Q. 1. Write a note on errors in Analysis. Classify them.
Ans. The basis of analytical chemistry is reliability, reproducibility and accuracy. Though the
measurements made are systematically and carefully but there is some degree of uncertainty
or error. Error refers to Numerical difference between a measured value and true value.
True Value:- True value of any quantity is that value which we never know but uncertainty
in this value is less then uncertainty in something else which is being compared.
% composition of a standard sample is certified by National Institute of Standard and
Techniques (NIST earlier and Now NB National Bureau of standard). Difference between
the standard value of Result by new method is error.
Classification of errors
Determinate errors
In-determinate errors
1. Determinate Errors: or constant errors
These are the errors which can be avoided or whose magnified can be determined . These
are further sub-classified into:
(a) Operational and personal errors: These errors are due to factors for which an
individual analyst is responsible. These are not connected with method or any
procedure. These are physical in nature and occurs when sound analytical techniques
are not followed.
Eg. Mechanical loss of materials in various steps of analysis:- under-washing/over
washing of ppts.; ignition of ppts at incorrects temperatures. Allowing hydroscopic
materials to absorb moisture; some persons are unable to judge the colour charges
sharply at end point etc.
(b) Instrumental and Reagent error: These arise from faulty construction of balances,
use of uncalibrated or improperly calibrated weights, Graduated glassware’s, other
instruments; attack of reagent on glassware’s/ porcelain etc. results in introduction of
foreign matter.
(c) Errors of Method: These arise from incorrect sampling and from incompleteness of
reaction. In titrimetric analysis there may be failure of reaction to proceed to
completion, occurrence of side reaction, reaction of substances other than the
constituent being determined.
(d) Additive and proportional errors:
Eg. Additive errors are loss of weight of a crucible in which ppt i s ignited and errors
in weighing. By taking samples of different weights error can be revealed. They may
arise from an impurity in a standard substance which lead to an incorrect value for
normality of standard solution.
2. Intermediate or accidental errors:
These errors have slight variations that occur in successive measurements. These are
made by same observer due to cause over which analyst has no control Eg. If a
sufficiently large no. of observations are to be taken then the errors lie on a curve of the
form
small errors occur more frequently than larger ones. Large errors occur relatively
infrequently. These are treated by statistics.
