Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 118

some of the curved portion at each end.
In Table 2.4 are shown results obtained in alkalinity determinations on non saline
water samples by the manual titration method and the two automated methods discussed
above.
The titration method has greater reproducibility than automated methods, but it is much
slower. About 8h are required for a set of 80 samples, whereas for both the automated
methods analysis takes less than 2h for a set of 80. Also, once the standardisation is
completed, the automated instruments require almost no attention. Both instruments were
equally reproducible, i.e. the coefficient of variation was 2% for four or more samples
analysed on separate days. Special precautions were needed in the automated methods to
avoid carry over effects between one sample and the next.
2.14.2.2 Segmented flow analysis
The National Water Council (UK) [154] has described three methods for determining
alkalinity in river water. These comprise the determination of alkalinities in the range 20-
100mg L −1 as calcium carbonate, the determination of alkalinity at low levels, ie below
20µg L −1 as calcium carbonate, and the use of a continuous flow air-segmented system
for automated determinations in the range 20-300mg L −1 as calcium carbonate in
polluted waters and certain types of trade waste waters. Each of the methods described is
empirical in nature but correct adherence to the stated procedure will enable reproducible
results to be obtained. In the preamble, a number of alternative systems commonly used
to express alkalinity values are compared and some examples of the amounts of alkalinity
contributed by the sodium salts of weak acids are tabulated.
2.14.2.3 Flow injection analysis
Canate et al. [155] have described a flow injection analysis system for the simultaneous
determination of the alkalinity, pH and total ionic concentration of potable water. The pH
measurements are performed by means of a glass calomel microelectrode inserted into
the water stream. The alkalinity and total ionic concentration are determined by flow
injection analysis titrations, acid base reactions and spectrophotometric detection. In
addition, the determination of the total ionic concentration requires the incorporation of
an ion exchange minicolumn. The results obtained are in agreement with those obtained
by conventional methods for the determination of these parameters and are obtained with
higher degrees of automation, which results in a smaller reagent consumption, greater
convenience and higher reproducibility and sampling frequency. A microcomputer is
installed for on-line data collection and the treatment and display of results.
The apparatus consists of a Pye Unicam SP6-500 spectrophotometer connected to a
Radiometer REC-80 recorder, a FIA tron 721 flow cell with a glass calomel
microelectrode connected to a Beckman 3500 pH meter for continuous monitoring of the
pH, a 150×3mm Omnifit minicolumn packed with Dowex 50-X8 cationic resin in proton
form, a Gilson Minipuls-2 peristaltic pump, a Hellman 178.10QS flow cell, two Tecator
L100-1 injection valves, a Rheodyne 5301 three-way valve and a Hewlett