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

OutlineTheory 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.

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.

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.

The precipitation gravimetry Ba2+ tSO42- BaSO4 BaSO4 Defined Precipitate Precipitatesubstance formed (gravimetric) weighed

The precipitation gravimetry 6- t2Fe3+ 2Fe(OH)3 Fe2O3 Defined Precipitate Precipitatesubstance formed (gravimetric) weighed

The precipitation gravimetry

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+ + H2OCO2 + 2NaOH Na2CO3 + H2O.

Indirect method volatilization gravimetry. In indirect methods define weight of the rest of substance after full removal of a defined volatile component.

BaCl22H2O BaCl2 + 2H2O.

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

Advantages and defects of the gravimetric analysis

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 substanceWeighing of defined substanceDissolution of defined substanceChoice of the precipitantCalculation of quantity of precipitantPrecipitationFiltering the precipitateRinsing the precipitateDrying the precipitateWeighingCalculation

1. Calculation of weight of defined substance

The gravimetric factor shows a part of a defined component into the weighed (gravimetric) form.

The weight of weighed (gravimetric) form is determined:By error of weighing machine (scales)

By optimum weight of the precipitates form.

Weight of the precipitates form for different types of precipitates

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.

Dissolution Dissolution in acids (diluted or concentrated)!Choice of the precipitantThe requirements to precipitates formThe precipitates form should be enough low solubility (Ksp10-8).The precipitate should be whenever possible largely crystal.The precipitates form should turn easily enough in weighed (gravimetric) form.

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.

Importance of the low contents of defined substance into the weighed form

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+ + 3S2O32- + 3H2O = 2Al(OH)3 + 3S + 3SO2.Organic precipitantsdimethylgloxime8-oksihinolin oxalic acid-nitrozo--naphtolInorganic precipitants:Alkalis or ammoniaHydrogene sulphide or sulphidesSulphatic acidPhosphatic acid

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.

Calculation of quantity of precipitantIn the gravimetric analysis a precipitation is considered practically full if in a solution defined substance is in limits of accuracy of weighingit is less than 0,0002-0,0001 gTake 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)

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).

Conditions of precipitation of amorphous precipitateA 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.

Avoiding Impurities

Precipitation gravimetry is based on a known stoichiometry between the analytes 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 precipitates matrix must be removed before obtaining its weight.

Types of impurities inclusionocclusiondigestionadsorption

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

Schematic of an occlusion by entrapment of supernatant solution

Surfaceadsorption of excess C

Filtering the precipitate

Papers filters (weight of ashes 0,00003 0,00007 g)Glass filters or filtering crucibles.

Rinsing the precipitateRinsing on the filter (for amorphous precipitate)Rinsing by decantation (for crystal precipitate)

Filtering crucibles

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.

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

Choice of a rinsings liquidCrystal precipitates with low solubility are rinsed by waterAmorphous precipitates are rinsed by solutions of volatile electrolytes to avoid of peptization of a precipitatePrecipitates with high solubility are rinsed by solutions of electrolytes which contain the same ion with a precipitate

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

Drying the precipitateDrying 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

Calculation

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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).

Thanks for your attention!