3.2 Gravimetry - GRAVIMETRIC ANALYSIS(Part(Part 2 1 SAMPLE...

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Unformatted text preview: GRAVIMETRIC ANALYSIS (Part (Part 2) 1. SAMPLE PREPARATION STEPS STEPS IN GRAVIMETRIC ANALYSIS Solids Solids are dried, weighed and dissolved in a dissolved suitable solvent Preliminary Preliminary elimination of interfering materials Purpose of solution preparation • To maintain low solubility of the precipitate • To obtain precipitate suitable for filtration • To help to mask potential interferences 1. Sample Preparation (Drying, weighing (Drying, and dissolving sample) 2. Precipitation (Adding precipitant) (Adding 3. Digestion (Coagulating using heat) (Coagulating 4. Filtration (Separating from mother (Separating liquor) 5. Washing the precipitate (Peptization) (Peptization) 6. Drying/Ignition and Weighing to constant weight 7. Data Calculation (By weight) (By Factors influencing solution preparation – Solution volume during precipitation – Concentration range of substance – Presence of other constituents – Temperature – pH 1 2 Steps in Precipitate formation 2. 2. PRECIPITATION (Forming Precipitate) Analyte + Precipitant An An analytical precipitate for gravimetric perfect analysis should consist of perfect crystals large enough to be easily washed and filtered - The perfect crystal would be free from free impurities impurities and be large enough so that it presented presented a minimum surface area onto which foreign ions could be adsorbed The The precipitate should also be ‘insoluble’ ‘insoluble’ - Low solubility such that loses from dissolution would be minimal Supersaturation (High (High Solute in Solution > @ equilibrium) Nucleation A Process where molecules in solution join randomly randomly to form a small aggregate called nucleus Particle growth or Crystal growth More particles are added to the nucleus to form larger particles, diameter 1-100 nm 1Colloids (1-100nm) (1- Fine Crystals Coarse Crystals (>10µm, Best) 4 3 • Induction period - The time before nucleation occurs after the addition of the precipitating agent to the solution (Ranges from milliseconds to several min.) (Ranges • Nucleation • Formation of small, stable aggregates or nuclei of precipitate • Nuclei have sizes down to ~1 nm, composed of a few atoms, and may be up to 1010 nuclei per few mole mole of analyte • Excess ions from solution collect around the nuclei • Particle growth - Particles grow with the addition of ions of the precipitate until system comes to equilibrium 5 Relative Relative Supersaturation The von Weimarn ratio = (Q-S)/S (Q• Von Weimarn showed that particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation • Relative supersaturation = (Q-S)/S (Q– Where Q is the molar concentration of the mixed mixed reagents before any precipitation occurs and S is the molar solubility of the product (precipitate) when the system has reached equilibrium – For the best possible results, conditions need to be adjusted such that Q is low as is possible and S is large is 6 Summary of Supersaturation Conditions for Analytical Precipitation To produce large, pure, insoluble perfect crystals that are easily washed and filtered by adjusting conditions so that (Q ↓) and (S ↑) Precipitation Precipitation from dilute solution (Q ↓) dilute Precipitate Precipitate with stirring (Q ↓) stirring [Stirring [Stirring prevents local excesses of the reagent] Precipitation Precipitation at high temperature (S ↑) high [The [The solubility of precipitates increases with T] Precipitating Precipitating at acidic (low) pH acidic Digesting Digesting the precipitate Precipitating Precipitating from homogeneous solution homogeneous Large (QLarge (Q-S) X High relative supersaturation - Nucleation is favored - Many particles but small size (small crystals, high surface area) - Colloidal precipitates form Small (Q-S) (QLow relative supersaturation - Particle growth is predominant - Few particles but larger size (large crystals, low surface area) - Crystalline precipitates form 7 8 • Precipitation from hot solution – The molar solubility (S) of precipitates increases with an increase in temperature – An increase in S decreases the supersaturation and increases the size of the particle • Precipitation 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 slower so the rate of precipitation is slower • 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.) • 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) Examples of Homogeneous Precipitation (a) Slow increase of pH Example: Example: Urea (NH2CONH2) generates (OH-) ions (NH2)2CO + 3H2O → CO2 + 2NH4+ + 2OH− - The pH is increased (more alkaline) - Liberated ammonia react with metal ions to form metal hydroxide precipitates, under controlled heating heating just below 100oC for 1-2 hrs - To precipitate hydroxides of Al, Ga, Th, Bi, Fe, Sn • Precipitation From Homogeneous Solution (A technique to increase precipitation) A Technique where a precipitating agent is not added but slowly generated throughout but the solution by a slow homogeneous chemical reaction Advantages: • No local excesses of precipitating agent of • Since (Q-S) is kept low dense and more (Qpure precipitates are formed • Substances precipitate as well-formed well(as crystals (as apposed to amorphous solid) Other examples: (b) Cation release (c) Precipitation from mixed solvents (eg change the pH or polarity of solvents) (d) (d) Anion release (e) Valency change 11 12 Example Example of Precipitation Mechanism ‘Formation Of AgCl(p) from AgNO3 and Cl-’ Ag Cl • AgNO3 is added to an acidic solution of Clacidic solution Cl • AgCl(s) nuclei form with a surface layer of ions • Primary adsorbed ions (Ag+) and Secondary counter ions adsorbed and Secondary (NO3-) form an electrostatic layer around the nucleus electrostatic • The counter ions tend to aggregate around counter [AgCl:Ag] the [AgCl:Ag]+center because these centers +ve (excess have a net +ve charge (excess Ag+) and thus additional additional -ve charge is required to maintain electrical neutrality • Counter ions are less tightly held than the primary primary adsorbed ions and the counter ion layer layer is diffuse and contains ions other than those of the counter ions • The layers of charged ions associated with the surface of the nuclei are known as the ‘ELECTRIC DOUBLE LAYER’ Example: Illustration of the formation of BaSO4(s) as BaCl2 is added into Na2SO4 solution Some Some terms… • Adsorption is a process in which a substance (gas, (gas, liquid, or solid) condenses onto the surface of a solid (eg Ag+ on AgCl(s)) (eg electric • The electric double layer consists of a layer of charge associated with the surface of the particles and a layer with a net +ve charge in in the solution surrounding the particles • A colloid is a finely divided particle (typically colloid (typically with diameters from 10 nm to 1 mm) that forms a stable dispersion within a medium (air or liquid) Ba2+:Primary layer - Excess Ba2+ from the Ba is precipitant, BaCl2 is attracted to SO4 SO - Layer is +ve charged Cl-: Secondary layer - as the BaSO4 ppt is BaSO formed, the Cl- ions Cl (from (from BaCl2) are repelled and form a 2nd layer - Layer is -ve charged 16 Impurities In Precipitates 3. Digestion • Heating 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 (Ostwald ripening) – This process helps produce larger crystals that that are more easily filtered from solution ∆T Impurities in precipitates are sources of gravimetric interference that occur by 1. Co-Precipitation - Soluble compounds are 1. Cocarried out of solution onto the precipitate Types Types of co-precipitation: co(a) Surface adsorption (b) Inclusion (Mixed-crystal formation) (c) Occlusion (Mechanical entrapment) 1. Post Precipitation - A foreign compound precipitates on top of the desired precipitate Example: Post precipitation of magnesium oxalate magnesium occurs if a precipitate of calcium oxalate is allowed calcium to stand too long before being filtered 18 • SURFACE ADSORPTION - Unwanted material is adsorbed onto the surface of the ppt - To overcome problem: - Digestion of a precipitate of reduces reduces the relative surface surface area, so as the areas areas available for adsorption of impurities - Washing helps to remove impurities bound to the surface of precipitates • CO-PRECIPITATION CO- is the precipitation of an unwanted species along with the analyte of interest - occurs to some degree in every gravimetric analysis - It is a major factor for precipitations of BaSO BaSO4 and those involving hydrous oxides oxides - It cannot be avoided, but may be minimized by careful precipitation and a thorough washing of the precipitate 3 Particle Volum = 4/3(πr ) e 2 Particle Surface Area = 4(πr ) Particle Volum e Particle Surface Area 0 2 4 6 8 10 12 Particle Radius (A.U.) Continued… Continued… 19 Examples of Surface Adsorption (1) BaSO4 precipitate adsorb its own common adsorb ions (Ba2+ or SO42-) depending on which is (Ba present in excess in the solution (2) Zn, Cd and Mn impurities may form Fe(OH) hydroxides with precipitation of Fe(OH)3 (3) Cl- (instead of Ag+) may be adsorbed on the AgCl precipitate surface AgCl precipitate surface when AgNO3 is added added to excess KCl (Figure b in next slide) (Figure - The surface of the precipitate has a minus charge because of the Cl- The Cl- ions will attract ions of opposite charge (or counter ions) such as K+, which counter then surround the precipitate particles Continued… Two cases of error may occur: • Co precipitation of a high MW counter ion gives a (eg precipitate weight that is too high (eg NO3>Cl- ) • Co precipitation of a low MW counter ion gives a result of precipitate weight that is too low 21 Example Question In determining SO42- in a sample, the SO42- was precipitated as BaSO4, but the precipitate was contaminated. 22 INCLUSION (MixedINCLUSION (Mixed-crystal formation) (a) What is the precipitant used: Ba2+ (BaCl2) Ba (BaCl (b) What is the primary layer contaminant: Ba2+ Ba (c) What is the secondary layer contaminant: ClCl (d) Name the type of contamination: Co-precipitation by surface adsorption Co(e) (e) Suggest ways to removed/reduced contamination Washing, digestion, etc…(refer to later slides) 23 • Interferences are incorporated into the incorporated precipitating crystal either as - substitute elements on a lattice site, or - extra elements between sites (interstitial) • Can occur if two ions/cpds have the same charge, charge, type of formula or sufficiently close ionic ionic diameters to fit into the same crystal lattice (eg Sr2+=113 pm and Ba2+=135 pm) (eg One One compound replaces part of another in mixed a crystal, forming mixed crystals • Inclusions occur throughout the crystal (not only on the surface), thus changes in particle size will not affect the extent of inclusion 24 14 • POST-PRECIPITATION POST- Sometimes a precipitate in contact with the mother mother liquor is contaminated by the pptn of is an impurity • OCCLUSION - occurs if crystal growth is crystal too rapid, some counter ions do not have time to escape from the surface so trapped it becomes trapped (occluded) within a growing growing crystal PEPTIZATION* AgCl(s) AgCl(s) → AgCl(colloid) - A procedure where the precipitate is washed & filtered, but part of the precipitate precipitate reverts to the colloidal colloidal form because supporting electrolyte is missing - Cooling the system with ice minimizes water minimizes loss of precipitate due to dissolution • MECHANICAL ENTRAPMENT - occurs if two crystals grow two together and trap a species in the space between them (These crystals lie close together during growth) 26 25 Increasing Purity Of Precipitates • Precipitation in the presence of electrolyte - Coulombic repulsion is diminished in the presence of electrolyte because of a compression of the volume of the ionic environment • Re-precipitation Re– a procedure including washing away the mother liquor, redissolving the precipitate, and precipitating the product again • Drying the solid – Generally the solids are dried at ~120 oC, but conditions for drying can vary considerably – To determine the correct drying regime, a thermogravimetric (TGA) balance may be used. • Digestion - Raising the temperature will increase the collision energy for colloidal particles and overcome Coulombic repulsion, leading to formation of larger particles (aka coalescence) Continued… 4. FILTERING PRECIPITATES • • • - Filtration help separate the precipitate from impurities and the mother liquor ‘Ashless’ Filter Paper (for crystalline precipitates) Very little ash from the paper is left behind after it is burnt, leaving mostly dried precipitate Sintered Glass Filter Crucibles (suitable for curdy precipitates) precipitates) The filter can withstand up to 500oC of drying Sintered Porcelain Filter Crucibles The filter can withstand temperatures up to 1000oC The choice of a medium depends on the type of precipitate and on the temperature at which the precipitate is to be heated 29 Apparatus For Suction Filtration • Buchner funnel • Sintered Glass Crucibles • Filter paper • Water aspirator • Heavy-walled rubber Heavytubing • Filter flask 30 5. WASHING PRECIPITATES (Peptization) Testing the completeness of washing WASHING WASHING removes adsorbed impurities and the mother liquor Example: We determine Cl- (analyte) by We precipitating AgCl(s) using AgNO3 as the precipitant (precipitating reagent) Crystalline Crystalline precipitates (large) are washed by rinsing with small amounts of pure water water (1) The filtrate (liquid collected after filtration) is (liquid tested for unreacted or excess Ag+ (from AgNO AgNO3) by adding NaCl or dilute HCl Colloids Colloids are washed with volatile electrolyte electrolyte solution (eg. HNO3, NH4OH for AgCl, but KNO3 is not used because it is nonvolatile) (2) If the solution becomes cloudy it means the washing process is not yet complete (because (because AgCl still forms) * Washing colloids with pure water will dilute and remove foreign ions, and the counter ions will occupy a larger volume PEPTIZATION is a process whereby the charge of the primary ions repel each other and revert back to the colloidal form (This results in the loss/removal of colloids through the filter) (3) Washing is continued until the test is negative (absence of precipitate) (absence 31 32 APPARATUS and TECHNIQUE FOR DRYING 6. DRYING/IGNITION AND WEIGHING Filtered Filtered precipitates are heated (ignited) to produce a constant weight of precipitate Drying Drying removes the solvent(s) and any volatile species carried down with the precipitate Drying Drying is done by heating at 110oC-120oC for 1-2 1hours (for the precipitate to be a form suitable for (for weighing) weighing) If If a precipitate must be converted to a more suitable form for weighing, ignition at a much higher temperature is required After After heating, the precipitate is cooled to room temperature in a DESICCATOR prior to weighing DESICCATOR Heating, Heating, cooling and weighing is repeated until a (or constant weight is achieved (or the difference between two consecutive weighing is <0.0002 g) Desiccators Set up for drying Solids in the oven 33 34 7. CALCULATING RESULTS Examples of Gravimetric Factor (GF) Gravimetric The result of a gravimetric determination is reported as percentage analyte, %A Analyte CaO FeS UO2(NO3)2 Cr2O3 a, b = moles according to formula Fwt = atomic wt or molecular wt of the analyte (# of molecules or atoms in the numerator and denominator must be chemically equivalent) GF = gravimetric factor* PPT CaCO3 BaSO4 U 3 O8 Ag2CrO4 GF CaO/CaCO3 FeS/BaSO4 3UO2(NO3)2/U3O8 Cr2O3/2Ag2CrO4 FW or MW is used as constant values to determine GF 35 36 Sample Problem 2 Sample Sample Problem 1 A mixture containing FeCl3 and AlCl3 weighs 5.95g. and They are converted to oxides and ignited to Fe2O3 and Al2O3. The oxide mixture weighs 2.62g. Calculate the % of Fe and Al in the original mixture. 37 Other Other Gravimetric Techniques ‘Gravimetric Combustion Analysis’ Determines the carbon and hydrogen content of organic compounds burned in excess O2 H2O trap CO2 trap Guard tube prevents H2O and CO2 entering from CO entering the the air 38 ‘Thermal Gravimetric Analysis (TGA)’ • Evaluate thermal decomposition and stability of materials eg polymers, resins, rubbers, explosives • Information on bulk composition of compounds • Precisely monitors weight loss of a sample in a given gas environment environment (atmosphere) as a function of temperature function and/or time • Atmosphere: N2,O2, air or He • Temp: ambient to 1000°C 1000° • Records the first derivative of the mass loss ...
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