Analytical Chemistry Flashcards
Amount of one chemical specie reacting stoichiometrically with another chemical specie
Equivalents
Equivalents per liter
Normality
Why we grade chemicals?
Quality of chemicals must be consistent with its intended purpose
Substances whose exact solution concentration can be determined by dissolving known weight of a reagent
Primary Standards
Chemical with highest purity (>95%)
American Chemical Society Reagent (ACS)
Chemical with high purity but no established specifications.
Reagent Grade /Analytical Reagent (AR)
Chemicals not adequate for reagent use / unfit for analysis
USP / NF
(US Pharmacopoeia / National Formulary)
Chemicals with high quality by unknown levels of impurities
Laboratory / Chemically Pure (CP)
Chemicals with low quality (<90%) with no established standard set of quality
Commercial / Technical Reagents
Sample Sizes
Hint: mass and volume
Ultra micro: < 1 mg
Micro: 1-10 mg ; < 50 uL
Semi micro: 10-100 mg ; 50-100 uL
Macro: >100 mg ; >100 uL
Sample Constituents
Hint: %mass
Ultratrace: <1ppb
Trace: 1 ppb
Minor: 0.01% - 1%
Major: 1% - 100%
Elimination of an interferent by converting it to a non-interfering form
Masking
Explain Le Chatelier principle
“The position of an equilibrium always shifts in such a direction to relieve an applied stress.”
Explain Common-Ion Effect
“The solubility of an ionic precipitate decreases when a
another solute containing a similar ion is added to the solution”
Explain autoprotolysis
Self-ionization of a solvent to produce both a conjugate acid and a conjugate base
Ideal Precipitate
Low solubility
Filterable
Inert
Known chemical composition
Differentiate:
Occlusion, Inclusion, Adsorption
INCLUSION is an impurity that occupies lattice sites in a crystal.
OCCLUSION is impurity is trapped in a crystal.
ADSORPTION is impurity adsorbed to precipitate surface
Why are many ionic precipitates washed with electrolyte solution instead of pure water?
To preserves the electric double layer and prevents peptization.
Many ionic precipitates are washed with electrolyte solutions instead of pure water to avoid leaching of the electrolytes responsible for the coagulation, which is called:
Peptization
Desiccants in general have which important property?
high chemical or physical affinity for water
Heating of precipitates in the precipitation solution to give the crystals a chance to dissolve and re-precipitate under equilibrium conditions
Digestion
3 Benefits of Digestion
- Redissolve colloids and reprecipitate them-crystal size grows
- Slow recrystallization (less impurities)
- Agglomeration occurs
Supersaturation is an unstable state in which a solution contains higher solute concentration than a saturated solution. Supersaturation is relieved by ____
precipitation of excess solute
Ratio used to predict if precipitate growth is more favored than nucleation.
Supersaturation Ratio
SR = (Q-S)/S
Defines the relationship between mass of sample and precipitate
Gravimetric factor
Gravimetric method results are directly calculated therefore ____
Gravimetric method does not required calibration or standardization
A _____ ionizes totally when dissolved in water.
strong electrolyte
The __ the pKa, the weaker the ___ and the stronger the ___
Higher pKa = weaker acid = stronger conjugate base
Calculation:
pH of Strong Acids and Bases
ACIDS: pH=-log(C,acid)
BASES: pH=14-log(C,base)
How to get pH of:
WEAK ACIDS and BASES
ACIDS: pH=-1/2 log(KaC,acid)
ACIDS: pH=14+1/2 log(KbC,base)
How to get pH of:
BUFFER SOLUTIONS
Henderson-Hasselbalch
pH=-log(Ka) - log(acid/basic component)
Buffer capacity is at a maximum when:
pH = pKa
Also, higher weak acid and conjugate base conc. = higher buffer capacity (more based needed to neutralize)
BUFFERS: Explain the effects of dilution on pH buffers
pH independent of dilution unless concentrations equal to Kw
Physical Characteristics of Acids and Bases
Acids:
sour, sticky, red litmus
Bases:
bitter/sweet, slippery, blue litmus
Water at room temperature usually has a slightly acidic pH. This is largely attributed to what compound(s)?
Carbonic acid
When benzoic acid dissolves in water, it partially reacts with water and ionizes to form benzoate and hydronium ions. In this reaction, what is the conjugate acid?
Hydronium ions
Define a salt in terms of acid and base.
Salt - compound (other than water) produced by the reaction of an acid and a base.
Different Ways to Classify Acids and Bases
- Arrhenius: Acids donates H+, Bases donates OH-
- Bronsted Lowry: Acids donates proton to Bases
- Lewis: Bases donates e- to Acids
Acids donates H+, Bases donates OH-
Arrhenius Acid and Base
Acids donates proton to Bases
Bronsted Lowry Acid
Type of Acid-Base pair in terms of “conjugates”
Bronsted Lowry
The stronger acid / base the ___ the conjugate base / acid
weaker
Bases donates e- to Acids
Lewis Base
Type of Acid-Base pair explaining the formation of complex ions
Lewis acids and bases
Differentiate Electrolytes and Nonelectrolytes
Electrolytes: can conduct electricity when dissolved in water. Lightbulb lights up
Nonelectrolytes: CANNOT conduct electricity when dissolved in water. Lightbulb does not light up
When a can of soda is opened and CO2 is released to the atmosphere, what is the resultant pH of the soda?
Increases
Natural rainfall is slightly acidic at a pH of about 5.5. Why is the pH of natural rain acidic?
H2CO3 forms when CO2 dissolves in rainwater
What is the primary basis in choosing the best indicator for an acid-base titration?
a. acid strength
b. base strength
c. pH at equivalence point
d. temperature
pH at equivalence point
For a strong acid/strong base titration, the pH at the equivalence point is ___
pH is equal to 7 (becomes completely neutral)
For a weak acid/strong base titration, the pH at the equivalence point is ___
pH is more than 7 (becomes conjugate base)
For a weak base/strong acid titration, the pH at the equivalence point is ___
pH is less than 7 (becomes conjugate acid)
The best indicator for the titration of a weak acid and a strong base
Phenolphthalein
Differentiate endpoint and equivalence point
Both are the same thing.
Endpoint = color change
Equivalence point = all analyte reacted
Standard Molarities:
HCl
H2SO4
NaOH
a) HCl - 12.1 M
b) H2SO4 - 18 M
c) NaOH - 19.4 M
Titrant pH Transitions:
Bromocresol Green
Methyl Red
Methyl Orange
Phenolphthalein
Bromocresol Green: 3.8 to 5.4
Methyl Red: 4.2 to 6.3
Methyl Orange: 3.1 to 4.4
Phenolphthalein: 8.3 to 10
Titrant pH Transitions:
Bromocresol Green
Bromocresol Green: 3.8 to 5.4
Titrant pH Transitions:
Methyl Red
Methyl Red: 4.2 to 6.3
Titrant pH Transitions:
Methyl Orange
Methyl Orange: 3.1 to 4.4
Titrant pH Transitions:
Phenolphthalein
Phenolphthalein: 8.3 to 10
Titrant pKa’s:
Bromocresol Green
Methyl Red
Methyl Orange
Phenolphthalein
Bromocresol Green - 4.66
Methyl Red - 5.00
Methyl Orange - 3.46
Phenolphthalein - 9.00
Titrant pKa’s:
Bromocresol Green
Bromocresol Green - 4.66
Titrant pKa’s:
Methyl Red
Methyl Red - 5.00
Titrant pKa’s:
Methyl Orange
Methyl Orange - 3.46
Titrant pKa’s:
Phenolphthalein
Phenolphthalein - 9.00
Measure of acid-neutralizing capacity of water
Alkalinity
Sources of alkalinity
OH-, HCO3- and CO3,2-
Which of the following pairs will form a buffer when mixed together in an aqueous solution?
a. KCl and KH2PO4
b. HCl and KOH
c. Ca(OH)2 and NaOH
d. HF and NaF
HF and NaF
When mixed together, all of the following pairs can form buffers EXCEPT
a. H3PO4 and NaH2PO4.
b. NaH2PO4 and Na2HPO4.
c. CH3CO2H and NaOH.
d. HCl and NaCH3CO2.
e. NaI and NaOH.
NaI and NaOH.
The number of moles of a strong acid (or a strong base) that causes 1.00 L of a buffer to undergo a 1.00-unit change in pH.
Buffer capacity
All half-cell potential (∆E) use the _________ as the reference
hydrogen electrode
Enumerate Weak Acids
Acetic Acid, Hydrofluoric (HF), Cyanic (HCN), Sulfuric (H2SO4), Phosphoric (H3PO4) and organic acids
What are Weak Bases
Ammonia and its derivatives
[NH3, etc]
Chemicals that acts as an acid or base
Amphoteric or Amphiprotic
Determination of nitrogen
Kjeldahl Method
OH-, HCO3- and CO3- Sol’n:
Write Sample Composition and Titrant Volumes
Na2CO3: Vph = Vbcg
NaOH: Vbcg = 0
NaHCO3: Vph = 0
Na2CO3 + NaOH: Vph > Vbcg
Na2CO3 + NaHCO3: Vph < Vbcg
Equivalents:
Acetic acid
1 eq
Equivalents:
Ammonium Sulfate (NH₄)₂SO₄
2 eq
Equivalents:
CN- : Ni
4 eq / mole Ni
Tetracyanonickelate
Equivalents:
For Liebig
CN- : Ag+
2 CN- : 1 Ag,+
[Ag(CN)2],-1
Dicyanosilver ion
OH-, HCO3- and CO3- Sol’n:
NaOH Titrant
Phenolphthalein
OH-, HCO3- and CO3- Sol’n:
NaHCO3 Titrant
Bromocresol Green / Methyl Orange
Start and Endpoints:
Volhard Method
Fajans Method
Mohr Method
Mohr Method: White to Brick Red PPT
Fajans Method: White to Pink PPT
Volhard Method: White to Bloody Red PPT
Indicators:
Mohr Method
Fajans Method
Volhard Method
Mohr Method: Na2CrO4
Fajans Method: Dichlorofluorescein
Volhard Method: FeSCN2+
Titrants:
Mohr Method
Fajans Method
Volhard Method
Mohr Method: AgNO3 (direct)
Fajans Method: AgNO3 (direct)
Volhard Method: SCN-
What sets Volhard Method apart from other precipitation titrations?
a) Excess SCN- titrant
b) AgNO3 back-titration
c) Needs filtration
Why EDTA has its own kind of precipitation reaction?
EDTA forms stable 1:1 complexes with virtually all multivalent metal ions.
Explain chelating agent
Organic compound that contains electron-donor groups to complex a cation.
Explain ligand or coordinating agent
Molecule or ion containing the donor atom in coordination chemistry
Product resulting from the reaction between a metal ion and a ligand
coordination compound or complex ion
Difference between Galvanic and Electrolytic Cells
Galvanic cells, chemical energy is converted to electrical energy to do electrical work such as running an electric motor.
Electrolytic cells require external electric source to bring about nonspontaneous redox reactions.
Oxidation occurs at the _____
While reduction occurs at the ____
Anode, Cathode
What is the role of graphite in dry cell?
Cathode materials in the dry cell
Describe salt bridges
Device that provides electrical contact but prevents mixing of dissimilar solutions in an electrochemical cell.
Describe reductants
AKA reducing agents
Brings about the reduction of another substance while the reductant is oxidizing.
Why do we bubble H2 in electrochemical cells?
1) Make sol’n saturated with H2
2) Hence results are reproducible
Electrode Potential VS Titration:
When are either one used?
Titration: to determine amount of analyte in sample
Electrode potential: to determine activity of analyte in sample
Recite Thermodynamics of Redox Reactions
dG0=-nFE0,cell = -RT lnK
E = E0 - (RT lnQ)/nF
Complete Description (Titrants, Analytes, Equivalents)
Permanganimetry
Iodometry
Iodimetry
Titrants:
- Permanganimetry: Potassium Permanganate (KMnO4)
- Iodometry: Sodium Thiosulfate (Na2S2O3)
- Iodimetry: Iodine (I2)
Analytes:
- Permanganimetry: analytes oxidizable by KMnO4
- Iodometry: I2 oxidized by analyte (I3- liberated ang tina-titrate)
- Iodimetry: analytes oxidizable by I2
Equivalents:
*Permanganimetry (MnO4-permanganate):
–𝑝𝐻 < 2: 5 eq (acidic condition)
–5<𝑝𝐻<9: 3 eq (basic condition)
*Iodometry (S2O3,2- thiosulfate) : 1 eq
*Iodimetry (I2 iodine) : 2 eq
*Fe,2+ and Ce,4+: 1 eq
*Dichromate Process (Cr2O7,2-): 6 eq
Titrants:
Permanganimetry
Iodometry
Iodimetry
Permanganimetry: Potassium Permanganate (KMnO4)
Iodometry: Sodium Thiosulfate (Na2S2O3)
Iodimetry: Iodine (I2)
Analytes:
Permanganimetry
Iodometry
Iodimetry
Permanganimetry: analytes oxidizable by KMnO4
Iodometry: I2 oxidized by analyte (I3- liberated ang tina-titrate)
Iodimetry: analytes oxidizable by I2
when used as a titrant in strongly acidic solution, can serve as its own indicator
Permanganate KMnO4
Standard electrode potential equation
E0 = E,cathode - E,anode
(Tip: reduced-oxidized)
Explain standard reduction potential, E0
Voltage associated with reduction reaction at an electrode when all solutes are 1M and all gases are at 1atm.
Explain cell potential
- The electric current between anode and cathode with difference in electric potential energy
- Measure of tendency of cell reaction to take place
Explain Ion-electron calculation
- Write eqns of the two half-reactions including elections
- Balance reactions, add H2O to balance O&H.
Why changing the stoichiometric coefficients of a half-cell reaction does not affect the value of E0?
Electrode potentials are intensive properties
The more positive standard electrode potential E0 is, the _____
The more positive E0 is, the greater the tendency for the substance to be reduced.
Dumas Method
Jean Bapiste Dumas
Determination of N (crude protein) in food
Winkler’s Method
Determination of Dissolved Oxygen (DO) in water
Wij’s Method
Determination of Iodine number in oils and fats
Metallic reductor:
Jones reductor
- Clemens Jones
- zinc-mercury amalgam
- reduce metal ions to very low oxidation state
Metallic reductor:
Walden reductor
- George Walden
- metallic silver with HCl
- reduce metal (except Cr and TiO) to lower oxidation state
Redox Titrations:
Differentiate Jones and Walden Reduction
Walden reductor is weaker but more selective than Jones reductor
Jones and Walden:
Fe3+ to Fe2+
Ce4+ to Ce3+
MnO4- (Permanganate) to Mn2+
Cr2O7- (Dichromate) to Cr3+
Jones only (Walden can’t):
Cr3+ to Cr2+
Redox Titrations:
Equivalents of the following titrants
Permanganimetry
Iodometry
Dichromate Process
Fe,2+ and Ce,4+
MnO4- (permanganate for Permanganimetry):
–𝑝𝐻 < 2: 5 eq (acidic condition)
–5<𝑝𝐻<9: 3 eq (basic condition)
S2O3,2- (thiosulfate for Iodometry) : 1 eq
I2 (iodine for Iodimetry) : 2 eq
Fe,2+ and Ce,4+: 1 eq
Cr2O7,2- (Dichromate Process): 6 eq
Metallic reductor:
Devarda’s Alloy
- Arturo Devarda
- reduce NO3- to NH3+
When a molecule absorbs/emits a photon, it ____
It is promoted/demoted to an excited/ground state.
Relationship between
absorbance and transmittance?
A = -log T = -log (P/P0)
Relationship between concentration and molar absorptivity
A = 𝜺𝒄b
Which of the following statements is/are correct?
(I) oxidation occurs at the anode
(II) reduction occurs at the cathode
(III) a reducing agent during a redox reaction
all
Which of these 0.1 M solution will give the highest boiling at 1 atm.
(a) table salt solution
(b) sugar solution
(c) barium chloride
(d) potassium chloride
barium chloride
The best conductor of electricity is ____
silver
When ferromagnets are heated above a critical temperature, its ability to posses permanent magnetism disappears. This temperature is called
Curie temperature
Acids donates H+, Bases donates OH-
Arrhenius Acid
