1. Gravimetric Analysis
By: Dr. O. Rajabi (Pharm.D.- Ph.D.) Associate Professor of Chemistry Department of Medicinal Chemistry Mashad University of Medical Sciences

2. gravi – metric (weighing - measure)
Definition: a precipitation or volatilization method based on the determination of weight of a substance of known composition that is chemically related to the analyte
analyte - chemical element or compound of interest

3. Reaction: aA + rR -----> AaRr ppt where:
a is # of moles of analyte A
r is # of moles of reagent R
AaRr is a pure, insoluble precipitate which we can dry and weigh or ignite to convert to something we can weigh
ppt=precipitate

4. 1914 Nobel Prize to T.W.Richards (Harvard University) for the atomic weights of Ag, Cl, and N
Richards and his group determined atomic weights of 55 of the 92 known elements using gravimetry

5. T.W.Richards:
“Every substance must be assumed to be impure, every reaction must be assumed to be incomplete, every method of measurement must be assumed to contain some constant error, until proof to the contrary can be obtained.”

6. 7 Steps in Gravimetric Analysis
Dry and weigh sample
Dissolve sample
Add precipitating reagent in excess
Coagulate precipitate usually by heating
Filtration-separate ppt from mother liquor
Wash precipitate (peptization)
Dry and weigh to constant weight

7. Precipitation:
Dissolve sample
Add ppt’ing reagent
Filter
Dry
Weigh

8. Suction Filtration Filter flask Buchner funnel Filter paper Glass frit
Filter adapter
Heavy-walled rubber tubing
Water aspirator

9. Mother liquor

10. Identify insoluble form
Two considerations:
Minimize errors due to limited precipitate solubility
Minimize errors due to precipitation process
Finite solubility of precipitate
ideally, Ksp = 0 (i.e., completely insoluble)
Some come close: ~10-38 for Fe(OH)3 ~10-50 for Ag2S
For AgCl, Ksp = 1.78 x 10-10

11. For a 0.1000 g AgCl precipitate in 200 ml H2O:
For example:
what would be the % error introduced in gravimetric analysis by the solubility of AgCl?
For a g AgCl precipitate in 200 ml H2O:
Note: Error is independent of mass of precipitate,  relative error will decrease as precipitate mass increases (i.e., 0.038% error for g AgCl)

12. Precipitation process
ideally, we’d like a precipitate that forms quickly. This implies:
Large, pure crystals
Low solubility
Easily filtered
Easily washed
How does precipitation occur?
As Ksp is exceeded, solution becomes “supersaturated”
At some point nucleation begins
At the same time, crystal growth begins

13. Two points to remember:
Crystal growth is independent of degree of supersaturation
Nucleation increases with degree of supersaturation
Minimization of supersaturation will produce the largest particles
Two particle size classes
Colloids
very small
difficult to handle experimentally
Crystals
“large” (~ 10-1 mm)
easily and rapidly filtered
high purity

14. What affects degree of supersaturation?
Ksp
Temperature
solubility  as T 
Reagent addition speed
slower addition gives precipitation a chance to begin at lower supersaturation levels
Solution concentration
low reagent concentration equals low supersaturation
But, even with the above precautions, we will often obtain colloid instead of a crystal!

15. Keys to successful colloid precipitation:
Add precipitant slowly and in slight excess
Digest precipitate (Heat, stir, sit)
What about crystalline precipitate?
Similar to colloids:
Dilute solution
Slow precipitant addition
Elevated temperature
Heat unstirred
Contaminants can escape from crystal lattice
Increase crystal bridges

16. Particle Size / Filterability
produce particles large enough to be 'caught‘
ideally, produce crystals
avoid colloidal suspension particle size = nm

18. Precipitate Formation
crystallization
nucleation: particles join to produce aggregates
crystal growth aggregate grows and 'fall out' of solution  
We want a few big chunks of precipitate! supersaturation: more solute than should be present in solution relative supersaturation: a measure of supersaturation, (Q-S)/S Q = actual solute concentration S = equilibrium solute concentration

19. Controlling Precipitation
Increase S
Increase temperature
Decrease Q
Dilute solution
Well mixed (stirring)

20. What Do We Get Out of Gravimetry?
% of analyte, % A
%A = weight of analyte x weight of sample

21. How Do We Get %A? G.F. = a FW[analyte] b FW[precipitate]
% A = weight of ppt x gravimetric factor (G.F.) x weight of sample
G.F. = a FW[analyte] b FW[precipitate]
G.F. = # gms of analyte per 1 gm ppt

22. Gravimetric Factor X apples + Y sugar = Z apple pies
What is this relationship in chemistry?

23. The Gravimetric Factor
G.F. = a FW[analyte] b FW[precipitate]
Analyte ppt G.F. CaO CaCO3 FeS BaSO4 UO2(NO3)2.6H2O U3O8 Cr2O3 Ag2CrO4

24. Analyte. ppt. G. F. CaO. CaCO3. CaO/CaCO3 FeS. BaSO4
Analyte ppt G.F. CaO CaCO3 CaO/CaCO3 FeS BaSO4 FeS/BaSO4 UO2(NO3)2 U3O8 3UO2(NO3)2/U3O8 Cr2O3 Ag2CrO4 Cr2O3/2Ag2CrO4

25. Problem
Consider a g sample containing 75% potassium sulfate (FW ) and 25% MSO4. The sample is dissolved and the sulfate is precipated as BaSO4 (FW ). If the BaSO4 ppt weighs , what is the atomic weight of M2+ in MSO4?
ANS: Mg2+

26. Answer
The hard part is setting up the correct equation (good stoichiometry skills are essential here!):
Rearranging and solving:

27. Problem
A mixture of mercurous chloride (FW ) and mercurous bromide (FW ) weighs 2.00 g. The mixture is quantitatively reduced to mercury metal (At wt ) which weighs 1.50 g. Calculate the % mercurous chloride and mercurous bromide in the original mixture.
ANS: g

28. Answer Again, important to set up correct equation:
Rearranging and solving:

29. Homogeneous Precipitation
(NH2)CO + 3 H2O + heat 
HCOOH + OH- + CO2 + 2 NH4+

30. High Electrolyte Concentration to Aid Precipitation
Excess charge on colloid creates ionic atmosphere around particle

31. Composition by Gravimetric Analysis
Ni2+ (aq) + H2DMG  Ni(DMG)2 + 2 H+
A g org sample produced g of bis(dimethylglyoximate) nickel (II) (FW = g/mol). Find the nickel content.
Explain how to create a large, filterable precipitate.

32. Combustion Analysis
Find the empirical formula for a mg organic sample that produced 6.97 mg of water and mg of carbon dioxide

33. Gravimetric Overview Simple Cheap Specific Timely (1/2 day) Accurate
Glassware
Reagents
ovens, etc.
Balances
Specific
Timely (1/2 day)
Accurate
Precise ( %)
Sensitive