3 Gravimetric Analysis

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How to Perform a Successful Gravimetric Analysis

Gravimetric Analysis1Gravimetric Analysis- What is It?Definition:a precipitation or volatilization method based on the determination of weight of a substance of known composition that is chemically related to the analyte

analyte - chemical element or compound of interestGravimetric Analysis- What is It?Reaction:aA + rR -----> AaRr pptwhere:a is # of moles of analyte Ar is # of moles of reagent RAaRr is a pure, insoluble precipitatewhich we can dry and weigh or ignite to convert to something we can weighppt=precipitateT.W.Richards1914 Nobel Prize to T.W.Richards (Harvard University) for the atomic weights of Ag, Cl, and N

Richards and his group determined atomic weights of 55 of the 92 known elements using gravimetry

T.W.RichardsEvery substance must be assumed to be impure, every reaction must be assumed to be incomplete, every method of measurement must be assumed to contain some constant error, until proof to the contrary can be obtained. As little as possible must be taken for granted.6How to Perform a Successful Gravimetric AnalysisWhat steps are needed?Sampled dried, triplicate portions weighedPreparation of the solutionPrecipitationDigestionFiltrationWashingDrying or ignitingWeighingCalculation7Gravimetric Analysis Gravimetric Analysis one of the most accurate and precise methods of macro-quantitative analysis.Analyte selectively converted to an insoluble form.Measurement of mass of material Correlate with chemical compositionWhy?SimpleOften required for high precision

8Determination of mass

Direct By differenceNaHCO3 + H2SO4CO2 + H2O +NaHSO3Determination of NaHCO3 10Desirable properties of analytical precipitatesReadily filtered and purifiedLow solubility, preventing losses during filtration and washingStable final form (unreactive)Known composition after drying or ignition

Suction FiltrationFilter flaskBuchner funnelFilter paperGlass fritFilter adapterHeavy-walled rubber tubingWater aspirator

Suction FiltrationMother liquor

13Kinds of Precipitating reagents:Selective Ag+ + Halides (X-) AgX(s) Ag+ + CNS- AgCNS(s)SpecificDimethylglyoxime (DMG)2 DMG + Ni2+ Ni(DMG)2(s) + 2 H+14Mechanism of Precipitation15

Fig. 10.2. Representation of silver chloride colloidal particleand adsorptive layers when Cl- is in excess. Cl- adsorbs on the particles when in excess (primary layer).A counter layer of cations forms. The neutral double layer causes the colloidal particles to coagulate.Washing with water will dilute the counter 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). Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

16Filterability of Precipitates

Colloidal suspensions10-7 to 10-4 cm diameterNormally remain suspendedVery difficult to filterCrystalline precipitates> tenths of mm diameterNormally settle out spontaneouslyReadily filterable 17R.S.S = (Q-S)/S

Precipitate formation affected by:Solubility of PrecipitateTemperature Concentration of reagensRate of mixing-Relative Super Saturation(R.S.S) R.S.S = (Q-S)/SQ = Instantaneous Concentrations of the mixed reagents S = Equilibrium Solubility of PrecipitateSmaller R.S.S leads to crystalline precipitates.Q S18Conditions for Analytical PrecipitationPrecipitation from hot solution The molar solubility (S) of precipitates increases with an increase in temperatureAn increase in S decreases the supersaturation and increases the size of the particle. Precipitation from dilute solution This keeps the molar concentration of the mixed reagents low. Slow addition of precipitating reagent and thorough stirring keeps Q low. (Uniform stirring prevents high local concentrations of the precipitating agent.) 19Conditions for Analytical PrecipitationPrecipitation at a pH near the acidic end of the pH range in which the precipitate is quantitative. Many precipitates are more soluble at the lower (more acidic) pH values and so the rate of precipitation is slower. Digestion of the precipitate. The digestion period can lead to improvements in the organization of atoms within the crystalline nuclei, such as expulsion of foreign atoms (or other impurities). 20 Important Factors for Gravimetric AnalysisNucleation (RSS)nIndividual ions/atoms/molecules coalesce to form nucleiParticle Growth (RSS)nCondensation of ions/atoms/molecules with existing nuclei forming larger particles which settle outCompetition21Important Factors for Gravimetric AnalysisColloidal SuspensionColloidal particles remain suspended due to: small size adsorbed ions giving a net + or charge(Brownian motion)

22 Important Factors for Gravimetric AnalysisCoagulation, agglomerationSuspended colloidal particles coalesce to form larger filterable particles by:Heatingstirring adding inert electrolyte

232425AgCl (s)AgCl (colloid)

Peptization

Re-dissolution of coagulated colloids by : washing and removing inert electrolyte26DigestionHeating the precipitate within the mother liquor (or solution from which it precipitated) for a certain period of time to encourage densification of nuclei.During digestion, small particles dissolve and larger ones grow. This process helps produce larger crystals that are more easily filtered from solutionDT27

Fig. 10.1. 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. Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)

28Ideal Analytical PrecipitationIn an ideal world, an analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered. The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed. The precipitate should also be "insoluble" (i.e., low solubility such that loses from dissolution would be minimal).

29Increasing PurityRe-precipitationa procedure including washing away the mother liquor, redisolving the precipitate, and precipitating the product againStructure of a ColloidWhy do two colloid particles resist aggregating to form a crystal?If two colloid particles, each with a negative charge, come close to one another, they will repel!So colloid particles are stable and resist crystal formation.30Coagulating a ColloidWhat can be done to overcome this colloid stability and force crystals to form?High heat, stirring, only a slight excess of the excess reagent, and the addition of an electrolyte can force a colloid to coagulate into crystals.31Coagulating a ColloidHigh heat, initially with stirring, is thought to lower the thickness of the double layer, thus making it easier for two colloid particles to collide and coagulate. The higher kinetic energy will also helpthem gain enough energy to overcome the repulsion.32Coagulating a ColloidIf too much of the excess reagent is added, then the double layer increases in volume as more of the excess solute ions will be adsorbed to the surface, which in turn requires a larger counter-ion layer.33Coagulating a ColloidSo it is important to make sure that there is only a slight excess of the excess reagent. Thus the diameter of the double layer will be minimized, enabling neighboring colloids to coagulate.34Coagulating a ColloidOn the other hand, the addition of a suitable electrolyte like nitric acid or hydrochloric acid may also lower the diameter of the double layer. Now the high concentration of the appropriate ion will make it easier to form the counter-ion layer and its thickness will be reduced. Again, two neighboring colloids can get closer together, making it easier to coagulate.35DigestingOnce a colloid starts to coagulate, it is best to digest the solution. Digestion is when the heated solution with the coagulating crystals sits undisturbed for an hour or more.36DigestingTypically, the colloidal suspension is stirred with heating until crystals start to coagulate. Then stirring is stopped, and the solution is heated to almost boiling for at least 10 minutes. Finally, the solution is allowed to cool slowly and sit undisturbed for several hours.Digestion results in larger, purer crystals which are easier to filter.37FiltrationOnce the crystals have formed and digested, they need to be filtered.The washing step can be a problem, as peptization of the coagulated colloid may occur. This means that the coagulated colloid reverts to a smaller colloidal particle.38FiltrationWashing with pure water often causes this problem as this lowers the concentration of counter-ions, which then causes the double layer to increase in volume, and the coagulated solid may break back into smaller colloids. These colloids will then go right through the filter, and the filtrate may look cloudy.39FiltrationTypically, the wash solvent is a dilute solution of the electrolyte. This keeps the double layer intact, minimizing peptization. This electrolyte will then volatilize during the drying step.The filtered and washed crystals are then dried to constant mass.40Coprecipitation of ImpuritiesDuring the precipitation process, other soluble compounds may also be removed from the solution phase.These other compounds are carried out of solution by the desired crystals. They are impurities and they are said to have coprecipitated. These are NOT other insoluble compounds, but by several mechanisms, have been carried out of solution.41Copr