The potentiometric method also surpasses the others for speed, simplicity, precision and accuracy as
indicated in Table 12.1. Furthermore, it is particularly suited to the continuous monitoring of fluoride
levels in drinking water. The spectrophotometric methods are lengthy because of the time required to
develop a stable colour (up to 1 hour), the alizarin red-S complex being especially poor in this respect.
It was noted, however, that for the three bleaching methods (1–3) the rate of change of absorbance by
the blank closely followed that of solutions containing fluoride, i.e. the difference between the blank
and a sample absorbance is nearly constant.
The final laboratory report favoured the use of the fluoride electrode for routine determinations and the
alizarin-complexone procedure as an alternative or referee method. These two showed the most
consistent agreement during the analysis of a variety of water samples from rivers, wells and reservoirs,
the former giving 100% recoveries with 'spiked' samples.
Analysis of a Competitive Product
Example 12.2.2
A manufacturer of motoring products planned to market a carburettor and combustion chamber cleaner
which would remove oily and other deposits. As a preliminary step in formulating the product, a
complete analysis of a cleaner already marketed by a competitor was required. The competitive
formulation was stated to include aromatic petroleum distillates and butyl cellosolve.
On receiving the sample, its appearance and odour were carefully noted as the inferences drawn at this
stage can frequently save the analyst considerable time and effort and may help to establish the
qualitative composition. In this case, the sample consisted of an orange-brown free-flowing liquid with
an odour both of ammonia and of an organic solvent reminiscent of paint-thinners. Agitation of the
sample caused foaming, suggesting the presence of a soap or other surfactant. On the basis of these
observations, an analytical scheme was initiated which ultimately involved fourteen steps or
determinations. In some instances, decisions to run particular determinations were taken only after the
results of earlier analyses were known. The scheme therefore evolved and was modified partly as a
result of planning and partly by hindsight. A summary of the completed scheme together with the
results is given in Table 12.2. The ammonia was considered to be present almost entirely as the
ammonium salt of oleic acid which thereby functioned as a detergent, the odour of ammonia suggesting
that it was present in slight excess. The proportion of aromatic petroleum distillate, which was
classified as a solvent naphtha, was determined by difference. In view of the surprisingly high water
content, a duplicate formulation was prepared to confirm that a stable one-phase system could be
formed from the constituents. The composition of the competitive product by weight was found to be: