ERRORS
Chemical analyses are affected by at least
two types of errors.
One
type,
called
random
(or
indeterminate) error, causes data to be scattered
more or less symmetrically around a mean value.
Random,
or
indeterminate,
errors
affect
measurement precision.
A second type of error, called systematic
(or determinate) error, causes the mean of a data
set to differ from the accepted value. Systematic,
or determinate, errors affect the accuracy of
results.
A third type of error is gross error. Gross
errors differ from indeterminate and determinate
errors. They usually occur only occasionally, are
often large, and may cause a result to be either
high or low. Gross errors lead to outliers, results that
appear to differ markedly from all other data in a
set of replicate measurements.
An outlier is an occasional result in replicate
measurements that differs significantly from the
other results.
SYSTEMATIC ERRORS
Systematic errors have a definite value, an
assignable cause, and are of the same magnitude
for replicate measurements made in the same
way. They lead to bias in measurement results.
Bias measures the systematic error
associated with an analysis. It has a negative sign
if it causes the results to be low and a positive sign
otherwise.
There are three types of systematic errors:
Instrumental errors are caused by nonideal
instrument behavior, by faulty calibrations, or by
use under inappropriate conditions.
Method errors arise from nonideal chemical
or physical behavior of analytical systems.
Personal errors result from the carelessness,
inattention, or personal limitations of the
experimenter.
Of the three types of systematic errors
encountered in a chemical analysis, method errors
are usually the most difficult to identify and correct.
Systematic errors may be either constant or
proportional.
Constant errors are independent of the size
of the sample being analyzed. The effect of a
constant error becomes more serious as the size of
the quantity measured decreases.
Proportional errors decrease or increase in
proportion to the size of the sample. A common
cause of proportional errors is the presence of
interfering contaminants in the sample.
DETECTION OF SYSTEMATIC METHOD ERRORS
a. Analysis of Standard Samples
The best way to estimate the bias of an
analytical method is by analysing standard
reference materials (SRMs), materials that contain
one or more analytes at known concentration
levels.
b. Independent Analysis
If standard samples are not available, a
second independent and reliable analytical
method can be used in parallel with the method
being evaluated
c. Blank Determinations
A blank contains the reagents and solvents
used in a determination, but no analyte.
d. Variation in Sample Size
Statistical Treatment of Random Errors
1. Samples and Populations
2. Properties of Gaussian Curves
a. The Population Mean m and the Sample
Mean x
b. The Population Standard Deviation s
c. Areas under a Gaussian Curve
3. The Sample Standard Deviation: A Measure of
Precision
a. An Alternative Expression for Sample 4.
Standard Deviation
a. Standard Error of the Mean
4. Reliability of s as a Measure of Precision
*Pooling Data to Improve the Reliability of s
5. Variance and Other Measures of Precision
a. Variance (s2)
b. Relative Standard Deviation (RSD) and
Coefficient of Variation (CV)
c. Spread or Range (w)
BLUE - STARCH
Starch has a component called amylose. If
the amylose has a free iodine, it forms a complex
that is why a blue color can be seen.
SPECTROSCOPY
ARGENTUMETRIC TITRATION
Method
Analyte
Mohr
Cl-
Volhard
Cl-
Titration
Reaction
Ag+ + Cl- =
AgCl(s)
Ag+ + Cl- =
AgCl(s) + x’s Ag+
Indicator
CrO42-
Fe3+
x’s Ag+ + SCN- =
AgSCN(s)
Fajan’s
Cl-
Mohr Method:
Titrations are performed only in neutral or
slightly basic medium to prevent silver hydroxide
formation (at pH >10).
2Ag+ + 2OH = 2AgOH(s)= Ag2O(s) + H2O
Or the formation of Chromic Acid at pH < 7
CrO42- + H3O = HCrO4- + H2O
2CrO42 + 2H3O+ = Cr2O72 + H2O
WATER PARAMETERS
-Density
-Temperature
-Dissolved Oxygen
-Total Solids
-Water Hardness
-Salinity
SINGLE-BEAM
DOUBLE-BEAM
Indicator
Reaction
2Ag + CrO42- =
Ag2CrO4(s)
End Color
Red
Fe3+ + SCN- =
FeSCN2+
Red
Dichlorofluorescein
Red
SPECTROSCOPY
Region
UV
(160375nm)
Visible
(3202500nm)
Infrared
Region
Light
Source
Deuterium
Lamp
Sample
Holder
Quartz
Light
Detector
Phototube
Tungsten
Lamp
Glass
Phototube
NaCl, KBr
HIGH SIGNAL TO NOISE RATIO
High signal to noise ratio would mean
greater value for signal. Noise depicts any factors
that would alter tge accuracy negatively. Thus, a
high ratio would mean more accurate readings.