of results against sample weight. Take care in selecting the scale to ensure that the trends are not
obscured. The graph (Figure 2.3) shows clearly that the error is negative and constant (see Figure 2.1).
Figure 2.3
Plot of analyte content against sample weight to
investigate a determinate error.The estimation of the overall precision of a method from its unit operations A frequent problem in
analysis is the estimation of the overall precision of a method before it has been used or when
insufficient data are available to carry out a statistical analysis. In these circumstances the known
precision limits for the unit operations of the method (volume measurement, weighing, etc.) may be
used to indicate its precision. Table 2.6 gives the normal precision limits for 'Grade A' volumetric
equipment.
If the absolute standard deviations for a set of unit operations are a, b, c,... , then s, the overall
standard deviation for the method is given by:
when the individual measurements are combined as a sum or difference (Table 2.1). Conveniently, the
relative standard deviations may be used to express the overall standard deviation when the individual
measurements are combined as a product or quotient.
Example 2.6
Consider as an example the standardization of a solution of hydrochloric acid by titration against a
weighed amount of sodium carbonate. The strength of the hydrochloric acid will be computed from