ERT207 ERT207 Analytical ChemistryAnalytical Chemistry

Gravimetric Analysis and Gravimetric Analysis and Precipitation EquilibriaPrecipitation Equilibria

Pn Syazni Zainul KamalPn Syazni Zainul Kamal

CO2 : Ability to classify and use CO2 : Ability to classify and use separation techniques and gravimetric separation techniques and gravimetric methods for mass determinationmethods for mass determination

IntroductionIntroduction

The term gravimetric pertains to a The term gravimetric pertains to a Weight Weight Measurement.Measurement.

Gravimetric method is one in which the analysis Gravimetric method is one in which the analysis is completed by a weighing operation. is completed by a weighing operation.

Gravimetric Analysis is a group of analytical Gravimetric Analysis is a group of analytical methods in which methods in which the amount of analyte is the amount of analyte is determined by the measurement of the mass of determined by the measurement of the mass of a pure substance containing the analyte.a pure substance containing the analyte.

Gravimetric Methods can also be defined as Gravimetric Methods can also be defined as quantitative methodsquantitative methods based on the determining based on the determining the mass of a pure compound to which the the mass of a pure compound to which the analyte is chemically related.analyte is chemically related.

Analyte = constituents to be determined

There are two main types of gravimetric analyses

A) Precipitation

B) Volatilization

analyte must first be converted to a solid (precipitate) by precipitation with an appropriate reagent. The precipitates from solution is filtered, washed, purified (if necessary) and weighed.

In this method the analyte or its decomposition products are volatilised (dried) and then collected and weighed, or alternatively, the mass of the volatilised product is determined indirectly by the loss of mass of the sample.

Example for Precipitation:-Example for Precipitation:-

Calcium can be determined gravimetrically by Calcium can be determined gravimetrically by precipitation of calcium oxalate and ignition of precipitation of calcium oxalate and ignition of the oxalate ion to calcium oxide.the oxalate ion to calcium oxide.

CaCa2+2+ + C + C22OO442-2- →CaC →CaC22OO44

CaCCaC22OO44 → CaO + CO → CaO + CO22 + CO + CO

The precipitate thus obtained are weighed and The precipitate thus obtained are weighed and the mass of calcium oxide is determined.the mass of calcium oxide is determined.

Example for Volatilisation:-Example for Volatilisation:-

The analyte or its decomposition products are volatilised at The analyte or its decomposition products are volatilised at a suitable temperature. The volatile product is then a suitable temperature. The volatile product is then collected and weighed, i.e. the mass of the product is collected and weighed, i.e. the mass of the product is indirectly determined from the loss in mass of the sample.indirectly determined from the loss in mass of the sample.

Example Example Water can be separated from most inorganic compounds by Water can be separated from most inorganic compounds by ignition, the evolved water can then be absorbed on any ignition, the evolved water can then be absorbed on any one of several solid desiccants. The weight of water evolved one of several solid desiccants. The weight of water evolved may be calculated from the gain in weight of the absorbent.may be calculated from the gain in weight of the absorbent.

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

Solubility

Filterability

Chemical Composition

Other Desirable Properties

The product must be sufficiently insoluble to prevent the loss of weight.

Precipitate formed should be adoptable to simple and rapid filtration methods.

The product must be of known chemical composition.

Other factors effecting the stability and purity of the precipitate.

For a successful determination in gravimetric analysis the following criteria should be met :-

(1)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.

(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.

For a successful determination in gravimetric analysis the following criteria should be met :-

Gravimetric AnalysisGravimetric Analysis

Gravimetric analysis is potentially more accurate Gravimetric analysis is potentially more accurate and more precise than volumetric analysis.and more precise than volumetric analysis.

Gravimetric analysis avoids problems with Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and temperature fluctuations, calibration errors, and other problems associated with volumetric other problems associated with volumetric analysis.analysis.

But there are potential problems with gravimetric But there are potential problems with gravimetric analysis that must be avoided to get good results.analysis that must be avoided to get good results.

Proper lab technique is criticalProper lab technique is critical

Steps in a Gravimetric AnalysisSteps in a Gravimetric Analysis

1. Preparation of the solution1. Preparation of the solution2. Precipitation2. Precipitation3. Digestion3. Digestion4. Filtration4. Filtration5. Washing5. Washing6. Drying or ignition6. Drying or ignition7. Weighing7. Weighing8. Calculation8. Calculation

1.1. Preparation of analyte solutionPreparation of analyte solution

Gravimetric analysis usually involves Gravimetric analysis usually involves precipitation precipitation of analyte from solutionof analyte from solution..

11stst step – prepare the analyte solution step – prepare the analyte solution May need preliminary separation to separate May need preliminary separation to separate

potential interferences before precipitating analytepotential interferences before precipitating analyte May need adjustment of solution condition to May need adjustment of solution condition to

maintain low solubility of precipitatemaintain low solubility of precipitate May require pH/temp/volume (of sol.) and/or other May require pH/temp/volume (of sol.) and/or other

adjustments to maximize precipitate formation. Eg adjustments to maximize precipitate formation. Eg Calcium oxalate insoluble in basic mediumCalcium oxalate insoluble in basic medium

2. Precipitation2. Precipitation The precipitating reagent is added at a The precipitating reagent is added at a

concentration that favors the formation of a "good" concentration that favors the formation of a "good" precipitate.precipitate.

This may require low concentration, extensive This may require low concentration, extensive heating (often described as "digestion"), or careful heating (often described as "digestion"), or careful control of the pH. control of the pH.

The precipitate shouldThe precipitate should Be sufficiently insolubleBe sufficiently insoluble Have large crystals (Easier to filter large Have large crystals (Easier to filter large

crystals)crystals) Be free of contaminantsBe free of contaminants

Precipitation process :Precipitation process :When solution of precipitating agent (AgNOWhen solution of precipitating agent (AgNO33) ) added into testing solution (chloride solution) to added into testing solution (chloride solution) to form AgCl precipitate, form AgCl precipitate, Supersaturation Supersaturation : the solution phase contains : the solution phase contains more dissolved salt than at equilibrium. The more dissolved salt than at equilibrium. The driving force will be for the system to approach driving force will be for the system to approach equilibrium (saturation).equilibrium (saturation).- Nucleation- Nucleation : initial phase of precipitation. A min : initial phase of precipitation. A min number of particle precipitate will gather number of particle precipitate will gather together to form a nucleus of particle precipitate together to form a nucleus of particle precipitate (solid phase). Higher degree of supersaturation, (solid phase). Higher degree of supersaturation, the greater rate of nucleationthe greater rate of nucleation

Ag+Cl- Ag+Cl-Ag+Cl-

Ag+Cl-Ag+Cl-Ag+Cl-

Ag+Cl-Ag+Cl-Ag+Cl-

nucleation involves the formation of ion pairs and nucleation involves the formation of ion pairs and finally a group of ions formed.finally a group of ions formed.

- - Particle growthParticle growth : particle enlargement process. : particle enlargement process. Nucleus will grow and forming a crystal of a Nucleus will grow and forming a crystal of a specific geometric shape.specific geometric shape.

Von weimarn discover – the particle size of Von weimarn discover – the particle size of precipitates is inversely proportional to the precipitates is inversely proportional to the relative supersaturation of the sol. during the relative supersaturation of the sol. during the precipitation process.precipitation process.

The von Weimarn RatioThe von Weimarn Ratio

von Weimarn ratio = (von Weimarn ratio = (QQ – – SS)) SS

A measure of relative supersaturation or A measure of relative supersaturation or supersaturation ratiosupersaturation ratio The lower the betterThe lower the better

If high, get excessive nucleation, lots of If high, get excessive nucleation, lots of small crystals, large surface areasmall crystals, large surface areaIf low, get larger, fewer crystals, small If low, get larger, fewer crystals, small surface areasurface area

SS = solubility of precipitate at equilibrium = solubility of precipitate at equilibrium

Keep it high with high temperatures, Keep it high with high temperatures, adjusting pHadjusting pH

QQ = concentration of mixed reactants before = concentration of mixed reactants before precipitationprecipitation

Keep it low by using dilute solutions, stir Keep it low by using dilute solutions, stir mixture well, add reactants slowlymixture well, add reactants slowly

Can lower Can lower SS later by cooling mixture after later by cooling mixture after crystals have formedcrystals have formed

3. Digestion of the Precipitate3. Digestion of the Precipitate

Let precipitate stand in contact with mother Let precipitate stand in contact with mother liquor (the solution from which it was liquor (the solution from which it was precipitated), usually at high temp. precipitated), usually at high temp. This process is called This process is called digestiondigestion, or , or OstwaldOstwald ripeningripening. The small particles tend to dissolve . The small particles tend to dissolve and precipitate on the surfaces of the larger and precipitate on the surfaces of the larger crystals crystals Large crystal (small surface area) have lower Large crystal (small surface area) have lower free energy than small crystal (large surface free energy than small crystal (large surface area)area)Digestion make larger crystals, reduce surface Digestion make larger crystals, reduce surface contamination, reduce crystals imperfectioncontamination, reduce crystals imperfection

4. Filtration4. Filtration

Sintered glass crucibles are used to filter the Sintered glass crucibles are used to filter the precipitates. precipitates. The crucibles first cleaned thoroughly and then The crucibles first cleaned thoroughly and then subjected to the same regimen of heating and subjected to the same regimen of heating and cooling as that required for the precipitate. cooling as that required for the precipitate. This process is repeated until constant mass has This process is repeated until constant mass has been achieved, that is, until consecutive been achieved, that is, until consecutive weighings differ by 0.3 mg or less.weighings differ by 0.3 mg or less.(you may see the proper technique of filtration in (you may see the proper technique of filtration in chapter 2)chapter 2)

5. Washing5. Washing

Coprecipitated impurities esp those on Coprecipitated impurities esp those on surface can be removed by washing the surface can be removed by washing the precipitateprecipitateWet precipitate with mother liquor and Wet precipitate with mother liquor and which will also be remove by washingwhich will also be remove by washingNeed to add electrolyte to the wash liquid Need to add electrolyte to the wash liquid bcoz some precipitate cannot be wash bcoz some precipitate cannot be wash with pure water, peptization occur.with pure water, peptization occur.

Eg HNOEg HNO33 for AgCl precipitate for AgCl precipitate

6) Drying or ignition6) Drying or ignition

To remove solvent and wash electrolytesTo remove solvent and wash electrolytesDone by heating at 110 to 120°C for 1 to 2 Done by heating at 110 to 120°C for 1 to 2 hrs.hrs.Converts hygroscopic compound to non-Converts hygroscopic compound to non-hygroscopic compoundhygroscopic compoundMay used high temp if precipitate must be May used high temp if precipitate must be converted to a more suitable form before converted to a more suitable form before weighingweighing

Eg MgNHEg MgNH44POPO44 convert to pyrophosphate convert to pyrophosphate MgMg22PP22OO77 by heating at 900°C. by heating at 900°C.

7) Weighing7) Weighing

After the precipitate is allowed to cool After the precipitate is allowed to cool (preferably in a desiccator to keep it from (preferably in a desiccator to keep it from absorbing moisture), it is weighed (in the absorbing moisture), it is weighed (in the crucible). crucible).

Properly calibrated analytical balanceProperly calibrated analytical balance

Good weighing techniqueGood weighing technique

Organic Precipitates Organic Precipitates

Organic precipitating agents have the Organic precipitating agents have the advantages of giving precipitates with very advantages of giving precipitates with very solubility in water and a favorable gravimetric solubility in water and a favorable gravimetric factor.factor.

Most of them are chelating agents that forms Most of them are chelating agents that forms slightly insoluble, uncharged chelates with the slightly insoluble, uncharged chelates with the metal ions.metal ions.

Organic Precipitates Organic Precipitates

in gravimetric method – the analyte (solute) is converted to precipitate which is then weight.From the weight of the precipitate formed in a gravimetric analysis, we can calculate the weight of the analyte

Gravimetric Analysis: Weight Gravimetric Analysis: Weight RelationshipRelationship

Gravimetric factor (GF) = weight of analyte per unit weight of precipitate. Obtain from ratio of the F Wt of the analyte per F Wt precipitate, multiplied by moles of analyte per mole of precipitate obtained from each mole of analyte

GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) f wt precipitate (g/mol) b = g analyte / g precipitate

Example 1

If Cl2 in a sample is converted to chloride and precipitated as AgCl, the weight of Cl2 that gives 1 g of AgCl is???

F wt Cl = 35.453

F wt AgCl = 107.868

GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate)

f wt precipitate (g/mol) b

= g analyte / g precipitate

GF = f wt analyte (g/mol) x a (mol analyte/moprecipitate)

f wt precipitate (g/mol) b

= g analyte / g precipitate

g Cl2 = g AgCl x f wt analyte (g/mol) x a

f wt precipitate (g/mol) b

= 1 AgCl x 70.906 x 1

143.321 2

= 0.2474 g

percent composition by weight of the analyte percent composition by weight of the analyte

in the sample :in the sample :

% A = g% A = gAA x 100% x 100%

ggsamplesample

ggA A – grams of analyte (the desired test substance)– grams of analyte (the desired test substance)

ggsample sample – grams of sample taken for analysis– grams of sample taken for analysis

EXAMPLE 2

A 0.3516 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 1000ºC. This residue weighed 0.2161 gm. Calculate the percent P (FW = 30.974) in the sample.

g P = 0.2161 g x 30.974 x 2g P = 0.2161 g x 30.974 x 2 222.57 1222.57 1

= 0.0601g= 0.0601g

% A = g% A = gAA x 100% x 100%

ggsamplesample

= 0.0601 g x 100%= 0.0601 g x 100% 0.3516 g0.3516 g= 0.1709 %= 0.1709 %

GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate) f wt precipitate (g/mol) b = g analyte / g precipitate

Orthophosphate (POOrthophosphate (PO443-3-) is determined by ) is determined by

weighing as ammonium phosphomolybdate weighing as ammonium phosphomolybdate (NH(NH44)PO)PO44.12MoO.12MoO33. Calculate the percent P . Calculate the percent P

and the percent Pand the percent P22OO55 if 2.1771g precipitate if 2.1771g precipitate

(ppt) were obtained from a 0.3588g sample. (ppt) were obtained from a 0.3588g sample.

[F wt P = 30.97], [F wt P.molybdate = 1876.5], [F wt P = 30.97], [F wt P.molybdate = 1876.5], [F wt P[F wt P22OO5 5 = 141.95]= 141.95]

g P = 2.1771g x 30.97 x 1 molg P = 2.1771g x 30.97 x 1 mol

1876.5 1 mol1876.5 1 mol

= 0.0359g= 0.0359g

% P = 0.0359g x 100%% P = 0.0359g x 100%

0.3588g0.3588g

= = 10.01 %10.01 %

g Pg P22OO55 = 2.1771 g x 141.95 x 1 mol = 2.1771 g x 141.95 x 1 mol

1876.5 2 mol1876.5 2 mol

= 0.0823g= 0.0823g

% P% P22OO55 = 0.0823g x 100% = 0.0823g x 100%

0.3588g0.3588g

= = 22.94%22.94%

Precipitation ReactionsPrecipitation Reactions

Solubility of a compound = Solubility of a compound = concentrations of a concentrations of a soluble species at equilibrium with its insoluble soluble species at equilibrium with its insoluble form.form.If the compound is sparingly soluble, it will If the compound is sparingly soluble, it will produce cation & anion.produce cation & anion.

Eg PbClEg PbCl2 2 slightly dissolved in water. So PbClslightly dissolved in water. So PbCl22 has a specific solubility, s = solid phase aq = has a specific solubility, s = solid phase aq = aqueous phaseaqueous phase

PbClPbCl22 (s) Pb (s) Pb2+2+ (aq)+ 2Cl (aq)+ 2Cl-- (aq) (aq)

The equilibrium constant for the reaction is The equilibrium constant for the reaction is known as solubility product constant.known as solubility product constant.

KKspsp (PbCl (PbCl22) = [Pb) = [Pb2+2+][Cl][Cl--]]22

Concentration of any solid (PbClConcentration of any solid (PbCl22) is ) is

constant and is combined in the constant and is combined in the equilibrium constant to give Kequilibrium constant to give Kspsp

ExampleExample

Calculate the Calculate the concentration concentration of Agof Ag++ and Cl and Cl-- in a saturated solution of AgCl, and the in a saturated solution of AgCl, and the molar solubilitymolar solubility of AgCl. K of AgCl. Kspsp for AgCl at for AgCl at

25°C is 1.0 x 1025°C is 1.0 x 10-10-10..

Solution :Solution :

AgCl AgAgCl Ag++ + Cl + Cl--

s s s=solubility (M)s s s=solubility (M)

Ksp = [AgKsp = [Ag++] [Cl] [Cl--]]

= (s)(s)= (s)(s)

= 1.0 x 10= 1.0 x 10-10-10

s = s = 1.0 x 101.0 x 10-5-5 M = [Ag M = [Ag++] = [Cl] = [Cl--]]

Molar solubilityMolar solubility of AgCl is equal to concentration of of AgCl is equal to concentration of [Ag[Ag++] or [Cl] or [Cl--] so molar solubility of AgCl= ] so molar solubility of AgCl= 1.0 x 101.0 x 10-5-5 M M

Common ion effect Common ion effect

If there is an excess of one ion over the If there is an excess of one ion over the other :other :

- the concentration of the other is - the concentration of the other is suppressedsuppressed

- solubility of precipitate is decreased- solubility of precipitate is decreased

Example Example

Calculate the molar solubility of AgCl in (a) Calculate the molar solubility of AgCl in (a) water and (b) in 0.10M KCl solution. Ksp for water and (b) in 0.10M KCl solution. Ksp for AgCl is 1.7x10AgCl is 1.7x10-10-10..

SolutionSolution

(a)(a) Solubility of AgCl in waterSolubility of AgCl in waterAgCl AgAgCl Ag++ + Cl + Cl--

s s s s s=solubility(M)s=solubility(M)

Ksp = [AgKsp = [Ag++ ] [Cl ] [Cl-- ] ] = (s)(s)= (s)(s) = 1.7x10= 1.7x10-10-10

s = 1.3x10s = 1.3x10-5 -5 MMSolubility of AgCl in water = 1.3x10Solubility of AgCl in water = 1.3x10-5-5 M M

b) Solubility of AgCl in 0.10M KClb) Solubility of AgCl in 0.10M KClAgCl AgAgCl Ag+ + + Cl + Cl--

s s+0.10s s+0.10Ksp = [AgKsp = [Ag++ ] [Cl ] [Cl-- ] ]

= (s)(s+0.10)= (s)(s+0.10) = 1.7x10= 1.7x10-10-10

Since s is small compared to 0.10M, it can be neglectedSince s is small compared to 0.10M, it can be neglected

Ksp = (s)(0.10)Ksp = (s)(0.10) = 1.7x10= 1.7x10-10-10

s = 1.7x10s = 1.7x10-9-9 M MSolubility of AgCl in the presence of 0.10M Solubility of AgCl in the presence of 0.10M KCl is 1.7x10KCl is 1.7x10-9-9

Precipitation only occur when ionic product Precipitation only occur when ionic product greater than Kgreater than Kspsp value value

If ionic product equal KIf ionic product equal Kspsp, all the ions will , all the ions will

be in the solution without formation of be in the solution without formation of precipitateprecipitate