LECTURE 3 & 4

• Gravimetric are quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.

• Mass measurements made with an analytical balance, yields highly accurate and precise data.

• Some analytical methods that based on mass measurement are: Precipitation gravimetry Volatilization gravimetry Electrogravimetry

Precipitation GravimetryPrecipitation Gravimetry

•The analyte is separated from a solution of the samples as a precipitate.•Is converted to a known composition that can be weighed.

Volatilization GravimetryVolatilization Gravimetry

•The analyte is separated from other constituents of a sample by conversion to a gas of known chemical composition.

ElectrogravimetryElectrogravimetry

•The analyte is separated by deposition on an electrode by an electrical current.

• The analyte is converted to a sparingly soluble precipitate.• Filtered, washed free of impurities, converted to a product

of known composition by suitable heat treatment and weighed.

• Example : a precipitation method for determining calcium in natural waters.

• An excess of oxalic acid, H2C2O4, is added to the solution of the sample.

• Then, ammonia is added, neutralizes the acid and cause the calcium to precipitate as calcium oxalate. The reaction are:

• The precipitate is filtered using a weighed filtering crucible, then dried and ignited.

• Converts the precipitate to calcium oxide.

• After cooling, the crucible and precipitate are weighed.• Mass of calcium oxide is determined by subtracting the

known mass of the crucible.

• Generally, there are 4 steps in precipitate gravimetric:

1. A sample substance of unknown composition is dissolved in water a react with another substance to form precipitate.

2. The precipitate is filtered off, dried and weighed. 3. Knowing the mass & chemical formula of precipitate,

the mass of a particular chemical component of the original sample can be calculated.

4. Finally, determine the % composition by mass of the component in the original compound.

• A gravimetric precipitating agent should react specifically or at least selectively with analyte.

• Specific reagentSpecific reagent: react only with a single chemical species.• Selective reagentsSelective reagents: react with a limited number of species.• The ideal precipitating reagent would give a product that is:

easily filtered & washed free of contaminants sufficiently low solubility that no significant loss of the analyte unreactive with constituents of the atmosphere known chemical composition after it is dried

• AgNO3 is an example of selective reagent. The common ions that it precipitates from acidic solution are Cl-, Br-, I- & SCN-

• Large size generally desirable - easy to filter, wash free of impurities and usually purer than precipitates made up of fine particles.

• Particles size of solid formed varies enormously.

• Colloidal suspensions, whose tiny particles are invisible to naked eye (10-7 to 10-4 cm in diameter).

• Colloidal particles show no tendency to settle from solution & not easily filtered.

• Filtering medium must be so small but time wasting, however, the individual colloidal particles can be made to stick together with suitable treatment, thus giving a filterable mass.

Colloidal suspensionsColloidal suspensions

•10-7 to 10-4 cm diameter•Normally remain suspended•Very difficult to filter

Crystalline suspensionsCrystalline suspensions

• tenths of mm diameter• Normally settle out spontaneously• Readily filterable

• Particle size of a precipitate is influenced by:

i. Precipitate solubilityii. Temperatureiii. Reactant concentrationsiv. Rate at which the reactants are mixed.

• Precipitate formation is affected by relative supersaturation,

where Q is the conc. of the solute at any instant and S is its equilibrium solubility.

• When (Q-S)/S is large, the precipitate tends to be colloidal, when (Q-S)/S is small, crystalline solid is more likely.

• The effect of relative supersaturation can be explained if we assume that precipitates form in two ways:

i. by nucleationii. by particle growth

• A few ions, atoms, or molecules come together to form a stable solid or nuclei.

• Form on the surface of suspended solid contaminants, such as dust particles.

• Further precipitation involve a competition between additional nucleation and growth on existing nuclei (particle growth).

• If nucleation predominates, a precipitate containing a large number of small particle results

• If growth predominates, a smaller number of larger particles is produced.

• When precipitate is formed at high relative supersaturation, nucleation is the major precipitation mechanism.

• At low relative supersaturation, the rate of particle growth tends to predominate, thus deposition of solid on existing particles occur; a crystalline suspension results.

Increasing SIncreasing S = elevate temp. & pH control

Decrease QDecrease Q = use dilute solution, slowly add reagent & stirring the solution.

• Individual particles are so small

• Fortunately, we can coagulate, or agglomerate the individual particles of most colloids to give a filterable product.

Coagulate of Colloids

• Can be hastened by heating, stirring, and by adding an electrolyte to the medium.

• Colloidal suspension are stable because all of the particles of the colloid are either positively or negatively charged.

• This charge result from cations or anions that are bound to the surface of particles

• Colloidal particles will migrate when placed in an electrical field.

• The process by which ions are retained on the surface of a solid is known as adsorption.

• Adsorption = process in which a substance (gas,liquid or solid) is held on the surface of the solid.

• Absorption = involves retention of substance within the pores of a solid.

• Example: when AgNO3 is added to solution containing chloride ion (Cl-), the colloidal particles of precipitate are negatively charged due to the adsorption of excess chloride ions.

• The charge become positive after enough AgNO3 added to provide an excess of silver ions.

• Attached directly to the solid surface is the primary adsorption layer, which consists mainly of adsorbed silver ions.

• Surrounding the charge particles is a layer of solution, called counter-ion layer, which contains sufficient excess of negative ions to balance the charge on the surface of the particle.

• Primary adsorbed silver ions and negative counter-ion layer constitute an electric double layer, imparts stability to the colloidal suspension.

• This double layer exerts an electrostatic repulsive force that prevents particles form colliding and adhering.

• Coagulation of colloidal suspension can occur by:

i. Heating and accompanied by stirring

ii. Increase the electrolyte concentration

• Heating the colloid while stirring Heating the colloid while stirring will decreases the number of absorbed ions per particle & reduce the size of counter ion layer, easier for the particles to approach each other.

• Increasing the electrolyte concentration Increasing the electrolyte concentration will reduce the volume of solution that contains sufficient ions of opposite charge to neutralize the particle.

• The result is the smaller structure is formed.

• Is the process by which a coagulated colloid reverts to its original dispersed state.

• During washing of the precipitate, some electrolyte which responsible for its coagulation is leached.

• Removal of this electrolyte will increasing the volume of counter ion layer.

• The repulsive forces are reestablished cause colloid to form its original state.

Dilemma??• Washing = needed to minimize contamination• Washing = risk of losses resulting from peptization

How to solve?

•Washing the precipitate with a solution containing an electrolyte that volatilizes when the precipitate is dried or ignited.

•Example: a dilute nitric acid is use to wash silver chloride. The nitric acid is volatilized during the drying step.

Treatment of Colloidal Precipitates

•Digestion = Heating the solution for an hour without stirring after precipitate formation. This helps to remove weakly bound water.

• Aging = Storing the solution, unheated, overnight.

• a phenomenon in which otherwise soluble compounds are removed from solution during precipitate formation.

• There are 4 types of coprecipitation:

i. Surface adsorptionii. Mixed-crystal formationiii. Occlusioniv. Mechanical entrapment.• Surface adsorption and mixed-crystal formation are

equilibrium process

• Occlusion and mechanical entrapment arise from kinetics of crystal growth.

• The major source of contamination in coagulated colloids but no significance in crystalline precipitates.

• The surface of our precipitate will contain some primary absorbed ions. As a result, counter ion will also present

• Example: in AgCl example, the precipitate will commonly contain some nitrate.

• This process occurs in crystalline and colloidal precipitate.

• Worse for colloids because the surface area is significantly larger.

1. DigestionWater is expelled from the solid to give a denser mass that has a smaller surface area for adsorption.

2. Washing• Using solution containing volatile electrolyte.• Does not remove much of the primary adsorbed ions

because the attraction between these ions and surface is too strong.

Coagulated colloid is always contaminated to some degree, even after extensive washing.

The error introduced into the analysis from this source can be as low as 1 to 2 ppt.

3. Reprecipitation

• Effective way to minimize the effects of adsorption.

• The filtered solid is redissolved and reprecipitated.

• Only a small portion of the contaminate makes it into the second solution.

• The second solution will be purer.

• This approach is one of last resort since it significantly adds to the analysis time.

• A type of co-precipitation in which a contaminant ion replaces an ion in the lattice of a crystal.

• For this exchange to occur, the two ions have the same charge and their sizes differ by no more than about 5%.

• Example: barium sulfate formed by adding barium chloride to a solution containing sulfate, lead and acetate ion. It is found that lead sulfate formed during the reaction.

• The lead ions replace some of the barium ions in the barium sulfate crystal.

• This problem is encountered with both colloidal suspensions and crystalline precipitates.

• When encountered, your only choices are to remove the interferences prior to precipitation or to select a different reagent.

• Occlusion = a compound is trapped within a pocket formed during rapid crystal growth.

• The amount of occluded material is greatest in that part of a crystal that form first.

• Mechanical entrapment occurs when crystal lie close together during growth.

• Several crystals grow together and in doing so, trap a portion of the solution in tiny pocket.

• While the solvent can be removed, the trapped ions will remain after drying.

• The best way to deal with these problems is to slow things down.

• Using dilute, warm solutions during precipitation gives counter ions time to leave and helps break up pockets.

• Digestion and aging the precipitate provides additional times for this to occur as well.

• A technique in which a precipitating agent is generated in a solution of the analyte by a slow chemical reaction.

• Homogenously formed precipitates are better suited for analysis than a solid formed by direct addition of precipitating reagent.

• Urea is often used for the homogenous generation of hydroxide ion.

• Urea is particularly valuable for the precipitation of hydrous oxides or basic salts.

• Example: hydrous oxides of iron(III) and aluminium, formed by direct addition of base, are bulky and gelatinous masses that are heavily contaminated and difficult to filter.

• In contrast, by using homogenous generation of hydroxide ion, they are dense and easily filtered and have considerably higher purity.

• After filtration, precipitate is heated until its mass becomes constant.

• Heating removes the solvent and any volatile species carried down with the precipitate.

• Some precipitates are ignited to decompose the solid and form a compound of known composition.

• This new compound is often called the weighing form.

• The temperature required for drying the precipitate until constant mass is varies from precipitate to precipitate.

Example:

• Computed from two experimental measurements: the mass of sample and the mass of a product of known composition

• Example:

• Determination of inorganic anions and cations.

• Determination of variety of organic substances.

• Example: determination of nicotine in pesticides and lactose in milk products.

• Gravimetric methods do not require a calibration or standardization step because the results are calculated directly from the experimental data and atomic masses.

• Thus, when only one or two samples are to be analyzed, a gravimetric procedure may be the method of choice.

• It requires less time and effort than a procedure that requires preparation of standards and calibration.

These reagents typically form slightly soluble salts or hydrous oxides with the analyte.

• Two types of organic reagents.

• One forms slightly soluble non-ionic products called coordination compounds.

• The other forms products in which the bonding between the inorganic species and the reagent is largely ionic.

8-Hydroxyquinoline (oxine)8-Hydroxyquinoline (oxine)

• reacts with over 20 different metallic cations

• pH can be used to control selectivity

Dimethylglyoxime (DMG)Dimethylglyoxime (DMG)•Will form complexes with:

Palladium – yellow, weak complex Nickel – bright red, very stable complex

• Since Pd is not very common, DMG is considered specific for Ni.

Sodium TetraphenylborateSodium Tetraphenylborate•Near-specific precipitating agent for potassium and ammonium ions.•The precipitates have stoichiometric composition and contain 1 mole of K+ or NH4

+ for each mole of tetraphenylborate ion.

• Commonly for determining water and carbon dioxide.

• Water is quantitatively distilled from many materials by heating.

• Direct determination, water vapor is collected on any of several solid desiccants, and its mass is determine from the mass gain of the desiccant.

• In direct method, amount of water is determined by the loss of mass of the sample during heating.

• Determination of sodium hydrogen carbonate content of antacid tablet is an example for volatilization of carbon dioxide.

• A weighed sample of finely ground tablets is treated with dilute sulfuric acid to convert the sodium hydrogen carbonate to carbon dioxide:

• This reaction is carried out in a flask connected to a weighed absorption tube containing the absorbent Ascarite II, which consist of sodium hydroxide absorbed on a nonfibrous silicate.

• This sorbent retains carbon dioxide by the reaction:

• Titrimetry – determination of analyte by reaction with measured amount of standard reagent

• Standard Solution (titrant) – reagent of known concentration

• Titration – slow addition of titrant to analyte solution from a volumetric vessel (buret)

• Equivalence Point – reached when amount of added titrant is chemically equivalent to amount of analyte present in the sample.

• End Point – the occurrence of an observable physical change indicating that the equivalence point is reached. Might differ from EquivalencePoint.

• By accurately measuring the volume of titrant that is added, the amount of sample can be determined.

• A buret is used to control and measure the amount of titrant that is added.

Titration- What are the requirements?

1. Reaction must be stoichiometric2. Reaction should be rapid3. No side reactions4. Marked change in some property of the solution

when reaction is complete5. Equivalence point 6. Reaction should be quantitative

• Standard solutions Standard solutions play a central role in all titrimetric methods on analysis.

• The ideal standard solution for a titrimetric method will:

1. Be sufficiently stable so that it is necessary to determine its concentration once.

2. React rapidly with the analyte so that the time required between additions of reagent is minimized.

3. React more or less completely with the analyte so that satisfactory end points are realized.

4. Undergo a selective reaction with the analyte that can be describe by a simple balance equation.

• Primary Standard Primary Standard – highly purified compound used as a reference material

in titrimetry

• Properties: High purity Stable in air Independent of relative humidity Readily availability at modest cost Reasonable solubility in the titration medium Large molar mass so that the relative error associated with

weighing the standard is minimized.

• Secondary Standard Secondary Standard

– do not meet requirements for a primary standard but are available with sufficient purity and properties to be generally acceptable

• Desirable properties of a Standard Solution:

Prepared from primary standard Stable Reacts rapidly and completely with analyte Reacts selectively with analyte

PrimaryPrimary• Potassium Acid Phthalate• KHC8H4O4 (FW 204.23)

• Benzoic Acid• C6H5COOH (FW 122.12)

• Na2CO3 , KH(IO3)2

• Arsenious Oxide (As2O3)

• Sodium Oxalate (Na2C2O4)

• KI , K2Cr2O7 , Fe(pure)

SecondarySecondary• NaOH , KOH , Ba(OH)2

• HCl , HNO3 , HClO4

• Sulfamic Acid (HSO3NH2)

• KMnO4 , Na2S2O3

• Ce(HSO4)4 (FW 632.6)