Exam 2 Flashcards
advantages of gravimetric
- not many measurements
- mass is easy to measure
- fairly accurate
- inexpensive
disadvantages of gravimetric
- prone to error
- meticulous
- time consuming
- not suited to many trials
3 types of gravimetric
precipitation
combustion analysis
thermogravimetric analysis
TGA curve
percent original mass as a function of temperature (gravimetric)
methods of precipitating non-water-soluble analytes for gravimetric
- in concentrated acids
- wet ashing
- dry ashing
- fusion
aqua regia
HCl + HNO3
wet ashing
heat + acid
metals converted to metallic cations
dry ashing
heating to extremely high temps in air
metals converted to oxides soluble in acid
fusion
used for rocks and minerals
mix with Na2CO3 and heat to high temp
Na2SiO3 is formed, soluble in water
_________ agent is added in excess in gravimetric precipitation
precipitating
characteristics of good precipitates for gravimetric
- stable at high temp
- not colloidal - settle out
- filterable - large particles
- purity
- very insoluble
- known composition
explain primary adsorbed layer
outer layer of precipitate particle
usually made of ion in excess in solution (precipitating agent)
gives a surface charge, and opposite charged spectators are attracted
explain counter-ion layer
layer of spectator ions attracted to primary adsorbed layer of precipitate
______ held counter-ion layer produces a colloidal suspension
loosely
explain saturation during precipitation
requires supersaturation, but not too much, because supersaturation favors nucleation but not crystal growth
2 steps of precipitate formation
- nucleation - formation of small nuclei of prec.
- crystal growth - combination of nuclei upon collision
how to achieve correct supersaturation in precipitate formation
- high temp
- don’t use overexcess of prec agent
- stir with addition of prec agent
- use large volumes of faily dilute solutions
- homogenous precipitation
explain homogenous precipitation
precipitating agent is formed by a slow reaction in situ, and released slowly
explain digestion (Ostwald ripening)
product is allowed to sit in mother liquor for long period at high temp
increases purity and crystal size
2 methods of precipitate filtration
filter paper
porous crucibles
washing precipitate with a volatile electrolyte solution (HCl or HNO3) does what?
prevents peptization (conversion to colloid)
electrolytes replace primary adsorbed layer, and heat then drives them off
main problem with gravimetric analysis
coprecipitation
3 types of coprecipitation
- occlusion
- inclusion
- adsorption
explain occlusion
type of error
mechanical entrapment of impurities within crystal
(+) error
explain inclusion
type of error
contaminant ions of similar size/charge as desired ions are incorporated into crystal
(+) or (-) error
explain adsorption (coprecipitation)
type of error
contaminants are attracted to surface of crystal
(+) error
most difficult type of coprecipitation to correct for
inclusion
must redissolve and try again
characteristics of rxn suitable for titration
- instantaneous
- equilibrium lies far toward products (high Keq)
- need indicator of completion
titration error
difference between endpoint and eq point
how is titration error estimated?
with a blank titration
2 types of indicators
visual
graphical/instrumental
First derivative curve
𝚫pH/𝚫vol as a function of Vavg
Gran plot
V10^-pH as a function of V titrant
the end point on a gran plot is where…
slope changes
good qualities of a primary standard
pure
stable
low cost
soluble
large MW (smaller error)
explain back titration
analyte + excess reagent –> product + unreacted reagent
unreacted reagent + titrant –> product
moles reacted reagent in backtitration =
moles total reagent - moles unreacted reagent
general category of precipitation titrations
argentometric methods
3 argentometric methods
Volhard
Mohr
Fajan’s
Volhard method
_____ is indicator, _____ is titrant, ______ is analyte
Fe3+ is indicator (blood red)
SCN- is titrant
cation is analyte
Mohr method
_____ is indicator, ______ is titrant, _______ is analyte
CrO4 2- (yellowish) is indicator
Ag+ is titrant
bromide, chloride and cyanide are analytes
Fajan’s method
_______ is indicator, _______ is titrant, ______ is analyte
anionic organic dye (adsorptive) is indicator
Ag+ is titrant
anions are analytes
in Fajan’s method we want colloidal particles because…
more surface area for dye means more vibrant endpoint
acid-base reaction with no H2O involved
neutralization
acid-base reaction related to Arrhenius concept, making water and salt
dissociation
strong acids (6)
HCl
H2SO4
HI
HBr
HNO3
HClO4 (sort of)
strong bases (2)
hydroxides of Group 1 & 2 metals
oxides of group 1 & 2 metals
Kw =
(all)
1.0 x 10^-14 @25°C
[H3O][OH]
KaKb
when is x negligible in ICE table?
when 500Ka is < the concentration of HA
when 500Kb is < the concentration of B-
