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Chem 220 - Analytical Chemistry > Classical Analytical Techniques > Flashcards

Classical Analytical Techniques Flashcards

1
Q

define gravimetric analysis

A

quantitative determination of analyte through mass measurement

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2
Q

what are the 4 types of gravimetric analysis?

A
  1. precipitation (analyte precipitated out)
  2. electrogravimetry (final product formed with electrodeposition on either cathode or anode)
  3. volatilization (thermal/chemical energy used to separate compounds to measure mass)
  4. particulate (analyte is already part of the solution and can be separated out from the matrix)
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3
Q

what are the step for gravimetric anaylsis?

A

sample prep > precipitation > digestion > collecting precipitate > drying

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4
Q

explain the precipitation step of gravimetric analysis

A

The accuracy of the analysis depends of the precipitation.

-An excess of precipitating agent is added
- the solution should be cooled when filtering
- changing the polarity of the solvent

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5
Q

explain the crystal growth step of gravimetric analysis

A
  1. first step is nucleation, minimun number of particles must come together to produce microscopic nuclei of solid particles
  2. next is particle growth, nuclei with grow by depositing other precipitate particles to format a crystal

boost growth:
- higher concentration of solution (approaching super-saturation)
- increasing temperature
- adding precipitant in slowly
- having large solution volumes

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6
Q

define homogenous precipitation

A

precipitate is generated slowly by chemical reaction

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7
Q

define precipitation in the presence of an electrolyte

A

ionic compounds are usually precipitated in the presence of an electrolyte. + charged surface attracts anions and repels cations from the ionic atmosphere in the liquid surrounding the particle. (+ charge on the surface due to absorption of cations)

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8
Q

explain the digestion step of gravimetric analysis

A

allowing the precipitation to stand in contact with the supernatant for a long period of time. this promotes slow recrystallization of te precipitate. this increases size and impurities are expelled from crystal.

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9
Q

define supernatant

A

the solution from which the crystal was precipitated from

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10
Q

how can impurities happen in gravimetric analysis?

A

co and post-precipitation

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11
Q

explain co-precipitation

A

this may occur by absorption, where the impurity is bounded to the surface of the crystal.

  • occlusion (impurities are enclosed in crystal)
  • inclusion (impurities occupies a site in the crystal lattice)

minimize:
- washing
- recrystallization
- digestion
- separating of offending co-precident before precipitation

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12
Q

explain post-precipitation

A

slow equilibrium process. it happens when the precipitate is allowed to stand in contact with the supernatant and a second substance slowly from a precipitate with the agent.

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13
Q

explain the collection step in of gravimetric analysis

A

the two main crucibles are:
- sintered (has a disc used to separate precipate, paired with a filter flask being connected to vacuum pump. heat in oven at 95C)
- porcelain (no filter system, must be filtered first with gravity filtration. heat in meker burner at 1200C)

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14
Q

explain the product composition/heating step of gravimetric analysis

A

final precipitate must have known, table composition.
- if hygroscopic, it can absorb water from the atmosphere (water must be removed to weigh accurately)
- ignition is used to change the chemical form (when no constant form after drying)

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15
Q

define equilibrium constant

A

K = concentration of products/reactants (no units)

-reaction favoured if K>1
- solutes = mol/L
- gases = bar
- pure solids and liquids are not included in K expressions

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16
Q

how to manipulate K

A
  • reaction is reversed = recipricol of K
  • reactions are added = K values are multiplied
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17
Q

define solubility product

A

Ksp, specific eq, constant for a reaction where solid salt dissolves to its ions in solution (ex. Hg2Cl2)
- solids are not included in Ksp expression

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18
Q

define the common ion effect

A

a salt is less soluble if one of its constituent ions is already in solution (applying le chateliers principle)

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19
Q

define protic

A

transferring one H+ molecules to another

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20
Q

define acid and bases

A

acid - proton donor, [H3O] increase when added to water

base - proton accepted, [H3O] decrease when added to water

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21
Q

define autoprotolysis

A

when it acts as either an acid or a base, self-ionization

Kw = 1.0 x 10-14

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22
Q

define strong and weak acid/bases

A

strong = completed react to produce H+/OH-
- Ka/Kb is large

weak = partial reaction to produce H+/OH-
- Kb/Ka is small

23
Q

what are the common strong acids to know

A

HCl
HBr
HI
H2SO4
HNO3
HClO4

24
Q

what are the common strong bases to know

A

NaOH
LiOH
CsOH
KOH
RbOH

25
Q

define polyprotic

A

compounds that can donate and accept more than one proton

  • have different K values at each dissociation (K1,K2,K3)
26
Q

define ionic strength

A

measure of the total concentration of ions in the solution

mu = 1/2 (sum of x^2)

27
Q

define activity coefficients

A

concentration x activity coefficient

28
Q

what are the relations with activity and ionic strength

A
  • as IS increase, AC descrease
  • AC approach 1 as IS approaches 0
  • as charge increase, AC go away from 1
29
Q

define interpolating activity coefficients

A

estimating a number between two values on the table

unknown AS interval/ delta AS = known IS interval/ delta IS

30
Q

define titrant and analyte

A

titrant: a reagent, in burette, known concentration
analyte: in the volumetric flask below with the unknown concentration

31
Q

define equivalence point

A

when the reaction is stoichiometry complete

32
Q

define end point

A

where there is a detectable change in the colour of the analyte due to an indicator

33
Q

what is the difference between the endpoint and equivalence point called?

A

titration error. It is poosible to eliminate this with blank titration, where titrant is added to a solution identical to analyte, but not analyte.

34
Q

what are the 3 key points needed for a useful titration?

A
  • a single well defined reaction, known stoich
  • Keq needs to be very large, reaction to go to completion
  • need to select a method of end point detection, indicator
35
Q

what are the different classification of titrations?

A

acid-base: protons being transferred
oxidation: electrons being transferred
complexometric: A and T combine to form a stable complex ion
precipitation: ions combine to form insoluble precipiate

36
Q

what are the 2 ways a titrant can be delivered?

A

direct titration: titrant is directly added to analyte until end point

back titration: excess titratn to rule out analyte, titrated with know concentration solution. where the concentration of an unknown compound is determined by reacting with a known amount of excess reagent.

37
Q

define primary standard

A

reagant that is pure enought to be weighed out and used directly to provide a known amount of moles. titirant is made from this.
-must be 99.9%pure
- should not decompose in storage
- should be stable when drying or heating
- should not absorb atmospheric gases
- should have high molecular weight

38
Q

define secondary standard

A

the true cocnentration must be determine experimentally. is done by reacting with known second reagant and this process is standardization.

39
Q

what are the 4 curves used to determine an end point?

A

typical: pH vs. volume fo titratnt
1st derivative: delta y/delta x
2nd derivative: delta (delta y/delta x)/delta x
Gran Plot: used for linear approx of non-linear relationship , uses titration data before or after the end point, end point is when y=0 (hockey stick)

40
Q

how do indicators work?

A

they are either a weak acid or base that is the protonated form of the indicator is a different color than the unprotonated for of the indicator. change in colour occurs over range of hydrogen ion concentrations. Should choose one where the colour change range is close to equivalence point, and pKa is close to the pH of the equivalence point of titration.

41
Q

what do you have to make sure you have for detecting multiple endpoints?

A

you need to have 2 distinct endpoints.

42
Q

define complexometric titration

A

-complex is any species in solution formed by combo of 2+ simpler species
-focus on metal complexes
- metals ions react with elctron pair donor to form complex

43
Q

define ligand

A

a complex that consist of a central atom or ion bound to an atom or group of atoms

monodentate: bind through 1 atom
multidentate/chelates: binds though multiple atoms, more stable

44
Q

what is EDTA?

A
  • common chelating agent
  • forms 1:1 complexes
  • it is tetraprotic
  • has different Ka values fro all 6 of the binding sites
45
Q

define formation constant

A
  • equilibrium constant for the reaction of a metal with a ligant
  • to e effective, it needs to go to completion
    -can be effected by pH
  • want to be very large

Kf = PRODUCT CONCENTRATION/ [METAL] x [LIGAND]

46
Q

define metal ion indiciator

A

It i a compound whose colour canges when it binds to a metal ion. Most common are Eriochrome Balck T and Xylenol Orange.

Mg-Indicator + EDTA —–> Mg-EDTA + Indicator
RED + colourless —-> colourless + blue (penguins)

47
Q

what are teh 4 titration techniques to use with EDTA?

A

direct, back, displacement, and indirect

48
Q

describe direct titration with EDTA

A

-analye is buffered to appropraite pH, at which the reaction with EDTA is complete

49
Q

describe back titration with EDTA

A
  • excess EDTA is added to analyte and then titrated with standard solution of metal ions
50
Q

describe displacement titration with EDTA

A
  • for metal ions that dont have a satisfactory indicator
  • analyte is treated with excess Mg(EDTA) to displace Mg, which is later titrated with standard EDTA
51
Q

describe indirect titration with EDTA

A
  • for anions that preciaptee metal ions
  • anion can be precipated with excess metal ions
52
Q

define precipitation titrations

A
  • titrations with formation of percipiate
  • endpoint is observed when there is a physical property change
  • argentometric titrations (AgNOS is common titrant)
53
Q

explain the 2 kinds of precipitation titrations

A

Volhard: indirect, uses standardized solutions of AgNOS and Thiocyante to determine concentration of halide. Involves back titration

Fajans: direct method for determining halide, uses anionic dye as adsorption indicator to mark end point/

54
Q

explain how a percipitation titration works in a mixture

A
  • solubilities are significantly different
  • ion that forms the less soluble precipitate with form first
  • if an ion selective electrode is used, then can plot data to see the two breaks in the curve = end point values

Chem 220 - Analytical Chemistry (10 decks)

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