1. Associate prof . L.V. Vronska Associate prof . M.M. Mykhalkiv
L e c t u r e 5
Gravimetric Analysis
Associate prof . L.V. Vronska
Associate prof . M.M. Mykhalkiv

2. Outline
Theory of the gravimetric analysis and classification of its methods.
Precipitation gravimetry: techniques of performance, the requirement to precipitations.
Use of the gravimetry in the pharmaceutical analysis.

3. The Gravimetric analysis is a method of the quantitative chemical analysis which is based on exact measurement of weight of defined substance or its components allocated in chemically pure compound or in the form of corresponding compounds (precisely known constant structure).
Gravimetric methods:
Particulate gravimetry.
Precipitation gravimetry.
Volatilization gravimetry.

4. The particulate gravimetry is a method of the gravimetric analysis which is based on exact measurement of weight of the defined substance allocated in the pure state.
The particulate gravimetry is a gravimetric method in which the mass of a particulate analyte is determined following its separation from its matrix.
The precipitation gravimetry is a method of the gravimetric analysis which is based on exact measurement of exact weight of the defined substance besieged in the form of a chemical compound with precisely known structure.
The precipitation gravimetry is a gravimetric method in which the signal is the mass of a precipitate.

5. The precipitation gravimetry
Ba t
SO BaSO4 BaSO4
Defined Precipitate Precipitate
substance formed (gravimetric)
weighed

6. The precipitation gravimetry
2Fe Fe(OH)3 Fe2O3
Defined Precipitate Precipitate
substance formed (gravimetric)
weighed

7. The precipitation gravimetry

11. CaCO3 + 2H+ CO2↑ + Ca2+ + H2O CO2 + 2NaOH → Na2CO3 + H2O.
The volatilization gravimetry is a method of the gravimetric analysis which is based on measurement of exact weight of volatile defined component. The volatilization gravimetry is a gravimetric method in which the loss of a volatile species gives rise to the signal.
Direct method of volatilization gravimetry. A defined volatile component absorb a specific absorber and on increase of the weight of the last calculate the weight of a volatile defined component.
CaCO3 + 2H CO2↑ + Ca2+ + H2O
CO2 + 2NaOH → Na2CO3 + H2O.

12. Indirect method volatilization gravimetry
Indirect method volatilization gravimetry. In indirect methods define weight of the rest of substance after full removal of a defined volatile component.
BaCl2∙2H2O BaCl2 + 2H2O↑.

13. Often recognise that a relative error of weighing should be
In indirect method volatilization gravimetry calculate the optimum weight of defined substance under the formula:
In calculations are necessarily supervised by preliminary set relative error of weighing:
Often recognise that a relative error of weighing should be

14. Advantages and defects of the gravimetric analysis
1. High accuracy (0,005-0,1 %)
2. High reproducibility
3. Simplicity of performance
Defects
1. Duration
2. Labour input

15. Precipitation gravimetry: techniques of performance, the requirement to precipitations.
The scheme of the analysis and the main operations of a precipitation gravimetry are the such:
Calculation of weight of defined substance
Weighing of defined substance
Dissolution of defined substance
Choice of the precipitant
Calculation of quantity of precipitant
Precipitation
Filtering the precipitate
Rinsing the precipitate
Drying the precipitate
Weighing
Calculation

16. 1. Calculation of weight of defined substance
The gravimetric factor shows a part of a defined component into the weighed (gravimetric) form.

17. The weight of weighed (gravimetric) form is determined:
By error of weighing machine (scales)
By optimum weight of the precipitate’s form.

18. Weight of the precipitate’s form for different types of precipitates
Precipitate type
Molar weight , g/mol
Weight of precipitate, g
Amorphous
(Fe2O3  n H2O)
до 100
0,07 – 0,10
Easy crystal (CaCO3)
до  100
0,10 – 0,15
Heavy crystal (BaCO3)
 200
0,2 – 0,4
Very heavy crystal (PbSO4)
 300
0,5

19. Weight weighed (gravimetric) form :
For crystal precipitates - 0,5 g
For amorphous precipitates – 0,1 g
!!!! It is necessary to remember: the more weight defined substance, the above relative accuracy of results of the analysis.

20. Choice of the precipitant The requirements to precipitate’s form
Dissolution
Dissolution in acids (diluted or concentrated)!
Choice of the precipitant
The requirements to precipitate’s form
The precipitate’s form should be enough low solubility (Ksp10-8).
The precipitate should be whenever possible largely crystal.
The precipitate’s form should turn easily enough in weighed (gravimetric) form.

21. Requirements to the weighed form
Exact conformity of structure to the chemical formula (precipitate form Fe(OH)3  Fe2O3  xH2O, and the weight form! Fe2O3).
Chemical stableness of the weighed form.
The contents of a defined element in the weighed form should be as it is possible smaller.

22. Importance of the low contents of defined substance into the weighed form
Cr2O3
Weighed form BaCrO4
152 g Cr2O g Cr
1 mg Cr2O х mg Cr
Х = 1041 / 152 =
0,7 mg (Cr)
Loss
253,3g BaCrO g Cr 1 mg BaCrO4 - х mg Cr
Х = 521 / 253,3 =
0,2 mg (Cr)

23. Requirements to precipitants:
It is desirable, that a precipitant was volatile compound.
A precipitant should be specific – to precipitate a defined ion in the presence of others ions.
2Al3+ + 3S2O H2O = 2Al(OH)3 + 3S + 3SO2.
Organic precipitants
dimethylgloxime
8-oksihinolin
oxalic acid
-nitrozo--naphtol
Inorganic precipitants:
Alkalis or ammonia
Hydrogene sulphide or sulphides
Sulphatic acid
Phosphatic acid

26. Advantage of organic precipitants consists in the following:
Solubility of precipitate with organic precipitants is less.
Precipitates with organic reagents are crystal.
Precipitates with organic reagents are purer as on their surface impurity are less adsorbed.
Organic precipitant have higher selectivity and specificity.
The gravimetric factor at definition with organic reagents on much less so, accuracy of definition increases.

27. Calculation of quantity of precipitant
In the gravimetric analysis a precipitation is considered practically full if in a solution defined substance is in limits of accuracy of weighing
it is less than 0,0002-0,0001 g
Take a volatile precipitant in 2-3 times more than calculated quantities (count on the reaction equation)
Take a nonvolatile precipitant in 1,5 times more than calculated quantities (count on the reaction equation)

28. Conditions of precipitation of crystal precipitates:
A precipitation are carried from enough diluted solutions by the diluted solution of precipitant.
Add a precipitant very slowly, on drops.
A solution are mixed continuously by a glass stick to avoid strong local satiation at addition of precipitant.
Conduct precipitation from a hot solution, and sometimes heat up also a solution of precipitant (to increase solubility).
Filter a precipitate only after cooling of solution.
Addition at precipitation of substances which raise solubility of a precipitate (for example, acids).

29. Conditions of precipitation of amorphous precipitate
A precipitation conduct from the concentrated solutions by the concentrated solutions of precipitant.
A precipitation conduct from hot solutions.
A precipitation conduct in the presence of electrolyte - coagulant.
A precipitate quickly filter and do not leave under a matrix solution.

30. Avoiding Impurities
Precipitation gravimetry is based on a known stoichiometry between the analyte’s mass and the mass of a precipitate. It follows, therefore, that the precipitate must be free from impurities. Since precipitation typically occurs in a solution rich in dissolved solids, the initial precipitate is often impure. Any impurities present in the precipitate’s matrix must be removed before obtaining its weight.

31. Types of impurities
inclusion
occlusion
digestion
adsorption

32. Schematic of a chemically adsorbed inclusion or a physically adsorbed occlusion in a crystal lattice, where C and A represent the cation–anion pair comprising the analyte and the precipitant, and is the impurity

33. Schematic of an occlusion by entrapment of supernatant solution

34. Surface adsorption of excess C

35. Filtering the precipitate
Paper’s filters (weight of ashes 0,00003 – 0,00007 g)
Glass filters or filtering crucibles.
Rinsing the precipitate
Rinsing on the filter (for amorphous precipitate)
Rinsing by decantation (for crystal precipitate)

36. Filtering crucibles

37. Proper procedure for filtering solids using filter paper
Proper procedure for filtering solids using filter paper. The filter paper circle in (a) is folded in half (b), and folded in half again (c). The filter paper is parted (d), and a small corner is torn off (e). The filter paper is opened up into a cone and placed in the funnel (f). Note that the torn corner is placed to the outside.

39. Procedure for filtering through a filtering crucible
Procedure for filtering through a filtering crucible. The trap is used to prevent water from a water aspirator from backwashing into the suction flask.

40. Choice of a rinsings liquid
Crystal precipitates with low solubility are rinsed by water
Amorphous precipitates are rinsed by solutions of volatile electrolytes to avoid of peptization of a precipitate
Precipitates with high solubility are rinsed by solutions of electrolytes which contain the same ion with a precipitate

41. Concentration of impurities Сn which remained in a precipitate after n rinsings :

42. Drying the precipitate
Drying in porcelain and glass filtering crucibles
!!! Drying of weighed form are leaded to its constant weight, that is the difference between its parallel weighing will not exceed 0,0002 г

43. Calculation ,

44. Use gravimetry in the pharmaceutical analysis Determination of moisture in drugs (indirect volatilization gravimetry). Determination of water (direct volatilization gravimetry). Determination of the dry rest in extracts, tinctures (indirect volatilization gravimetry). Determination of sulphatic ashes and ashes (particulate gravimetry). Determination of drugs (precipitation and particulate gravimetry).

45. Thanks for your attention!