Page 171 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

THE HUMBER OF REPLICATE DETERMINATIONS 4.15

of parallel determinations, a reasonably good assessment can be made by
establishing the variation of the value for the absolute error A obtained for an
increasing number of determinations.

The value for t is taken from the 95 per cent confidence limit column of the
t-tables for n - 1 degrees of freedom.
The values for A are used to calculate the reliability interval L from the
equation:
100A
L=-- per cent
Z
where z is the approximate percentage level of the unknown being determined.
The number of replicate analyses is assessed from the magnitude of the change
in L with the number of determinations.

Example 8. Ascertain the number of replicate analyses desirable (a) for the
determination of approximately 2 per cent Cl- in a material if the standard
deviation for determinations is 0.051, (b) for approximately 20 per cent Cl- if
the standard deviation of determinations is 0.093.
(a) For 2 per cent Cl-:
- - -
Number of ts 100A Difference (per cent)
determinations A=- L=-
n z

(b) For 20 per cent Cl-:

Number of ts 100A Difference (per cent)
determinations A=- L=-
n Z

In (a) the reliability interval is greatly improved by carrying out a third
analysis. This is less the case with (b) as the reliability interval is already narrow.
In this second case no substantial improvement is gained by carrying out more
than two analyses.
This subject is dealt with in more detail by Eck~chlager,~ and Shewells
has discussed other factors which influence the value of parallel determinations.