Anions in potable waters - Determination of Anions in Natural and Treated Waters - page 473

Source: Reproduced with permission from the Royal Society of
Chemistry [3]
This corresponds to the change in the width (∆t) at a height h as a function of log C
(C=analyte concentration).
Alkalinity is measured by titration with hydrochloric acid to the methyl red end-point.
Similar to a conventional titration, the reaction is monitored after the equivalence point
so that the blank signal (distilled water) shows a higher peak width. The greater the
alkalinity, the smaller is the peak width. The analytical signal was ∆t blank —∆t sample , so
that the slopes of the calibration graphs were positive. To determine total ionic
concentration the salts present in the water sample are transformed into their
corresponding acids on passing through the cationic resin in the H+ form. The acids are
then titrated with sodium hydroxide and phenolphthalein. In this method the minicolumn
is placed in the loop of valve S ′ (Fig. 7.3) which is inserted into the circulating stream in
the emptying position. In the filling position, the minicolumn is regenerated with a
sodium hydroxide stream and washed with distilled water, thus allowing continuous use
for 4-5h.
The reaction is monitored at 552nm (after the equivalence point in the conventional
titration) so that, as in the above system, the peak width is wider for the blank than for the
samples.
As the insertion of a glass calomel microelectrode into the flow injection analysis
configuration introduced no species that disturb the system, pH monitoring can be readily
carried out by inserting the sensor prior to the reagent injection (Fig. 7.3) and calibrating
it with standard buffer solutions at the working flow rate.
Table 7.2 summarises the features of the simultaneous determination of alkalinity, pH
and total ionic concentration.
Canate et al. [3] tested the validity of their procedure by analysing a series of synthetic
samples, the composition and results of which are shown in Table 7.3. In general, the
errors in the determination of alkalinity are higher than those of the total ionic
concentration.
The method was also applied to the determination of these parameters in potable water.
The results (Table 7.4) have been compared with those of conventional manual methods
(eg for alkalinity titration with hydrochloric acid and methyl orange as indicator and for
total ionic concentration, passage of the water sample through a cationic ion exchange
column in acidic form and titration of the eluate with sodium hydroxide and
phenolphthalein as indicator). The results obtained were in good agreement with these
conventional methods.
Hemmings and Shaw [4] have described a flow injection analysis method, based on
methyl orange for the determination of 10-200mg L −1 alkalinity in potable waters with a
detection limit of 4.2mg L −1 . Basic ions such as carbanate and hydroxide cause a
reduction of the red acid components of the buffered indicator solution. Total alkalinity is