Chemical stoichiometry: Measurement based on
exact knowledge of chemical combination.
Titration: Employs a reaction of known
stoichiometry between a reagent of known
concentration with an analyte of unknown
concentration. Quantity of reagent needed to
completely react with analyte is measured to
determine analyte concentration.
• Acid-base titration
• Redox titration
• Chelometric (complexometric) titration
• Potentiomentic titration
• Spectrophotometric titration
• Amperometric titration
• Conductiometric titration
• Precipitation titration
All methods that involve end point determination
measure volume or mass or no. coulombs of reactant.
Volume
Molarity
No. of moles
mL(1) ×
mL(2) ×
M(1)
M(2)
= mmoles (1)
= mmoles (2)
for 1:1 stoichiometry
mL(1) × M(1) = mL(2) × M(2)
a useful relationship:
M(1) = [M(2) x mL(2)]/mL(1)
Creating a reagent of known concentration:
Primary Standards
A primary standard is a highly purified compound
that serves as a reference material for titrimetric
methods of analysis.
1. High purity
2. Stable in atmosphere
3. Reproducible composition (easily dried, etc.)
4. Modest cost
5. Reasonable solubility
6. Relatively large molar mass
The National Institute of Standards and Technology
(NIST) provides lists of primary standards and is also
a source of these standards.
Why NaOH(s) is not a primary standard.
1.
2.
The purity of NaOH(s) is not high.
Pellets of NaOH will take moisture out of the
atmosphere (enough to form droplets of NaOH solution).
3.
4.
NaOH(s) readily reacts with CO2 in the
atmosphere:
2NaOH(s) + CO2(g) → Na2CO3(s) + H2O
The molar mass of NaOH is only 40.0.
However, solutions of NaOH (protected from CO2
and H2O in the atmosphere) are excellent secondary
standards and are widely used. Therefore, the
preparation and standardization of NaOH solutions
with KHP is a very common laboratory procedure.
H
Potassium Hydrogen Phthalate (KHP)
Molecular Mass, M= 204.23
Potassium Hydrogen Phthalate (KHP)
C8H5O4K or C6H4(COOH)(COOK)
molecular mass, 204.22, is a primary standard.
KHP standardization of NaOH solutions:
Dissolution
C6H4(COOH)(COOK)(s) →
C6H4(COOH)(COO)-(aq) + K+(aq)
Titration
C6H4(COOH)(COO)- + OH- → C6H4(COO)22- + H2O
Must be able to know exactly when the equivalents of
NaOH added equals the equivalents of KHP present.
How is this 1:1 stoichiometry achieved?
The KHP titration with NaOH in Experiment 2 uses
phenolphthalein (HIn) as an indicator to detect the
end point.
At the start of the titration, the solution is acidic due
to the KHP and the indicator is in its colorless form
(HIn). In base, the indicator (In-) turns pink.
Two reactions take place during the titration:
HP- + OH- → P2- + H2O
HIn + OH- → In- + H2O
(1)
(2)
Obviously, the indicator must be much less
concentrated than KHP to achieve its role of
indicating the stoichiometric point of (1).
Errors in Chemical Analyses
All measurements have associated errors and
uncertainties (see Heisenberg Uncertainty Principle).
It is important to understand these uncertainties and
to know the maximum error/uncertainty that a
measurement can tolerate.
The variability of a measurement cannot be
determined from a single measurement, therefore
multiple measurements (replicates) are made to check
reproducibility.
Precision = reproducibility of a measurement
The replicate measurements are very rarely exactly
the same. Therefore, a central value is taken as the
best estimate.
For measurements xi, where i = 1,2,3…, the mean or
average value, x , is given by
⎛ N ⎞
⎜ ∑ xi ⎟
x = ⎝ i =1 ⎠
N
Common terms used to convey precision, such as
standard deviation, variance, and coefficient of
variation, are based on deviation of measurements
from the mean:
d i = xi − x
Accuracy = closeness of a measurement to the true
or accepted value, xt
Absolute error, E, is given by
E = xi − xt
Relative error, Er, is given by
Er
(
xi − xt )
=
× 100
xt
Types of errors:
Random (or indeterminant): data are scattered more
or less symmetrically about the central value
(precision reflects random error)
Systematic (or determinant): constant deviation (in
sign and magnitude) from central value
(accuracy affected by systematic error)
systematic errors lead to bias in results whereby a
series of replicate measurements may all be high or
low
Kjeldahl Determination of Nitrogen:
a) Decompose organic sample in hot conc. H2SO4
b) Converts N is sample to NH4+HSO4c) Remove excess H2SO4, add NaOH and collect
NH3(g) in H3BO3 soln.
d) Determine NH3 by acid-base titration
Often used to determine N content of fertilizers, soils
Pyridine ring is more difficult to convert to NH4+
than other organic forms of N. Requires added
catalyst and more digestion time. This can give rise
to systematic error.
Analyst 1 - good precision, good accuracy
Analyst 2 - poor precision, good accuracy
Analyst 3 - good precision, poor accuracy
Analyst 4 - poor precision, poor accuracy
A systematic error is apparent in the data of analysts
3 and 4, probably due to poor reactivity of nicotinic
acid.