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Lecture 4 - Gravimetric Analysis
- Precipitation Methods – dissolved analyte converted to sparingly soluble precipitate.
a. readily filtered b. low solubility c. converted to product of known composition (heat)
Ex. Excess of oxalic acid (H 2 C 2 O 4 2-) added carefully to measured volume of Ca2+. (1) In basic sol’n:
Ca^2 +( aq ) +C 2 O 4 ( aq )→CaC 2 O 4 ( s )
(2) CaC 2 O 4 (s) is collected in a filtering crucible then dried
(3) CaC 2 O 4 ignited to produce calcium oxide:
CaC 2 O 4 ( s )∆→CaO(s)+CO (g)+CO 2 ( g )
(4) CaO(s) cooled, weighed
(5) Original concentration of Ca2+^ computed
- Volatilization Methods
a. Analyte is volatilized at suitable temperature b. Volatile product is collected and weighed
- Precipitates – Particle Size & Filterability
a. Colloids – (d = 10-7^ to 10-4^ cm) -invisible to naked eye -not easily filtered, don’t settle out of solution
b. Particles – (0.10 mm or greater) -spontaneously settle out of solution -readily filtered and washed free of impurities -more desirable
*Size of particles influenced by relative supersaturation of the solutions in which is formed:
Relative Supersaturation = S
Q − S
Where Q = concentration of solute, S = solute’s equilibrium constant
S
Q − S high – rate of nucleation increases
S
Q − S low – particle growth dominates, excluding
nucleation
*Nucleation dominates – results in a large # of very fine particles
*Particle growth dominates – small # of larger particles
- Gravimetric Calculations
x 100 W
% Analyte^ W sample
= analyte
CaCl 2 + 2 AgNO 3 → 2 AgCl 2 ( s )+Ca(NO 3 ) 2
2 molAgCl
1 mol CaCl x FWAgCl
FW CaCl wt CaCl 2 =wtAgCl x^22
Gravimetric Factor (F)
F – Relates mass of product to mass of analyte, stoichiometry
b(FWof substance B)
F =a(FWof substance^ A)
where a and b are the coefficients of A and B, respectively
Ex. Calculate the % Phosphorus in a 0.3516 g detergent sample. Final yield is 0.2161 g Mg 2 P 2 O 4
%Analyte = x 100 MassSample
MassAnalyte
a. Mass P =
1 mol P
x30.97gP 1 mol Mg 2 P 2 O 4
x^2 molP 222.57g Mg 2 P 2 O 4
0.2161 gMg P O x^1 mol^ Mg^2 P^2 O^4 2 2 4
Mass product Gravimetric Factor
= 0.0614 g P
% P = =
3516 g sample
0614 g P 17.10 %
or…
Ex1:Phosphate is precipitated from its solution with
ammonium molybdate, as (NH 4 ) 3 [PMo 12 O 40 • xH 2 0]. Since the precipitate does not have a constant composition with regard to water content, it is dissolved in ammonia and the molybdate is precipitated with Pb(NO 3 ) 2 , as PbMoO 4.
a) What is the value of the gravimetric factor for the calculation of %P?
b) If the final precipitate weighs 0.100 g, what is the weight of P in the initial sample?
Ex2: A 0.2025 g sample consisting of only BaCl 2 and KCl required 20.25 mL of 0.1200 M AgNO 3 solution for the quantitative precipitation of chloride. Calculate the %Ba and %K in the sample.
Ex3: A 0.4994 g sample of a hydrate of CuSO 4 • xH 2 O, is heated to a constant weight of 0.3184 g (total loss of water). Calculate the value of x.
