Chem 121 Lesson 04 Gravimetric Analysis (s).pdf

Chem 121 Lesson 04 GRAVIMETRIC ANALYSIS AND PRECIPITATION EQUILIBRIA  Gravimetry - measures the amount of analyte present in a given sample in terms of the mass of insoluble pure compound that is chemically related to the analyte. - one of the most accurate and precise methods of...

Chem 121 Lesson 04 GRAVIMETRIC ANALYSIS AND PRECIPITATION EQUILIBRIA  Gravimetry - measures the amount of analyte present in a given sample in terms of the mass of insoluble pure compound that is chemically related to the analyte. - one of the most accurate and precise methods of macro-quantitative analysis but one that is extremely tedious  Gravimetry Can be of different types: A. Precipitation Gravimetry B. Volatilization Gravimetry C. Gravimetric Titrimetry D. Electrogravimetry E. Atomic Mass Spectrometry Types of Gravimetric Analysis A. Precipitation Gravimetry - the analyte is converted to an insoluble precipitate which is then filtered, washed free of impurities, and then converted to a product of known composition by a suitable heat treatment. Types of Gravimetric Analysis B. Volatilization Gravimetry - the analyte or its decomposition products are volatilized at a suitable temperature - moisture determination is the most common example Types of Gravimetric Analysis B. Volatilization Gravimetry - Moisture determination - Can be direct or indirect moisture determination. Types of Gravimetric Analysis B. Volatilization Gravimetry - Moisture determination - Direct determination – water is collected on any of several solid dessicants and its mass is determined from the mass gain of the dessicant. - Indirect determination – the amount of water is determined by the loss of mass of the sample during heating. Types of Gravimetric Analysis B. Volatilization Gravimetry Forms of Water in the Solid - can be essential or non-essential water - essential water is an integral part of a solid in stoichiometric amount - non-essential water is retained by a solid due to physical forces; not in any sort of stoichiometric proportion Types of Gravimetric Analysis B. Volatilization Gravimetry Forms of Water in the Solid A. Types of essential water (1) water of crystallization is water in hydrated salts such as BaCl2 2H2O (2) water of constitution is water found in compounds that yield stoichiometric amount of water when heated or decompose such as water in Ca(OH)2 or KHSO4 Types of Gravimetric Analysis B. Volatilization Gravimetry Forms of Water in the Solid B. Types of non-essential water (1) adsorbed water is water retained on the surface of solids which is dependent on humidity, temperature, and specific surface area of the solid. Types of Gravimetric Analysis B. Volatilization Gravimetry Forms of Water in the Solid B. Types of non-essential water (2) sorbed water is water encountered with many colloidal substances such as starch, protein, charcoal, zeolite minerals, and silica gel. Types of Gravimetric Analysis B. Volatilization Gravimetry Forms of Water in the Solid B. Types of non-essential water (3) occluded water is water entrapped in microscopic pockets spaced irregularly throughout solid crystals. Types of Gravimetric Analysis B. Volatilization Gravimetry Carbonate Determination - The sample is ordinarily decomposed by acids to give CO2 which is readily evolved from solution by heat. CO2 is can be absorbed by absorbent. Types of Gravimetric Analysis B. Volatilization Gravimetry Sulfides And Sulfites Determination - The H2S or SO3 evolved from the sample through the addition of acid is collected on a suitable absorbent. Types of Gravimetric Analysis B. Volatilization Gravimetry Combustion Products (H2O AND CO2) - H2O and CO2 are collected selectively on weighed absorbents and the increase in mass serves as the analytical parameter. Types of Gravimetric Analysis C. Gravimetric Titrimetry - the mass of a reagent/ titrant required to react completely with the analyte provides the information needed to determine the concentration of the analyte Types of Gravimetric Analysis D. Electrogravimetry - the analyte is separated by deposition on an electrode by an electrical current Types of Gravimetric Analysis E. Atomic Mass Spectrometry - uses a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter.  Steps in Gravimetric Analysis 1. Sample 2. Precipitation of 3. Separation and 4. Weighing of the dissolution to a the Analyte Drying of the Analyte suitable (Addition of Analyte solvent. precipitating agent) The weight of the precipitate constitutes the measurement signal.  Gravimetric Analysis 1. Sample Dissolution - done to render the analyte in the sample in liquid form - considers factors like appropriate solvent, concentration range of the analyte, presence and concentration of other constituents, temperature, pH, etc.  Gravimetric Analysis 2. Precipitation - selecting the appropriate precipitating reagent is crucial; one that should react specifically with the analyte - guided by the precipitation process involving particle growth and nucleation  Gravimetric Analysis 2. Precipitation - Nucleation is a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid. - results to a large number of very fine particles. - Rate of nucleation increase enormously with increasing RSS  Gravimetric Analysis 2. Precipitation - particle growth: the subsequent growth after nucleation. - results to a few number of large particles - rate of particle growth is only moderately enhanced by high relative supersaturations.  Gravimetric Analysis 2. Precipitation - Precipitating agent should react specifically or selectively with the analyte - Specific reagent – reacts only with a single chemical species - Selective reagent – reacts with only a limited number of species  Gravimetric Analysis 2. Precipitation - precipitating reagent should react with the analyte to give a product/precipitate that is: i. of sufficiently low solubility ii. readily filtered and washed free of contaminants iii. unreactive with constituents of the atmosphere iv. of known composition after it is dried  Gravimetric Analysis 2. Precipitation i. of sufficiently low solubility - prevents the precipitate from redissolving in the solvent getting the most analyte possible.  Gravimetric Analysis 2. Precipitation ii. readily filtered and washed free of contaminants - prefers crystalline over colloidal precipitate - uses the Von Weimarn ratio or Relative Supersaturation (RSS)  Gravimetric Analysis 2. Precipitation ii. readily filtered and washed free of contaminants - Relative Supersaturation (RSS) !"# (RSS) = # Q – initial concentration of the solute S – solubility of the solute at equilibrium  Gravimetric Analysis 2. Precipitation ii. readily filtered and washed free of contaminants - low RSS yields to lower surface area forming few but larger (crystalline) precipitate - high RSS yields to higher surface area forming many small (colloidal) precipitate  Gravimetric Analysis 2. Precipitation ii. readily filtered and washed free of contaminants - low RSS leads to particle growth yielding crystalline but few precipitates - high RSS leads to nucleation yielding many but colloidal precipitates  Colloidal Particles: A Review - are so small that they are not retained by ordinary filter paper - are stable because all of the particles are either negatively or positively charged - The charge on a colloidal particle is determined by the charge of the lattice that is in excess when the precipitation is complete.  Colloidal Particles: A Review - a heterogenous mixture - suspended particles are molecules or ions ranging in size from 1 to 1000 nm - exhibits Tyndall effect  Colloidal Particles: A Review - classified according to the states of the dispersed phase and the dispersing medium.  Colloidal Particles: A Review Why do the particles remain suspended rather than forming larger aggregates and precipitating out? The answer is mainly due to electrostatic repulsion. A representation of two colloidal particles.  Colloidal Particles: A Review Colloidal particles form ionic barriers with the same charge, they repel each other and do not easily aggregate A representation of two colloidal particles.  Colloidal Particles: A Review How to neutralize or destroy the ionic barriers in colloids? - Heating with stirring - decreases the number of adsorbed ions per particle à decrease in the thickness of the electric double layer - Adding a suitable electrolyte - decreases the volume of solution that contains sufficient counter-ions to balance the charge of the primary adsorbed layer à shrinkage of the counter-ion layer  Treatment of Colloidal Precipitates - Digestion involves heating the solution for about an hour or more in the solution from which it was formed (the mother liquor). This helps to remove weakly bound water from the precipitate. - Aging involves storing the solution, unheated, overnight. This allows trapped contaminates time to “work their way out”. - Both can result in a denser precipitate that is easier to filter.  Peptization of Colloidal Precipitates Peptization - a coagulated colloid returns to its dispersed state Washing a coagulated precipitate to get rid of excess counter ions or trapped impurities can result in peptization The use of volatile electrolyte to wash the precipitate is helpful. The volatile electrolytes displace the less volatile counter ions. Heating the precipitate during drying can remove the volatile electrolytes.  Gravimetric Analysis 2. Precipitation iii. unreactive with air, atmosphere or water - precipitate must not be hygroscopic in order to keep the integrity of the precipitate  Gravimetric Analysis 2. Precipitation iv. known chemical composition - precipitate must be of known identity since it may necessitate further stoichiometric calculations  Gravimetric Analysis 2. Precipitation - ends in digestion - the process wherein when the precipitate is allowed to stand where the larger crystals grow at the expense of smaller ones (Ostwald ripening)  Gravimetric Analysis 2. Precipitation - ends in digestion - the improves purity and crystallinity of the precipitate - generally done at room, or relatively high temperature Precipitation from Homogeneous Solution When a precipitating agent is added as a solution to the analyte there will always be locally high concentrations. Precipitation from homogenous solution is the approach to solve this problem. The precipitate is formed by slow generation of a precipitating reagent homogenously throughout the solution.  Precipitation from Homogeneous Solution Basis of homogenous precipitation: - The reagent is added in an unreactive form to the sample solution which permits a complete mixing of all materials. - Some property of the solution is changed to slowly convert the reagent to a reactive form. - The solution slowly and uniformly changes.  Gravimetric Analysis C. Separation of the Analyte - done mainly by filtration at room or cooled temperature - washing removes any impurities obtained together with the precipitate during filtration  Gravimetric Analysis C. Separation of the Analyte - washing must be carried out using a solvent that keeps the precipitate insoluble but renders the impurities soluble - completeness of washing must be ensured  Gravimetric Analysis D. Weighing the Analyte - employs pre-weighing procedure like drying, ignition or combination of both - Drying is done to remove any solvent molecules adhering to the precipitate acquired from the wash liquid usually done at 110OC to 120OC for 1 to 2 hours  Gravimetric Analysis D. Weighing the Analyte - employs pre-weighing procedure like drying, ignition or combination of both - Ignition at a much higher temperature is done when the precipitate needs to be converted into a more suitable form before weighing Problems Associated with Precipitation Method Coprecipitation - It is a process in which normally soluble compounds are carried out of solution by a precipitate. This does not include materials that would normally be insoluble. Problems Associated with Precipitation Method Types of coprecipitation 1.Surface adsorption 2.Mixed-crystal formation 3.Occlusion 4.Mechanical entrapment  Coprecipitation 1.Surface adsorption – the surface of the precipitate contain some primary adsorbed ions. The adsorbed ions become surface contaminants. Dealing with surface adsorption: - Washing with water – slight effect - Washing with volatile electrolyte – better approach - Reprecipitation – last resort since it is very tedious  Coprecipitation 2. Mixed-crystal formation – occurs when similar ions replace the analyte ion in the crystal lattice during precipitation Similar ions à have the same charge and have sizes within 5% of the analyte ion or if their ionic diameters are sufficiently close to fit into the same crystal lattice  Coprecipitation 2. Mixed-crystal formation Ions that interfere with each other K+, 133 pm NH4+, 148 pm Sr2+, 113 pm Ba2+, 135 pm Mn2+, 80 pm Cd2+, 97 pm  Coprecipitation 2. Mixed-crystal formation - remove interferences prior the precipitation process - select a different precipitating agent that would be more specific.  Coprecipitation 3. Occlusion – a compound is trapped within a pocket formed during rapid precipitation. Foreign ions in the counter-ion layer may become trapped or occluded within the growing crystal.  Coprecipitation 4. Mechanical entrapment – occurs when crystals lie close together during growth and in doing so trap a portion of the solution in tiny pockets. While the solvent can be removed, the trapped ions will remain after drying.  Coprecipitation 4. Mechanical entrapment Both occlusion and mechanical entrapment are at minimum when the rate of precipitate formation is low, i.e. precipitating at warm, dilute solutions. Digestion and aging also lessen the chance of the two above problems.  Impurities in Precipitates - occur as a result of coprecipitation and can form through: 1. Occlusion 2. Surface Adsorption 3. Isomorphous Replacement 4. Post-precipitation  Impurities in Precipitates 1. Occlusion - process wherein foreign materials which are not part of the crystal structure becomes trapped within the crystal  Impurities in Precipitates 2. Surface Adsorption - adherence of contaminants forming a primary adsorbed layer on the surface of the precipitate usually as a result of an excess ion  Impurities in Precipitates 3. Isomorphous Replacement - the crystal to be precipitated gets replaced by another ion of the same type of formula and crystallize in the same geometric form  Impurities in Precipitates 4. Post-precipitation - occur when the mixture is allowed to stand too long that another substance slowly forms another precipitate with the precipitating agent  Summary: Gravimetric Analysis Gravimetric analysis exhibits the following characteristics: - A relatively slow method of analysis. - Minimal requirements. Major equipment is a good balance and an oven. - No calibration is required. - Accuracy is 1-2 parts per thousand. - Sensitivity is quite low. The analyte concentration should be over 1%. - Not very specific but can be made reasonably selective. Application of Gravimetric Methods - Gravimetric methods have been developed for most inorganic anions and cations as well as neutral species such as water, sulfur dioxide, carbon dioxide, and iodine. - A variety of organic substances can also be easily determined gravimetrically. Examples: lactose in milk products, salicylates in drug formulations, nicotine in pesticides etc.  Gravimetric Analysis GRAVIMETRIC FACTOR (GF) - a stoichiometric factor representing the weight of the analyte per unit weight of the precipitate and is given by the equation: !"!"!#$%& $ 𝐺𝐹 = x !" '(&)*'*%!%& % - where a and b – stoichiometric ratio of the analyte and the precipitate  Gravimetric Analysis WEIGHT OF THE ANALYTE - usually is the substance sought in gravimetric analysis and is computed from the mass of the weighed precipitate and the gravimetric factor Weightanalyte = Weightprecipitate x GF  Gravimetric Analysis WEIGHT OF THE ANALYTE - when expressed as percent, this is equal to: "&'()*!"!#$%& Percent Analyte = "&'()* +!,'#& x 100  Sample Problem Set up the gravimetric factors (GF) for the following: Sought Weight GF Fe Fe2O3 2MWFe/MWFe2O3 Fe as Fe3O4 Fe2O3 2MWFe3O4/3MWFe2O3 MgO Mg2P2O7 2MWMgO/MWMg2P2O7  Sample Problem What weight of Fe3O4 will furnish 0.5430 g Fe2O3?  Sample Problem An iron ore was analyzed by dissolving a 1.1324-g sample in concentrated HCl. The resulting solution was diluted with water, and the iron(III) was precipitated as the hydrous oxide Fe2O3 · xH2O by the addition of NH3. After filtration and washing, the residue was ignited at a high temperature to give 0.5394 g of pure Fe2O3 (159.69 g/mol). Calculate (a) the % Fe (55.847 g/mol) and (b) the % Fe3O4 (231.54 g/mol) in the sample.  Sample Problem A 0.2356-g sample containing only NaCl (58.44 g/mol) and BaCl2 (208.23 g/mol) yielded 0.4637 g of dried AgCl (143.32 g/mol). Calculate the percent of each halogen compound in the sample.  Sample Problem A mixture of calcium oxide and barium oxide weighs 0.6411 and yields 1.1201 g of mixed anhydrous sulfates. Find the %Ba and % Ca in the original mixture.  Sample Problem In the analysis of a 0.5 g sample of feldspar, a mixture of sodium and potassium chlorides is obtained weighing 0.1180 g. Subsequent treatment with AgNO3 furnishes 0.2451 g of AgCl. What is the %Na2O and %K2O in the sample?  THANK YOU! I hope you learn something new today!