated in Gravimetric analysis?

A. Supersaturation

B. Nucleation

C. Particle growth

D. None of the above

4. For gravimetric analysis Rate of Nucleation is fast in?

A. Small ppt

B. Large ppt

C. A and B

D. None of the above

5. When more than two ions in solution and simultaneously second ion is also precipitated it is called a

A. Co precipitation

B. Post precipitation

C. A and B

D. None of the above

6. OSTWALD RIPENING is

A. Re precipitation

B. Dissolved small ppt

C. Produced larger ppt

D. All of the above

Answer key

1. C

2. C

3. B

4. A

5. A

6. D

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