Introduction - Determination of Anions in Natural and Treated Waters - page 26

and recording electrode potential in millivolts, using a high-impedance millivoltmeter
and plotting a calibration graph on semilogarithmic graph paper (or using a direct reading
pH/ion meter which plots the calibration graph internally).
In potentiometric titration techniques, the electrode is simply used to determine the
end-point of a titration, much as a coloured indicator would be used.
Direct potentiometry is an accurate technique but the precision and repeatability are
limited because there is only one data point. Electrodes drift and potential can rarely be
reproduced to better than ±0.5mV so that the best possible repeatability in direct
measurement is usually considered to be ±2%.
Orion, the leading manufacturers of ion selective electrodes, supply equipment for both
direct potentiometry (EA 940, EA 920, SA 720 and SA 270 meters) and potentiometric
titration (Orion 90 autochemistry system).

Ion selective electrode equipment: Orion direct potentiometry meters
A review of these four meters, in Table 1.6, shows that only the EA 940 has a facility for
multiple point calibration and this places it at the top of their range of direct-
potentiometry meters. This instrument automatically prints out results. It has a memory
for storing calibration information for all the electrodes.
The EA 920 is a lower-priced instrument for two-step calibration. It also has a memory
for storing calibration information. The SA 720 and the portable SA 270 are relatively
inexpensive bottom of range instruments with more limited capabilities.

The Orion 960 autochemistry system (direct potentiometry—potentiometric titration)
This is the top-of-the-range instrument. In addition to direct potentiometry and
potentiometric titrations it has other features not previously incorporated in
potentiometric analysers.
The 960 uses 12 basic analytical techniques. To do an analysis one of techniques is
chosen and modified to suit the requirements of the particular sample. The memory will
accommodate up to 20 methods.
KAP™ analysis is a time-saving technique that eliminates sample preparation and
calibration. Simply weigh sample into a beaker, add water and measure. Aliquots of one
standardising solution or reagent are added automatically to the sample and sample
concentration is determined from the changes in potential observed after each addition.
Every step is performed in one beaker.
Results from KAP analysis are automatically verified in two ways. First a check for
electrode drift and noise is performed at the beginning of each

Table 1.6 Orion pH/ISE meter features chart

Orion pH//Se meters

Feature EA

940

EA
920

SA
720

SA
270

P H

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