Page 159 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

MINIMISATION OF ERRORS 4.5

4.5 MINIMISATION OF ERRORS
Systematic errors can often be materially reduced by one of the following
methods.
1. Calibration of apparatus and application of corrections. Al1 instruments
(weights, flasks, burettes, pipettes, etc.) should be calibrated, and the
appropriate corrections applied to the original measurements. In some cases
where an error cannot be eliminated, it is possible to apply a correction for
the effect that it produces; thus an impurity in a weighed precipitate may be
determined and its weight deducted.
2. Running a blank determination. This consists in carrying out a separate
determination, the sample being omitted, under exactly the same experimental
conditions as are employed in the actual analysis of the sample. The object
is to find out the effect of the impurities introduced through the reagents and
vessels, or to determine the excess of standard solution necessary to establish
the end-point under the conditions met with in the titration of the unknown
sample. A large blank correction is undesirable, because the exact value then
becomes uncertain and the precision of the analysis is reduced.
3. Running a control determination. This consists in carrying out a determination
under as nearly as possible identical experimental conditions upon a quantity
of a standard substance which contains the same weight of the constituent
as is contained in the unknown sample. The weight of the constituent in the
unknown can then be calculated from the relation:
Result found for standard Weight of constituent in standard
-
-
Result found for unknown x
where x is the weight of the constituent in the unknown.
In this connection it must be pointed out that standard samples which
have been analysed by a number of skilled analysts are commercially available.
These include certain primary standards (sodium oxalate, potassium
hydrogenphthalate, arsenic(II1) oxide, and benzoic acid) and ores, ceramic
materials, irons, steels, steel-making alloys, and non-ferrous alloys.
Many of these are also available as BCS Certified Reference Materials
(CRM) supplied by the Bureau of Analysed Samples Ltd, Newham Hall,
Middlesborough, UK, who also supply EURONORM Certified Reference
Materials (ERCM), the composition of which is specified on the basis of
results obtained by a number of laboratories within the EEC. BCS Reference
Materials are obtainable from the Community Bureau of Reference, Brussels,
Belgium. In the USA similar reference materials are supplied by the National
Bureau of Standards.
4. Use of independent methods of analysis. In some instances the accuracy of a
result may be established by carrying out the analysis in an entirely different
manner. Thus iron may first be determined gravimetrically by precipitation
as iron(II1) hydroxide after removing the interfering elements, followed by
ignition of the precipitate to iron(II1) oxide. It may then be determined
titrimetrically by reduction to the iron(I1) state, and titration with a standard
solution of an oxidising agent, such as potassium dichromate or cerium(1V)
sulphate. Another example that may be mentioned is the determination of
the strength of a hydrochloric acid solution both by titration with a standard