1. Steps in Gravimetric Analysis
Lecture 17
Gravimetric Analysis
Steps in Gravimetric Analysis

2. Gravimetric Analysis
In this technique, the analyte is converted to an insoluble form which can then be washed, dried, and weighed in order to determine the concentration of the analyte in the original solution. Gravimetry is applied to samples where a good precipitating agent is available. The precipitate should be quantitative, easily washed and filtered and of a suitable quantity for accurate weighing.

3. Therefore, gravimetry is regarded as a macro analytical technique
Therefore, gravimetry is regarded as a macro analytical technique. However, it is considered, when appropriately done, one of the most accurate analytical techniques. Also, Gravimetry is one of a few analytical methods that do not require standard solutions as the weight of precipitate is the only important parameter in analyte determination.

4. Properties of Precipitates
1. Low solubility
No significant loss of the analyte occurs during filtration and washing
2. High purity
3. Known composition
After drying and ignition, known chemical composition
4. Easy to separate from reaction mixture
5. Stable under atmospheric conditions 4

5. Some interfering species
Species analyzed
Precipitated form
Form weighed
Some interfering species K+
KB(C6H5)4
same
NH4+, Ag+, Hg2+, Tl+, Rb+, Cs+
Mg2+
Mg(NH4)PO4.6H2O
Mg2P2O7
Many metals except Na+ and K+
Ca2+
CaC2O4.H2O
CaCO3 or CaO
Many metals except Mg2+, Na+, or K+
Ba2+
BaSO4
BaSO4 or BaS
Ca2+, Al3+, Cr3+, Fe3+, Sr2+, Pb2+
Cr3+
PbCrO4
Ag+, NH4+
Mn2+
Mn(NH4)PO4.H2O
Mn2P2O7
Many metals
Ni2+
Ni(dimethylglyoximate)2
Pd2+, Pt2+, Bi3+, Au3+

8. Steps in a Gravimetric Analysis
After appropriate dissolution of the sample, the following steps should be followed for a successful gravimetric procedure:
1. Preparation of the Solution: This may involve several steps including adjustment of the pH of the solution in order for the precipitate to occur quantitatively and get a precipitate of desired properties, removing interferences, adjusting the volume of the sample to suit the amount of precipitating agent to be added.

9. 2. Precipitation: This requires addition of a precipitating agent solution to the sample solution. Upon addition of the first drops of the precipitating agent, supersaturation occurs, and then nucleation starts to occur where every few molecules of precipitate aggregate together forming a nucleous. At this point, addition of extra precipitating agent will either form new nuclei or will build up on existing nuclei to give a precipitate. This can be predicted by Von Weimarn ratio where, according to this relation the particle size is inversely proportional to a quantity called the relative supersaturation where:
Relative Supersaturation = (Q – S)/S

10. The Q is the concentration of reactants before precipitation, S is the solubility of precipitate in the medium from which it is being precipitated. Therefore, in order to get particle growth instead of further nucleation we need to make the relative supersaturation ratio as small as possible.
The optimum conditions for precipitation which make the supersaturation low are:

11. Precipitation using dilute solutions to decrease Q
Slow addition of precipitating agent to keep Q as low as possible
Stirring the solution during addition of precipitating agent to avoid concentration sites and keep Q low
Increase solubility by precipitation from hot solution
Adjust the pH in order to increase S but not a too much increase as we do not want to lose precipitate by dissolution
Usually add a little excess of the precipitating agent for quantitative precipitation and check for completeness of the precipitation

12. 7. Precipitation from Homogeneous Solution
In order to make Q minimum we can, in some situations, generate the precipitating agent in the precipitation medium rather then adding it. For example, in order to precipitate iron as the hydroxide, we dissolve urea in the sample. Heating of the solution generates hydroxide ions from the hydrolysis of urea. Hydroxide ions are generated at all points in solution and thus there are no sites of concentration. We can also adjust the rate of urea hydrolysis and thus control the hydroxide generation rate. This type of procedure can be very advantageous in case of colloidal precipitates.

13. 3. Digestion of the Precipitate: The precipitate is left hot (below boiling) for 30 min to 1 hour in order for the particles to be digested. Digestion involves dissolution of small particles and reprecipitation on larger ones resulting in particle growth and better precipitate characteristics. This process is called Ostwald ripening. An important advantage of digestion is observed for colloidal precipitates where large amounts of adsorbed ions cover the huge area of the precipitate. Digestion forces the small colloidal particles to agglomerate which decreases their surface area and thus adsorption.

14. Ostwald ripening During digestion at elevated temperature:
Small particles tend to dissolve and reprecipitate on larger ones.
Individual particles agglomerate.
Adsorbed impurities tend to go into solution.
Ostwald ripening

15. Lecture 18
Gravimetric Analysis, Cont….
Gravimetric Calculations

16. 4. Filtering and Washing the Precipitate: It is crucial to wash the precipitate very well in order to remove all adsorbed species which will add to weight of precipitate. One should be careful not to use too much water since part of the precipitate may be lost. Also, in case of colloidal precipitates we should not use water as a washing solution since peptization would occur. In such situations dilute nitric acid, ammonium nitrate, or dilute acetic acid may be used. Usually, coagulated particles return to the colloidal state if washed with water, a process called peptization.

17. Washing of Precipitates

18. Adsorption and Particle size
You should know that adsorption is a major problem in gravimetry in case of colloidal precipitate since a precipitate tends to adsorb its own ions present in excess, forming what is called a primary ion layer which attracts ions from solution forming a secondary or a counter ion layer. Individual particles repel each other keeping the colloidal properties of the precipitate. Particle coagulation can be forced by either digestion or addition of a high concentration of a diverse ions strong electrolytic solution in order to shield the charges on colloidal particles and force agglomeration.

19. Representation of silver chloride colloidal particle
and adsorptive layers when Cl- is in excess.

21. Cl- or Ag+ adsorbs on the particles when in excess (primary ion layer).
A counter layer of cations forms. Washing with water will dilute the counter ion layer and the primary layer charge causes the particles to revert to the colloidal state (peptization) So we wash with an electrolyte that can be volatilized on heating (HNO3).

22. 5. Drying and Ignition: The purpose of drying (heating at about oC in an oven) or ignition in a muffle furnace at temperatures ranging from oC is to get a material with exactly known chemical structure so that the amount of analyte can be accurately determined.

28. 6. Calculations:
The last step in the gravimetric procedure is performing the analytical calculations, as will be seen shortly.

29. Impurities in Precipitates
1. Occlusion: Some constituents of the precipitation medium may be trapped in the crystal structure resulting in positive or negative errors. The trapped materials can be water, analyte ions, precipitating agent ions, or other constituents in the medium. Slow addition of precipitating agent and stirring may avoid occlusion but if it does occur, dissolution of precipitate and reprecipitation may have to be done.

30. 2. Inclusion: If the precipitation medium contains ions of the same charge and size as one forming the crystal structure of the precipitate, this extraneous ion can replace an ion from the precipitate in the crystal structure. For example, in the precipitation of NH4MgPO4 in presence of K+, ammonium leaves the crystal magnesium ammonium phosphate and is replaced by K+ since both have the same charge and size. However, the FW fro NH4+ is 18 while that of K+ is 39. In this case a positive error occurs as the weight of precipitate will be larger when K+ replaces NH4+. In other situations we may get a negative error when the FW of the included species is less than the original replaced species

32. 3. Surface Adsorption: This always results in positive errors in gravimetric procedures. See previous discussion on colloidal precipitates.
4. Postprecipitation: In cases where there are ions other than analyte ions which form precipitates with the precipitating agent but at much slower rate then analyte, and if the precipitate of the analyte is left for a long time without filtration then the other ions start forming a precipitate over the original precipitate leading to positive error. Examples include precipitation of copper as the sulfide in presence of zinc. Copper sulfide is formed first but if not directly filtered, zinc sulfide starts to precipitate on the top of it. The same is observed in the precipitation of calcium as the oxalate in presence of magnesium.

33. Name Student No. In the reaction 2A + B D C Calculate the concentrations of A, B, and C at equilibrium when 0.2 mol of A are mixed with 0.6 mol of B. The equilibrium constant is 2.5*10-7.

34. Gravimetric Calculations
The point here is to find the weight of analyte from the weight of precipitate. We can use the concepts discussed previously in stoichiometric calculations but let us learn something else. Assume Cl2 is to be precipitated as AgCl, then we can write a stoichiometric factor reading as follows: one mole of Cl2 gives 2 moles of AgCl. We can define a quantity called the gravimetric factor (GF) where:
GF(analyte) = (a/b)*(FW analyte/FW ppt)
a,b are stoichiometric coefficients
GF for Cl2 = (1 mol Cl2/2 mol AgCl) * (FW Cl2/FW AgCl)

35. (mg Cl2/FW Cl2) = 1/2 (mg AgCl/FW AgCl)
Weight of substance sought = weight of precipitate x GF
One can also consider the problem by looking at the number of mmoles of analyte in terms of the mmoles of the precipitate where for the precipitation of Cl2 as AgCl, we can write
Cl2 D 2 AgCl
mmol Cl2 = 1/2 mmol AgCl
(mg Cl2/FW Cl2) = 1/2 (mg AgCl/FW AgCl)

36. Example
Calculate the mg analyte to mg precipitate for the following: P (at wt =30.97) in Ag3PO4 (FW = ), Bi2S3 (FW ) in BaSO4 (FW = )
Solution
P D Ag3PO4
mmol P = mmol Ag3PO4
Mg P/30.97 = mg Ag3PO4/711.22
Mg P/mg Ag3PO4 = 30.97/ =

37. Bi2S3 D 3 BaSO4
mmol Bi2S3 = 1/3 mmol BaSO4
mg Bi2S3/FW Bi2S3 = 1/3 mg BaSO4/FW BaSO4
mg Bi2S3/ = 1/3 mg BaSO4/233.40
mg Bi2S3/ BaSO4 = 1/3 (514.15/ ) =

38. First we set the mol relationship between analyte and precipitate
Phosphate in a g sample was precipitated giving g of (NH4)2PO4.12 MoO3 (FW = ). Find percentage P (at wt = 30.97) and percentage P2O5 (FW = ) in the sample.
Solution
First we set the mol relationship between analyte and precipitate
P D (NH4)2PO4.12 MoO3
mmol P = mmol (NH4)2PO4.12 MoO3
mg P/at wt P = mg (NH4)2PO4.12 MoO3/ FW (NH4)2PO4.12 MoO3
mg P = at wt P x (mg (NH4)2PO4.12 MoO3/ FW (NH4)2PO4.12 MoO3)
mg P = (1.1682x103 / ) = mg
% P = (19.280/271.1) x 100 = 7.111%

39. The same procedure is applied for finding the percentage of P2O5
P2O5 D 2 (NH4)2PO4.12 MoO3
mmol P2O5 = 1/2 (NH4)2PO4.12 MoO3
mg P2O5/FW P2O5 = 1/2 (mg (NH4)2PO4.12 MoO3/ FW (NH4)2PO4.12 MoO3)
mg P2O5 = 1/2 x FW P2O5 x (mg (NH4)2PO4.12 MoO3/ FW (NH4)2PO4.12 MoO3)
mg P2O5 = 1/2 x (1.1682x103/1876.5) = mg
% P2O5 = (44.185/271.1) x 100 = 16.30%

40. Gravimetric Calculations Precipitation Equilibria
Lecture 19
Gravimetric Calculations
Precipitation Equilibria

41. mg Mn2O3 / FW Mn2O3 = 3/2 (mg Mn3O4/FW Mn3O4)
Manganese in a 1.52 g sample was precipitated as Mn3O4 (FW = 228.8) weighing g. Find percentage Mn2O3 (FW = 157.9) and Mn (at wt = 54.94) in the sample.
Solution
3 Mn2O3 D 2 Mn3O4
mmol Mn2O3 = 3/2 mmol Mn3O4
mg Mn2O3 / FW Mn2O3 = 3/2 (mg Mn3O4/FW Mn3O4)
mg Mn2O3 = 3/2 FW Mn2O3 (mg Mn3O4/FW Mn3O4)
mg Mn2O3 = 3/2 x ( 126/228.8) = 130 mg
% Mn2O3 = (130/1520) x 100 = 8.58%

42. The same idea is applied for the determination of Mn in the sample
3 Mn D Mn3O4
mmol Mn = 3 mmol Mn3O4
mg Mn / at wt Mn = 3 (mg Mn3O4/FW Mn3O4)
mg Mn = 3 at wt Mn (mg Mn3O4/FW Mn3O4)
mg Mn = 3 x ( 126/228.8) = 90.8 mg
% Mn = (90.8/1520) x 100 = 5.97%

43. mg S/at wt S = mg BaSO4/FW BaSO4
What weight of sulfur (FW = ) ore which should be taken so that the weight of BaSO4 (FW = ) precipitate will be equal to half of the percentage sulfur in the sample.
Solution
S D BaSO4
mmol S = mmol BaSO4
mg S/at wt S = mg BaSO4/FW BaSO4
mg S = at wt S x ( mg BaSO4 / FW BaSO4)
mg S = x (1/2 %S/233.40)
mg S = %S
%S = (mg S/mg sample) x 100
%S = %S/mg sample) x 100
mg sample = %S x 100 /%S = mg

44. A mixture containing only FeCl3 (FW = 162. 2) and AlCl3 (FW = 133
A mixture containing only FeCl3 (FW = 162.2) and AlCl3 (FW = ) weighs 5.95 g. The chlorides are converted to hydroxides and ignited to Fe2O3 (FW = 159.7) and Al2O3 (FW = ). The oxide mixture weighs 2.62 g. Calculate the percentage Fe (at wt = 55.85) and Al (at wt = 26.98) in the sample.
Solution
 g FeCl3 + g AlCl3 = 5.95
g Fe2O3 + g Al2O3 = 2.62

45. Fe D FeCl3
1 mol Fe = 1 mol FeCl3
g Fe/at wt Fe = g FeCl3/ FW FeCl3
Rearrangement gives
g FeCl3 = g Fe (FW FeCl3/at wt Fe)
In the same manner
g AlCl3 = g Al ( FW AlCl3/at wt Al)

46. g FeCl3 + g AlCl3 = 5.95
g Fe (FW FeCl3/at wt Fe) + g Al ( FW AlCl3/at wt Al) = 5.95
assume g Fe = x, g Al = y then:
x (FW FeCl3/at wt Fe) + y ( FW AlCl3/at wt Al) = 5.95
x (162.2/55.85) + y (133.34/26.98) = 5.95
2.90 x y = (1)

47. The same treatment with the oxides gives
2 Fe D Fe2O3
mol Fe = 2 mol Fe2O3
g Fe/at wt Fe = 2 (g Fe2O3/FW Fe2O3)
g Fe2O3 = 1/2 g Fe (FW Fe2O3/at wt Fe)
In the same manner
g Al2O3 = 1/2 g Al (FW Al2O3/at wt Al)
g Fe2O3 + g Al2O3 = 2.62
1/2 g Fe (FW Fe2O3/at wt Fe) + 1/2 g Al (FW Al2O3/at wt Al) = 2.62
1/2 x (159.7/55.85) + 1/2 y (101.96/26.98) = 2.62
1.43 x y = (2)

48. 2.90 x y = (1)
1.43 x y = (2)
from (1) and (2) we get
x = 1.07
y = 0.58
% Fe = (1.07/5.95) x 100 = 18.0%
% Al = (0.58/5.95) x 100 = 9.8%

49. 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 g, what is the atomic weight of M2+ in MSO4?
K2SO4 g BaSO4
MSO4 g BaSO4

50. Rearranging and solving:
mmol BaSO4 = mmol K2SO4 + mmol MSO4
Rearranging and solving:

51. A mixture of mercurous chloride (FW 472
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.
Hg2Cl2 g 2Hg
Hg2Br2 g 2Hg

52. Answer mmol Hg = 2 mmol Hg2Cl2 + 2 mmol Hg2Br2
Rearranging and solving:

53. A 10.0 mL solution containing Cl- was treated with excess AgNO3 to precipitate g of AgCl. What was the concentration of Cl- in the unknown? (AgCl = g/mol)
mmol Cl- = mmol AgCl
Concentration of Cl-

54. Precipitation Equilibria
Inorganic solids which have limited water solubility show an equilibrium in solution represented by the so called solubility product. For example, AgCl slightly dissolve in water giving Cl- and Ag+ where
AgCl (s) D Ag+ + Cl-
K = [Ag+][Cl-]/[AgCl(s)]
However, the concentration of a solid is constant and the equilibrium constant can include the concentration of the solid and thus is referred to, in this case, as the solubility product, ksp where
Ksp = [Ag+][Cl-]

55. It should be clear that the product of the ions raised to appropriate power as the number of moles will fit in one of three cases:
 1. When the product is less than Ksp: No precipitate is formed and we have a clear solution
2. When the product is equal to ksp : We have a saturated solution
3. When the product exceeds the ksp : A precipitate will form
It should also be clear that at equilibrium of the solid with its ions, the concentration of each ion is constant and the precipitation of ions in solution does occur but at the same rate as the solubility of precipitate in solution. Therefore, the concentration of the ions remains constant at equilibrium.

56. A 1:1 salt is less soluble than a nonsymmetric salt with the same Ksp
The molar solubility depends on the stoichiometry of the salt.
A 1:1 salt is less soluble than a nonsymmetric salt with the same Ksp

57. Three situations will be studied: 
a. Solubility in pure water
b. Solubility in presence of a common ion
c. Solubility in presence of diverse ions
Two other situations will be discussed in later chapters
Solubility in acidic solutions
solubility in presence of a complexing agent  

58. Solubility in Pure Water
Example
Calculate the concentration of Ag+ and Cl- in pure water containing solid AgCl if the solubility product is 1.0x10-10.
Solution
First, we set the stoichiometric equation and assume that the molar solubility of AgCl is s.
AgCl(s) D Ag+ + Cl-

59. Ksp = [Ag+][Cl-] 1.0x10-10 = s x s = s2 s = 1.0 x 10-5 M
[Ag+] = [Cl-] = 1.0 x 10-5 M

60. Example
 Calculate the molar solubility of PbSO4 in pure water if the solubility product is 1.6 x 10-8
Solution
Ksp = [Pb2+][SO42-]
1.6 x 10-8 = s x s = s2
s = 1.3x10-4 M

61. Ksp = [Pb2+][I-]2 7.1x10-9 = s x (2s)2 7.1x10-9 = 4s3 s = 1.2x10-3 M
Calculate the molar solubility of PbI2 in pure water if the solubility product is 7.1 x 10-9.
Solution
Ksp = [Pb2+][I-]2
7.1x10-9 = s x (2s)2
7.1x10-9 = 4s3
s = 1.2x10-3 M 

62. If we compare the molar solubilities of PbSO4 and PbI2 we find that the solubility of lead iodide is larger than that of lead sulfate although lead iodide has smaller ksp. You should calculate solubilities rather than comparing solubility products to check which substance gives a higher solubility. In presence of a precipitating agent, the substance with the least solubility will be precipitated first.

63. Precipitation Equilibria, Cont…
Lecture 20
Precipitation Equilibria, Cont…
Acid-Base Equilibria

64. What must be the concentration of Ag+ to just start precipitation of AgCl in a 1.0x10-3 M NaCl solution.
Solution
AgCl just starts to precipitate when the ion product just exceeds ksp
Ksp = [Ag+][Cl-]
1.0x10-10 = [Ag+] x 1.0 x 10-3
[Ag+] = 1.0x10-7 M

65. What pH is required to just start precipitation of Fe(III) hydroxide from a 0.1 M FeCl3 solution. Ksp = 4x10-38.
Solution
Fe(OH)3 D Fe OH-
Ksp = [Fe3+][OH-]3
4x10-3 = 0.1 x [OH-]3
[OH-] = 7x10-13 M
pH = 14 – pOH
pH = 14 – 12.2 = 1.8

66. Solubility in Presence of a Common Ion
10 mL of 0.2 M AgNO3 is added to 10 mL of 0.1 M NaCl. Find the concentration of all ions in solution and the solubility of AgCl formed.
First we find mmol AgNO3 and mmol NaCl then determine the excess concentration
mmol Ag+ = 0.2 x 10 = 2
mmol Cl- = 0.1 x 10 = 1
mmol AgCl formed = 1 mmol
mmol Ag+ excess = 2 – 1 = 1
[Ag+]excess = mmol/mL = 1/20 = 0.05 M

67. We can find the concentrations of NO3- = 0.2*10/20 = 0.1 M
[Na+] = 0.1*10/20 = 0.05 M
Chloride ions react to form AgCl, therefore the only source for Cl- is the solubility of AgCl; but now in presence of 0.05 M excess Ag+

68. Ksp = [Ag+][Cl-]
1.0x10-10 = ( s) (s)
Since the solubility product is very small, we can assume 0.05>>s
1.0x10-10 = 0.05 (s)
s = 2.0x10-9 M
Relative error = ( 2.0x10-9 / 0.05) x 100 = 4x10-6% which is extremely small, therefore:
[Cl-] = 2.0x10-9 M
[Ag+] = x10-9 = 0.05 M
Look at how the solubility is decreased in presence of the common ion.

69. 25 mL of 0. 100 M AgNO3 are mixed with 35 mL of 0. 050 M K2CrO4
25 mL of M AgNO3 are mixed with 35 mL of M K2CrO4. Find the concentration of each ion in solution at equilibrium if ksp of Ag2CrO4 = 1.1x10-12.
Solution
2 Ag2+ + CrO42- g Ag2CrO4(s)
mmol Ag+ = x 25 = 2.5
mmol CrO42- = x 35 = 1.75
From the stoichiometry we know that two moles of silver react with one mole of chromate, and chromate will be in excess, therefore:

70. I prefer that you work out the concentration of chromate left in as follows:
mmol CrO42- left = initial mmol CrO42- - mmol CrO42- reacted
mmol CrO42- left = initial mmol CrO42- - ½ mmol Ag+
mmol CrO42- left = 0.05*35 – ½*0.1*25 = 0.5

71. mmol CrO42- excess = 1.75 – 1.25 = 0.50
[CrO42-] excess = mmol/mL = 0.5/60 = M
[NO3-] = mmol/mL = x 25/60 = M
[K+] = mmol/mL = 2 x x 35/60 = M
The silver was consumed in the reaction and the only way to calculate its concentration is through the solubility product:

72. Relative error = (5.76x10-6/0.0083) x 100 = 0.07%
Ksp = [Ag+]2 [CrO42-]
1.1x10-12 = (2s)2 * ( s)
However, s is very small since the equilibrium constant is very small. Therefore, assume >>s
1.1x10-12 = (2s)2 x
s = 5.76x10-6 M
Relative error = (5.76x10-6/0.0083) x 100 = 0.07%
[Ag+] = 2s = 2x5.76x10-6 = 1.15x10-5 M
[CrO42-] = s = x10-6 = M

73. Solubility in Presence of Diverse Ions
As expected from previous information, diverse ions have a screening effect on dissociated ions which leads to extra dissociation. Solubility will show a clear increase in presence of diverse ions as the solubility product will increase. Look at the following example:
Example
Find the solubility of AgCl (ksp = 1.0 x 10-10) in 0.1 M NaNO3. The activity coefficients for silver and chloride are 0.75 and 0.76, respectively.

74. Solution
AgCl(s) D Ag+ + Cl-
We use activities instead of concentrations since the solution contains diverse ions where we have:
Ksp, Th = aAg+ aCl-
Ksp, Th = [Ag+] fAg+ * [Cl-] fCl-
1.0x10-10 = s x 0.75 x s x 0.76
s = 1.3x10-5 M

75. The solubility of AgCl in pure water can be calculated to be 1
The solubility of AgCl in pure water can be calculated to be 1.0x10-5 M. If we compare this value to that obtained in presence of diverse ions we see
%increase in solubility = {(1.3x10-5 – 1.0x10-5)/1.0x10-5}x 100 = 30%
Therefore, once again we have an evidence for an increase in dissociation or a shift of equilibrium to right in presence of diverse ions.