Chemistry Reference
In-Depth Information
Fig 2.8
Manifold for the simultaneous determination of pH, alkalinity and total
ionic concentration, MR=methylred, pp=phenol phthalein (for details
see text)
Source: Reproduced with permission from the Royal Society of
Chemistry [155]
Packard HP-85 microcomputer with HP 3478A analogue to digital converter and an HP-
1B 82937 interface.
Manifold
Fig. 2.8 shows the reversed FIA configuration used. It involves the use of two detectors; a
potentiometric detector ME (a glass microelectrode) located before the confluence of the
sample with any reagent for the continuous monitoring of pH, and a spectrophotometric
detector for the monitoring of the acid base reactions. Two serial injection valves are
placed along the water stream. The first of these, S
′
, is aimed at incorporating the ion
exchange microcolumn (IE) into the stream in order to determine the total ionic
concentration. This valve is synchronised with the selecting valve S and in the filling
position of valve S
′
, selects channel 1 (methyl red in an acidic medium) which fills the
loop of V
i
(ordinary injection valve 1126µL). The alkalinity of the water sample is
determined in this position. In the emptying position, S
′
incorporates the mini-column IE,
S selects channel 2 (phenolphthalein in a basic medium) and the determination of the
total ionic concentration is carried out. The flow rate adopted was 0.75ml minL
−1
and the
reactor lengths L
1
=2.40cm and L
2
=110cm.
The microcomputer collects the data from both detectors and, by means of the
computer program described below, processes the data and gives the pH, alkalinity and
total ionic concentration for each sample.
Computer program
The microcomputer collects the pH and absorbance values (measurement frequency,
0.2s) which are introduced into a BASIC program. The data acquisition process starts at a
time from the reagent injection equal to the travel time, so that as little memory as
possible is used. Data treatment is
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