1. Gravimetric Analysis and Precipitation Equilibria
Dr. A.K.M. Shafiqul Islam
&
Dr. Zarina Zakaria

2. Introduction The term gravimetric pertains to a Weight Measurement.
Gravimetric method is one in which the analysis is completed by a weighing operation.
Gravimetric Analysis is a group of analytical methods in which the amount of analyte is determined by the measurement of the mass of a pure substance containing the analyte.
Gravimetric Methods can also be defined as quantitative methods based on the determining the mass of a pure compound to which the analyte is chemically related.

3. There are two main types of gravimetric analyses
A) Precipitation
A chemical reaction causes the formation of a sparingly soluble substance that precipitates from solution is filtered, washed, purified (if necessary) and weighed.
B) Volatilisation
In this method the analyte or its decomposition products are volatilised and then collected and weighed, or alternatively, the mass of the volatilised product is determined indirectly by the loss of mass of the sample.

4. Example for Precipitation:-
Calcium can be determined gravimetrically by precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide.
Ca C2O42- →CaC2O4
CaC2O4 → CaO + CO2 + CO
The precipitate thus obtained are weighed and the mass of calcium oxide is determined.

5. Example for Volatilisation:-
The analyte or its decomposition products are volatilised at a suitable temperature. The volatile product is then collected and weighed, i.e. the mass of the product is indirectly determined from the loss in mass of the sample.
Example
Water can be separated from most inorganic compounds by ignition, the evolved water can then be absorbed on any one of several solid desiccants. The weight of water evolved may be calculated from the gain in weight of the absorbent.

6. Other Desirable Properties
Not all insoluble precipitates are well suited for gravimetric analysis.
It is important to consider what properties are required in order that a precipitate be applicable for a quantitative precipitation method:-
Solubility
Filterability
Chemical Composition
Other Desirable Properties

7. Other Desirable Properties
Solubility
The product must be sufficiently insoluble to prevent the loss of weight.
Filterability
Precipitate formed should be adoptable to simple and rapid filteration methods.
Chemical Composition
The product must be of known chemical composition.
Other Desirable Properties
Other factors effecting the stability and purity of the precipitate.

8. For a successful determination in gravimetric analysis the following criteria should be met :-
The desired substance must be completely precipitated. In most determination the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the common ion effect, this further decrease the solubility of the precipitate.
E.g. When Ag+ is precipitated out by addition of Cl-
Ag+ + Cl- = AgCl
The low solubility of AgCl is reduced further by the excess of Cl- which is added force to the reaction to proceed towards right side.

9. For a successful determination in gravimetric analysis the following criteria should be met :-
(2)The weighed form of the product should be of known composition.
(3)The product should be pure and easily filtered. It is usually difficult to obtain a product which is pure or which is free from impurities. This could be reduced by careful precipitation and sufficient washing.

10. Gravimetric Analysis
Gravimetric analysis is potentially more accurate and more precise than volumetric analysis.
Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis.
But there are potential problems with gravimetric analysis that must be avoided to get good results.
Proper lab technique is critical

11. Steps in a Gravimetric Analysis
Preparation of the solution
Precipitation
Digestion
Filtration
Washing
Drying or ignition
Weighing
Calculation

12. 1. Preparation of analyte solution
Gravimetric analysis usually involves precipitation of analyte from solution.
May need to separate potential interferences before precipitating analyte
May require pH and/or other adjustments to maximize precipitate formation
When possible, select precipitating agents that are selective (or specific, if possible).

13. 2. Precipitation The precipitate should
Be insoluble, but not too insoluble
Have large crystals
Easier to filter large crystals
Be free of contaminants
The smaller the surface area the better
The smaller the solubility, S, the greater the tendency to form lots of small crystals that trap contaminants and are difficult to filter
The greater the solubility, the more analyte is left unprecipitated
We need to strike a balance
(but not the analytical balance, please)

14. The von Weimarn Ratio von Weimarn ratio = (Q – S)/S
A measure of relative supersaturation
The lower the better
If high, get excessive nucleation, lots of small crystals, large surface area
If low, get larger, fewer crystals, small surface area
Q = concentration of mixed reactants before precipitation
Keep it low by using dilute solutions, stir mixture well, add reactants slowly
S = solubility of precipitate at equilibrium
Keep it high with high temperatures, adjusting pH
Can lower S later by cooling mixture after crystals have formed

15. Digestion of the Precipitate
Very small crystal with a large specific surface area have a higher surface energy and a higher apparent solubility than large crystals. This is an initial rate phenomenon and does not represent the equilibrium condition, and it is one consequence of heterogeneous equilibria. When the precipitate is allowed to stand in the presence of mother liquor (the solution from which it was precipitated), the larger crystal grow at the expanse of the small ones. This process is called digestion, or Ostwald ripening. The small particles tend to dissolve and precipitate on the surfaces of the larger crystals

16. Digestion of Crystalline Precipitates –(1)
When a crystalline precipitate is allowed to stand in contact with the solution from which it was precipitated (mother liquor), it is more easily filtered and requires a less dense filtering medium than the freshly formed precipitate.
This improvement in filterability is accelarated by heating, common practice is to treat crystalline precipitates in this manner prior to filtration. The process is called DIGESTION.
Considerable experimental evidence indicates that the digestion process consists of a cementing together of crystals. The resulting aggregates are appreciably larger than the individual crystals and are therefore more easily retained.

17. Digestion of Crystalline Precipitates –(2)
Ions making up a precipitate are in dynamic equilibrium with their counterparts in the solution. As a result, solution and reprecipitation of the solid take place constantly.
The latter process can occur in such a way as to form bridges between adjacent solid particles thus leading to crystalline aggregates.
At elevated temperatures, where the solubility is greater, the process would be expected to take place at an accelerated rate, this is in keeping with the experimental observation that the effectiveness of digestion is greater at higher temperatures.

18. Digestion of Crystalline Precipitates –(3)
The improved filterability of precipitates after digestion has also been attributed wholly or in part to growth of large crystals at the expense of small ones. Such process might occur since theoretically small crystals should be more soluble than large ones.
Therefore, digestion of a mixture of crystals results in the disappearance of the more soluble smaller particles and growth of the larger and more insoluble ones.
There was however some experimental evidence that the very smallest crystals of the solid may have disappeared during the digestion.

19. Impurities in Precipitation
Precipitates tend to carry down from the solution constituents that are not normally soluble, causing the precipitate to become contaminated. This process is called co-precipitation.

20. Co-precipitation
At least four types of co-precipitation can be recognized
Occlusion and Inclusion
Surface Adsorption
Isomorphous Replacement
Postprecipitation

21. a. Occlusion and Inclusion
Occlusion is a type of coprecipitation in which the soluble impurity is enclosed or entrapped within the crystal structure of the solid. As such it acts as a seat of imperfection within the crystal.
Generally, occluded substances are distributed in a non homogeneous fashion throughout the crystal.
In addition, the amount of occluded substance is greatly influenced by the conditions and manner of precipitate formation.

22. a. Occlusion and Inclusion
Inclusion occurs when ions, generally of large size or charge, are trapped within the crystal lattice (isomorphorous inclusion, as with K+ in NH4MgPO4 precipitation)
Occlusion and inclusion impurities are difficult to remove. Digestion is effective in reducing the quantity of entrapped contaminant.

23. b. Surface Adsorption
All precipitates carry down soluble impurities by adsorption
on their surfaces.
However, only where the specific surface areas are great does
coprecipitation of this type have an appreciable effect on the
weight of the solid.
Therefore contamination by surface adsorption will be
important only when dealing with colloidal precipitates.
Colloidal particles formed under analytical conditions will
always have adsorbed on their surfaces one of the constituent
ions of the precipitate.

24. b. Surface Adsorption Let’s use AgCl as an example
Primary adsorption layer
Ion in excess adsorbs on the surface of the precipitate
Crystal becomes positive if Ag+ is in excess
Crystal becomes negative if Cl- is in excess
Counter-ion layer (counterlayer)
Charged crystal attracts ions of opposite charge
For example, NO3- attracted to positive crystal, results in a layer of silver nitrate around the AgCl
For example, Na+ attracted to negative crystal, results in a layer of NaCl around the AgCl
The more dilute the solution, the thicker is the counter-ion layer

25. Colloidal Precipitates (Such as AgCl
When S is very small (very insoluble precipitates), cannot avoid high degree of nucleation and very small crystals
Colloidal particles 1 to 100 μm in diameter
Surface area as high as 3,000 ft2/g
High potential for surface adsorption
Other problems besides surface adsorption
Colloidal precipitates cause difficulties in filtering and washing as well

26. Colloidal Particle in Solution
Ag+ H+
NO3−
Counter-ion
layer of solution
with excess anions
Homogeneous solution
(charges balanced)
Colloidal solid
Positively charged primary adsorption
layer on colloidal particle
Coagulation: aggregation of colloidal particles into larger filterable masses

27. b. Surface Adsorption
Several techniques are adopted to reduce the magnitude of error arising from adsorption of contaminants on a colloidal precipitate.
Among these are the use of elevated temperatures and dilute solutions, digestion, washing and reprecipitation.

28. c. Isomorphous Replacement
Two compound are said to be isomorphous if they have the same type of formula and crystallize in similar geometric forms. When their lattice dimension are about same, one ion can replace another in a crystal, a resulting in a mixed crystal. This process is called isomorphous replacement.
For example, in the precipitation of Mg+2 as magnesium ammonium phosphate, K+ has nearly the same ionic size as NH4+ and can be replaces it to form magnesium potassium phosphate.
Isomorphous replacement is very serious and very little can be done about it.

29. d. Post Precipitation
A type of contamination wherein the impurity is an insoluble compound that precipitates after all or a major part of the analytical precipitate has formed.
Post precipitation arises only when the attempt is made to separate two ions on the basis of the rate at which they precipitate.
This will come down if the solution is allowed to stand too long before being filtered.

30. 3. Digestion (Ostwald Ripening)
Let precipitate stand in contact with solution, usually at high temperature
Large crystals (small surface area) have lower free energy than small crystals (large surface area)
Digestion makes larger crystals, reduces surface contamination, reduces crystal imperfections
Digestion coagulates a colloid (causes the particles to agglomerate, or stick together), but surface area does not decrease as much as if larger crystals actually grew

31. Hydrophilic vs. Hydrophobic Colloids
Hydrophilic (water-loving) colloids form gels and do not coagulate well
Hydroxides (Al(OH)3, Fe(OH)3, for example) often form gels and are very difficult to filter
Hydrophobic (water-fearing) colloids coagulate readily
Luckily, AgCl is hydrophobic
Gravimetric analysis of silver or chloride by AgCl precipitation is effective in spite of colloidal nature of precipitate

32. 4. Filtration
Sintered glass crucibles are used to filter the precipitates. These are marked as fine, medium and coarse porosities (marked f, m and c).
The crucibles first cleaned thoroughly and then subjected to the same regimen of heating and cooling as that required for the precipitate. This process is repeated until constant mass has been achieved, that is, until consecutive weighings differ by 0.3 mg or less.
Colloidal precipitates present filtering problems if particles are too small
Can plug filter paper or glass or pass right through the filter if not coagulated well
Hydrophobic colloids generally filter better than hydrophilic colloids

33. 5. Washing Removes the mother liquor
May also remove some of the coprecipitated compounds
Can cause peptization of colloids
Diluting the counter-ion layer causes it to get larger, forcing coagulated colloidal particles apart
And THERE THEY GO right through the filter
Wash with a solution of a volatile electrolyte that will be removed in drying step

34. 6. Drying or Igniting
Dry the precipitate to remove water and volatile electrolytes from wash solution
Ignition (very high-temperature drying) converts precipitates to compounds more suitable for weighing
Removes water of hydration
Converts hygroscopic compound to non-hygroscopic compound
e.g., MgNH4PO4 is decomposed to the pyrophosphate Mg2P2O7 by heating at 900 oC
Sometimes it even converts the filter paper to ash

35. Obtaining a Final Known Composition
Ignition is often used to drive off water or give an oxide form
Thermogravimetry is a form of gravimetric analysis where mass is measured as a function of temperature:

36. Thermogravimetry

37. Combustion Analysis
Absorbs water
Absorbs CO2
Find the empirical formula for a mg organic sample that produced 6.97 mg of water and mg of carbon dioxide

38. 7. Weighing Properly calibrated analytical balance
Good weighing technique
Avoid static electricity
Review important points on p. 51 periodically
And now, step 8, the calculations
Chem 105 stoichiometry, anyone?

40. Organic Precipitates
Organic precipitating agents have the advantages of giving precipitates with very solubility in water and a favorable gravimetric factor.
Most of them are chelating agents that forms slightly insoluble, uncharged chelates with the metal ions.

41. Organic Precipitates

42. Gravimetric Analysis: Weight Relationship
From the weight of the precipitate formed in a gravimetric analysis, we can calculate the weight of the analyte from the weight relationship of the precipitate and analyte.
The weight of the analyte is calculated from the weight of the precipitate.

43. E.g. If we consider calculating the percentage of Cl2 by weighing
the AgCl precipitate, then
Cl > 2AgCl (we get 2 moles of AgCl from 1 mol Cl2)
Calculation of the corresponding weight of Cl2 that would be
contained in the AgCl sample will be:-
gm Cl2 / gm AgCl = [F. Wt. Cl2 ] / [F. Wt. AgCl
x [1 mol Cl2 / 2 mol AgCl]
 gm Cl2 = [gm AgCl] x [ gm Cl2 / 1 mol Cl2] x
[1 mol AgCl / gm AgCl] x
[1 mol Cl2 / 2 mol AgCl]

44. We can consider Gravimetric Factor (GF) as: -
Then,
gm Cl2 = gm AgCl x GF(gm Cl2/gm AgCl)
GF = f. wt. of substance sought / f. wt. of substance weighed
x a/b (mol sought / mole weighed)

45. #1 A gm sample of commercial phosphate detergent was ignited at a red heat to destroy the organic matter. The residue was then taken up in hot HCl which converted P to H3PO4. The phosphate was precipitated with Mg2+ followed by aqueous NH3 to form MgNH4PO4.6H2O. After being filtered and washed, the precipitate was converted to Mg2P2O7 (FW=222.57) by ignition at 1000º C. This residue weighed gm. Calculate the percent P (FW = ) in the sample.
Gravimetric factor, GF = f. wt. of substance sought / f. wt. of substance
substance weighed x a/b (mol sought / mol weighed)
GF = gm / gm x 2/1 = 0.278
Amount of P = GF (0.278) x Wt. of precipitate (0.2161) = gm
 % P = gm P / gm sample x 100 = 17.11%

46. #2 A 0. 4960 gm sample of a CaCO3 is dissolved in an acidic solution
#2 A gm sample of a CaCO3 is dissolved in an acidic solution. The calcium is precipitated as CaC2O4.H2O and the dry precipitate is found to weight gm. What is the percentage of CaO in the sample? (FW CaO = 56.08; FW CaC2O4.H2O = )
GF = gm / gm X 1/1 =
Wt of CaO = Wt. of ppt. X GF = gm X
= gm
% of CaO in CaCO3 = gm / gm x 100
= 47.87%

47. Precipitation Reactions
Two fully dissolved and dissociated compounds are mixed and a “precipitate” (solid product) forms
The mixture becomes opaque due to the formation of solid

48. Precipitation Reactions
Barium Iodate in Water:
At Equilibrium:
Solubility Product (in general):
saturated solution contains excess solid
Ksp = [Ba2+][IO3−]2
for salt MxAy: Ksp = [M]x[A]y

49. Precipitation Reactions
How many grams of Ba(IO3)2 dissolve in 500 mL of water
At 25 C if Ksp = 1.57 × 10−9 S S 2S
Ksp = [Ba2+][IO3−]2 = [S][2S]2 = × 10−9
4S3 = × 10−9
S (molar solubility of Ba(IO3)2) = x 10−4 mol L−1
You must learn how to use your calculator!

50. Precipitation Reactions
How many grams of Ba(IO3)2 dissolve in 500 mL of water
At 25 C if Ksp = 1.57 × 10−9
S = 7.32 × 10−4
mmol Ba(IO3)2 = S × Vml = (7.32 × 10−4)(500 mL) = mmol
Wt. Ba(IO3)2 = mmol × 487 mg mmol−1 = 178 mg  g
This could cause a significant error in a gravimetric analysis

51. Common Ion Effect
The presence in solution of an excess of one ion from a slightly soluble ionic substance depresses its solubility (more complete precipitation).
How many grams of Ba(IO3)2 dissolve in 500 mL of M Ba(NO3)2 at 25 C if Ksp = 1.57 × 10−9 S S 2S
[Ba2+][IO3−]2 = (S )(2S)2 = × 10−9
Assumption: S << M
(0.0200)(2S)2 = × 10−9  S = 1.40 × 10−4 M
Wt. Ba(IO3)2 = 1.40 × 10−4 M × 500 mL × 487 mg mmol−1 = mg

52. Theory of Precipitation
Precipitation occurs when the ion product exceeds the solubility product
Two Steps in Precipitation
Nucleation: small groups of ions form stable solid masses (nuclei)
Particle Growth: ions from solution add on to nuclei to form particles
For MxAy: CMx × CAy > Ksp

53. Theory of Precipitation
The amount by which the ion product exceeds the solubility product is expressed qualitatively as supersaturation.
Large supersaturation favors nucleation
Small supersaturation favors particle growth
Supersaturation = Q − S
Q: instantaneous concentration
S: equilibrium concentration (molar solubility)

54. Minimizing Supersaturation
Supersaturation = Q − S
Elevated Temperature (increase S)
Dilute Solution (decrease Q)
Slow Addition, good stirring (decrease Q)

55. TUTORIAL
Read Gary text book on 314 to 320 and understand the topics. Extract the objective/purpose of steps in gravimetrics analysis.
Do questions number 16, 22, 27 and 32 complete with the calculations.