1
Errors in Chemical Analysis
• Examples:
1. Three measurements were made of the percent by
mass of chloride in an impure salt sample: 60.53%,
60.37%, 60.29%. Calculate the mean, the mean
deviation and the sample standard deviation.
2 A
l
i
i d
t
l
th t
2. An analysis was carried out on a sample that
contained exactly 20.0 ppm iron. The results
obtained by the method gave:
19.4, 19.5, 19.6, 19.8, 20.1, 20.3 ppm
Find the mean, mean deviation, standard deviation,
CV and relative error of the results.
1
Module 1
Introduction to Analytical Methods
•
Basic definitions and explanations
•
Different ways of reporting results:
– %(w/w), % purity, concentration
2
SI units
Quantity
Units
Abbreviation
Mass
kilogram
kg
Length
meter
m
Time
second
s
3
Temperature
Kelvin
K
Amount of substance
mole
mol
Electric current
ampere
A
Luminous intensity
candela
cd

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2
Common units
•
Litre (L)
– 10
-3
m
3
•
Weight
– Mass x gravity
•
Molarity (M)
– Moles of solute per litre of solution
– mol/L
•
Density (
d
)
– Mass/volume
4
Common units
• Concentration
–
Analytical concentration
(
c
)
•
Concentration of substance initially present
•
Mass/volume, moles/volume, etc.
–
Equilibrium concentration
(
[
species
]
)
Equilibrium concentration
(
•
Concentration of substance in a solution when it has
come to equilibrium
•
Units of molarity are indicated by square brackets
around name of species
–
If a substance does not dissolve in the solution, what
would it’s equilibrium concentration be?
5
Common units
•
Percent concentration
–
Weight percent (
w/w
)
•
Mass solute/mass solution X 100%
–
Volume percent (
v/v
)
V l
l t /
l
l ti
X 100%
•
Volume solute/volume solution X 100%
–
Weight per volume percent (
w/v
)
•
Mass solute/volume solution X 100%
6