Page 160 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

4 ERRORS AND STATlSTlCS

solution of a strong base and by precipitation and weighing as silver chloride.
If the results obtained by the two radically different methods are concordant,
it is highly probable that the values are correct within small limits of error.
5. Running parallel determinations. These serve as a check on the result of a
single determination and indicate only the precision of the analysis. The
values obtained for constituents which are present in not too small an amount
should not Vary among themselves by more than three parts per thousand.
If larger variations are shown, the determinations must be repeated until
satisfactory concordance is obtained. Duplicate, and at most triplicate,
determinations should suffice. It must be emphasised that good agreement
between duplicate and triplicate determinations does not justify the conclusion
that the result is correct; a constant error may be present. The agreement
merely shows that the accidental errors, or variations of the determinate
errors, are the same, or nearly the same, in the parallel determinations.
6. Standard addition. A known amount of the constituent being determined is
added to the sample, which is then analysed for the total amount ofconstituent
present. The difference between the analytical results for samples with and
without the added constituent gives the recovery of the amount of added
constituent. If the recovery is satisfactory Our confidence in the accuracy of
the procedure is enhanced. The method is usually applied to physico-chemical
procedures such as polarography and spectrophotometry.
7. Interna1 standards. This procedure is of particular value in spectroscopic and
chromatographic determinations. It involves adding a fixed amount of a
reference material (the internal standard) to a series of known concentrations
of the material to be measured. The ratios of the physical value (absorption
or peak size) of the internal standard and the series of known concentrations
are plotted against the concentration values. This should give a straight line.
Any unknown concentration can then be determined by adding the same
quantity of internal standard and finding where the ratio obtained falls on
the concentration scale.
8. AmpliJication methods. In determinations in which a very small amount of
material is to be measured this may be beyond the limits of the apparatus
available. In these circumstances if the smali amount of material can be
reacted in such a way that every molecule produces two or more molecules
of some other measurable material, the resultant amplification may then
bring the quantity to be determined within the scope of the apparatus or
method available.
9. Isotopic dilution. A known amount of the element being determined,
containing a radioactive isotope, is mixed with the sample and the element
is isolated in a pure form (usually as a compound), which is weighed or
otherwise determined. The radioactivity of the isolated material is measured
and compared with that of the added element: the weight of the element in
the sample can then be calculated.

4.6 SlGNlFlCANT FIGURES AND COMPUTATIONS
The term 'digit' denotes any one of the ten numerals, including the zero. A
significant figure is a digit which denotes the amount of the quantity in the
place in which it stands. The digit zero is a significant figure except when it is
the first figure in a number. Thus in the quantities 1.2680 g and 1.0062 g the