CHEM 220 - Midterm 2

2 Classical Analytical Techniques

Gravimetric Analysis — where the mass of product is used to calc the quantity of the og analyte, most accurate and precise

Volumetric Analysis — chemical procedure used for determining the concentration of a solution

• In a titration, a reagent (titrant) is volumetrically measured to determine the amount needed to complete a chemical reaction with the analyte.

Types of Gravimetric Analysis

Precipitation Gravimetry: analyte of interest precipitated out following a chemical rxn

Electrogravimetry: final product is formed through electrodeposition

Volatilization Gravimetry: thermal or chemical energy is used to separate compounds so that their masses can be measured

Particulate Gravimetry: when the desired analyte is already part of the sol and can be separated out from the matrix

Steps of Gravimetry Analysis

The analyte is selectively converted to an insoluble form.

Sample prep → precipitation → digestion → collection of precipitate → drying

Precipitation

Steps to ensure max separation

• An excess of precipitating agent is added

• The sol should be cooled while filtering, solubility of most compounds decreases w/ decreased temp

• Change the polarity of solv. polar salts precipitate in nonpolar solv and vice versa

• Substance is pure, has known comp, easily filterable, & stable

Crystallization

Precipitation comes about from the growth of crystals after a precipitating agent has been added to sol. In precipitation process

1. Nucleation - min # of particles must come together to produce microscopic nuclei of solid particle

• Higher [ ] of sol will promote rate of nucleation

2. Particle growth — nuclei will grow by depositing other precipitate particles to form a crystal. Technique to promote particle growth:

• Increasing temp of sol to increase the solubility of sol and decrease supersaturation

• Adding precipitant slowly to decrease formation of super-saturation pockets

• Large sol V

Homogeneous Precipitate & Ionic compounds

In Precipitation process

In homogeneous precipitation, a precipitate is generated slowly by a chemical rxn.

Ionic compounds are usually precipitated in the presence of an electrolyte.

Digestion

• Precipitation to stand in contact w// the mother liquid/supernatant for some period of time

• Promotes slow recrystallization of the precipitate which allows particle size to increase and impurities to be expelled

• Contamination of the precipitate can come about by two processes: co-precipitation and post-precipitation

Co-Precipitation

Can occur through adsorption — where impurity is bound to the surface of the crystal. Part of digestion process

• occlusion - as particle grows in size, the impurities are enclosed in the crystal

• inclusion — impurity occupies a site in the crystal lattice

To min co-precipitation: washing, recrystallization, digestion, method of addition of 2 reagents, and separation of the offending co-precipitant before precipitation

Post-precipitation

Part of Digestion process

• Slow eq process

• Occurs when precipitate is allowed to stand in contact w/ the supernatant & 2nd substance slowly forms a precipitate w/ the precipitation agent

• Involves supersaturated impurity that crystallizes only after sol has been allowed to stand

• If know that post-precipitation will occur, no digestion should be done

Collection of Precipitate

Once the precipitate has been formed, it needs to be separated from the supernatant. Different crucibles can be used to accomplish this. The two main types are sintered and porcelain

• Sintered — small glass contained w/ fritted disc that separates precipitate from sol + vacuum pump

  • 3 types: fine, medium and coarse

  • Heating is done in oven

• Porcelain — solid w/ no filtering system so sample sol must be filtered using gravity filtration b/w transfer to porcelain

  • Heating is done in Meker burner

Product Composition

Part of collection of precipitate

If precipitate is hygroscopic — can absorb water form atmosphere.

Ignition (strong heating) is used to change the chemical form of some precipitates that do not have a constant comp after drying at moderate temps

Volumetric Titration

Equivalence point: the point at which the rxn is stoichiometry complete

End point: the point where a detectable change is observed

• Detectable change: change in voltage, observing color change, and absorbance of light

Titration error: the diff b /w the end point and the equiv point

Blank titration: possible to estimate titration error w/ blank titration. It is when the titrant is added to a sol where is identical to the analyte sol but contains no analyte

Must be a single and well-defined rxn b/w analyte and titrant, Keq must be large (Keq » 1), and need to select method of end point detection as close to equivalent as possible

Classification of titrations - types + titrant

Types of rxns:

• acid base: protons are transferred b/w analyte and titrant

• oxidation: e- are transferred b/w analyte and titrant

• complexometric: analyte and titrant combine to form stable complex ion

• precipitations: ions in sol combine to form an insoluble precipitate

  • also called aregntoemtric titration

How the titrant is delivered

• Direct titration: the titrant is directly added to the sol of analyte until end point is detected

• Back titration: a known excess of titrant/reagent is added, the unreacted reagent is then titrated w/ a second sol of known [ ]

Primary vs Secondary Standard

Primary Standard — reagent that is pure enough to be weighed out and used directly to provide a known # of moles. The titrant is prepared from a 1 standard.

Secondary Standard — used as a the titrant. [ ] is determined w/ a 1 standard. The process where the true [ ] of a reagent is determined by reacting it w/ a known second reagent is known as standardization.

Requirements for a primary standard

• Must be pure (99.9% or better)

• Must be readily available

• Should not decompose under ordinary storage conditions

• Should be stable when dried by heating or vacuum - drying is required

• Should not absorb atmospheric gases

• Should have a high MW

Optimal requirements: should dissolve in water & completely dissociate in aq sol

Correct way to standardize if the titrant is the primary standard (solution we know the [ ] of):

1. Prime burette w/ primary standard

2. Transfer some of the 2nd standard to a clean, dry beaker

3. Use appropriate volumetric pipet to get an aliquot and transfer to 250 mL Erlenmeyer flask

4. Add some indicate and some DI water

5. Titrate to end point

6. Dispose of 2nd standard that was in the beaker, remix sol and then take a small amount again

7. Repeat steps 3-6 until there are at least 3 trials in agreement (+- 0.2 mL)

Correct way to standardize if the titrant is the secondary standard (solution we do not know the concentration of)

1. Prime burette w/ 2 standard

2. Transfer some of the 1 standard to a clean, dry beaker

3. Use appropriate volumetric pipet to get an aliquot and transfer to 250 mL Erlenmeyer flask

4. Add some indicate and some DI water

5. Titrate to end point

6. Dispose of 2nd standard that was in the beaker, remix sol and then take a small amount again

7. Repeat steps 3-6 until there are at least 3 trials in agreement (+- 0.2 mL)

Solubility Product (Ksp) & Common Ion Effect

Solubility Product (Ksp): The specific eq. constant for a rxn in which a solid salt dissolved to give its constitute ions in sol

• solids are emitted

Common Ion Effect: salt is less soluble if one of its constituent ions is already present in sol, application fo le Chatelier’s Principle

Strong Acids and Strong Bases

Strong acids: HCl, HBr, HI, H2SO4, HNO3, HClO4

Strong bases: NaOH, LiOH, CsOH, KOH, RbOH, R₄NOH

Protic vs Autoproteolysis vs Polyprotic

Protic: transfer of a proton (H+) b/w molecules

Autoproteolysis: self-ionization to act as acid or base

Polyprotic: compounds that can donate or accept more than one proton

Ionic Strength & Activities

Ionic Strength (u): measure of total [ ] of ions in sol

The eq constant does not take into effect the ionic strength on a chemical rxn. So, [ ] can be replaced w/ activities (A) - [ ] multiplied by its activity coefficient (opposite y)

• As u goes up, y goes down

• The smaller the ion size, the more important A effects become

Interpolate: estimate a # b/w 2 values on a table

Monoprotic Definitions

Weak acids/bases: react only ‘partially’ to produce H+ / OH-

Conjugate acid-base pairs: an acid & base that differ only through the loss or gain of a sing proton (conjugate base of WA is WB)

Formula [ ] (F): total # of mols of a compound dissolved in a liter

Zwitterion - molecule that contains an equal # of positive and negative charged functional groups

Buffers

Mixture of acid and base — there must be a comparable amount of conjugate acid and base to have buffering effect

If you mix A moles of WA w/ B moles of its conj. base, then the moles of acid remain close to A, and the moles of base remain close to B

• little rxn occurs to change either [ ]

Isoionic Point vs Ioselectric Point pH

Isoionic Point: the pH produced by the pure, neutral, polyprotic acid

Isoelectric Point: the pH at which the avg charge of the polyprotic acid is 0

• for a diprotic AA, the isoelectric pH is halfway b/w the 2 pKa values\

• Utilized in 2D electrophoresis

Water Hardness

Titration 1: Red + EDTA → Blue

• acquire known V of water sample; contains both Mg2+ and Ca2+

• buffer to pH 10, then titrate w/ standardized EDTA

• uses eriochrome black T and titrate from red to blue endpoint

• total H2O hardness, [Ca2+] + [Mg2+] is known

Titration 2: Water + NaOH → Red + EDTA → Blue

• Taken known V of water sample and add NaOH

• Base reacts w/ Mg2+ to form Mg(OH)2, pH increases to 12

• Use hydroxynapthol blue & titrate from red to blue endpoint

• [Ca2+] via titration and [Mg2+] via subtraction

Volhard Titration

Type of precipitation titration

Indirect method & involves back titration

1. Excess AgNO3 is added to sample to precipitate halide

2. Precipitate filtered off & filtrate is kept

3. Fe indicator, titrated w/ SCN- to orange end point

4. One drop past equiv point gives red color

Fajans Titration

Type of precipitation titration

Discrete method for determining halides, uses adsorption dye (negative indicator)

1. before equiv point, there is excess halide

• some halide ions will be adsorbed on surface of precipitate

• surface change of particles is negative

2. after equiv point, there is excess of silver ions

• some ions will adsorb onto precipitate

• causes a positive surface charge

• anionic dye adsorbed onto surface of particles & color changes

Yellow → Pink

Titration Curves

Typical Curve: pH vs V of titrant, end point is where slop is the greatest/ steepest

1st derivative plot: end point of titration is inflection point

2nd derivative plot: end point is b/w the positive and negative point

Gran plot: end point V is where y = 0

Complexometric Titrations

Complex - any species formed by the combo of 2+ similar species, that can also exist independently in sol

• Most metal ions react w/ e- pair donors — ligands

Monodentate binds to metal ions through only 1 atom

Multidentate binds to metal ions through more than one atom — chelating ligands/agents

EDTA

• Acid commonly used for complexometric titration

• Hexadente ligation (6 potential binding sites)

• All metal ions form 1:1 w/ EDTA

• Neutral EDTA is tetraprotic, H4y

• For titration to be effective, needs to go to completion which is affected by pH

  • There is a min pH needed for titration of metal ions to be complete (assuming 0.01 M EDTA)

2 common metal ion detectors: eriochrome black T & Xylenol orange

EDTA titration techniques

Direct Titration: analyte buffered to appropriate pH & when titrated w/ standard EDTA

Back Titration: excess EDTA is added to analyte, then excess EDTA is titrated w/ standard sol of metal ions

Displacement Titration: analyte titrated w/ excess Mg(EDTA)2-, which displaces Mg2+, which can then be titrated w/ EDTA standard

Indirect Titration: used for anions w/ excess metal ion, titration w/ excess EDTA, then back titrate EDTA w/ standard